Year

1991

Degree Name

Doctor of Philosophy

Department

Department of Biology

Abstract

The need for studies of the roles of sexual and asexual reproduction in the life-history of species able to use both modes of reproduction is well recognised. This study uses a combination of approaches to investigate the life-history strategy of the freshwater ostracod Candonocypris novaezelandiae. This species was selected for study because it is common in a range of temporary and permanent water bodies, and includes populations with and apparently without males (and hence may have the capacity for sexual and asexual reproduction). Thus its life-history strategy is potentially directly comparable with the relatively well documented 'Daphnia model' of the roles of sexual and asexual reproduction in freshwater environments. This study represents the first attempt to use the Ostracoda to test theoretical predictions about the ecological correlates of sexuality. It is also a detailed survey of the population genetic structure of an Australian freshwater invertebrate.

There may be at least four divergent lineages within C. novaezelandiae. The greatest genetic (electrophoretic) differences were detected between two sets of populations characterised by distinctive but slightly overlapping size distributions. This distinction was between a large-green and small-brown morph. Both morphs included sexually and asexually reproducing forms. The two reproductive forms within each morph displayed characteristic genetic differences and the sexuals tended to be smaller than the asexuals.

C. novaezelandiae proved to be obligately parthenogenetic in most sampled populations. Extensive surveys of spatial and temporal genetic variation within the large-green morph and, to a lesser extent the small-brown morph, indicated that local populations in south-eastern Australia are usually maintained by asexual reproduction. This seemed to be true regardless of the size or degree of permanency of the sampled water bodies. Laboratory breeding experiments showed that females from clonal populations reproduce by apomictic parthenogenesis, both in the presence and absence of conspecific males. Laboratory hatching experiments confirmed that, in contrast to many facultative asexuals, the resistant eggs of parthenogenetic C. novaezelandiae are asexually generated.

Electrophoretic data suggest that there is relatively little differentiation in the population structure of the large-green morph in south-eastern Australia in either space or time. The same few four-locus genotypes were numerically dominant in most local populations, even though samples were collected from a wide geographic area and a range of environments. Similarly, the multi-locus genotypic composition of local populations was generally stable over periods of up to several years. In fact, genotypes generally persisted in water bodies even throughout periods of complete dehydration. The broad temporal and spatial distributions of four-locus genotypes may imply that clones have broad ecological tolerances and a good capacity for colonisation. However, this interpretation may be undermined by the fact that four-locus genotypes provide only a weak test of clonal identities.

Despite the fact that the large-green morph may be strictly asexual in south-eastern Australia, local populations were usually multi-clonal. Nevertheless, similar levels of diversity were detected across a range of environments and there was no strong evidence of clonal extinction through competitive exclusion. Dispersal of clones among water bodies,followed by good persistence in colonised water bodies, was inferred to be an important source of diversity within local populations. However, the exact mechanisms by which multiple clones coexist within single water bodies are not clear.

C. novaezelandiae does sometimes reproduce sexually. For the large-green morph, evidence of sexual reproduction was restricted to a population in a large, temporary swamp in Western Australia. This population is probably exclusively sexual. In partial contrast, there was a sexually reproducing form of the small-brown morph as well as smallbrown and large-green parthenogens in a large permanent lake in southern Victoria (Lake Purrumbete). The sexually reproducing form was numerically dominant but was apparendy being rapidly displaced by the small-brown parthenogens. Such an occurrence, especially in a species-rich, spatially heterogeneous environment such as Lake Purrumbete, is at odds with the predictions of some theorists.

Although there are some parallels, the life-history strategy of C. novaezelandiae generally provides a contrast with that of Daphnia. This ostracod showed no clear association between sexual reproduction and temporary (unstable) water bodies, and there was little evidence of ecological diversification of clones. In particular, the population genetic structure of the large-green morph in south-eastern Australia appears to largely reflect the recent and expansive spread of a relatively small number of generalist clones. Since C. novaezelandiae is common in farm dams and natural water bodies used for watering livestock, this spread is probably partly linked to farming activities. On the other hand, sexually reproducing forms of this ostracod showed very restricted distributions.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.