-A98 .3W6>b

Bulletin 465

September, 1942

Cytological and Genetic Studies of Sterility
in Inbred and Hybrid Maize

Frances J. Clark

(Eonnctticut
^0Wttlt«ral ^xperintcttt Station

CONTENTS

Page

Introductiox 705

asynapsis 709

Inversions 711

Traxslocations 716

POLLEX AXD OVCLE SeMI-LeTHALS 718

Degexerativk Changes 720

Discussion 723

Summary 724

Literature Citeu 725

Gytological and Genetic Studies of Sterility
in Inbred and Hybrid Maize'

Frances J. Clark

A DISCUSSION of sterilitj'' in maize has applications to both practical
and theoretical problems. One aspect is related to the increased
growing of hybrid corn in recent years entailing more extensive
breeding programs for the production of inbreds and hybrids. It is
of practical value to determine and eliminate causes of sterility in in-
bred lines in order that uniform production may be assured in hybrid
corn.

Partial or complete sterility may be manifested in corn by poorly
filled ears, aborted pollen, and defective seeds, or a combination of
these effects (Figures 1 and 2). Hereditary and non-hereditary fail-
ure of seed formation have been described by Mangelsdorf (22). Cer-
tain environmental conditions, such as relative growth rates of tassels

Figures 1 and 2.

Figure 1. Photomicrograph of pollen, stained with a weak solution of iodine,
showing two types of aborted pollen grains. Smaller grains are darker in the pho-
tograph, and empty grains have no included starch. The large grains are normal.

Figure 2. Semi-sterile ear (right) and normal ear of maize. The semi-sterile
ear has about 50 percent aborted ovules.

and ears, insect injury, diseases and growing conditions, may also
change the development of ears and tassels. Usually such factors
may be separated from sterility caused by aberrant chromosome be-
havior or genetic factors for lethal effects on gametes. However, if
the silks receive insufficient pollen, the resulting ear often cannot
be distinguished from a partially sterile one. Sterility which is in-
herent in the genetic or chromosomal constitution of the plant will be
transmitted to succeeding generations through the pollen or egg cells

1 This investigation was supported in part by a gra.nt from the Rockefeller Foun-
dation.

TOG Connecticut Expemnent Station Bulletin 465

or tlutuigli lioth. iuul usually results in relatively consistent percent-
ages of aborted ovules or pollen in the progeny.

The results of inbreeding and of crossing inbred lines have been
discussed by Jojies (14. 15, 1()), Jones and Mangelsdorf (18). Jenkins
(13). Si^rague (37) and others. The problem of hybrid vigor and
aspects of heterosis have been reviewed by East (10). Singleton (35)
and Jones (IT). A decrease in size and vigor of a ])lant generally
accompanies inbreeding and the approach to homozygosity. Certain
lines of maize become difficult to maintain because of such weakening,
and are particularly susceptible to unfavorable growing conditions.
Hybrids from such inbreds may, however, be quite vigorous. This
fact has been given theoretical consideration by Jones (IT). Causes
of poorW filled ears of corn and particularly M^eak plants have been
determined for some stocks by detailed genetic and chromosomal anal-
yses, but such methods have not been generally ap])lied in corn
breeding programs.

Problems of theoretical interest are also associated with a cyto-
logical study of sterility in maize. Singleton (35) and Dobzhansky
and Rlioades (9) have suggested cj'tological and genetic teclmiiiue^
for determining and locating factors responsible for heterosis. Burn-
ham and Cartledge (T) have outlined a method for locating factors
involved in resistance to disease. Theoretical significance can also be
given to the variations found among a large number of plants, as in a
breeding program. Ty])es of plan.ts that are aberrant in leaf growth,
chloropliyll ])atterns. ])lant form and fertility of ganu^tes are often
observed. The frecpiency of occurrence of such changes constitutes a
si^ecial juolilem to be attacked only in experiments designed for the
purpose. However, when large numbers of ])lants are gi-own it is pos-
sible to observe the kinds of changes whicli occui- and to inv(>stigate
the causes.

The folh)wing i'e|)ort considers cytological and genetic studies of
>om(' types of sterility encoimtei'cd in iul)reds and hybrids. Most of
the i'ej)ort is based on progenies from semi-stei'ile ears discox'ered in a
fieh] coin yield test in li)3.S. Open-pollinated ears wei-e harvested
i'roiii \iiri('(i('s iind h\brids grown in the test ploi for calculations of
yield. .V total of ll.'.)l(') eai-s wei-c liarNc^tcd I'roui ;'.ll viirieties and
hybrids included in this test. Oi" this nunibci'. -Jf. including 'J'J dif-
ferent liybi'ids were classified as semi-stei'ile on the basis of the general
aj)|K'arance of the eai's. Tweuty-foui' of these were grown in tlu' next
genei'ation. Of this number it was found th:d the sterility was trans-
nntted to succeeding genei'ations in \-2 of the progenies: niiu' of the
progenies li;id no t r;insniission (d' llie sterility: three of the progeiues
had p(»ssible transmission (d' the stciility. and two hiivc not as yet been
grown. These I'esults gi\<' ;i niaxiniuni occnn-eni'c of ch:in,'ies ciuisini:
stei'ility (d" IT in I l.!>ir,. nv idiout I per l.:i(iii. Thi- cnnnot be used a'-
a measure cd' the IVe(|iiciicy <d' mutntions. the term being iise(l in a
broad sense, since the dala were obtnineil from so m;iiiy dillei-«'nt lines,
but is an iip|)ro.\inuit ion (d' t he frefpiemy. .Vs such, it may be compared

Sterility in Inhred and Tlyhrtd Maize 707

in a <>-eneral way only with values found in experiments (lesi<>-ned to
determine the frequencj'^ of naturally oceurrino- mutational chanfjes.
The cytological and genetic analyses of the progenies from the semi-
sterile ears were made in order to determine the kinds of naturally
occurring changes which wva^j be detected more ])articularly with
cytological methods.

Another series of semi-sterile ears used in this investigation was
obtained from various hybi-ids grown in other test plots. Six of these
were from hybrids with the inbred Connecticut 243 ; five were from
other field corn hj^brids, and one was from a sweet corn hybrid. The
sterility was found to be transmitted to six of the progenies grown.
Five of the progenies showed no transmission of the sterility, and one
a possible transmission of the sterility. The progeny from the semi-
sterile ear having questionable transmission in this group of hybrid
ears is difficult to classify both genetically and cytologically since this
hybrid, one Avith Connecticut 243, segregates a striped plant which is
partiall}'^ ear-sterile, does not always mature and is not inherited as a
simi3le recessive.

