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Cell Biology International (2005) 29, 993–1004 (Printed in Great Britain)
Endopolyploidy as a morphogenetic factor of development
Alim P. Anisimov*
Department of Cell Biology, Far Eastern National University, 8 Sukhanov str., Vladivostok 690950, Russia


Abstract

This paper summarizes the works published by author and his co-workers in the Russian journal Tsitologiya concerning endopolyploidy in mollusks and appraises this phenomenon in general. Both ontogenetic and phylogenetic aspects of endopolyploidy have been studied. In the snail Succinea lauta, a complex examination of endomitosis has been performed. A regular replacement of the normal (complete) proliferative mitosis by abnormal (incomplete) restitutional mitosis, and then by Geitler's classic endomitosis has been demonstrated. We examined 29 bivalve and 82 gastropod species for the presence of polyploid cells in glandular tissues and ganglia. In the bivalve species, ordinary diploid cells form various tissues, while in the gastropods, the role of polyploidy in tissue development appears to increase in phylogenesis. The rise of endopolyploidy and cell giantism in histogeneses of a variety of animal and plant species is widely known. It is believed to be a regular event in the evolution of certain groups. To give a universal interpretation of endopolyploidy, we proposed that a single polyploid cell be better considered as an endoclone. In this case, evolutionary transformation of diploid cell clones into polyploid endoclones may be viewed as Dogel's oligomerization applied to cell-tissue level. From this viewpoint, major properties of an oligomerized system (intensification of function, functional efficiency (ergonomy), increased genomes reliability, simplification of the intra- and supersystem regulations, and acceleration of development) can be considered as principal peculiarities of polyploid growth strategy. The above peculiarities allow one to consider endopolyploidy as an additional means of integrative onto(histo)genetic regulations and correlations and as an important evolutionary factor (coordinations) acting through natural selection. Thus, in general, endopolyploidy is an adaptive morphogenetic factor, but its concrete role may differ in different tissues and organisms depending on cell specialization and histogenetic particularities.


Keywords: Development, Endomitosis, Endopolyploidy, Evolution, Mollusk, Morphogenesis, Oligomerization, Ontogenesis, Phylogenesis.

*Fax: +7 4232 429510.


1 Introduction

Cell endoreproduction, leading to cell endopolyploidy (somatic polyploidy and polyteny), is involved in the program of numerous animal and plant histogeneses and occasionally becomes an important additional factor, affecting postnatal growth (Geitler, 1953; Nagl, 1978; Brodsky and Uryvaeva, 1985; D'Amato, 1989; Zybina and Zybina, 1996; Barow and Meister, 2003). Moreover, endopolyploidy leading to aneuploidy and general genome instability is peculiar to various cancers (Mamaeva, 1998; Atkin, 2000; Storchova and Pellman, 2004). In recent years, an increasing number of publications on endopolyploidy concern its molecular and genetic aspects. Thus, certain insights have been gained into the role of cell cycle genes and their proteins (cyclins, cyclin-dependent kinases, their inhibitors, DNA-topoisomerase II, and others) in the transformation of diploid cells into endopolyploid ones (Nagl, 1995; Datta et al., 1996, 1998; Boer and Murray, 2000; Larkins et al., 2001; Ravid et al., 2002; Dhawan and Gopinathan, 2003; Cortes et al., 2004; Inze, 2005; Lilli and Duronio, 2005). However, cytological mechanisms and the general significance of endopolyploidy remain essentially obscure.

Numerous researches concerning endopolyploidy and its genetic regulation have been conducted using models such as megakaryocytes, cardiomyocytes, hepatocytes, trophoblast giant cells, and cancer cells in mammals and men, embryonic and larval cells in Drosophila, and specialized cells of various organs in seed plants. Mechanisms of cell polyploidization may differ. Megakaryocytes and other mammalian cells undergo a restitution (incomplete) mitosis, i.e. the cells enter mitosis but skip anaphase B and cytokinesis (Ravid et al., 2002). This is usually called endomitosis though this process does not conform to classic (insect) endomitosis (Geitler, 1939, 1953). In Drosophila and plants as well as in mammalian trophoblast, chromonema endoreduplication leading to polyteny takes place. In the near future, interesting results about endocycle regulation may be expected on such models as silkworm Bombyx among insects (Dhawan and Gopinathan, 2003), Daphnia among crustaceans (Beaton and Hebert, 1999), and Oikopliura among chordates (Ganot and Thompson, 2002). It is curious that there are not such works on mollusks, though in some snails the salivary glands and neurons have been noted for high-level endopolyploidy for a long time. We studied both ontogenetic and phylogenetic aspects of endopolyploidy in mollusks. This paper reviews our results and discussions published during the last 10 years in the Russian journal Tsitologiya and presented at a number of conferences. An attempt to formulate a general concept of endopolyploidy and its role in onto- and phylogenesis has also been undertaken.

