UNBALANCED SOMATIC CHROMOSOMAL
VARIATION IN CREPIS

BY
M. NAVASHIN

University of California Publications in Agricultural Sciences

Volume 6, No. 3, pp. 95-106, plates 4 and 5, 2 figures in text

Issued March 19, 1930

University of California Press
Berkeley, California

Cambridge University Press
London, England

UNBALANCED SOMATIC CHROMOSOMAL
VARIATION IN CREPIS

BY

M. NA YASHIN

INTRODUCTION

It is a well-known phenomenon that the chromosomal constitution
may vary during the somatic cycle of development. Numerous
instances have been described for various representatives of the two
organic kingdoms. The majority of such phenomena may be divided
into two principal groups, viz., chromosomal chimeras and chromo-
somal variegation. In chromosomal chimeras, entire shoots, branches,
groups of similar tissues, different tissues, or finally, groups of cells
within the same tissue, differ in their chromosomal composition.
Various kinds of chimeras may be of occasional occurrence, whereas
in certain cases a chimeral condition constitutes a constant typical
feature of the organism. Thus, the tetraploid shoots known to occur
in Datura (Blakeslee and Belling, 1924) and the so-called mosaics in
Drosophila (Morgan, Bridges and Sturtevant, 1925) represent chimeral
conditions of occasional occurrence. Constant differences in the
chromosomal constitution of different tissues of certain insects (Fro-
lowa, 1928) or in the root tips of hemp (Breslawetz, 1926) may be
referred to as typical features of certain organisms.

Extreme chimeral conditions, in which separate little groups of
cells of different chromosomal constitution are occasionally found in
normal tissues, represent transitional stages to chromosomal variega-
tion. The distinction between the two kinds of variation depends
obviously upon the degree of mutability. If variation occurs infre-
quently, the resulting chromosomal constitution becomes fixed in long
lineages of cells and a chimera arises. If, on the contrary, the fre-
quency of variation is very high, no definite lineage of cells can
possibly arise, and variegation is the result.

In Crepis investigations, many cases of vegetative chromosomal
variation have been observed, the great majority of them being located
in the root tips. They were mostly of the balanced type, i.e., they
represented various grades of polyploidy of definite sectors of cells or

96 University of California Publications in Agricultural Sciences [Vol. 6

of individual cells. Tetraploid cells were found, for instance, in the
root tips of C. dioscoridis (M. Navashin, 1926) and of a hybrid deriva-
tive of C. biennis xC. setosa (Hollingshead, 1928a). Giant polyploid
cells containing as many as 128n chromosomes were found in C. tec-
torum (M. Navashin, loc. cit.). Many instances of various grades of
polyploidy of chimeral type were found by the present writer and by
Miss Gerassimova in several Crepis species and in interspecific hybrids.
Finally, it was found in haploid individuals of C. capillaris (Hollings-
head, 19286 ; Babcock and Navashin, 1930) that a very considerable
proportion of cells become diploid, the latter circumstance even lead-
ing to the formation of diploid shoots (Hollingshead, loc. cit.).

As was stated above, the majority of observations concern the
root tips. It was found many times by the present writer that
different roots taken from the same plant differed in their chromo-
somal constitution. Thus in many instances entire tetraploid roots
were found to occur, together with normal ones, in diploid individuals
of C. tectorum and C. capillaris. Furthermore, in plants possessing
small extra fragments or other chromosomal abnormalities, certain
roots were found to be entirely normal as regards their chromosomes.
Finally, cases were found when a single plant yielded roots of three
different chromosomal types; and hybrid derivatives possessing one
foreign chromosome produced some roots without that extra chromo-
some. All these and similar observations, however, were mostly
explained as experimental errors, i.e., as due to occasional confusion
of roots belonging to different individuals, a circumstance eventually
taking place. All these cases, however, could by no means be explained
as mere results of errors. On the contrary, by taking special precau-
tions it was found that the majority, if indeed, not all of them should
be attributed to actual variation in somatic cycle.

In the cultures of the past year (1929) one plant was found which
was undoubtedly of unbalanced chimeral type. Not only its roots but
the upper parts of the shoots as well differed in their chromosomal
constitution. The description of this interesting chimera, the dis-
cussion of its mode of origin and of the bearing of chromosomal varia-
tion on some problems of general importance constitute the principal
subject of this paper.

It gives the writer special pleasure to acknowledge the excellent
help given during the course of the investigation by Miss II. N.
Gerassimova.

1930] NavasJmi: Unbalanced Somatic Chromosomal Variation in Crepis 97

AN UNBALANCED CHROMOSOMAL CHIMERA IN
C. tectorum L.

