The role of triploid hybrids in the evolutionary dynamics of mixed-ploidy populations

Theory suggests that the evolution of autotetraploids within diploid populations will be opposed by a minority-cytotype mating disadvantage. The role of triploids in promoting autotetraploid establishment is rarely considered, yet triploids are often found in natural populations and are formed in experimental crosses. Here, I evaluate the effects of triploids on autotetraploid evolution using computer simulations and by synthesizing research on the evolutionary dynamics of mixed-ploidy populations in Chamerion angustifolium (Onagraceae). Simulations show that the fate of a tetraploid in a diploid population varies qualitatively depending on the relative fitness of triploids, the ploidy of their gametes and the fitness of diploids relative to tetraploids. In general, even partially fit triploids can increase the likelihood of diploid–tetraploid coexistence and, in some cases, facilitate tetraploid fixation. Within the diploid–tetraploid contact zone of C. angustifolium , mixed populations are common (43%), and often (39%) contain triploids. Greenhouse and field studies indicate that triploid fitness is low (9% of diploids) but variable. Furthermore, euploid gametes produced by triploids can be x , 2 x or 3 x and contribute the majority (62%) of new polyploids formed in each generation (2.3 × 10⁻³). Although triploid bridge, alone, may not account for the evolution of autotetraploidy in C. angustifolium , it probably contributes to the prevalence of mixed-ploidy populations in this species. Therefore, in contrast to hybrids in homoploid species, triploids may actually facilitate rather than diminish the fixation of tetraploids by enhancing the rate of formation.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 82 , 537–546.     

Distribution and reproductive capacity of natural triploid individuals and occurrence of unreduced eggs as a cause of polyploidization in the loach,Misgurnus anguillicaudatus

Loaches (Misgurnus anguillicaudatus) were collected from 35 localities in Japan and assayed by flow cytometry to determine ploidy status. No tetraploids were found, with samples from 33 localities having no or few (1.2–3.2%) triploids. Samples collected from Ichinomiya Town, Aichi Prefecture, showed a relatively high rate of triploidy (7.7%). Samples collected from a fish farm in Hirokami Village, Niigata Prefecture, also showed high proportions of triploids (2.0–15.8%), these triploid males being sterile, but the females producing both large-sized triploid and small-sized haploid eggs. Such eggs developed bisexually rather than gynogenetically, giving rise to viable tetraploid and diploid offspring after normal fertilization. Of eight diploid females obtained from the same locality, one produced a high incidence of viable diploid gynogens (55%) after gynogenetic induction by fertilization with UV-irradiated spermatozoa. These observations indicated the presence of diploid fish which produced both diploid and haploid eggs. Thus, triploid and diploid individuals were also produced after fertilization with haploid spermatozoa. These results suggested that the occurrence of such unreduced eggs may be a cause of natural polyploidization in this species.     

Evidence for a monophyletic origin of triploid clones of the Amazon molly, Poecilia formosa

Asexual reproduction in vertebrates is rare and generally considered an evolutionary dead end. Asexuality is often associated with polyploidy, and several hypotheses have been put forward to explain this relationship. So far, it remains unclear whether polyploidization in asexual organisms is a frequent or a rare event. Here we present a field study on the gynogenetic Amazon molly, Poecilia formosa. We used multilocus fingerprints and microsatellites to investigate the genetic diversity in 339 diploid and 55 triploid individuals and in 25 P. mexicana, its sexual host. Although multilocus DNA fingerprints found high clonal diversity in triploids, microsatellites revealed only two very similar clones in the triploids. Phylogenetic analysis of microsatellite data provided evidence for a monophyletic origin of the triploid clones of P. formosa. In addition, shared alleles within the triploid clones between the triploid and diploid genotypes and between asexual and sexual lineages indicate a recent origin of triploid clones in Poecilia formosa.     

