Chromosome numbers in the genus Formica with special reference to the taxonomical position of Formica uralensis Ruzsk. and Formica truncorum Fabr.

Hereditas 92: 321-325 11980)

Chromosome numbers in the genus Formica with special reference to
the taxonomical position of Formica uralensis Ruzsk. and Formica
truncorum Fabr.

MARITA ROSENGREN 1 . RAINER ROSENGREN 2 and VERONICA SODERLUND 1

Department of Genetics 1 and Department of Zoology 1 ,
University of Helsinki, Finland

ROSENGREN. M., ROSENGREN, R. and SODERLUND, V. 1980. Chromosome numbers in the genus
Formica with special reference to the taxonomical position of Formica uralensis Ruzsk. and Formica
truncorum Fabr Hereditas 92: 321-325. Lund, Sweden. ISSN 0018-0661. Received January 25. 1980

The chromosome numbers of the species Formica aquilonia Yarr., Formica uralensis Ruzsk. and
Formica pressilabris Nyl. were all found to be n = 26. The chromosome number off uralens:s thus does
not accord with the view that this species belongs to be subgenus Serviformica. Chromosomal polymor-
phism was found in Formica truncorum Fabr., one population having the haploid number n = 28, instead
of n=26. The chromosome numbers obtained for South Finnish populations of Formica polyctena
Foerst., Formica pratensis Retz.. Formica lugubris Zett., Formica rufibarbis Fabr. and Formica
transkaucasica Nas. were the same as those reported from Central Europe.

Marila Rosengren, Department of Genetics, University of Helsinki,
P. Kautatiekatu 13, SF-00100 Helsinki 10, Finland

The systematics of the genus Formica is still un-
clear, especially with respect to Formica sensu
stricto (= the "rufa group" as defined by BETREM
1960). Not only are the morphological criteria
commonly used in delimiting species within the
subgenus Formica rather vague but the same spe-
cies may be included in the subgenus or not, or
arbitrarih given the status of a"true" member of
the rufa group or not. A case in point is the
mound-building species Formica uralensis Ruzsk.
This species is usually considered a member of the
subgenus Formica s.str. or the rufa group (BETREM
I960; GOSSWALD et al. 1965; KUTTER 1977). but
DLUSSK>! (1967. and personal communication I in-
cludes it in the subgenus Serviformica (the "fusca
group"). Another unclear case is represented by
Formica truncorum Fabr.. which is included in the
rufa group by BETREM (1960) and KUTTER (1977),
but not by COLLINGWOOD (1979). Various problems
of the n fci-group taxonomy can be tackled suc-
cessfr'!; by unconventional methods: gel electro-
phoic.-iis of enzymes (PAMILOCI al. 1979). analysis
of volatile compounds (BERGSTROM and LOFQVIST
1973). application of numerical taxonomy (DouwES
1979) and use of behavioural critera (ROSENGREN
and CHERIX 1980). Chromosomal studies should
naturally have a central position in this context.
The haploid chromosome number of ants varies

between n=3 and n = 84, with a genus frequency
peak at 14 (CROZIER 1975). A high degree of vari-
ability in chromosome number may occur within a
single genus (e.g. the genus Camponotus with ha-
ploid numbers varying between 9 and 26), but
within the genus Formica the chromosome num-
bers appear to be ver> uniform (CROZIER 1975), and
the information given by it is, consequently, of
limited value, unless complemented by studies of
the karyotype. Unfortunately, karyotype analyses
are very difficult in Formica due to the extreme
smallness of the chromosomes. Our intention was
to use banding techniques, but the methods we
tried did not give satisfactory results.

The chromosome numbers of the species For-
mica aquilonia Yarr., Formica uralensis Ruzsk.
and Formica pressilabris Nyl. are reported here
together with that of a possible new Formica spe-
cies described by COLLINGWOOD ( 1979. p. 152)
under the name Formica "nylanderi". Chromo-
somal polymorphism occurs in many ant species
(CROZIER 1975; IMAI et al. 1977). but has so far not
been observed in the genus Formica. We have
now found chromosomal polymorphism in Formi-
ca truncorum Fabr. In other cases, however,
(Formica polyctena Foerst., Formica pratensis
Retz.. Formica lugubris Zett.. and Formica trans-
kaucasica Nas.) the chromosome counts made bv

322 M ROSENGREN ET AL

Hereditas 92 1 1 980)

us on Finnish populations are in close agreement
with the chromosome numbers previously report-
ed for Central European populations of the same
species by HAUSCHTECK-JUNGEN and JUNGEN (1976).