The transmissible sterilities identified in the two groups of semi-
sterile ears were found to be due to three general causes: (a) trans-
location between non-homologous chromosomes, (b) inversions of re-
gions of chromosomes and (c) genetic factors for pollen or ovule semi-
lethals in which no cytological change could be found. A summary
of the results obtained in these series of semi-sterile ears is given in
Table 1. Six diiferent translocations have been found, one inversion
and three definite pollen or ovule semi-lethals, with the addition of
three possible semi-lethals. The progenies in which transmission of
the sterility was definitely established showed semi-sterility in ap-
proximatel}^ 50 percent of the plants, whereas in progenies with
doubtful transmission of the sterility about 25 percent of the plants
were classified as segregating defective gametes.

The translocations and the inversion that have been identified are
different from any that have been previously reported. This indi-
cates that the changes are of a spontaneous nature and not due to any
contamination from genetic stocks grown at the same time.

Five inbred lines of dent corn have also been examined. Hy-
brids with one of the inbreds were found to be semi-sterile, and two
of the inbreds Avere characterized by some pollen abortion. K c3^to-
logical examination was made to determine the causes of the sterility.
Two inbreds were found to have variable amounts of asynapsis; one
was found to have a small inversion : one was found to be homozygous
for a translocation, and one was found to be mosaic for a chromoso-
mal change. The last inbred is also characterized by degenerative
changes in ear development, but any relation between such develop-
ment and the cytological changes has not been established. One of
the inbreds having asynapsis has also been found to segregate a plant
with a striped chlorophjdl pattern, but the segregation is not that of a
Mendelian recessive.

ro8

Connecticut Experiment Station

Bulletin 465

'^

u

fo

M

Uh

«

c

.9

2I

K-J

t-^ c; — = — 0

-r

c

0!

c

"O

— 0

-O

■§1

0 ^i 0 0 '4-1 ^

^

-t- -^1

0)

0 g

Q TO

x:

^

O

-TD

>~£;

^_

1 •4— +-

■o

-1- — '■ r- 1 — ^ iJ^

w a, in

oo ^i c

l/l (13

c

0

zl

CN

-. 0000 3

0 ^ ~^ ^

0)

■*"

CO

On

c
— 0

•♦-'

^'ui

Q)

^ in

"1 0.

^^

■§1.
0 C

ro

0 ON 0 0 CO 0

o

r^ <M

u

■t

"D

0)

i-l.

■a

-^

oc

-J- — ■*-

?1

<-< ij-j -^ -t- CI 00

E-cF

<U 1^ in

CO ^ ro

on *i: c

m fp

S !2 = > §

5 r3 ^ •^' 5

a;

M-ffl

«j

ts

and
of
ster

eth;

Its

call

0 c

ui 0

u.

0!

plan

len

ent

s

lie 1
plar
logi

u

- — 0 UJ J- ii 0

— ' 0.

_^ 0 u -^ 5 > ,^ i;

G S C. M'-5 I/) ° 0 6'
C f' E l-

<l

0

u

4>

^ J3 S nJ S ./,;''= ^ C 1

r->.*^ V,,— ^ •- QJ ^'^

c

E ?^ « >=S S

.-• < u. i 0 .--

y,'A < ^^^y, 1

b/: -^

-73 "ti

g

0:

to

bo

(i>

rt

K

X

^

<i>

M <D

u >

0

r- M

.ti i*

•I I-l

O 10

m

Sterility in I fibred and Hyhrkl MaiEe ■ 709

The results of the cytological and j:>enetic study of the iiibreds and
hybrids will be discussed under the following divisions: asynapsis,
inversions, translocations, pollen and ovule semi-lethals, and degener-
ative changes. Unless otherwise noted, all preparations made were
aceto-carmine smears from anthers fixed in a fixing solution composed
of 3 parts 95 percent ethyl alcohol and 1 part glacial acetic acid, ac-
cording to the method of McClintock (24).

ASYNAPSIS

The failure of the chromsomes to maintain close synapsis in the
meiotic divisions has been found in a number of plants (3, 11, 12, 19,
20, 28, 31). Beadle (1, 2) found such a condition in maize to be in-
herited as a simple recessive gene {as). Variable numbers of gametes
are aborted as a result of the action of this gene since one or more
chromosomes fail to be included in the nuclei of part of the develop-
ing gametes. Aii examination of the pollen in inbred C243 revealed
that abou.t 50 percent of the pollen was of aborted grains — small or
without any included starch grains. A cytological examination of
meiosis indicated that the cause of this partial pollen sterility was in
the lack of complete synapsis of the chromosomes. It is doubtful
whether this condition is the same as that reported by Beadle, referred
to above, since tlie asynapsis is not as complete, and the ears show very
little if any sterilitj?'. Counts of microsporocytes at the diakinesis
stage of meiosis are given in Tables 2 and 3.

Table 2. Counts of the Numbers of Cells Having Univalents at Diakinesis
AND Metaphase. Fkom Two Plants of Inbred C243.

Number of Number of Percent of

cells with cells with Total cells with

Plant 10 bivalents univalents cells asynapsis

38-392-] '■

Diakinesis 107 234 341 68.6

Metaphase 25 138 163 84.7

42G 17-3

Diakinesis 26

34 60

56.7

Table 3. The 406 Cells From Column
TO THE Number

Two of Table 2 Classified
OF Univalents.

According

Plant 2

Number of univalents
4 6 8 10

12

20

38-392-1 178
42G 17-3 20

130 48 13 1
10 3 1 0

1
0

1

0

It is evident that most of the cells had only one or two chromo-
somes which did not maintain their synapsis up to the time of diakine-
sis and metaphase. In manj^ cells in which complete separation of
some homologues was observed there were also one or more of the
bivalents associated only at the ends. The number of chromosomes
with partial asynapsis had no relation to the number of univalents in
the cell. When univalents are present in the meiotic divisions, they are

10

Connect'fCKt h'.rjni'nDciif Station

liullctrn 465

often not inchulod in either ol' the two (hiutihter niielei due to their
lagging on the spindle or being left otf the spindle figure. Small
mieronuclei Avhich become pycnotic develop from univalents excluded
from developing telophase nuclei. This is simihir to the l)ehavior of
fragment chromosomes (2()). Quartets that were counted with ref-
erence to the presence of micronuchM are tab\dated in Tal)le 4.

Tablk 4. Numbers of Quartets with Microxuclei axd the Xumhek ok Mkim-
xucLEi IX the Microspores.