2 Materials and methods

The snail Succinea lauta (Gastropoda: Pulmonata) was chosen as a model object. Morphological data were mainly obtained through examining squashed and Feulgen- or Ghimza-stained preparations. The studies of endomitotic cycles and ontogenetic and phylogenetic changes of somatic cell ploidy were performed using electron microscopy, autoradiography ([3H]thymidine, [3H]uridine) and DNA cytophotometry. For comparative purposes, nuclear DNA content in various somatic cell types of 29 bivalve and 82 gastropod species was estimated using a scanning microdensitometer and computerized image analyzer.

3 Results and discussion

3.1 Polyploid cells of Succinea and their cambial reserves

In Succinea, many cell types in various tissues and organs are to a different degree polyploidized (Anisimov et al., 1995). As distinct from other cells, polyploid cells are large or giant in size (Fig. 1). Especially large are secretory tegument (subepidermal) cells and CNS neurons.


Fig. 1

Schematic morphology of some organs of Succinea lauta, where endopolyploidy is well expressed. (A) Subepidermal glands. (B) Salivary gland. (C) Digestive gland. (D) Intestine. (E) Albumen gland. (F) Prostate. (G) Nervous ganglion. Bar: 50μm.


The ploidy levels of Succinea cell nuclei are illustrated by DNA content frequency distribution histograms (Fig. 2). Here and below, “C” corresponds to haploid DNA amount, and “N”, to the haploid number of chromosomes. Usually, DNA content in mucous and albumen secretory cells reaches up to 16–64C levels. In albumen cells of the salivary gland, it reaches 128C. In the same subepidermal gland cells in the mantle, lung, and ped, it may approach 256C DNA. Neurons of different CNS ganglia form continuous series of ploidy levels from 2C to 2, 4, 8, or 16 thousands of haploid DNA amounts.


Fig. 2

Frequency distribution of nuclear DNA content (ploidy levels, C) in the organs of Succinea lauta. (A) Sperm cells (standard). (B) Subepidermal glands. (C) Esophagus. (D) Intestine. (E) Digestive gland. (F) Pharynx gland. (G) Salivary gland. (H) Gonad epithelium. (I) Albumen gland. (J) Oviduct epithelium. (K) Prostate. (L) Spermatheca. (M) Neurons of CNS ganglia.


Along with polyploid cells, the tissues retain their cambial reserves (presumptively stem cells) and ability to regenerate. Thus, in the digestive gland, discarding and mitotically dividing cells are observed. Small cambial cells are labeled by [3H]thymidine (Fig. 3; unpublished data).


Fig. 3

[3H]thymidine autoradiographs of the digestive gland of Succinea lauta. (A) General view of the digestive tube epithelium. (B) Fragment with mitosis. cc, cambial cells (some of them are labeled); dc, digestive cells; m, mitosis; sc, secretory polyploid cells. Bars: 10μm.


Another example of this ambivalent state is the albumen gland, an auxiliary gland of the female sexual tract (Anisimov, 1994a,b). During sexual maturation of the snails, all secretory cells of this gland polyploidize up to 16C–32C (Fig. 4; N 1–8). Some cells rich the 64C-ploidy level (N 9, 10). At the same time, in mature glands, diploid cells, able to proliferate, are still to be found (N 9, 10). During egg laying, cell proliferation terminates, some high-ploidy cells die off, and their proportion decreases (N 11). Later on, activation and proliferation of diploid cells are observed. These cells give rise to repeated secretory polyploid cell generations (N 12, 13). Therefore, according to Leblond (1964), epithelium of the albumen gland develops initially as “growing” tissue, whose growth is accompanied by endopolyploidy. However, during its active functioning it displays properties of a renewing tissue. This can be paralleled to histogenetic behavior of the mammalian liver. The difference is in that, in the snail albumen gland, cell differentiation is obligatorily accompanied by endopolyploidy and is terminated at the 16C–32C ploidy level, whereas in the mammalian liver, polyploidy occurs facultatively and the cells can function, being diploid.