Table 1. Each row in the table corresponds to a nest. Column
"No. nuclei" = the total number of haploid metaphase nuclei in
which chromosomes were counted (the number within paren-
thesis is the number of nuclei with the most common chromo-
some number). Column "n" = the most common chromosome
number

Species and nest

Locality No. nuclei n

Material and methods

AH the chromosome numbers reported in this
paper were counted on haploid cells of the testes,
although the brains of workers and males were
also studied in many of the species. Mitotic divi-
sions in brain cells were, as a rule, found in pre-
pupae, but not in pupae, as was also observed by
IMAI (1966) while meiotic figures in the testes were
observed mainly in early pupae (the eyes unpig-
mented or only weakly pigmented). In addition to
cells with haploid numbers the testes contained
some diploid and polyploid nuclei (e.g. triploid).

The material was dissected in colchicine-hypo-
tonic solution and in most cases incubated for
about 20 min in the same solution (see IMAICI al.
1977 for procedure). We used exclusively the
air-drying technique described by IMAI et al.

phase contrast microscope and stained in Giemsa
solution diluted 1:24 with Sorensen's pH 6.8 buf-
fer. Mounts were prepared with cover slips and
neutral mounting medium (Gurr). According to
IMAI et al. (1977) this technique should give
C-banding without subsequent treatment, but we
were unable to obtain satisfactory C-banding,
though replacement of incubation in colchicine so-
lution with incubation in pure water appeared to
give slightly better results. Attempts to induce
G-or C-banding with trypsin or barium hydroxide
also failed.

Our material of Formica species was collected
along the coast of Southern Finland from Sibbo
east of Helsinki to the Hitis archipelago west of
Hango (for locality of each of the nests, see Table
1.). The species were determined with the aid of
widely used identification guides (e.g. DLUSSKY
and PISARSKI 1971; KUTTER 1977). In addition,
workers from each nest were investigated in the
scanning electron microscope (ISM-U3) at 13
KW, after coating with gold.

Results and discussion

All Formica species investigated, except F. rufi-
barbis and the Hitis nest of F. truncorum had the

F. (Formica) polyciena

F. (Formica) aquilonia

F. (Formica) aquilonia

F. (Formica) lugubris

F. (Formica) "nylanderi"

F. (Formica) prftiensis

F, (Formica) truncorum

F. (Formica) truncorum

F. (Formica?) uralensis

F. (Serviformica} rufibarbis

F. (Ser\iformica) transkaucasica

F. (Captoformica) pressilabris

Grankulla 58(65) 26

Esbo 11(11) 26

Vanda 37(37) 1C

Vanda 22(22) 2v,

Sibbo 28(28) 26

Grankulla 37(34) 26

lng 10(7) 26

Hitis 75(65) :

Vanda 102(861 2(

Lappvik 10(5) 2',

Siikajiirv! 14(13) 26

haploid number n=26 (Table !). The material of F.
rufibarbis was too small and the chromosome
number too variable (n = 25-28) to permit any de-
finite conclusion, but n = 27 was the most common
haploid number for this species and corresponds

ior r. rufibarbis and five other European Servi-
formica species by HAUSCHTECK-JUNGEN and JUNG-
EN ( 1976). The number 27 seems to be the rule for
the haploid set, having been reported for Nearctic
and Asian Serviformica as well (HUNG and IMAI,
cited in CROZIER 1975: cf. FRANCOEUR 1973). The
only known exception is F. transkaucasica (syno-
nyme: F. picea Nyl.), for which HAUSCHTECK-
JUNGEN and JUNGEN (1976) obtained 2n=52 in
Switzerland (brain tissue from workers). This ac-
cords with the haploid number (n=26, testis)
found by us for this species in Finland.

DLUSSKY ( 1967) considers F. uralensis a member
of the subgenus Serviformica, but BETREM (I960)
and KCTTER ( 1977) refer it to the subgenus Formi-
ca. The view of DLUSSKY (1967. and personal
communication) is based on the fact that the
mandibles of uralensis males are of the same sup-
posedly primitive dentate type as in some Servi-
formica. Other features of uralensis diagnostic of
Serviformica are the complete lack of outstanding
hairs on the eyes of all the castes, including the
males, the dullness of the frontal triangle on the
head and possibly the narrowness of the para-
meres of the male genitalia (DLUSSKY. persona!
communication: see also KUTTER 1977). In the
Finnish populations of F. uralensis the male
mandibles are indeed of a markedly dentate type
compared with those in Formica s.str. (in which

Here dilas 92 (1980)