Plant

38-392-1

42C 17-1

42C 17-2

Quartets with

micronuclei

30

113

110

Quartets with no

micronuclei

20

25

10

Microspores witli :

0 micronuclei

119

249

104

1

01

227

210

2

12

60

82

3

6

11

21

4

1

5

3

5

1

0

0

Percent of microspores

with micronuclei

40.5

55.0

65.8

Tlie number of micronuclei in a (juartet is indicative of the auu)unt
of pollen abortion to expect, although a chromosome left out in a pre-
vious division is distributed at random to one of four microspores and
may l)e inclnded in a developing nucleus. Two pollen counts were
made of two plants of C243, one having 76 i)ercent small and empty
grains, the other having 47 percent small and empty grains. The ])ol-
len abortioi) resulting froui spores which faih^d to receive a t-omplete
clii-omosouie couij)UMuent uiay be variable if tiu' auu)unt of asynapsis
is (l('|)('n(h'nt upon envii-onuiental couditious. Souic iiulication of this
was found in couipnrlng |)lauts growu iu tiie Held and in the green-
house. l*owers aiul Dald {•2!>) also foiuid e\idence that the amount of
asj'^napsis in uuii/.e may (lc|)('nd on cn\ironnicn(al conditions.

In addition to the plants of C'24.'5 which wei-e I'ound to have
asynapsis as described aboNc. iiil)r('d C14. three [)lauts of which were
examined, was found to ha\c sonic asynapsis. Four i)lant> oi" the hy-
brid ('24;i X C14 were I'ound to ha\e conipleto ])airing of thi' chrouu)-
somes, ami likewise complete symqisis was found in one plant each of
('14 X ('<>77, Ldg X 02-^'^^ and"d!4:'> X miniatui-e seed. 'I'hesc results
show that tlu^ asynai)sis of both ( '"2 b' ami CI I disappeai- upon hy-
lu'idi/ation. If it is gene-conlrollcd. it niii>t be rcccs^ixc ami the two
lines must have dill'erent genes I'or asvnapsi-.

The transmission (d' t!i<' tendency toward asynapsis was tested by
e.xamining pollen of plants ol' the progeny of hackcros.ses with C'24;i.
The ba<kcr(».ss, 'I\'.\ •'' ("Jl:'. ~- II) ga\c -j:'. plants with normal pollen
;ind H; plants with ahoiteil pollen. The backcross 24:^ X I •'^f (<)^0
X 24;> I ga\c UI> phiiits with noi'iiial polh'U and eight plant- with

Sterility i)i Inhred and Ilybrid Maize 111

aborted pollen. A 1 :1 ratio would be expected from a simple gene
ratio. Transmission is effected through both pollen and eggs but,
since these numbers are small, it cannot be stated definitely that the
inheritance is that of a simple recessive gene. Partial asynapsis seems
to occur frequently in inbred plants and may well have some physi-
ological basis for expression.

INVERSIONS

Two inversions were found during the examination of the inbreds
and hybrids. One of these included the centromere, as was found by
the prophase synapsis of the chromosomes and by the absence of
bridges and fragments in the meiotic divisions. The other inversion
does not include the centromere, as was shown by the cytological ex-
amination. Types of crossovers in inversions, and gametes produced
are discussed by McClintock (26), Darlington (8) and Sturtevant and
Beadle (39).

The inbred Iowa La has been grown for about 14 years. Cyto-
logical examination of plants of this inbred indicated that a small
inversion was present although the inbred might have been expected
to be free from aberrations after the period of inbreeding. One plant
was examined cytologically in the summer of 1938 to determine the
morphology of the chromosomes. Bridges and small fragments were
found at the meiotic divisions, and it was concluded that the plant
was heterozygous for a small inversion. The progeny of this plant
was grown and 10 plants of the next generation were also found to be
heterozygous for an inversion since bridges and fragments were
found. It was concluded that the inversion is a short one since the
fragment is so small, but the location on the chromosomes was not
observed. There is some evidence that it may be the small terminal
inversion, not including the centromere, on the short arm of chromo-
some 8 (previously reported by McClintock, 25), and the amount of
crossing over is similar to that observed for the chromosome 8 in-
version. The frequency of crossing over in this inversion was deter-
mined by counts of the types of bridges and fragments observed at the
meiotic divisions (Table 5).

Table 5. Counts of Bridges and Fragments at the First Meiotic Division in
Four Plants Having a Small Heterozygous Inversion.

Percent

Plant

Non-cross-

Bridge plus

Bridge, no

Fragment,

Two

cross-

no.

overs

fragment

fragment

no bridge

fragments

Total

overs

3

2"

4

1 + 1?=^

67

29.9

3

P

2

0

40

15.0

■?

0

0

0

9

22.2

8

1-M?

2

0

50

22.0

Anaphases

836-6 46

-7 34

-9 7

-11 38

Telophases

-7 322 17 1 48 T 390 17.4

1 Fragment possibly attached to bridge.

2 No bridge; bridge in one?

2 One with bridg-e ; one with no bridge.

712

Connecticut E^-periment Station Bulletin 465

C.V\

mV.

%

%.

10

#♦*

-^r*-^-^

■jrj^ ■"■■-■'■ ■■ " -ti*'^

Ijm i:i,.. 1(M3.
(l'.\lil;inali'iii on uiipusilf ]);iL;t'.)

Stenllty in Iiibrecl arul Hybiid Maize 713

A single crossover within the inverted region is represented in
these counts by a bridge and a fragment. If the fragment is included
in one of the developing telophase nuclei, only a bridge is observed; if
the bridge should break early, only a fragment Avould be observed.
A double crossover (4-strand) is represented by a bridge and two
fragments at the first meiotic division. The occurrence of other types
of crossovers cannot be demonstrated from the division I configura-
tions, but the presence of bridges in the second meiotic division indi-
cates that they may occur in this small inversion (Figures 10 and 11).
Six cells were found (of a total of 140 counted) having a bridge in
one of the sister cells resulting from the first meiotic division. A three-
strand double crossover, one crossover in the inversion and one cross-
over outside the inversion, results in a bridge in one of the sister cells
at the second meiotic division. A triple crossover results in a bridge
in both sister cells of division II, one sporocyte of this type being ob-
served among the 140 counted. It is evident that numerous cross-
overs take place within the limits of this small inversion.

The second inversion was found during the examination of the
semi-sterile ears from the field corn test described above. The inver-
sion was found in the following hybrids: 4-8 X B,4, 540 X 4-8, 187-2
X 4-8A and Iowa Hybrid 13. Since 4-8 was one parent of the first
three hybrids, it may be that this inbred carried the inversion and
that it was found in the open pollinated ear of Iowa Hybrid 13
through contamination. Further examinations of the inbreds used in
the making of these hybrids will have to be made before it is certain
that the inbred 4-8 was heterozygous or homozygous for the inversion,
or whether it occurred spontaneously in a generation prior to making
the hybrids.