Triploid planarian reproduces truly bisexually with euploid gametes produced through a different meiotic system between sex

Although polyploids are common among plants and some animals, polyploidization often causes reproductive failure. Triploids, in particular, are characterized by the problems of chromosomal pairing and segregation during meiosis, which may cause aneuploid gametes and results in sterility. Thus, they are generally considered to reproduce only asexually. In the case of the Platyhelminthes Dugesia ryukyuensis, populations with triploid karyotypes are normally found in nature as both fissiparous and oviparous triploids. Fissiparous triploids can also be experimentally sexualized if they are fed sexual planarians, developing both gonads and other reproductive organs. Fully sexualized worms begin reproducing by copulation rather than fission. In this study, we examined the genotypes of the offspring obtained by breeding sexualized triploids and found that the offspring inherited genes from both parents, i.e., they reproduced truly bisexually. Furthermore, meiotic chromosome behavior in triploid sexualized planarians differed significantly between male and female germ lines, in that female germ line cells remained triploid until prophase I, whereas male germ line cells appeared to become diploid before entry into meiosis. Oocytes at the late diplotene stage contained not only paired bivalents but also unpaired univalents that were suggested to produce diploid eggs if they remained in subsequent processes. Triploid planarians may therefore form euploid gametes by different meiotic systems in female and male germ lines and thus are be able to reproduce sexually in contrast to many other triploid organisms.     

Inter-embryo competition in the progeny of autotriploid Aloineae (Liliaceae)

In reciprocal crosses between diploid and triploid Aloineae the progeny are largely diploid or diploid plus one or two chromosomes, but in reciprocal crosses between triploids and tetraploids they are tetraploid or nearly so. Thus the triploids contribute circa haploid gametes to the progeny when crossed with diploids but circa diploid gametes when crossed with tetraploids. These results are compared with those of a number of earlier workers. It is concluded that the bias in the frequency of progeny types towards diploidy or tetraploidy, depending on the ploidy level of the plant which is crossed with the triploid, is caused by inter-embryo competition. Those embryos with an endosperm/embryo factor of 1.5, the value found in normal diploid/diploid crosses having triploid endosperms, are selected in preference to those with factors higher or lower than 1.5.Inter-gamete competition also occurs among the euploid and aneuploid gametes produced by the triploids. This is more pronounced on the male side, because the degree of survival of aneuploid pollen from the triploids into the next generation is much lower than that of aneuploid egg nuclei.Non-reduction in the triploids gives rise to occasional pentaploid progeny in crosses with tetraploids, but it is more probable that in diploid/triploid crosses tetraploid progeny are the products of non-reduction in the diploid.     

Pollen production and spontaneous polyploidization in diploid populations of Anthoxanthum alpinum

The most common form of polyploidization is that which occurs via the fertilization of unreduced 2 n gametes, i.e. gametes that possess the somatic chromosome number of the species. However, very few data are available concerning the frequency of spontaneous polyploidization in diploid plant populations. In this study we have quantified both the frequency of In pollen production and the frequency of polyploid seed production in diploid populations of the grass Anthoxanthum alpinum. More than 6000 seeds from four different populations collected during two years of study were screened for ploidy using a flow cytometer. In parallel, the frequency both of plants with large pollen (LP) and of LP production on these plants was quantified in two populations. No tetraploid seeds were detected, although in each population a few triploid seeds were regularly produced at a frequency of two triploid seeds per thousand. The frequency of LP producers was similar in the four populations, as was the frequency of LP production. The frequency of LP was ten times that of triploid seeds. These finding suggest that the 'triploid block' is not enough to prevent the regular occurrence of triploids in diploid populations of this species. The role played by the hypothesized 'triploid bridge' in the genesis of autotetraploids is discussed in the light of these results.     

Fitness of alternative modes of reproduction: developmental constraints and the evolutionary maintenance of sex

Parthenogenesis, including facultative parthenogenesis, is common among orthopteroid insects. We investigated the fitness associated with sexual and asexual reproduction within a population of the facultatively parthenogenetic cockroach Nauphoeta cinerea. There is significantly reduced fitness for females reproducing parthenogenetically compared to sexually. Fewer than half of all females can reproduce parthenogenetically. In addition, tenfold fewer offspring are produced by parthenogenesis due to reductions in both the number of offspring produced per clutch and the number of clutches produced. Development and brooding of sexually or parthenogenetically produced first instar nymphs does not differ, although the production of the first parthenogenetic clutch is delayed relative to the first sexually produced clutch. The fitness of parthenogens is also lower than the fitness of sexually produced offspring. Parthenogens are less viable than sexually produced offspring even in the benign conditions of the laboratory. Development to adulthood of parthenogens is slower. Fewer parthenogens survive to adulthood and the adult life span of parthenogens is reduced. Individuals produced by parthenogenetic reproduction are unlikely to reproduce parthenogenetically themselves. Finally, parthenogenetically produced females produce fewer offspring by sexual reproduction than do sexually produced females. Since parthenogenetic reproduction is apomictic in N. cinerea and parthenogens are diploid, we suggest that asexual reproduction is developmentally constrained. Once meiosis has evolved, returning to a mitotic mode of reproduction may be difficult. Nauphoeta cinerea offers a system for testing how asexuality is constrained as modes of reproduction can be compared within a facultative parthenogen.     