Fig. 4

Frequency distribution of DNA content (ploidy levels, C) in [3H]thymidine labeled (black) and unlabeled (white) nuclei in the course of development of the albumen gland in Succinea lauta. Standard, spermatocytes-I (4C) and Sertoli cells (8C); 1–4, juvenile snails at the first summer after birth; secretory cells start to polyploidize; 5–8, further secretory cells polyploidization during sexual maturation of the snails in the second summer after birth; 9–13, appearance of new generation of the secretory cells during egg-laying at summer end.


In some snails, certain epidermal cells can regenerate into nervous tissue and even form a whole ganglion, as was shown by Gomot et al. (1990), Moffett (1995), and other authors, carried out on Helix, Melampus, and other snails.

3.2 Restitutional mitosis and endomitosis as mechanisms of endopolyploidy

We have revealed a universal mechanism of endoreproduction in gland cells and neurons of S. lauta (Anisimov, 1997a; Anisimov and Kirsanova, 2002). The mechanism suggests a regular change of the normal (complete) proliferative mitosis by abnormal (incomplete) restitutional mitosis, and then by Geitler's classic endomitosis (Geitler, 1939, 1953). These two processes, restitutional mitosis and endomitosis, must be considered as mutually compatible and successive mechanisms of cell endopolyploidy.

Thus, in the albumen gland, cambial, ciliated, and immature secretory cells are reproduced by normal mitosis (2C:2C) (Fig. 5A). Tetraploid secretory cells are mainly formed by abnormal mitosis (4C), whose portion reaches 40–60% of all mitoses as the gland begins to maturate (Anisimov, 1997a). Abnormal mitoses are dominated by metaphase blocks, whereas anaphase blocks occur rarely (Fig. 5B). Subsequent cycles in polyploid series 8C–16C–32C undergo endomitosis (Fig. 5C–F). Rare asymmetric mitoses (1C:3C) give rise to uncommon polyploid series 3C–6C–12C, and so on (Anisimov, 1994b).


Fig. 5

Mitoses and endomitoses in the gland cells (A–F) and neurons (G,H) of Succinea lauta. (A) Normal proliferative mitoses (4C). (B) Abnormal restitutional mitoses (4C). (C) Endomitoses (8C): pro-, meta-, anaphase. (D) Endomitoses (16C). (E) [3H]thymidine labeled interphase (16–32C). (F) Endomitosis (32C). (G) Endomitosis (64C). (H) Transition from endomitosis to interphase (64C). Squashed preparations were stained by Feulgen or Ghimza. Bar: 10μm.


In snail ganglia, normal mitoses are followed by 4C restitutional ones and, later on, by endomitoses of increasing ploidy (Fig. 5G,H; Anisimov and Kirsanova, 2002). Different ganglia differed in the proportion of different ploidy classes (Kirsanova and Anisimov, 2001). In the procerebral ganglia and dorsal bodies, ordinary diploid neurons predominated (Fig. 6A). They innervate mainly sensory organs. In other ganglia, large and giant neurons were numerous; their endomitotic polyploidy reached up to 2048–16384C (Fig. 6B). These correspond territorially to extensive clusters of diploid nerve cells (compare Fig. 6A and B).


Fig. 6

Nuclei with different ploidy (2C–2048C) in squashed preparations of the cerebral (A) and visceral (B) ganglia of Succinea lauta. In A, two clusters of diploid neurons forming dorsal bodies are seen. Stained by Ghimza. Bar: 100μm.


In Succinea neurons, DNA synthesis and endomitoses occur during the whole life-span following body growth (Fig. 7; Kirsanova and Anisimov, 2001). In young snails, the activity of neuronal DNA synthesis is highest, with the portion of [3H]thymidine labeled nuclei making up 50.2%. In older animals, a steady decrease in this parameter is observed down to 35.8% and 7.0% in small and large adult snails, respectively.


Fig. 7

Percentage of [3H]thymidine labeled (black) and unlabeled (white) nuclei with different ploidy levels (I–V) in the CNS ganglia during growth of Succinea lauta. (A) One-month-old juvenile snails. (B,C) One-year-old small and large snails. Cer, Ped, Vis, cerebral, pedal and visceral (complex) ganglia, respectively. I, 2C–4C; II, 4C–16C; III, 16C–64C; IV, 64C–256C; V, 512C and more. Vertical bars: 95% confidence intervals.