The limits of the inversions are shown diagramatically in Figure
3, and photographs are shown in Figures 12 and 13. Since the in-
version includes the centromere, crossovers do not result in bridges and
fragments at the meiotic divisions. However, duplicated and defi-
cient gametes are formed as a result of crossing over, and aborted
pollen grains and semi-sterile ears are characteristic. Inversions

Figure 10. Chromosome bridge at anaphase of the second meiotic division in
one of the sister cells. See text for explanation. Plant 40-836-5 (Pedigree Iowa
La.) Magnification X 500.

Figure 11. Bridges at anaphase of the second meiotic division in both sister
cells. See text for explanation. Plant 40-836-5. (Pedigree Iowa La). X 500.

Figures 12 and 13. Two sporocytes heterozygous for the inversion on chro-
mosome 1. The loop configuration is at the lower part of each photograph. The
arrows point to the centromeres. Plant 39-1428-2. (Pedigree 4-8x^4). V950.

Figure 14. Heterozygous translocation between chromosomes 1 and 2. A
drawing of this translocation is given in figure 9. Plant 38-382-1. (Pedigree 237
Q2L4A). X 750.

Figure IS. Photomicrograph of a sporocyte from the same plant which had the
sporocyte shown in figures 9 and 14. In this cell there was no translocation between
chromosomes 1 and 2. The numbers 1 and 2 'on the photograph are placed by the
respective chromosomes. The arrows point to the centromere regions. Plant 38-382-1.
(Pedigree 237 Q2L4A). X 750.

714

Connecticut Experiment Station Bulletin 405

i l_

Figure ^. A. Normal chromosome 1. The arrows
mark the approximate limits of the inversion.

B. Inverted chromosome 1.

C. Outline drawing of synapsis of normal and in-
verted homologues at mid-prophase of meiosis. Plant
42G 8-1 (Pedigree 540X4-8).

The centromere region in all drawings is indicated
by the ligliter open circle.

including centromeres are rarely reported (Sturtevant and Beadle
39, p. 189). They may occur as frequently as other inversions, but
the method of detecting them in maize by the presence of anaphase
bridges and fragments does not alloAv the identificatiou of those
inversions includin.g the centromere. The amount of jioHeu abortion
in ])]auts from two of tlie ]iyl)rids is given in Table (>.

Table 6.

Counts of the Pollen Produckd by Four Plants Having a Hktkr-
ozYGous Inversion on Chromosome 1.

Pedigree

Plant

Normal pollen
grains

Empty
grains

Small
■grains

Total

Percent
aborted

187-2 y 4-8A

40-899-15

691

647
786

269
252
318

6
12
11

966

*)1]

1115

28.5
29.0
29.5

540 X 4-8

42G 2-3

42G 8-1

8-3

539
768
813

365
146
227

38

942
914
1040

42.8
16.0
22.0

Since a >iiiM|(. ci-o.ssoNcr williiii llu'. inverted region results in two
normal clironial ids and two delicient cliromatids from one sporocyte,
the anioiinl of crossing o\-er is ap]»i'o\iinatel \' doubh^ th(> amount of
])ollen alioilion.

II a jilaiit lielero/ygoiis for (he inxcr^ioii nn cliiiimosome 1 is
-eir.Ml. (lie fesuHing progeny -lionld |ia\-e one foiii'tji plants with uor-
mal clirojuosonie 1, one-foiirlli liomo/.ygons foi- (he invcrled ciiroMU)-
some, 1 and oiie-lialf hetei'ozygous for (he inversion. If a |)lan( heter-
o/ygoiis for (he invei'sion is used as either (he male or female paicnt

Sterility In lvl>i'Cil (ind riijhrid Maize 715

Tablk 7. Progeny of Plants Heterozygous for Inversion on Chromosome 1.

No.

of plants

No

of semi-

Percent with

Type of cross

witho

Lit inversion

sterile plants

Total

inversion

Hybrid ear open-polinatec

4-8 X R4

11

s'

19

42.1

540 X 4-8

22

18'

40

45.0

187-2 X 4-8A

15

14^

29

48.3

Iowa Hybrid 13

22

3

25

12.0

Selfed ears from

187-2 X 4-8 A

2

15^

17

88.2

10

6

16

37.5

5

7

12

58.3

3

9

12

75.0

Heterozygous inversion

X or on normal

11

8

19

42.1

14

7

21

33.3

8

11

19

57.9

Totals

123

106

229

46.3

1 One plant examined cytologically.

2 Two plants examined cytologically.

in a cross with a normal stock, one-lialf the resulting progeny should
be heterozygous for the inversion and one-half should have normal
chromosomes. When any of these three tyi^es of crosses is made, there
should be one-half the progeny with normal pollen and one-half the
progeny with pollen segregating aborted grains, the latter represent-
ing plants heterozygous for the inversion. Ears of most plants
lieterozygous for this inversion are recognizable as semi-sterile. Trans-
mission of the inversion was tested by making the crosses described
above. The data are given in Table 7. Classitications were based on

Figure 4. Outline drawing of the synapsis at mid-
prophase of meiosis in a plant heteroz5'-gous for a trans-
location between chromosomes 1 and 2. An inbred
(696-3c) was found to be homozygous for the translo-
cation. Chromosome 1 has prominent chromomeres at
the end of the short arm and a heterozygous knob on the
short arm. Chromosome 2 has a homozygous knob on
the long arm. Plant 38-1210-8 (Pedigree 696-3c X
Pamunkey).

716

Connecticut Experiment Station

Bulletin 465

jDollen or ear examinations and in some instances Avere verified by
cytological examination. Crosses between the inversions from differ-
ent sources will be made to determine if thej^ are identical. A further
proof of this would be in findinf^ the inversion in the inbred 4-8.

TRANSLOCATIONS

Seven different translocations have been identified during tlie
analysis of the semi-sterile ears. One of the translocations is of par-
ticular interest since 696-3c, a Lancaster field corn inbred, was found
to be homozygous for it.

It was observed that all plants resulting from crosses with this
inbred had semi-sterile ears. The hybrid 696-3c X Pamunkey was
examined cytologicalh^, and the translocation was found to be present
in the heterozygous condition (Figure 4). Approximately 0.4 of the
sliort arm of chromosome 1 was exchanged with about 0.5 of the long
arm of chromosome 2. The translocation had occurred in the inbred
and became homozj^gous during inbreeding. Since no apparent growth
change occurred in the inbred as a result of the translocation it was
not detected until hybrids with it were grown and the translocation
became lieterozygous. Roberts ('32), however, found evidence stat-
istically that some homozygous translocations may affect the de-\-elop-
ment of the maize plant in the rate of maturing and total growth.