Aneuploidy and genetic variation in the Arabidopsis thaliana triploid response

Polyploidy, the inheritance of more than two genome copies per cell, has played a major role in the evolution of higher plants. Little is known about the transition from diploidy to polyploidy but in some species, triploids are thought to function as intermediates in this transition. In contrast, in other species triploidy is viewed as a block. We investigated the responses of Arabidopsis thaliana to triploidy. The role of genetic variability was tested by comparing triploids generated from crosses between Col-0, a diploid, and either a natural autotetraploid (Wa-1) or an induced tetraploid of Col-0. In this study, we demonstrate that triploids of A. thaliana are fertile, producing a swarm of different aneuploids. Propagation of the progeny of a triploid for a few generations resulted in diploid and tetraploid cohorts. This demonstrated that, in A. thaliana, triploids can readily form tetraploids and function as bridges between euploid types. Genetic analysis of recombinant inbred lines produced from a triploid identified a locus on chromosome I exhibiting allelic bias in the tetraploid lines but not in the diploid lines. Thus, genetic variation was subject to selection contingent on the final ploidy and possibly acting during the protracted aneuploid phase.     

Reproduction and the origin of polyploids in hybrid salamanders of the genus Ambystoma

Eggs and larvae produced by diploid, triploid, and tetraploid females collected from breeding ponds on Pelee Island in Lake Erie were studied to examine the reproductive mechanism. No instance of parthenogenesis was found as all examined females required sperm to produce viable progeny. Diploid females produced diploid and triploid larvae, triploid females produced triploid and tetraploid larvae, and tetraploid females produced triploid and tetraploid larvae. The majority of the eggs produced by hybrid females do not develop or do not complete embryogenesis. Electrophoretic examination of females and their offspring demonstrate that the male genome is being incorporated in reduced as well as unreduced eggs produced by all three ploidy classes of females. The elevation of ploidy among Pelee Island Ambystoma is attributed to sperm incorporation in unreduced eggs. Triploid as well as tetraploid individuals are constantly being produced. A critical examination of the literature on parthenogenetic or gynogenetic modes of reproduction in North America Ambystoma hybrids shows no conclusive evidence supporting these modes and it is suggested that the reproductive mechanism found among Pelee Island female hybrids may be more generally applied to other hybrid Ambystoma populations.     

Evolution of triploidy in Apios americana (Leguminosae) revealed by genealogical analysis of the histone H3-D gene

Autotriploidy is normally considered to be maladaptive in plants because of its association with high levels of sterility. Nonetheless, triploid individuals are found in many plant species and play important roles in plant evolution, in particular as a first step toward tetraploid formation. However, few studies have addressed the evolutionary potential of triploid lineages, which may principally suffer from the impossibility of combining useful mutations in a single genome due to their low fertility. Therefore, triploids acquire genetic diversity only via recurrent evolution and somaclonal mutation. This study evaluates the potential of multiple origins of polyploidy as a source of genetic diversity in Apios americana, a North American legume that possesses both diploid and triploid populations. Ploidy level determination via flow cytometry shows that triploids are mainly restricted to the portion of eastern North America that was covered by ice during the Wisconsinan glaciation 18,000 years ago. This distribution implies that either selection or postglaciation colonization played a role in shaping this cytogeographic pattern. A haplotype network of the single copy nuclear histone H3-D gene reconstructed using statistical parsimony, together with single-strand conformational polymorphism analysis, shows that autotriploidy evolved at least three times in this species and that heterozygosity is high in triploids. The genetic diversity found in A. americana resulting from recurrent evolution and fixed heterozygosity increases the likelihood of producing successful genotypes and may give the opportunity for triploids to be better fit than diploids in new habitats. This suggests that triploid lineages can exhibit evolutionary potential of their own, and do not serve solely as a first step toward tetraploid formation.     

