Genetics of sexual Ostracoda from a low Arctic site

1992

This study describes the results of laboratory breeding experiments designed to determine the mode of reproduction of female Candonocypris novaezelandiae from populations with and without males. Two morphological forms of this species were studied: a large-green and small-brown morph. Large-green females from populations without large-green males invariably generated genotypically identical daughters by parthenogenesis, regardless of the presence or absence of males in the laboratory cultures. In contrast large-green females from a population that included large-green males, when they had the opportunity for fertilization, usually produced genotypically diverse offspring of both sexes. These females were probably sexually reproducing. Unmated females from the mixed-sex population did not reproduce. The small-brown morph was collected from a single water body in which it was represented by both sexes and co-occurred with large-green parthenogenetic females. Most of the small-brown females tested reproduced sexually in the presence of males or did not reproduce in the absence of males. One small-brown unmated female, however, reproduced by parthenogenesis. Available evidence indicates that the field population of the small-brown morph includes both sexual and parthenogenetic females and that the relative abundance of the parthenogenetic females is increasing with time. The sexual and asexual forms within and between morphs are probably independent.

Section: Discussionmentioning

confidence: 99%

Variation in the mode of reproduction among individuals of the ostracod Candonocypris novaezelandiae

1992

“…Similarly, high levels of allele frequency divergence over microgeographical scales have been documented for ponddwelling species representing a range of taxa and breeding systems (e.g. Hebert & Payne, 1985;Havel et at., 1990). Those studies that have also included macrogeographical sampling indicate that the finescale divergence is only rarely accompanied by regional differentiation (Benzie, 1986;Schwartz & Hebert, 1987;Crease et at., 1990;Hebert et a!., 1993).…”

Section: Discussionmentioning

confidence: 87%

Genetic evidence of widespread dispersal in a parthenogenetic freshwater ostracod

Ayre

1997

We used an hierarchical analysis of allozyme variation to investigate for the freshwater ostracod Candonocypris novaezelandiae the relative contributions of sexual and asexual reproduction to recruitment into 42 local populations and to infer patterns of gene flow within and among four geographical regions (watersheds) in south-eastern Australia. Allele frequency variation among local populations was marked (mean FST = 0.228) but showed no regional differentiation. The allele frequency differences among local populations probably reflect the effects of stochastic processes, such as founder events, as well as variation in the success, and hence abundance, of particular clonal genotypes within water-bodies. Indeed, local populations were highly clonal, containing only females and displaying relatively low levels of genotypic diversity. Nevertheless, the distribution of genotypes within and among regions was surprising. The bulk of sampled individuals (88 per cent) were represented by just six common genotypes that were shared extensively among local populations and were geographically widespread. Individual samples contained a mean of 4.05, and up to 10, distinct four-locus genotypes and overall we detected a total of 26 electrophoretically distinct genotypes. In combination, our results suggest that either the south-eastern Australian populations of C. novaezelandiae arose through a recent colonization event (perhaps associated with an expansion of agricultural practices) or there is sufficient continuing gene flow between regions to prevent differentiation. However, the exact contributions of sexual and asexual reproduction to dispersal in this ostracod remain unclear.

“…Given that the genetic data of both previous breeding trials (Chaplin, 1991(Chaplin, , 1992 and this paper indicate that Mpi' is a reliable indicator of the sexual lineage, it is likely that there is an excess of females even within the sexual lineage of the smallbrown morph. Such excesses may, in fact, be common within strictly sexual Ostracoda (Havel et a!., 1990). In the case of the small-brown morph in Lake Purrumbete, the presence of parthenogens further increases the extent of the female bias of the sex ratio.…”

Section: Discussionmentioning

confidence: 99%

“…Large excesses of females are common in mixed-sex populations of ostracods. This is true even of populations for which their exclusive reliance upon sexual reproduction has been verified with allozyme data (Havel et al, 1990). Although the basis for these excesses of females has not been investigated experimentally for any species, it is often explained in terms of reduced longevity and sometimes reduced vitality of males (McKenzie, 1971;Tetart, 1975;Harten, 1983).…”

Section: Introductionmentioning

confidence: 99%

The local displacement of a sexually reproducing ostracod by a conspecific parthenogen

1993

Candonocypris novaezelandiae is a freshwater ostracod which can perhaps best be described as a species complex of sexual and parthenogenetic forms of each of two morphs. Genetically distinct sexual and parthenogenetic forms of one of these morphs (the small-brown morph) coexist in a large maar lake in south-eastern Australia. Repeated sampling has revealed that the sex ratio of the small-brown morph in this lake is highly significantly biased in favour of females and that, during a 27 month sampling period, the extent of this bias increased consistently from 2.76 females per male in the first sample to 6.9 females per male in the final sample. Associated with the increase in the relative abundance of females was a reduction in the level of multi-locus genotypic diversity relative to expectations for sexual reproduction and recruitment. The initial sample displayed 92 per cent of the multi-locus genotypic diversity expected for a sample from a population with the same underlying genetic composition but which relies exclusively upon sexually generated recruitment; only 50 per cent of the expected diversity was observed for the final sample. These sex ratio and diversity changes are consistent with an increase in the relative abundance of parthenogenetic females in this population. This interpretation is supported by the finding that during the sampling period the relative abundance of four putative clonal genotypes in samples increased from 3.8 to 29.41 per cent. The displacement of a sexual lineage by a closely related parthenogenetic form in a spatially heterogeneous, biotically diverse environment, such as that sampled, is at odds with theoretical predictions about the ecological and evolutionary roles of sexual reproduction but may be related to an increase in the degree of human disturbance of the lake habitat (or at least of the sampling site). In contrast to the lake population in which the sexual lineage appears, at least for the time being, to be numerically dominant, populations of this ostracod in two artificial farm ponds located within close proximity to the lake had a highly clonal structure.