Figures 5, 6 and 7.
Oulliiir drawings of syiiapscd chromosomes at tlu- niid-prupliasr of incio:

I'IGURE 5. Heterozygous translocation between chromosome 1 and C).
The translocation occurred near the ends of each cliromosome. Chromo-
some 0 is attached to the nucleolus and is cliaracterized in this plant by a
small and a large knob on the long arm. Plant 40-894-10. (Pedigree 682-8c
X 24.3).

i-'iGUUE 6. J fc'terozygfius translocation hetwren clnomosomes 2 and 5.
The translocation occurred about mid-way on the long arm of chromosonie
2 and very near the centromere on chromosome 5. t'lu-omosome S in this
plant was heterozygous f<ir a small knoIi on the lung arni. Plant ,V)-732-4.
(Pedigree ri()9-.SX2).

I''lGURE 7. Heterozygous translocation between chromosomes (> and 9.
The translocation occurred on chromosome 6 beyond the small knob and on
chromosome 9 near llie end <if the sliort arm. Plant .39-724-(i. (Pedigree
474-7X474-5).

Sterility in Inbred and Hyhrid Maize

717

t'M-

';|^«»4#^fo'-*w«*'. '?*^»|6

Figure 8. Photograph of "streaked dwarf" type of plant. The upper
leaves show the typical streaking and the lower leaves are normal. This
plant had not yet lost the upright habit of growth. Plant 38-15-8. (Pedi-
gree 243 X 14) .

(1"^ ConiKctirutE.rper'iment Station Bulletin 4Q5

In a breeding" i)r()£rrain siunll chaiiiios would proliably not influence
the selection within inbred lines.

The six other translocations were found in the progenies of the
semi-sterile ears. A sweet corn hybrid was found to have a translo-
cation between chromosomes 6 and 9. Translocations found among
the field corn hybrids were between chromosomes 1 and G. 1 and 7,
2 and 5, 4 and 6, and G and 8 (Figures 5, 6 and 7). The presence of
chromosome G in four out of six of the translocations occurring spon-
taneously may suggest that the naturally occurring breakage and reat-
tachment of chromosomes is not random. Each of the 10 chromo-
somes would be expected to occur 1.2 times in six translocations taking
place at random between the chromosomes. The presence of chromo-
some G in four of the translocations is significant (P<.02). but the
number of translocations should be increased before any general con-
clusion maj^ be drawn.

The disjunction of the chromosomes having translocated regions
usually results in approximated 50 percent of the gametes containing
duplications and deficiencies, although some low-sterile translocations
have been reported (4). The disjunction of translocated chromosomes
in wheat maj" not be ranchjm according to the report of Thompson and
Hutcheson (40). Their calculations of the effect of crossing over on
sterility, however, do not take into account that crossing over occurs
in a four-strand stage. A representative count of the pollen produced
by plants heterozA^gous for the translocation between chromosomes 6
and 4 is given in Table 8.

Tahi.e 8. Types of Pollen* Produced by a Plant Fro^i Iowealth Hybrid 129,
Heterozygous for a Translocation.

Plant

Normal pollen

Small grains

Empty

Total

Percent

aborted

42G n-1

11-2
12-1

A7U
f.64
470

248

235
154

231

188
170

955

1(187

sun

50
39
41

One of tlic cars har\ested in tlic licld corn test and chissitiod su-
perficially as .semi-sterile a]i|)eared to be a mosaic. Half the ear was
well filled with kernels while the other half was only partially filled.
Seed from both halves of the ear was planted, and although 11 plants
were examined cytologically (at diakin(\sis) and ]iollen was examined
from 20 ];lants, there was no c\'idonce of any semi-sterility. Kars
from these j>lants wei-c normal. It was concln(h'd that the apparent
semi-sterilit}- of ])art of the ear was (hie to environmental or physio-
logical ciinses.

POLLEN AND OVULE SEMI-LETHALS

Instances of a type of senii-stci-ilily in which cei'tain pi'oportions
ol' (lie ganietes arc dcrcc|i\e but in which no \isil)le chromosome aber-
rations can be dete<'ted have been rei)oi1c(| in mai/e, (2;>, liO, 33, 34,
30). These factors may be locatecl on the chromosomes by means of

iStcfUlty iih Inhred mid Ili/hrid Marie 719

lmka<ie with known genes, and may thenuselves be used as genetic
characters useful in locating other genes. Three types of senii-letlials
similar to those reported by others and not associated with detectable
chromosome aberrations were found in the inbred and hybrid ears ex-
amined.

One of the pollen -segregating types was from the variety Kato.
In the first generation from the open-pollinated ear there were 13
plants with normal pollen and one plant that was segregating empty
pollen. In succeeding generations there were obtained 11 plants with
segregating pollen (eight of these coming from a cross with the pollen
parent having the segregating type) and 11 plants with normal pol-
len. Three of the segregating plants were examined cytologically,
and it was found that the chromosomes were all apparently normal
with no evidence of any delicienc}^ inversion oi- translocation. Any
classification for the semi-sterility is based on ])ollen examinations
since plants segregating empty pollen have normally filled ears. The
pollen is about 30 percent aborted. Since it was found that the factor
causing aborted pollen is transmitted through the pollen, it is sus-
pected that a chromosomal aberration is present that may be small and
w^as not detected in the cytological examinations.

From the hybrid W209-13K X ^lultiple Leaming 1936, a small-
pollen type was isolated. In this instance plants that are segregating
the small pollen (smaller in size than normal pollen grains but well
filled with starch) also have some ovule abortion and produce ears
appearing semi-sterile. From the open-pollinated ear of the hybrid,
there were 11 plants with normal pollen and eight with segregating
pollen. There were 16 plants w^ith normal pollen and five with seg-
regating pollen in the next generation from a selfed ear. A cross bj^
mi gave 18 plants with normal pollen and two with segregating pollen.
As shown in Table 9. there is no alteration of the expected 1 :1 ratio

Table 9. Crosses of su X Two Plants Segregating Small Pollen and stt.

Plant Sn kernels stt kernels Total

41-559-3 202 177 379

-28 95 92 187

Total 297 269 565
Expected 288 288

indicating no linkage of the factor for small pollen and su. This
factor for small pollen is different then from that reported b}^ Single-
ton and Mangelsdorf, referred to above, as spi reported by them is
closely linked with 5-(/.

There was no evidence of any chromosomal deficiency, inversion
or translocation from a cytological examination of a plant segregating
the small pollen.