The effect of ploidy level on fitness in parthenogenetic flatworms

Although polyploidy plays an important role in speciation, its impact on fitness is still debated. One problem is that its adaptive significance can only be inferred by comparing forms with different ploidy that are identical in all other traits. This situation is uncommon, presumably because ploidy types often differ in reproduction mode, genetic background or habitat. Here we compare fitness in a system of triploid and tetraploid karyotypes of the planarian flatworm Schmidtea polychroa . Both types have the same type of sperm-dependent parthenogenesis and share the same genetic background and habitat. Hence, fitness differences, if any, can be attributed to different ploidy levels only. Contrary to the general assumption of a positive correlation between fitness and ploidy level, we showed that triploids produced 58% more offspring than tetraploids. Within each ploidy type, we identified groups of highly related clones using microsatellites. Significant variation among clonal groups in body size, offspring and cocoon number and hatching time indicated a genetic basis for variance in these traits. A small model shows that despite low fitness of tetraploids, stable coexistence of triploids and tetraploids can be explained by the recurrent origin of triploids from tetraploids and vice versa.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 85 , 191–198.     

Fitness differences among diploids, tetraploids, and their triploid progeny in Chamerion angustifolium: mechanisms of inviability and implications for polyploid evolution

Abstract. Theoretical models indicate that the evolution of tetraploids in diploid populations will depend on both the relative fitness of the tetraploid and that of the diploid-tetraploid hybrids. Hybrids are believed to have lower fitness due to imbalances in either the ploidy (endosperm imbalance) or the ratio of maternal to paternal genomes in their endosperm (genomic imprinting). In this study we created diploids, tetraploids, and hybrid triploids of Chamerion angustifolium from crosses between field-collected diploid and tetraploid plants and evaluated them at six life stages in a greenhouse comparison. Diploid offspring (from 2 x × 2 x crosses) had significantly higher seed production and lower biomass than tetraploid offspring (from 4 x × 4 x crosses). Relative to the diploid, the cumulative fitness of tetraploids was 0.67. In general, triploids (from 2 x × 4 x , 4 x × 2 x crosses) had significantly lower seed production, lower pollen viability, and higher biomass than diploid individuals. Triploid offspring derived from diploid maternal parents had lower germination rates, but higher pollen production than those with tetraploid mothers. Relative to diploids, the cumulative fitness of 2 x × 4 x triploids and 4 x × 2 x triploids was 0.12 and 0.06, respectively, providing some support for effect of differing maternal:paternal ratios and endosperm development as a mechanism of hybrid inviability. Collectively, the data show that tetraploids exhibit an inherent fitness disadvantage, although the partial viability and fertility of triploids may help to reduce the barrier to tetraploid establishment in sympatric populations.     

Reproductive capacity of triploid loaches obtained from Hokkaido Island, Japan

Reproductive capacity was investigated in naturally occurring triploid individuals of the loach Misgurnus anguillicaudatus collected from Memanbetsu Town, Abashiri County, Hokkaido Island, Japan. These triploids have been considered to appear by accidental incorporation of the haploid sperm genome from normal diploid into unreduced diploid eggs from the clonal lineage that usually reproduces unisexually. By fertilization with sperm from the normal male, one triploid female gave many inviable aneuploid (2.1–2.7n) and very few tetraploid progeny, whereas the other produced both diploid and triploid progeny. The results suggest that at least four different types of eggs can be formed in triploid females in this locality. In contrast, no progeny hatched when eggs of the normal female were fertilized with sperm or sperm-like cells obtained from triploid males. These gametes exhibited inactive or no motility after adding ambient water. They had larger head sizes than those of normal haploid sperm and had a short or no tail. Although their ploidy was triploid or hexaploid, a small number of haploid cells were detected in the semen by flow cytometry. Thus, triploid males were generally sterile, but they have a little potential for producing very few haploid sperm.     

