A TEST OF THE "GENERAL-PURPOSE" GENOTYPE HYPOTHESIS: DIFFERENTIAL TOLERANCE TO THERMAL AND SALINITY STRESS AMONG DAPHNIA CLONES.

Abstract

The advantages and disadvantages of sexual reproduction have been the subject of considerable debate (Maynard Smith 1978; Michod and Levin 1988) in both the plant (Bierzychudek 1985) and animal (Lynch 1984) literature. Recently, the phenomenon of (Vandel 1928, 1940) has been reexamined (Lynch 1984; Beaton and Hebert 1988; Bierzychudek 1989). Geographic parthenogenesis refers to the observation that differences exist in the geographic ranges between asexual taxa and their sexual relatives. Generally, the patterns indicate that asexuals tend to be distributed over broader latitudinal ranges (e.g., found at higher latitudes) than sexuals, and that asexuals tend to occupy more disturbed marginal habitats. The reasons for these differences are not clear (Glesener and Tilman 1978; Bell 1982; Lynch 1984), but the notion that perhaps asexual taxa possess more broadly adapted genotypes (i.e., general genotypes) than sexual taxa has gained some support (Bierzychudek 1989; Michaels and Bazzaz 1989). The notion is that asexual genotypes are transmitted intact (barring any significant mutational input) from generation to generation; any favorable broadly adapted gene complexes, which might allow an individual to track environmental changes, will not be disrupted. Selection will act on the entire composite genotype of an asexual, whereas with sexual organisms, recombination will break up any favorable broadly adapted gene complexes (segregational and recombinational loads; Crow 1988). Few experimental tests of the existence of genotypes within asexualsexual complexes have been conducted (Bierzychudek 1989; Michaels and Bazzaz 1989). In these cases, data are somewhat contradictory, thus, no firm conclusions can be drawn. Most other data are descriptive (Bell 1982; Lynch 1984), and possible alternative explanations have not been excluded (e.g., ploidy-level differences between sexuals and asexuals). The purpose of the present study is to examine whether obligately parthenogenetic clones of the cladoceran, Daphnia pulex, are more broadly tolerant of changes in environmental conditions (i.e., temperature and salinity) than closely related cyclically parthenogenetic clones. If this is true, then the data would lend support to the notion that truly asexual taxa possess general-purpose genotypes even in a limited comparative situation (i.e., compared with close relatives that have both mictic and amictic phases in their life cycle).