Another type of semi-sterilit}-, similar to the lethal ovule (lo)
factor reported by Singleton (33) and Singleton and Mangelsdorf

720 Connecticut ExpeHment Station Bulletin 465

(3G) was found in the hybrid G77 X 112-1. These inbred parents are
the same as those from which Biirnham isolated the ^?a gene (5, 6).
The semi-sterility found here was characterized by ears which are
semi-sterile in a^^pearance due to the abortion of about 50 percent of
the oA'ules. but the pollen produced b}^ these plants is normal. It
was determined that this is not the same lo previoush' reported by
making crosses with sa. There was no disturbance of the Su : su ratio
indicating no linkage with su on chromosome 4, The numbers ob-
tained were 227 jSu and 70 su (25.1 percent).

In addition to the three t^'pes of pollen and ovule lethals just
discussed, there were also three types which have not l:)een grown be-
yond the first generation from the open-pollinated ears. For this rea-
son ihej were classified in Table 1 as having questionable transmission
of the sterility factors. From the hybrid La (Idt) X Ki' (Osf)
there were obtained 12 plants with normal pollen and two plants hav-
ing some anthers with normal pollen and others with empty pollen.
Twenty-two normal plants and four plants segregating empty and
small grains were obtained from the hj'brid 244. From another hy-
brid, lowealth Hybrid AQ,, 18 norinal plants were obtained and nine
plants with small or empty pollen grains. Some of the latter plants
were very small with narrow leaves, and these may have represented
deficiencies of some sort since no mature seed was obtained from
them.

DEGENERATIVE CHANGES

In addition to a reduction in size and vigor of maize plants dur-
ing inbreeding, abnormalities such as small seeds, chlorophyll defic-
iencies, dwarfed plants, sterile tassels and silkless ears may be fountl.
Some of the vaidations indicate that mutational changes have oc-
curred since the inheritance is that of simple recessive or dominant
genes. When such types of plants occur during inbreeding, they may
or may not be eliminated depending upon whether plants carrying the
factors in the heterozygous condition are selected for continuing the
lines. Other aljnormalities occur sporadically, do not behave in in-
heritance as Mendelian recessives or dominants and may result ulti-
mately in the loss of a ])arti('ular inl)r('(l line.

An example of tlic latter type of change appeared in lino 1-7-1-2,
Connecticut 242, (10). A streaking of the leaves occurred, superficial-
ly resembling si diseased condition, and the line wa.s ultimately lost.
Another tNpe of sti'caking of the leaves has apjieared in the inbred
C243, Mhich also has some asyna])sis ol the (liiomosomes (Figuri* S).
The type of sti-eaking, liowever, does not .seem to be confined to this
particular inbred. The first five or six leaves of i)lants of this type
arc normal. Latci- leaves have shoi't white streaks, and seem longer
and less turgid than normal leaves. \\ hen the car shoot emerges, or
aftei' th(! silU's arc showinii'. the plant chai-acteristically bends so that
the to]) of the |)lant may l)c imraMcl to the siiid'acc of the ground. As
(he rjif niiiliircs. th<' pliiiit iii;iy rdntiniic to lirml. Thr inddiict ion ol"
;iny malur'c ^t-i^t] <ih the imi- di'|M'nd-~ nn (he dcgicc to which lh(

Stei'ility in Itibred and Tlyhrld Maize 721

stripping and bendino- are manifest. The plants are slioiter than
their normal sibs.

A preliminar}/- histological examination showed that cell divisions
were normal in young leaf sheaths and there was no evidence of any
small ring chromosome whose loss might account for the variegation
(27). It w^as observed that cells surrounding the vascular bundles in
the leaf may become abnormal in appearance and partially filled with
an opaque substance. The subsequent death of these cells could ac-
count for the streaking and lack of stiffness of the leaves since in later
stages some of the streaking resembles a necrosis.

Five plants which were the characteristic "streaked dwarf" type
have been examined cytologically. Three of these were from hybrids
with 243, two of which had 243 as the pollen parent (Ldg X 243 and
782-54 X 243) and one with 243 as seed parent (243 X 14). The
other two were in the inbred C237 and a related hybrid, 615-llD X
615-9, Pollen from these plants and three additional streaked plants
was essentially normal with no segregation type of pollen grain being-
present. The chromosomes were all apparently normal at the mid-
prophase of meiosis, and there was no evidence of any deficiency, in-
version or translocation. In addition two normal sib plants were ex-
amined cytologically. They were also found to have normally synap-
sed chromosomes and no chromosome aberration in the heterozygous
condition. Pollen from these plants was normal with the exception
of a few smaller pollen grains.

The "streaked dwarf" type of plant does not occur in a ratio ex-
pected on the basis of a simple recessive gene as shown in Table 10.
Seven streaked plants were selfed. In the progeny of these there
were 59 normal plants, and none was streaked.

Table 10. Numbers of Streaked and Normal Plants Resulting from Crosses
WITH THE "Streaked Dwarf" Type of Plant.

Fa

Backcross

Plant

Normal

Streaked

Normal

Streaked

40-127

4

0

.

174

10

0

-^

—

814-12

8

1

—

-^

815-9

38

1

—

—

814-6 X 2

—

—

14

3

Total

60

2

14

0

Expected

46.5

15.5

8.5

8.5

Another type of degenerative change was found in the line 1-T,
C237, (16). At the seventeenth generation of inbreeding two sib lines
were separated in this inbred. Ears of one line are more poorly filled
than those of the sib line, resulting in a significantl}^ lower jdeld of
grain. Plants with the poor ears are taller than the sib line. Two
plants of this poor-ear line were examined cytologically. One of these
had sporocytes in which no chromosomal changes could be observed,
and the synapsis of homologous chromosomes was normal. The other

C onnecticut E :i'penmerd Station

BuUethi 465

plant was exaiuiiietl more completely and there were foimd a few fi<j-
iires of heterozygous translocations. On eight preparations from dif-
ferent anthers, a total of 642 chromosomes could be identified in 137
sporocytes. Of these, five sporocytes had a heterozygous transloca-
tion between chromosomes 1 and '1 (P^igures 9. 14 and 15). and five
sj)oi-ocytes had heteroZA'gous translocations that Avere not definitely
identified, but were probably l)etween chromo.-omes 4 and 5 and
chromosomes 4 and 10.