Polyploidization increases the sensitivity to DNA-damaging agents in mammalian cells

Polyploidization occurs during normal development as well as during tumorigenesis. In this study, we investigated if the responses to genotoxic stress in cancer cells are influenced by the ploidy. Prolonged treatment of Hep3B cells with the spindle inhibitor nocodazole resulted in mitotic slippage, followed by re-replication of the DNA to produce polyploids. Reintroduction of p53 restored the checkpoints and suppressed polyploidization. Remarkably, a stable tetraploidy cell line could be generated from Hep3B by a transient nocodazole treatment followed by a period of recovery. Using this novel tetraploid system, we found that tetraploidization increased the cell volume without significantly affecting the cell cycle. Although tetraploidization was accompanied by an increase in centrosome number, the majority of mitoses in the tetraploid cells remained bipolar. Polyploidization sensitized cells to genotoxic stress inflicted by ionizing radiation and topoisomerase inhibitors without affecting the sensitivity to spindle inhibitors. Accordingly, more gamma-H2AX foci were induced by radiation in tetraploids than in normal Hep3B cells. Likewise, primary tetraploid human fibroblasts displayed higher gamma-H2AX foci formation than diploid human fibroblasts. An implication for chemotherapy is that some cancer cells can be sensitized to genotoxic agents by a preceding step that induces polyploidization.     

Polyploidy and aneuploidy induced by colcemid in Drosophila melanogaster

Colcemid was fed to Drosophila melanogaster larvae throughout most of the larval period. Surviving individuals were then mated with untreated flies, and their progeny were examined for polyploid flies or flies resulting from X-chromosome nondisjunction. A total of 251 polyploid offspring was recovered from the experimental matings, none from the control. All of the polyploids were evidently triploids, and all but one were obtained from colcemid-fed females: males produced significantly lower frequencies of triploid offspring than females. The highest average frequency of triploid offspring obtained from any treatment group was 18%. Nonrandom distributions of triploid offspring were observed among females raised identically, indicating tht polyploidization occurs mitotically, rather than meiotically, giving rise to clones of tetraploid oogonia. 9 colcemid-fed females produced exclusively triploid offspring. Colcemid also caused a significant increase in X-chromosome nondisjunction in females, though the frequencies of such offspring were at least several-fold lower than the frequencies of triploid offspring. Somatic polyploidy was apparently also indiced since patches of large cells were found on the wings of some flies raised on colcemid-containing food. Various teratological abnormalities were observed among the treated flies, including deformed or missing eyes and partially duplicated thoraxes.     

Can obligate apomixis and more stable reproductive assurance explain the distributional successes of asexual triploids in Hieracium alpinum (Asteraceae)?

• Although reproductive assurance has been suggested to be one of the most important factors shaping the differential distributional patterns between sexuals and asexuals (geographic parthenogenesis), it has only rarely been studied in natural populations of vascular plants with autonomous apomixis. Moreover, there are almost no data concerning the putative relationship between the level of apomictic versus sexual plant reproduction on one hand, and reproductive assurance on the other.
• We assessed the level of sexual versus apomictic reproduction in diploid and triploid plants of Hieracium alpinum across its distributional range using flow cytometric analyses of seeds, and compared the level of potential and realized seed set, i.e. reproductive assurance, between the two cytotypes under field and greenhouse conditions.
• Flow cytometric screening of embryos and endosperms of more than 4,100 seeds showed that diploids produced solely diploid progeny sexually, while triploids produced triploid progeny by obligate apomixis. Potential fruit set was much the same in diploids and triploids from the field and the greenhouse experiment. While in the pollination‐limited environment in the greenhouse apomictic triploids had considerably higher realized fruit set than sexual diploids, there was no significant difference between cytotypes under natural conditions. In addition, sexuals varied to a significantly larger extent in realized fruit set than asexuals under both natural and greenhouse conditions.
• Our results indicate that triploid plants reproduce by obligate apomixis, assuring more stable and predictable fruit reproduction when compared to sexual diploids. This advantage could provide apomictic triploids with a superior colonisation ability, mirrored in a strong geographic parthenogenesis pattern observed in this species.

     

Pathways to polyploidy: indications of a female triploid bridge in the alpine species <Emphasis Type="Italic">Ranunculus kuepferi</Emphasis> (Ranunculaceae)