Some authors proposed the presence of polytene chromosomes in giant neurons, proceeding from their presence in giant cells of mammalian trophoblast (Brodsky and Uryvaeva, 1985; D'Amato, 1989). However, we did not observe such chromosomes in pulmonate and opisthobranch gastropods.

Using autoradiography allowed one to prove that endomitosis is a real phase of the cell cycle, and not an artifact (Anisimov, 1997a). Whereas in the first hour after a single [3H]thymidine administration, unlabeled mitoses and endomitoses were observed, 4−8–24h after precursor injection, endomitoses, as well as ordinary mitoses, were labeled. Thus, we observed G2- and S-periods of endocycle.

At the ultrastructural level, we propose the following main features of endomitosis (Fig. 8; Anisimov, 1997b). Mitotic spindle is absent. Nuclear envelope remains intact. Nevertheless, the normal (mitotic) chromosome cycle occurs and is accompanied by chromosome compaction in endomitosis and differential decompaction during interphase. In anaphase and telophase, chromosomes split asynchronously, some of them remaining as diplochromosomes for a long time.


Fig. 8

Ultrastructure of endomitotic nuclei in Succinea lauta. (A) Interphase. (B) Endoprophase. (C) Endometaphase. (D) Fragment of (A). Note abundant euchromatin fibrils and granules around chromatid and chromonema structures. (E) Fragment of (C). Note non-numerous fibrils and granules at the chromosome surface. (F) Resting phase of the terminally differentiated cell; pseudoendomitosis. Bars: 1μm.


Nucleoli do not disappear during endomitosis (Fig. 8B). At the same time, giant complex nucleoli, usually formed in gland cells (albumen, salivary and other), may partially split into fragments, remaining in contact with chromosomes (Fig. 9D,G). As a whole, the total area and Ag-protein content of nucleoli change in proportion to gene dosage (Fig. 9), but this dependence may become non-linear during several endocycles. These irregular dynamics reflect a combined impact on the above parameters of several factors of nucleolar activity, namely, endomitotic polyploidy (gene dosage effect), type of cell differentiation, inhibition of transcription during terminal differentiation, its reactivation in mature cells, and others (Anisimova and Anisimov, 2001, 2002, 2005).


Fig. 9

Nucleoli (Ag-NOR) in various cells of Succinea lauta. (A) Diakinetic spermatocyte with 22 chromosomes (1N) and 8 NORs of different size. (B) Diploid spermatogonium nuclei. (C) 64C Sertoli cell nucleus. (D) The same nucleus during endomitosis. (E) 2C–4C nuclei in the immature albumen gland. (F) 16–32C nucleus with a complex nucleolus in the mature albumen gland. (G) The same nucleus during endomitosis. Squashed preparations were stained with AgNO3 and methyl green. Bar: 10μm.


3.3 Transcriptional activity of endomitotic chromosomes

RNA synthesis rate was estimated by means of [3H]uridine autoradiography in combination with ultrastructural data (Anisimov, 1997c). During mitosis, precursor incorporation into chromosomes temporarily ceased or remained at 0.5–0.7% of that in interphase diploid nuclei. At the same time, endomitotic chromosomes retained transcriptional activity of about 3–4% of maximum interphase activity. During endomitosis, the chromosomes became compacted, but retained a portion of euchromatin fibrils and granules, observed in abundance in interphase nuclei close to chromatid and chromonema fragments (Fig. 8D,E). Therefore, certain remaining transcriptional activity and the slightly fibrous appearance of chromosomes may be considered as distinctive features of endomitosis.

Noteworthy is that in the terminally differentiated polyploid cells, where DNA synthesis ([3H]thymidine incorporation) and endomitotic cycles are absent, chromosomes may retain large compacted regions (chromocenters), and interphase nuclei may thereby imitate endomitosis (Anisimov, 1997d). This is probably caused by some unknown processes related to cell differentiation. We observed such nuclei in both RNA synthesizing and resting cells of the albumen gland (Fig. 8F). Even degenerating nuclei contained chromatid fragments. We consider such a nucleus state as a pseudoendomitosis. A similar pattern of chromosome arrangement is typical for some other cases, which were described as endomitosis long ago (Kiknadze and Istomina, 1980; Therman et al., 1983). Since endomitosis is a reproductive process, we believe the term “stationary endomitosis”, offered by Therman et al. (1983) for such interphase nuclei, having a chromocentric structure, to be incorrect. Probably, disintegration of the polytene chromosome process, known from mammalian trophoblast cells (Zybina and Zybina, 1996), cannot be considered endomitosis; neither can the resultant chromosomes be called endomitotic.