Numerous progenies of triploids in C. tectorum have been grown
and subjected to investigation by the writer since 1926. Among several
cultures grown in 1929 in Moscow in the experimental garden of the
Timiriazev Federal Institute of Scientific Research, one, namely cul-
ture 29.1013, contained the plant which is described below. This
particular plant 29.1013-40 was investigated cytologically, and root
tips were taken from it in the young rosette stage. Altogether twenty-
six roots were investigated. Nineteen of them proved to be triplo-B
simple trisomic, the remaining seven showing wholly normal diploid

Fig. 1. Somatic metaphases: a, of the normal (diploid) component; b, of the
trisomic component (note the extra B-chromosome). Root tips. X 2400.

chromosomal complement (fig. 1). This first result of the investiga-
tion strongly suggested the conclusion that the plant in question was
a sectorial chimera with one part trisomic and the other normal diploid.
Further observations have shown the correctness of this conclusion.
After the shoots had developed it appeared that they were of two
clearly distinct types. Two of them were undoubtedly triplo-B simple
trisomic ("diamond"), the third being of typical normal appearance.
The trisomic shoots were (as is common for trisomies) about half as
tall as the normal component, and possessed the typical "diamond-
shaped" leaves which are the best morphological characteristics of
the triplo-B type. The flowering heads, buds, and fruiting heads were
much smaller in the dwarf component as compared with the tall one.
This difference in head size is again a constant distinction between
the triplo-B trisomies and normal plants. The cytological investiga-

98 University of California Publications in Agricultural Sciences [Vol. 6

tion of the young nieristem of the flower buds and of the root tips,
taken separately from the two components, proved beyond any ques-
tion that the plant really was a trisomic-normal, sectorial chimera.
Thus nine chromosomes were found in the buds and root tips of the
dwarf component whereas those of the tall component had only the
four pairs, typical of the somatic garniture in C. tectorum. (See
pis. 4 and 5 and fig. 1.)

As noted above, the two components of the chimera did not differ
morphologically in any way from the respective entire plants of cor-
responding chromosomal constitution. With regard to their physio-
logical behavior, the trisomic portion did not show any marked pecu-
liarities, but the "normal" diploid component was entirely different
from the thousands of diploid C. tectorum plants that had been pre-
viously seen. In contrast to them, it was of very low fertility, a
phenomenon never met with in normal individuals of C. tectorum.
The cause of this low fertility of the ' ' normal ' ' component could not
be detected. No irregularities in meiosis in P.M.C. were found.

MODE OF ORIGIN OF THE CHIMERA

The chimera described above occurred in the progeny of a triplo-A
simple trisomic plant derived from a triploid parent. The appearance
of an extra B-chromosome should be attributed, therefore, to elimina-
tion of the extra A-chromosome followed by duplication of the
B-chromosome, most likely through equational non-disjunction. Cases
of the latter sort were observed many times by the writer when cer-
tain chromosomes in hybrids were duplicated, obviously through equa-
tional non-disjunction. As for the origin of the chimeral condition,
two principal ways are feasible ; first, through diembryony, and
second, through somatic elimination of the extra chromosome sometime
during early development ; at any rate prior to the formation of
flowering shoots. The first manner of origin is highly improbable as
might be easily shown. Thus, in order to account for the pccurrence
of two cytologically different embryos in the same achene, one may
suggest the three following possibilities: the formation of an adven-
titious embryo from the vegetative tissue, or of two functional embryo
sacs in the same ovule, or finally of two functional ovules in the same
ovary. Occurrence of adventitious embryo is a phenomenon never
met with in C re pis and, furthermore, it could by no means account

1930] Navashin: Unbalanced Somatic Chromosomal Variation in Crepis 99

either for the formation of a normal diploid embryo or for that of a
triplo-B trisomic. It could give rise only to an individual identical
with the original plant, i.e., a triplo-A trisomic. Two embryo sacs in
the same ovule occasionally occur in Crepis; likewise instances have
been found where a double ovule is formed. In such cases, however,
the second embryo sac is situated at the chalazal end of the ovule and
could hardly be fertilized, for the simple reason that the pollen tube
could never reach it. Also, the double ovules are most probably
incapable of development ; never were embryos found to be contained
in them.

Thus it seems that the most probable way to explain the origin of
the chimera is to admit the somatic elimination of the extra chromo-
some which could give rise to the normal diploid shoot.1 Unbalanced
somatic chromosomal variation is known to occur in Datura (Blake.s-
lee and Belling, loc. cit.) and is strongly suspected in many other cases.

With regard to the mechanism of somatic chromosome elimination,
some observations of the present writer on phenomena found in root
tips in C. iectorum may throw light on the problem. The results of
these observations are given below.

CHROMOSOMAL VARIEGATION in C. tectorum

Among several hundred seedlings grown from seed obtained from
a normal C. tectorwm plant, one was found that showed a striking
peculiarity, namely, the meristematic cells of its root tip differed in
various ways one from another in their chromosomal constitution.

