Potamopyrgus antipodarum (4-7 mm) occurs at high densities in a wide range of freshwater habitats throughout New Zealand. Most populations of this gastropod consist solely of triploid parthenogenetic females, but many populations also contain diploid, sexual females and males. Thus there are many "mixed populations" in which sexual and asexual individuals coexist (Dybdahl and Lively 1995). It is the occurrence of these mixed populations that make the snail so valuable for testing theory on the maintenance of sex.
Genetic studies have shown that asexual lines are derived from sympatric sexual females; and that clonal diversity in mixed populations is very high (Dybdahl & Lively 1995). Specifically, clones tend to be unique to a particular lake population, and they share alleles that are present in the local sexual population. Clonal diversity is a monotonic function of sampling effort, with over 100 clones identified in some lakes. In addition, different sets of clonal genotypes tend to aggregate in different depth-stratified vegetation zones (Fox et al. 1996; Jokela et al. 1999). This later result is suggestive of niche partitioning among the clones (see Negovetic & Jokela 2000, 2001).
Recently, we tested the common assumption that, except for mode of reproduction, all else is equal between sexual and parthenogenetic females in natural populations. We found variation among the depth-stratified habitats in size(age) at first reproduction and brood size; but, within these habitats, sexual individuals did not differ significantly from parthenogens (Jokela et al., 1997a,b). Sexuals and asexuals also did not differ in the number of aborted embryos in the brood chamber (Jokela et al., 1997a,b). Hence, as far as we can tell, asexual females are capable of producing twice the number of daughters as sexual females, giving a full two-fold cost of sex. As such, it is especially curious that the asexuals have not eliminated the sexuals in mixed populations. It is also curious that such high clonal diversity is observed. Why have the most fecund clones not eliminated the others?
One possible explanation for the maintenance of sex and clonal diversity in mixed populations is frequency-dependent selection by parasites. P. antipodarum is the first intermediate host to over a dozen species of digenetic trematodes (e.g. Jokela & Lively 1995a). These trematode worms infect the snails as larvae, and asexual reproduction by these larvae invariably sterilizes infected snails. Hence there is at least the potential for strong selection. Most of our work has focused on an undescribed species of trematode (Microphallus sp), which is also the most common source of infection in the snail.
Dybdahl, M. F. and C.M. Lively. 1995. Diverse, endemic and polyphyletic clones in mixed populations of the freshwater snail Potamopyrgus antipodarum. Journal of Evolutionary Biology 8:385-398.
Fox, J.A., M.F. Dybdahl, J. Jokela, and
C.M. Lively. 1996. Genetic structure of coexisting sexual and
clonal subpopulations in a freshwater snail (Potamopyrgus antipodarum).
Jokela, J., M.F. Dybdahl, and C.M. Lively. 1999. Habitat-specific variation in life-history traits, clonal population structure, and parasitism in a freshwater snail (Potamopyrgus antipodarum). Journal of Evolutionary Biology 12:350-360.
Jokela, J. and C.M. Lively. 1995. Spatial
variation in infection by digenetic trematodes in a population
of freshwater snails (Potamopyrgus antipodarum).
Jokela, J., C.M. Lively, M.F. Dybdahl, and J.A. Fox. 1997a. Evidence for a cost of sex in the freshwater snail Potamopyrgus antipodarum. Ecology 78:452-460.
Jokela, J., C.M. Lively, J.A. Fox, and
M.F. Dybdahl. 1997b. Flat reaction norms and "frozen"
phenotypic variation in clonal snails (Potamopyrgus antipodarum).
Negovetic, S. and Jokela, J. 2000. Food choice behavior may promote habitat specificity in mixed populations of clonal and sexual Potamopyrgus antipodarum. Animal Behaviour 60: 435-441.
Negovetic, S. and Jokela, J. 2001. Life-history variation, phenotypic plasticity and maintenance of subpopulation structure in a freshwater snail. Ecology, in press.
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