Androdioecy (mixtures of males and hermaphrodites) is a rare mating system in both plants and animals. Theory suggests that high levels of inbreeding depression can maintain males in androdioecious populations if hermaphrodites commonly self-fertilize. However, if inbreeding depression (d) can be ‘purged’ from selfing populations, maintaining males is more difficult. In the androdioecious clam shrimp, Eulimnadia texana, d is estimated to be as high as 0.7. Previous work suggests that this high level is maintained in the face of high levels of inbreeding due to an associative overdominance of fitness-related loci with the sex-determining locus. Such associative overdominance would make purging of inbreeding depression difficult to impossible. The current experiment was designed to determine if d can be purged in these shrimp by tracking fitness across seven generations in selfing and outcrossing treatments. Evidence of purging was found in one of four populations, but the remaining populations demonstrated a consistent pattern of d across generations. Although the experimental design allowed ample opportunity for purging, the majority of populations were unable to purge their genetic load.

Therefore, d in this species is likely due to associative overdominance caused by deleterious recessive alleles linked to the sex determining locus.