Maintenance of androdioecy in the freshwater shrimp, Eulimnadia texana: Estimates of inbreeding depression in two populations

Androdioecy is an uncommon form of reproduction in which males coexist with hermaphrodites. Androdioecy is thought to be difficult to evolve in specie s that regularly inbreed. The freshwater shrimp Eulimnadia texana has recen tly been described as both androdioecious and highly selfing and is thus an omalous. Inbreeding depression is one factor that may maintain males in the se populations. Here we examine the extent of "late" inbreeding depression (after sexual maturity) in these clam shrimp using two tests: (1) comparing the fitness of shrimp varying in their levels of individual heterozygosity from two natural populations that differ in overall genetic diversity; and (2) specifically outcrossing and selfing shrimp from these same population s and comparing fitness of the resulting offspring. The effects of inbreedi ng differed within each population. In the more genetically diverse populat ion, fecundity, size, and mortality were significantly reduced in inbred sh rimp. In the less genetically diverse population, none of the fitness measu res was significantly lowered in selfed shrimp. Combining estimates of earl y inbreeding depression from a previous study with current estimates of lat e inbreeding depression suggests that inbreeding depression is substantial (delta = 0.68) in the more diverse population and somewhat lower (delta = 0 .50) in the less diverse population. However, given that males have higher mortality rates than hermaphrodites, neither estimate of inbreeding depress ion is large enough to account for the maintenance of males in either popul ation by inbreeding depression alone. Thus, the stability of androdioecy in this system is likely only if hermaphrodites are unable to self-fertilize many of their own eggs when not mated to a male or if male mating success i s generally high (or at least high when males are rare). Patterns of fitnes s responses in the two populations were consistent with the hypothesis that inbreeding depression is caused by partially recessive deleterious alleles , although a formal test of this hypothesis still needs to be conducted.