CHROMOSOME NUMBERS IN THE GENUS FORMICA 323

to

%! * -

'oid chromosome sets of a: f . "nylande,

um (Hitis) n=28; c: F

all the teeth on the mandibles, except the apical
one. are either absent or reduced to one or two
irregular knots) and the workers are differentiated
by the exceptionally strong sculpturing of the
frontal triangle (Fig. 2). Arguments for not includ-
ing F. uralensis in Serviformica are the facultative
social parasitism of uralensis females during nest
founding i : K.LTTER 1977). the structure and shape of
the nest (mounds of needles as in the subgenus
Formica) and the general type of foraging behav-
iour (RosENCREs 1969, 1971). The chromosome
number n = 26 found in the present study also dif-
fers from the common pattern in Serviformica,
There may thus be reason to consider that Formi-
ca uralensis represents a separate phylogenetic
lineage, although its enzymes provide evidence of
some relationship with Serviformica (PAMiLoet al.
1979).

F. pressilabris Nyl. has the chromosome num-
ber n = 26 (Table 1). This accords with results for
F. execta reported from Switzerland (brains of
workers. HAUSCHTECK-JUNGEN and JUNGEN 1976).
The same chromosome number is found also in
Nearctic Copioformica (Hu\G 1969).

HAUSCHTECK-JUNGEN and JUNGEN (1976) studied
five species of the subgenus Formica (rufa, polyc-
tena, lugubris, truncorum and pratensis) and
found 2n = 52 in all of them (brain tissue of work-
ers). We have confirmed this result with testis
preparations of polyctena lugubris and pratensis
(n = 26. Table 1) and in addition demonstrated that
this number also holds for F. aquilonia. The same
chromosome number is found in the F. "nylande-
ri" population, which, although showing affinities
tof. lugiihris in its worker morphology, has high-
ly deviant queens and can possibly be separated as
a new species (COLLINGWOOD 1979).

Some of the studied populations, e.g. F. "ny-
landeri" and F. aquilonia had a satellite chromo-
some (see Fig. I.).

The number n = 28 found for F. truncorum
(Table 1, Fig. I) is at variance both with the
chromosome number of the other, more eastern F.
truncorum population investigated by us in Fin-
land and with the chromosome number (2n = 52)
reported for a Swiss population of this species by
HAUSCHTECK-JUNGEN and JUNGEN (1976). We ob-
tained the same result with male pupae (testes)

324 M. ROSENGREN ET AL

Heretlilas 92 i 1980 1

- -/,

"\

/';

lt
V:

A \

Fig. 2 -d. Frontal mangle and adjacent pans on the heads of workers (SEM). a: F imncorum ln-26). b.

from this nest in both 1978 and 1979 (results from
the two years pooled in Table 1), but the material
from 1978 contained one pupa with n = 27 instead
of n = 28. Morphologically, the ants of this diver-
gent population do not differ clearly from the
normal (n=26) truncorum population. This was
confirmed by our scanning electron micrographs.
In both the Finnish truncorum populations the
workers had a very characteristic chitin micro-
structure on the frontal part of the head (Fig. 2).
According to our observations this type of micro-
structure is not found in any other species of the

subgenusforw/'ca, except Formica yessensis For.
(specimens sent to us by Dr. Seigo Higashi from
Sapporo. Japan). It is thus evident that F. trunco-
rum has a very special position within the sub-
genus Formica, a fact also indicated by other fea-
tures (KUTTER 1977; PAMILO et al. 1979). Our
chromosomal findings can hardly be correlated
with the taxonomic position of truncorum , unless
n = 28 is the rule in this species (which seems un-
likely at present). Chromosomal polymorphism
has been observed in several ant species, but has
not previously been reported for the genus Formi-

Hereditas 92 11980)

CHROMOSOME NUMBERS IN THE GENUS FORMICA 325

ca, and the only chromosome numbers known
ealier for Formica species are n = 26 and 27.

Due to the smallness of the chromosomes and
our failure to obtain a banding pattern, we were
not able to determine what kind of chromosomal
rearrangement had occurred to change the number
from 26 to 28. Nor do we know whether only one
or a few nests off. truncorum are characterized
by n = 28. or whether we have to do with a karyo-
type "race" extending over a substantial part of
the archipelago of the Gulf of Finland.

For the future, it seems important to develop
banding methods suitable for the genus Formica,
so that the chromosomes can be used more effec-
tively to investigate the taxonomic relations.

Acknowledgments. We wish to thank Pekka Pamilo, M.Sc.,
for valuable informations and for providing us with some ant
pupae, and Anna Damstrom, M.A.. for checking the language.

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