3.4 General view on polyploid cell as on oligomerized system

The role of endopolyploidy (including polyteny) is still obscure. Different authors emphasized different properties and functional aspects of endopolyploid cells in different tissues and organisms (Nagl, 1978, 1990; Barlow, 1978; Uryvaeva, 1981; Therman et al., 1983; Brodsky and Uryvaeva, 1985; D'Amato, 1989; Edgar and Orr-Weaver, 2001; Barow and Meister, 2003), and all might be right in one's own way. For example in various mammals, relationship of cell ploidy levels with body size and growth rate differs for hepatocytes and cardiomyocytes, suggesting different roles of endopolyploidy in two organs (Vinogradov et al., 2001; Anatskaya and Vinogradov, 2004). The authors suppose that in liver, somatic polyploidy can be considered a “cheap” solution to growth problems that appear when the organ is working at the limit of its capabilities while in heart, the additional genomes can serve for cell regeneration and as a defense against oxidative damage in those animals whose cardiomyocytes work at the limit of their metabolic capacity. Despite numerous discussions, a general concept of endopolyploidy has not been formulated.

To give a universal interpretation of endopolyploidy, we proposed that a single polyploid cell be better considered as an endoclone. This term emphasizes the endoreproductive mechanism of genome unification (incomplete mitosis, endomitosis, and endoreduplication), monoclonal origin of a polyploid cell and its functional equivalence to cell clone (or to its part). In this case, evolutionary transformation of diploid cell clones into polyploid endoclones may by viewed as V. Dogel's oligomerization (or as V. Franz's centralization) applied at the cell-tissue level (Anisimov, 1999c).

Basic properties of oligomeric endoclone arise from its united organization, which requires structural and functional centralization and integration. In contrast, a usual cell clone has polymeric organization and alternative properties. From this viewpoint, major properties of an oligomerized system can be considered as principal peculiarities of polyploid growth strategy. These peculiarities are: (1) intensification of function as opposition to extensive functioning of diploid cell clones; (2) functional efficiency or economy (ergonomy) as a result of the first; (3) increased genome reliability arising from genome multiplication and permission to endow with some gene copies; (4) simplification of the intrasystem (intracellular) regulation of homologous genomes united by common receptive field of the polyploid cell; (5) simplification of the supersystem (supercellular) regulation within tissue, organ, and organism by decreasing the necessary number of intercellular connections; (6) acceleration of development and differentiation of certain cell populations, tissues, and organs as a consequence of mitosis reduction (economizing on mitosis).

3.5 Morphogenetic roles of endopolyploidy in ontogenesis and evolution

The above peculiarities allow one to consider endopolyploidy as an additional means of integrative onto(histo)genetic regulations and correlations and as an important evolutionary factor acting through natural selection. The correlation principle, i.e. the principle of functional interdependency and concerted changes of parts in a developing organism, was formulated by G. Cuvier and developed by Russian biologists A. Sewertzoff and I. Schmalgauzen. The ontogenetic correlations consolidated by selection may be regarded as the corresponding coordinations in phylogenetic relations of organisms, as the use of the classic concepts would be continued after G. Cuvier, Ch. Darvin, I. Schmalgauzen, and others. Thus, in general, endopolyploidy is an adaptive morphogenetic factor, but its concrete role may differ in different tissues and organisms depending on cell specialization and histogenetic particularities.

This view on endopolyploidy is consistent with the data available on the properties and distribution of polyploid cells in histogeneses of various animal and plant species. We examined 29 bivalve and 82 gastropod species for the presence of polyploid cells in glandular tissues and ganglia (Anisimov et al., 1999, 2001, 2004; Zyumchenko and Anisimov, 2000, 2001a,b, 2002; Anisimov, 2003; Tabakova et al., 2005). Moreover, we analyzed all available literature about division and polyploidization of bivalve and gastropod somatic cells (Anisimov, 1998a,b,c,d, 1999a,b,c). The results are shown in Fig. 10.


Fig. 10

Phylogenetic relations of mollusk groups studied and ploidy levels of some cell types.


