1063-0740/05/3105- © 2005 Pleiades Publishing, Inc.
Russian Journal of Marine Biology, Vol. 31, No. 5, 2005, pp. 303–308.
Original Russian Text Copyright © 2005 by Biologiya Morya, Tabakova, Kirsanov, Anisimov.
The development of tissues in many groups of ani-
mals and plants entails so-called somatic (cellular, tis-
sual, endomitotic) polyploidy arising as a result of an
unnormed increase in the number of chromosome com-
plements in initially diploid cells . As a variant of
certain histogenenes, somatic polyploidy is a relevant
and, in some instances, a leading mechanism of organ
and tissue growth. The polyploid histogeneses of mol-
lusks have been revealed and studied well enough in
gastropods. In that group of animals, moderately poly-
ploid values of DNA mass (4
) were speciﬁc for
most of the secretory tissues [4, 9, 17, 18], and extreme
) were recorded in neurons
[3, 11, 21, 31]. Cell polyploidization is implemented
through the mechanism of incomplete mitosis and
endomitosis [4, 7]. In gastropod series, a pronounced
evolutionary tendency to an increase in the role of
somatic polyploidy was traced from prosobranch
(streptoneurous) to opistobranch and pulmonate
(euthyneurous) mollusks . These studies were of
considerable help in understanding the essence of the
phenomenon of somatic polyploidy, which was consid-
ered a phylogenic display of oligomerization of cellular
clones with corresponding consequences as allo- or
aromorph modiﬁcations of histogenenes .
The distribution of somatic polyploidy has practi-
cally not been studied in representatives of other classes
of mollusks, and this has considerably hindered under-
standing and interpretation of the evolutionary sense of
that cytological phenomenon. We carried out special
research on determination of nuclear ploidy in cells of
various tissues of some bivalves. In the present commu-
nication, data on nervous cells of ganglia of the central
nervous system of bivalves are submitted.
It is considered that the neurons of Bivalvia, in con-
trast to giant neurons of gastropods, are of usual sizes,
and it was also assumed that they are diploid [11, 28].
At the same time, many authors describing the histolog-
ical structure of ganglia discriminated between several
size groups of neurons, in which the sizes of the nuclei
are also considerably varied [22, 23, 28]. Meanwhile, it
is well known that the size of neurons and of their
nuclei correlates with neuron ploidy in the higher gas-
tropods [11, 30, 31, 34]. By Gillette’s assumption ,
an increase in the neuron size and of the ploidy is quite
probable in all classes of Molluska phylum, with max-
imum manifestation in pulmonates and opistobranchs.
Thus, the chance of the occurrence of somatic polyp-
loidy also in bivalves, phylogenic relatives of gastro-
pods, is not excluded. However, there is no reliable lit-
erature data on how the growth of nervous cells occurs
in bivalves, or whether this growth is connected to mul-
tiplication of the number of genomes.
MATERIAL AND METHODS
We examined 28 molluskan species of Bivalvia
class belonging to ﬁve existing subclasses. Classiﬁca-
tion is cited after Cohen
 with additions [1, 13,
15, 20, 35].
Morphological Variability and Levels of Nuclear Ploidy
in Neurons of the Central Nervous System of Bivalves Relative
to the Problem of Somatic Polyploidy
E. V. Tabakova, I. A. Kirsanov, and A. P. Anisimov
Far East State University, Vladivostok, 690950 Russia
Received March 24, 2005
—Ploidy of neurons was assessed by the method of nuclear DNA cytophotometry in ganglia of the
central nervous system (CNS) in 28 species of Bivalvia belonging to 5 existing subclasses and having various
bio-ecological characteristics. A signiﬁcant variation in the nucleus size and chromatin structure was revealed
on crush preparations and slides; however, almost all nuclei were diploid. Some species had nuclei with tetra-
ploid DNA value (from 0.2 up to 1%) and with condensed chromosomes, the probable mitotic fraction of cells.
It was concluded that variation in the sizes of neurons and of their nuclei in the given instance was of a func-
tional character, and polyploidy was untypical for the nervous system of bivalves. Apparently, the polyploidy,
as an evolutionary strategy of neuron growth, had no anatomical prerequisites in bivalves, which is similar to
CNS oligomerization in higher gastropods, as they are devoid of any functional adaptive sense.
bivalves, neurons, DNA content, somatic polyploidy, endopolyploidy.