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

Abstract

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.

Inbreeding and outbreeding depression in geographical parthenogensHeterocypris incongruensandEucypris virens(Crustacea: Ostracoda)

“…But, in geographic parthenogens, the spatial distribution of sexual and asexual forms is disjointed by definition (Bell 1982;Suomalainen et al 1987;Schwander & Crespi 2009). In southeastern Australia, recent and rapid expansion of parthenogenetic females resulted in the local displacement of sexual lineages of Candonocypris novaezelandiae (Chaplin 1993;Chaplin & Ayre 1997). In H. barbara we have not found significant differences in life history traits between sexual and asexual females that coexist in Lampedusa Island temporary ponds (Rossi et al 2007).…”

Heterocypris(Crustacea, Ostracoda) from the Isole Pelagie (Sicily, Italy): Population genetics

“…By direct comparison we have observed that both female morphotypes from Lampedusa share several alíeles at different loci with winter clones of H. incongrunes from northern Italy (Rossi et al, 1996). In this case, our data should provide a further genetic evidence of coexisting syngamic and clonal lineages in ostracods as shown for Candonocypris novaezelandiae (Chaplin, 1992(Chaplin, , 1993Chaplin & Ayre, 1997) and Eucypris virens (Rossi et al, 1998;Schön et al, 2000;Rossi et al, in preparation).…”

“…In geographic parthenogenetic species, such as H. incongruens, gene pool diversity among populations may be very high due to a wide geographic spread, fragmented habitat, low gene flow and high likelihood of bottlenecks. Among different clonal lineages of H. incongruens from the Po Plain, genetic similarity index ranged from 0.46 to 0.99 and was linked to morphological features and ecological specialisation (Rossi & Menozzi, 1990, 1993Rossi et al, 1993Rossi et al, , 1996. By direct comparison we have observed that both female morphotypes from Lampedusa share several alíeles at different loci with winter clones of H. incongrunes from northern Italy (Rossi et al, 1996).…”

“…The female-biased sex ratio, observed in these populations and commonly re-ported in freshwater ostracods, could be the result of the sympatric occurrence of parthenogenetic and amphimictic females (Chaplin et al, 1994). In Ostracoda, an example of a species complex that includes both sexual and asexual females which are genetically and morphologically distinct was described by Chaplin (1992Chaplin ( , 1993 and Chaplin & Ayre (1997) in Candonocypris novaezelandiae.…”

Genetic characterization of breeding systems, ploidy levels and species boundaries in Cypricercus (Ostracoda)

“…Past breeding system determinations have relied on gender ratio analyses, with parthenogenesis being inferred for all-female populations. Genetic studies have now established the general validity of breeding system assignments made using the approach (Havel & Hebert, 1989), as the presence of males, even at low frequency, has always been associated with sexual recruitment (Chaplin, 1993).…”