Polyploidy is one of the most important evolutionary processes in plants. In natural populations, polyploids usually emerge from unreduced gametes which either fuse with reduced ones, resulting in triploid offspring (triploid bridge), or with other unreduced gametes, resulting in tetraploid embryos. The frequencies of these two pathways, and male versus female gamete contributions, however, are largely unexplored. Ranunculus kuepferi occurs with diploid, triploid and autotetraploid cytotypes in the Alps, whereby diploids are mostly sexual, while tetraploids are facultative apomicts. To test for the occurrence of polyploidization events by triploid bridge, we investigated 551 plants of natural populations via flow cytometric seed screening. We assessed ploidy shifts in the embryo to reconstruct female versus male gamete contributions to polyploid embryo and/or endosperm formation. Seed formation via unreduced egg cells (B_III hybrids) occurred in all three cytotypes, while only in one case both gametes were unreduced. Polyploids further formed seeds with reduced, unfertilized egg cells (polyhaploids and aneuploids). Pollen was highly variable in diameter, but only pollen >27 μm was viable, whereby diploids produced higher proportions of well-developed pollen. Pollen size was not informative for the formation of unreduced pollen. These results suggest that a female triploid bridge via unreduced egg cells is the major pathway toward polyploidization in R. kuepferi, maybe as a consequence of constraints of endosperm development. Triploids resulting from unreduced male gametes were not observed, which explains the lack of obligate sexual tetraploid individuals and populations. Unreduced egg cell formation in diploids represents the first step toward apomixis.     

Tetraploid hybrids from crosses between diploid and tetraploidDactylis and their significance

Chromosome counts on the progeny of crosses between diploid and tetraploid races ofDactylis show that tetraploid hybrids are produced as well as the expected triploids. The relative proportions of 4x and 3x hybrids vary greatly in different crosses, and the data suggest that parental geno-type influences the result. Overall, the frquencies of 3x and 4x hybrids are about equal, with no indication of a difference between the reciprocal 2x×4x and 4x×2x cross-combinations except perhaps in the case of diploids and autotetraploids of the same subspecies. Rare triploid hybrids are found in crosses between diploid subspecies ofDactylis. The mechanisms by which a diploid plant could donate two genomes to its offspring are discussed in relation to theDactylis situation, and the evolutionary significance of 4x hybrids formed in this way is considered.     

A Complementary Method for Production of Tetraploid Crassostrea gigas Using Crosses Between Diploids and Tetraploids with Cytochalasin B Treatments

We present a new method to produce tetraploid Crassostrea gigas by cytochalasin B inhibition of polar body 2 expulsion in diploid females crossed with tetraploid males. This offers a means of direct introgression of genetic characters from selected diploid to tetraploid lines, avoiding a triploid step. Offspring larval ploidy shifted over time and depended on size, with tetraploids more frequent among the smaller larvae and triploids among the large. Viable tetraploids were found at 4 and 6 months, indicating the technique was successful. The possibility that gynogenesis occurred was tested by microsatellite analysis to confirm the presence of paternally inherited alleles. These were present in all animals of the 2n × 4n + CB (female first) cross. However, a 4n × 2n + CB cross produced triploids, including some gynogens. Our method illustrates for the first time that diploid C. gigas eggs, if selected for large size, can give viable tetraploid offspring.     

Escape from an evolutionary dead end: a triploid clone of Gyrodactylus salaris is able to revert to sex and switch host (Platyhelminthes, Monogenea, Gyrodactylidae)

Diploid parthenogenesis, with rare sex, is considered as the basic mode of reproduction among the hermaphroditic and viviparous Gyrodactylus. A particular strain of the monogenean parasite Gyrodactylus salaris (RBT clone) was recognized by an invariable, unique mitochondrial DNA haplotype in rainbow trout (Oncorhynchus mykiss) farms. The RBT clone was shown to be triploid and asexual by analyzing a 493 bp sequence of a nuclear DNA marker. Three alleles were present as heterozygous in all 237 individuals sampled in years 2001-2005 from five isolated Finnish farms. The triploid clone probably originated from a diploid oocyte fertilized by a non-self hermaphrodite, most probably in a fish farm. Identical mitochondrial COI gene (1606 bp) was also found in G. salaris parasites on landlocked salmon (Salmo salar) in two rivers draining to the lake Kuitozero, Russian Karelia. In the river Pisto, the clone was triploid, but the diagnostic "short" nuclear allele of the RBT clone was replaced by an allele typical for salmon specific parasites in the Lake Onega. The clone in the river Kurzhma was diploid, having lost the "short" allele, but still heterozygous for the other two alleles of the RBT clone. Evidently, the triploid parthenogenetic RBT clone had produced diploid oocytes, when (as a female) stimulated by a non-self mate in the new environment. The genetic reorganization coincided with a switch to the salmon host. Participation of triploids into the gene pool of the species is rarely reported in animals, and the triploidy is generally considered as an irreversible dead-end of the evolution. Liberalism in ploidy level may significantly add to the evolutionary options available for a parasite in ever-changing environments.