Furthermore, the most outstanding features of this individual was
that its chromosomes not only differed in number in different cells of
the root tip but that some of them were atypically built. These
atypical chromosomes, instead of being rod-shaped, were the shape of
rings or disks. In early prophases they appeared like broad rings
formed by a thread of the same diameter as the remaining normal
chromosomes of the same nucleus. In later stages the rings became
progressively contracted while thickening of the thread and narrow-

1 One could suppose, on the contrary, addition of the third B-chromosome
leading to the formation of trisomic shoots by a normal diploid plant. The
results of eytological investigation have shown, however, that the majority of
the roots were trisomic (see above). Moreover, two shoots were trisomic and
only one, diploid. All this makes it most probable that the plant in question
was originally a triplo-B trisomic and that in the course of development it
produced one diploid shoot through elimination of the extra chromosome.

100

University of California Publications in Agricultural Sciences [Vol. 6

ing of the central clear space took place. At the metaphase the space
within the rings usually disappeared entirely so that the rings assumed
the form of more or less regular disks. In no stages of development
did these structures differ in their staining reactions from the ordinary
chromosomes. In one case the two daughter strands composing a
normal chromosome failed to separate in the anaphase. In the few
telophases observed, the atypical chromosomes resulted in two identical

Fig. 2. Chromosomal variegation in C. tectorum. X 2400.

a, prophase showing one "ring"; b, metaphase showing eight chromosomes
plus one "disk," note the absence of the second satellited chromosome, which
has been either transformed into the ' ' disk ' ' or replaced by a non-homologous
chromosome; c, metaphase showing normal diploid complement plus one "ring"
and one "disk"; d, metaphase showing seven chromosomes plus one "ring";
e, incomplete tetraploid metaphase showing four "rings," two of them united in
a fashion of chain links; /, late anaphase showing one lagging chromosome which
failed to divide into daughter halves.

bodies situated like the ordinary telophasic chromosomes at the two
opposite poles of the cell. Thus there was hardly any doubt that one
was dealing with real chromosomes modified in their organization in
such a way that the ends of the chromosomes, instead of being free,
became united thus forming a closed ring.

These peculiar chromatin structures varied in number in different
cells. The number of the remaining normal chromosomes varied as

1930] Navashin: Unbalanced Somatic Chromosomal Variation in Crepis 101

well. There were found cells which contained normal diploid chromo-
some sets plus one or two ring-shaped chromosomes extra; those con-
taining nine normal chromosomes plus one "ring" extra; those with
seven normal chromosomes and one "ring" extra. Finally, polyploid
(probably tetraploid) cells were found which contained a varying
number of "rings" along with the normal chromosomes; sometimes
there were as many as five "rings." In addition to all these cases of
variable chromosomal composition, cells with entirely normal diploid
complement were not infrequent. (See fig. 2.)

The distribution of the cells of different chromosomal constitution
appeared to be more or less a random one. It was evident that the
process of chromosomal variation in this plant took place frequently
and probably often in opposite directions in subsequent generations
resulting from the same cell. Consequently no long lineage of cells
of identical chromosomal constitution could possibly be established,
and a typical variegation resulted.

DISCUSSION

On the basis of the above data, it may be considered as proved that
unbalanced somatic chromosomal variation actually takes place in
Crepis. The case described of chromosomal variegation in C. tectorum
shows that some disturbances in the chromosomal mechanism may lead
both to addition and subtraction of individual chromosomes even in
cells closely related by descent. Although only one actual case of
failure of daughter chromosomes to disjoin was observed (cf. fig. 2/),
the occurrence of cells of different chromosomal constitution in the
immediate progeny of the same cell indicates that such or some similar
process must be not infrequent. Otherwise one is compelled to admit
the possibility of the formation of chromosomes de novo or of the
disappearance of same, a statement which is against all cytological
knowledge. On the contrary, the most probable assumption is that
some chromosomal disorder, like that described above, makes it
impossible for the chromosomes to function normally, and leads from
time to time to addition (duplication) or to subtraction of certain
chromosomes. These conditions may arise only for a short period of
time, perhaps even only in one cell and may lead to the formation of
long lineages of cells of aberrant chromosomal constitution. A chim-
eral individual may arise in such a case. If, on the contrary, the dis-

102 University of California Publications in Agricultural Sciences [Vol. 6

turbance persists in the tissue, no definite chromosomal constitution
can be established and variegation takes place.