Figure 9. Hetfrozys^ous translocation be-
tween chromosomes 1 and 1 I'onnd in a few
sporocytes of an antlicr. A pliotograph of the
sporocyte from which the drawinj; was made
is shown in fij^ure 14. The translocation oc-
curred near the knob on the long arm of
chromosome 2 and about mid-way on the long
arm of chromosome 1. IManl 3S-,v'^2-l.
r Pedigree 2.37 Q21.4.\),

C'ytological cxaiiiiiialions were aUo made oi planls Ironi a cross
between tlic ixior car and nonnal sib lines. No chromosomal changos
\ver(W)bs('i\c(| in preparations Ifom four i)lan(s aUhough one of these
had some nnn homologous association. ( )ne sporocyte from prepar-
ations from another |)lant liad a hetei-o/ygous transh)cat ion that was
not completely ident ifieij. In preparations fi'om a sixth plant a trans-
location iii^ure w iis ob.semci I in one sporocyte at diakinesis and one fig-
ure (d' ;i I Ti t rnnsloeation was observed at pachytene. Pollen from
a plant in this progeny had \'-\ percent small and empty grains.
'I'liis is slightly more than the :'. to I percent aborted pollen i'onnd in
normal plants.

f^tcnflfij In. I iihi'cd (iiul Ili/hr'nl Mar^c iS-'t

Since a cytolo^-ical examination is made of i-elatively 1'evv spor-
ocytes, and part ()f tlie tassel is left to develoj) mature pollen, it is
difficult to estimate tlie frequency of occurrenc^e of any clii'omosomal
clianii'es takini>- place in sporocytes or pre-meiotic cells from an analy-
sis of the chromosomes. Ho^Yever, if any chan<i^es are present in the
pollen, they should be detected in the next <^eneration. A count was
made for any transmission of s'emi-sterilit}^ after using a plant from a
cross with the poor-ear line as the pollen parent. From plants of this
cross 50 normal ears were obtained and one semi-sterile. Another
cross had 30 normal ears, one semi-sterile and one with a portion of
the ear appearing- semi-sterile. These have not as yet been examined
cytologically to determine if any chromosomal change had been trans-
mitted. These numbers are still too small to enable a determination
of the frequency of occurrence of an}^ chromosomal changes to be
made.

DISCUSSION

In the foregoing account of sterility in inbred and hybrid maize
the emphasis has been placed on the kind of change observed rather
than on the frequency of occurrence. Any precise determination of
the frequency of mutational or chromosomal changes must be made in
experiments designed for such a purpose.

In each of the groups of semi-sterile ears used, 50 percent of the
ears had no transmission of the sterility or doubtful transmission to
succeeding generations (Table 1). Tliis is in agreement with the ob-
servation that many poorly filled ears of corn are manifestations of
environmental conditions and are not due to any inherent chromo-
somal changes which affect the fertility of the gametes. That chromo-
somal alterations do occur was shown by finding translocations and
inversions in the succeeding^ generations from half the progenies
grown.

The translocations reported here have arisen without any treat-
ment and are thus considered of naturally occurring origin. They are
different from any previously reported translocations. Burnham
(5, 6) found a sterility in maize in inbred material, and Lindstrom
(21) has emphasized that mutations may be readily isolated from in-
bred lines of maize. A comprehensive analysis of defective types of
endosperm development originating in varieties, inbreds and hj^brids
has been presentecl by Mangelsdorf (22).

The frequency of spontaneous chromosomal changes (transloca-
tions and inversions) is not known for maize, but Stadler (38) found
0.9 percent of the control plants in an experiment wath X-rays and
ultra-violet radiation to have segregation for defective pollen. In to-
talling the ears harvested in the field corn test reported in this dis-
cussion, there was found a maximum of 0.11 percent plants with semi-
sterile ears, those segregating defective ovules. Since many different
lines were totalled in determining this value, it cannot be considered
one derived from a "control".

724 Connecticut Experiment Station liuUctin 465

If there is no deticiency associated with a tran>h)catioii. or no
selection against gametes carrying it. lines homozygous for a change
are automatically establislied durmg inbreeding in the same manner
that a gene for a plant character is obtained homozygous in an in-
bred line. 'The inbred 696-3c was found to be homozj^gous for a
translocation between chromosomes 1 and 2. Another, a sweet corn
inbred, was previously found at the Connecticut Experiment Station
to produce all semi-sterile ears when crossed with other inbreds. This
was also homozygous for a translocation (identified by Burnham).
Inbreds homozygous for translocations are not detected, until crosses
are made with them imless there is some efi'ect on the growth of the
plants. They are useless in the production of hybrids since all the
resulting crossed progen}'^ is semi-sterile.

The type of sterility which is transmitted either through the
pollen or through the egg cells is also occasionally found in inbred and
hybrid stocks. When the pollen is segregating defective tj^pes but the
ears are normal, the condition cannot be recognized by field examin-
ation. All inbred carrying a factor for pollen abortion would be at no
serious disadvantage if it were normal in other respects and produced
adequate amounts of pollen in s])ite of the sterility. When a factor
for ovule abortion is present and the pollen is normal, the condition
would be easily recognized by the semi-sterile ears produced, and the
line would be discarded for commercial production.

Degenerative changes which occur sporadically in inbred material
are of interest in theoretical problems but are a hindrance in a prac-
tical breeding program. ]\Iany of these may have physiological or en-
vironmental bases for expression. They are dillicult to analyse gen-
etically and there is a possibility that the presence of modifying fac-
tors may affect the phenotype, particularly in instances in which the
ratios differ significantly from theoretical Mendelian ratios. When
sib lines differ after a number of years of inbreeding, as was found to
be the case in line C2;^>7, the inferences may lie drawn that the inbreds
may be very susceptible to critical environmental conditions, that hom-
ozygosity has not been attained or that the fre(|uency of changes af-
fecting the phenotype of the ])lant is relatively high. In the line
C237 there may also be a condition t'lnoiiuii- n lallicr high frequency
of chi'ftmosomal changes.

A cytological study of the inbreds and hybrids reported here has
shown that many of the changes had occurred without producing any
visible effect on the ])lant growth although the IVrtilily of the gam-
etes was altered. The analysis was uii(l('itMi<cii to determine the causes
of (he >(ci'iliti<'s and the tyjjcs of changes A\hi(h had oceiirred.

SUMMARY

.Vn <'.\!iniiii:il ion of {\\v progenies ol' ;)(• semi-sterile eai'S oecnrring
spontaneon.->ly iimong hybi-ids grown in (est plots showed that IS of
the pi-ogenies transmittecl the stei-ility to tlu; next generation, four had

Stenlity in Iiibred and Ilyhrid Maize 725

doubtful trausniission of the sterility and 14 had no transmission of
the sterilit}''.

Six translocations, an inversion and three factors for lethal
gametes were found among the progenies in which the sterility was
transmitted to succeeding generations.

Five inbred lines of dent corn were analyzed cytologically to de-
termine the causes of sterility in the inbrecls and in hybrids with
them. Two were characterized by variable amounts of asynapsis. One
was found to have a small inversion and one was found to be homozy-
gous for a translocation. The fifth inbred Avas studied in relation to a
degenerative type of ear development and w^as found to have anthers
with the majority of the sporocytes with normal chromosomes. A few
sporocytes were found having heterozygous translocations.