In all the bivalve species studied, CNS ganglia, digestive gland, epidermis, and subepidermal glands of labial palpes, mantle, and ped (byssus gland), as well as muscles and connective tissues, are formed by ordinary diploid cells. Nuclei with tetraploid DNA content occurred more or less regularly in the digestive gland of several species only. Their occurrence did not correlate with the species' phylogenetic position, lifetime, and other factors. In other organs, tetraploid nuclei occurred rarely and pertained probably to G2-cells, retaining reproductive capacity.

In contrast to Bivalvia, in Gastropoda the role of polyploidy in tissue development appears to increase in phylogenesis. Endopolyploidy appeared and propagated rather regularly in gastropod evolution. It is almost absent in Archaeogastropoda, present in some groups of Meso- and Neogastropoda among Prosobranchia, and widespread in phylogenetically later gastropods—Opisthobranchia and Pulmonata.

The results suggest that two groups of factors might cause endopolyploidy.

The first one is allomorphic ecological, including trophic, adaptations, which might appear in small taxa as well as in separate species. The organs involved in these adaptations may differ between species. Cell polyploidy in these organs is moderate (usually up to 4C–8C) and facultative. We observed such cells in the salivary and digestive glands in some prosobranch (Archaeo-, Meso-, Neo-) gastropod families and orders and in single bivalvian species as was noticed above. In all these cases, endopolyploidy seems to favor various fine adaptations via intensification of cell functioning and enhancement of functional efficiency.

The second reason for polyploid cells is some more or less aromorphic ontogenetic changes. They apparently lead to obligate and prominent polyploidy in all species within large molluskan groups. Thus, among Opisthobranchia and Pulmonata, polyploidy (up to 16C–64C) occur in both the salivary and digestive glands in all species. In certain species, salivary gland cells may also have a much greater DNA amount—up to 4096C in Tritonia and to 16,384C in Odostomia. In neurons of the above two subclasses, endopolyploidy occurs most regularly and is most prominent. Like Succinea in these mollusks, neurons are heteroploid small, middle, large, and giant cells, whose DNA amount is tens, hundreds, or thousands of times as large as that in gaploid cells. For example, in Aplysia or Tritonia, the ploidy of giant neurons reach up to 130–260 thousands C which corresponds to 16–17 cycles of cell endoreproduction.

Thus, in Opisthobranchia and Pulmonata, endopolyploidy must have deeper premises and play more important, morphogenetic, roles as compared with other mollusks. A distinctive feature of these taxa is CNS centralization, in particular, fusion of some ganglia (so-called euthyneury) (Fig. 11; Kirsanova and Anisimov, 2000). Based on this, some systematics unify Opisthobranchia and Pulmonata (and some small taxa) into complex taxon Euthyneura, or Pentaganglionata (see Haszprunar, 1988). We believe that anatomical CNS oligomerization (euthyneury) might be paralleled by oligomerization of cell clones. The latter oligomerization occurred via neuronal endopolyploidy and giantism with respective functional intensification, optimization of the intra- and intercellular regulations, and so on (see above). In particular, giantism and increase of axon diameter of snail neurons provided a basis for their new electrophysiological, metabolic, and topological properties (Shepherd, 1983; Gillette, 1991). These changes might provoke respective changes in the development of other organs, where large polyploid cells arose regularly too. Such dependence should be considered as an integrative ontogenetic correlation, as mentioned above.


Fig. 11

CNS of Succinea lauta as reconstructed from series of sections (1–10). Abd, Cer, Pal, Par, Ped, Pl, ProCer, abdominal, cerebral, pallial, parietal, pedal, pleural and procerebral ganglia, respectively; Db, dorsal body.


Gillette (1991) considers the neuronal giantism as an adaptation favoring innervation of large peripheral areas in large snails and slugs (Pulmonata and Opisthobranchia), which have simple behavioral and sensory systems. In contrast to this strategy, in the neogastropod Prosobranchia, “larger, more complex brains, with large numerous of small (diploid=A.A.) neurons, are associated with the development of sense organs for high-resolution analysis of the environment and greater complexity of behavior” (Gillette, 1991, p. 237). As has been shown above, in Succinea appertaining to Pulmonata, the majority of neurons in the procerebral ganglia and dorsal bodies which innervate mainly sensory organs remain small and diploid too (see Fig. 6A and Fig. 11 sections 1, 2). These examples illustrate evolutionary constraints of endopolyploidy in terms of morphogenesis as compared to a histogenesis strategy based on diploid cells. Histogenesis via regular mitoses and cell number increase is a precondition for histogenetic plasticity and successful complication of animal (or plant) anatomy in phylogenesis, whereas endopolyploidy plays an auxiliary morphogenetic role, that of allowing for certain adaptations and comprehensive morphofunctional cell-tissue correlations in the developing organism.

