THE LOCAL DISPLACEMENT OF A SEXUALLY REPRODUCING OSTRACOD BY A CONSPECIFIC PARTHENOGEN

Candonocypris novaezelandiae is a freshwater ostracod which can perhap s best be described as a species complex of sexual and parthenogenetic forms of each of two morphs. Genetically distinct sexual and partheno genetic forms of one of these morphs (the small-brown morph) coexist i n 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 h ighly 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 abu ndance of females was a reduction in the level of multi-locus genotypi c diversity relative to expectations for sexual reproduction and recru itment. The initial sample displayed 92 per cent of the multi-locus ge notypic diversity expected for a sample from a population with the sam e underlying genetic composition but which relies exclusively upon sex ually generated recruitment; only 50 per cent of the expected diversit y was observed for the final sample. These sex ratio and diversity cha nges are consistent with an increase in the relative abundance of part henogenetic females in this population. This interpretation is support ed by the finding that during the sampling period the relative abundan ce 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 rela ted parthenogenetic form in a spatially heterogeneous, biotically dive rse environment, such as that sampled, is at odds with theoretical pre dictions about the ecological and evolutionary roles of sexual reprodu ction but may be related to an increase in the degree of human disturb ance of the lake habitat (or at least of the sampling site). In contra st to the lake population in which the sexual lineage appears, at leas t for the time being, to be numerically dominant, populations of this ostracod in two artificial farm ponds located within close proximity t o the lake had a highly clonal structure.