The phenomenon of somatic chromosomal variation throws some
light upon certain problems of general interest. Thus it is very prob-
able that occasional addition or subtraction of individual chromosomes
may be responsible for "bud variation" of various kinds a.s has been
suggested by various writers (cf. review by Chittenden, 1927).
Balanced chromosomal variation may represent a legitimate condition
of specialization of certain tissues. Somatic variation of high fre-
quency like that described above for C. tectorum may result in chromo-
somal variegation. And, if genes of visible characters are involved
in it, genetic variegation would be the result. In certain cases true
somatic segregation may take place, namely, if one allelomorphic
chromosome be replaced by another through the addition of one
chromosome followed by the subtraction of another. Thus homozygous
branches may eventually be produced by a heterozygous individual
without any "reverse gene mutation" at all.

One feature of the described chromosomal chimera deserves special
emphasis. As was pointed out above, the "normal" component
possessed a greatly reduced fertility in spite of its normal chromo-
somal composition and completely healthy condition. It should be con-
cluded, therefore, that the trisomic component exerted some influence
on the "normal" component that led to the reduction of its reproduc-
tive power. This clearly indicates that not only the similarity or
dissimilarity of the genes contained in the allelomorphic chromosomes
but some other internal conditions as well may control fertility.
Investigation is in progress.

SUMMARY

1. Among many plants of a triploid progeny of Crepis tectorum L.
one sectorial chromosomal chimera was found. It consisted of three
shoots, two of which were triplo-B simple trisomic ("diamond"), the
third being normal diploid. The trisomic component did not differ in
any way from the complete trisomic plants of the corresponding type.
The "normal" component was morphologically identical with the
ordinary diploid plants of the same species but differed from them in
that its fertility was greatly reduced.

2. A case of "chromosomal variegation" in C. tectorum is described.
This phenomenon consists in a highly frequent variation in chromo-

1930] Navashin: Unbalanced Somatic Chromosomal Variation in Crepis 103

somal constitution leading to formation of scattered cells differing- in
various ways in their chromosome contents. The process of chromo-
somal variation goes on here in either direction, i.e., either addition or
subtraction of definite chromosomes takes place in subsequent cell
generations.

3. The origin of the described sectorial chimera is explained by loss
of the extra B-chromosome early in development. The variation in
chromosomal composition took place only once, thus establishing a
lineage of cells of new chromosomal type. Frequent variation making
it impossible for the cell progenies to establish certain definite stable
chromosomal constitution leads to variegation.

4. It is suggested that somatic chromosomal variation may lead
under some circumstances to "bud variation" or even to true somatic
segregation if one allelomorphic chromosome of a heterozygous indi-
vidual is replaced by another through a process similar to that
described for the "variegated" individual of C. tecto7-um. Genetic
variegation may possibly also depend upon chromosomal variegation.

LITERATURE CITED

Babcock, E. B., and Navashin, M.

1930. The genus Crepis. Bibliographia Genetica, 6:1-86.
BLAKESiiE, A. F., and Belling, J.

1924. Science, 60:19-20, cited from L. Sharp, An Introduction to Cytology

(New York, 1927).
Breslawetz, L.

1926. Polyploide Mitosen bei Cannabis sativa L. Ber. Deutseh. Bot. Ges.,

44:498-502.
Chiiienden, B. J.

1927. Vegetative segregation. Bibliographia Genetica, 3:355-442.
Frolowa, S.

1928. Die Polyploidie einiger Gewebe bei Dipteren. Zeitschr. Zellforsch.

mikr. Anat., '8:542-65.

HOLLINGSHEAD, L.

1928a. Chromosomal chimeras in Crepis. Univ. Calif. Publ. Agr. Sci., 2d2) :

543-54.
19286. A preliminary note on the occurrence of haploids in Crepis. Am. Nat.,
62:282-84.
Morgan, T. H., Bridges, C. B., and Sttjrtevant, A. H.

1925. The Genetics of Drosophila. Bibliographia Genetica, 2:1-262.
Navashin, M.

1926. Variability des Zellkerns bei Crepis-ATten in Bezug auf die Art-

bildung. Zeitschr. Zellforsch. mikr. Anat., 4:171-215.

««

PLATE 4

General view of the chimera 29.1013-40 of Crepis tectorum L. Two trisomic
shoots (left) and one normal shoot (right). Note the difference in the leaf
shape, in the habit of growth, and in the general development of the two
components. The plant was taken from the ground just before the photograph
was made. Natural size circa.

[104]

UNIV. CALIF. PUBL. AGR. SCI. VOL. 6

[NAVASHIN] PLATE 4

PLATE 5

Some details of the same ehimeral plant. Below, the lower part of the
chimera showing the robust stem of the normal component and the weak slender
stems of the trisomic component. The mode of connection between the two
components clearly indicates that the formation of the diploid (normal) shoot
must have taken place very early in the development.

Above, buds, flowering and fruiting eapitula of the trisomic component
(left) as compared with those of the normal component (right). Natural size
circa.

[106]

UNIV. CALIF. PUBL. AGR. SCI. VOL. 6

[NAVASHIN] PLATE 5