LITERATURE CITED

1. Beadle, G. W. Genetical and cytological studies of Mendelian asynapsis in Zea

mays. N. Y. (Cornell) Agr. Expt. Sta. Mem. 129. 1930.

2. — . Further studies in asynaptic maize. Cytologia 4 :269-287. 1933.

2>. Bergner, a. D., J. L. CartledgEj and A. F. Blakeslee. Chromosome beha-
vior due to a gene which prevents metaphase pairing in Datura. Cytologia
6:19-37. 1934.

4. BuRNHAM, C. R. An interchange in maize giving low sterility and chain con-

figurations. Natl. Acad. Sci. Proc. 18 :434-440. 1932.

5. . Abnormal genetic ratios in chromosome 1 of maize associated

with pollen abortion. Amer. Nat. 69:60 (Abstract). 1935.

6. . Cytogenetic studies of a case of pollen abortion in maize.

Genetics 26 :460-468. 1941.

7. and J. L. Cartledge. Linkage relations between smut resist-
ance and semi-sterility in maize. Amer. Soc. Agron. Jour. 31 :924-933. 1939.

8. Darlington, C. D. Recent advances in cytology. Second edition. P. Blakis-

ton's Son and Co. Philadelphia. 1937.

9. DoBzHANSKY, Th. and M. M. Rhoades. A possible method for locating favor-

able genes in maize. Amer. Soc. Agron. Jour. 30:668-675. 1938.

10. East, E. M. Heterosis. Genetics 21 :37S-397. 1936.

11. Goodspeed, T. H. Chromosome unbalance and the asynaptic condition as in-

duced in Nicotiana bv X-radiation. Sixth Int. Cong. Genetics Proc. 2 :67-69.
1932.

12. Hirschhorn, E. Observaciones sobre la meiosis normal y un caso de asinapsis

en Sisyrinchium. Rev. Argentina de Agron. 1 :66-75. 1934.

13. Jenkins, M. T. The effect of inbreeding and of selection within inbred lines

of maize upon the hybrids made after successive generations of selfing.
Iowa State Col. Jour. Sci. 9 :215-236. 1935.

14. Jones, D. F. The effects of inbreeding and crossbreeding upon development.

Conn. Agr. Expt. Sta. Bui. 207. 1918.

15. . The attainment of homozygositv in inbred strains of maize. Gen-
etics 9 :405-418. 1924.

16. . Continued inbreeding in maize. Genetics 24:462-473. 1939.

17. . Chromosome degeneration in relation to growth and hvbrid vigor.

Natl. Acad. Sci. Proc. 28 :38-44. 1942.

18. ■ — ■ and P. C. AIangelsdorf. The improvement of naturally cross-
pollinated plants by selection in self-fertilized lines. I. The production of
inbred strains of corn. Conn. Agr. Expt. Sta. Bui. 266. 1925.

19. Kollee, p. C. Asynapsis in Piswn sativum. Jour. Genet. 36 :27S-306. 1938.

20. Levan, a. The cytology of Allium amplectens and the occurrence in nature of

its asynapsis. Hereditas 26:353-394. 1940.

726 Connect icut Experiment Station Bulletin 465

21. LixDSTROM, E. W. Some new mutants in maize. Iowa State Col. Jour. Sci
9 :237-246. 1935.

12. M.VNGELSDORF^ P. C. The genetics and morphology of some endosperm char-
acters in maize. Conn. Agr. Expt. Sta. Bui. 279. 192().

25. . Mechanical separation of gametes in maize, lour. Hered.

23 :289-295. 1932.

24. AIcClintocKj B. A cytological demonstration of the location of an interchange

between two non-homologous cliromosomes in Zca mavs. Natl. Acad. Sci.
Proc. 16:791-796. 1930.

25. . Cytological observations of deficiencies involving known

genes, translocations and an inversion in Zca iiia\s. Mo. Agr. Expt. Sta.
Res. Bui. 163. 1931.

26. . The fusion of broken ends of sister half -chromatids follow-
ing chromatid breakage at meiotic anapliases. Mo. Agr. Expt. Sta. Res.
Bui. 290. 1938.

27. . The production of homozygous deficient tissues with mutant

characteristics by means of the aberrant mitotic behavior of ring-shaped
chromosomes. Genetics 23:315-376. 1938.

28. MvER.'^, W. ^L and L. Powers. ]\Ieiotic instability as an inherited character in

varieties of Triticum acstivum. Jour. Agr. Res. 56:441-452. 1938.

29. Powers, L. and A. O. Dahl. Failure of diakinesis and metaphase pairing and

the behavior during meiosis of univalent chromosomes in Zca maxs. Jour.
Agr. Res. 54:655-668. 1937.

30. RnaADES, M. M. and V. H. Rhoades. Genetic studies with factors in the tentli

chromosome in maize. Genetics 24:302-314. 1939.

31. Richardson, M. M. Meiosis in Crcpis. II. Failure of pairing in Crcpis ca-

peUaris. (L.)Wallr. Jour. Genet. 31 :1 19-143. 1935.
.32. Roberts, L. M. The effects of translocation on growth in Zca mays. Genetic.-^

27 :584-603. 1942.
?>i. SixGLETOX, W. R. Complete elimination of certain classes of gametes in Zca.

Sixth Int. Cong. Genet. Proc. 2:182-184. 1932

34. . Influence of female stock on the functioning of small

pollen male gametes. Natl. Acad. Sci. Proc. 26:102-104. 1940

35. . Hybrid vigor and its utilization in sweet corn breeding.

Amer. Nat. 75:48-60. 1941.

36. and P. C. Max(;i:i.si)orf. Gametic Icthals on tlie fourth

chromosome of maize. Genetics 25 :3()6-390. 1940.

y/. Sprague, G. F. Hvbrid vigor and growth rates in a maize cross and its re-
ciprocal. Jour. Agr. Res. 53:819-830. 1936.

38. Stadler, L. J. The comparison of uhraviolet and X-ray effects on mutation.
Cold Spring Harbor Sym. on Quant. Biol. 9:168-177. 1941.

.39. Stlktevant, A. H. and G. W. Beadle. An introduction lo genetics. W. B
.Saunders Co., Philadelphia. 1940.

40. Thompson, W. P. and I. Hutcheson. Cln-omosonu- hrhavior nnd I'erlilit) in
diploid wheat with translocation complexes of four and six cln-nmosomes.
Canad. Jour. Res. 20:267-281. 1942.

University of
Connecticut

Libraries

39153029000561