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

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.