To apply this view to other objects we may suppose the various adaptive factors to cause endopolyploidy in some organs of adult mammals (liver, heart, many glands, skin, connective tissue) and plants (leaf, root and other) where polyploid cells (4C, 8C, rare 16C) appear in addition to diploid cell populations (facultative endopolyploidy). In these cases, a somewhat functional intensification, increase of functional efficiency or genome reliability of the cells may be favored by endopolyploidy.

More important (morphogenetic proper) reasons for polyploid cell appearance may be perceived in many cases when high-level endomitotic polyploidy or endoreduplicative polyteny arises as a developmental “law” (obligate endopolyploidy). The well-known examples are mammalian megakaryocytes and trophoblast, salivary glands and other larval organs in Drosophila or Bombyx, fruit and, perhaps, some vegetative organs in plants, and others. Like the giant neurons of mollusks, these cells could possess the particular properties concerning intra- and supercellular regulations, rate of development and differentiation, topology of the cell populations, etc. It seems that selectively increasing the developmental rate in the polyploidizing cell populations could be of great morphogenetic importance during both ontogenesis and phylogenesis of organisms. Specifically, cell-tissue oligomery (i.e. endopolyploidy) could be helpful to coordinate both growth and functional stress between newly forming anatomical structures on the one hand and earlier formed organs and tissues on the other hand. This mechanism could be of particular importance when considerably complicating the ontogeneses in the groups progressing quickly, when specializing the species (for example in parasites), and when cenogenetically transforming the ontogeneses (embryonal and larval new-formations, heterochrony, heterotopy). It is no mere chance that the majority of examples for somatic polyploidy and polyteny are known in mammals among vertebrates, insects among invertebrates, and angiosperms among plants, whose evolution abounded into various ontogenetic complications. In many invertebrates, when digesting the unusual environments, endopolyploidy was accompanying the extraordinary histogeneses. They are, for example, the giant-cell organs (uterus, esophageal glands) in parasitic Ascaris (Anisimov, 1974, 1976) and the giant single-celled trophamnion in coelenterates Polypodium—a parasite of sturgeon oocytes (Raikova, 1987). In these cases, endopolyploidy could jointly or separately serve as both the means of concordance of developmental rates and the means of functional intensification. The same role of endopolyploidy may be attributed to cancer tumors where polyploid cells appear as a rule (if this role needs to be determined). The cell neoplasm aspires to survive in spite of body resistance, and polyploidy may assist in this tendency. Notable examples for topological regularity of endopolyploidy may be demonstrated with gradual patterning of the heteroploid cell distribution in various objects (see Kühn, 1955; Barlow, 1977; Brodsky and Uryvaeva, 1985; D'Amato, 1984, 1989; and from new works: Ganot and Thompson, 2002; Barow and Meister, 2003), although the direct meaning of this regularity remains unexplained as yet.

4 Conclusion

Thus, from a general viewpoint, the histogenetic essence of endopolyploidy may be determined as endoclonogenesis or oligomerization of initially diploid cell clones and its result represents an additional morphogenetic factor. The concrete role of this factor may differ in different tissues and organisms, but on the whole this role arises from general properties of an oligomerized system. As the adaptive morphogenetic factor, endopolyploidy favors the onto(histo)genetic regulations and correlations, and acquires an evolutionary significance as a coordination factor through natural selection.

Acknowledgements

This study was supported by the Ministry of Education and Science of Russian Federation (Grants No 94-10.8-15; 95-0-10.0-121; 97-0-10.0-43) and the United States Civilian Research and Development Foundation (Award No. REC-003). It was contributed to by under- and postgraduate students and senior lecturers of the Department of Cell Biology of the Far Eastern National University (Russia), namely, Anna A. Anisimova, Aleksandra A. Brovar, Natalya A. Galimulina, Irina A. Kirsanova, Evgenia V. Tabakova, Natalya P. Tokmakova and Natalya E. Zyumchenko.

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doi:10.1016/j.cellbi.2005.10.013


ISSN Print: 1065-6995
ISSN Electronic: 1095-8355
Published by Portland Press Limited on behalf of the International Federation for Cell Biology (IFCB)