Inbreeding depression in a self-compatible, androdioecious crustacean, Eulimnadia texana

The observation that offspring produced by the mating of close relatives ar e often less fit than those produced by matings between unrelated individua ls (i.e., inbreeding depression) has commonly been explained in terms of th e increased probability of expressing deleterious recessive alleles among i nbred offspring (the partial dominance model). This model predicts that inb reeding depression should be limited in regularly inbreeding populations be cause the deleterious alleles that cause inbreeding depression (i.e., the g eneric load) should be purged by regularly exposing these alleles to natura l selection. We indirectly test the partial dominance model using four high ly inbred populations of an androdioecious crustacean, the clam shrimp Euli mnadia texana. These shrimp are comprised of males and hermaphrodites, the latter capable of either self-fertilizing or mating with a male (i.e., outc rossing between hermaphrodites is impossible). Hermaphrodites are further s ubdivided into monogenics (produced via self-fertilization) and amphigenics (produced via self-fertilization or outcrossing). Electrophoretic evidence suggests significant differences in heterozygosity among populations, but that selfing rates were not statistically different (average s = 0.67). Add itional electrophoretic analyses reveal that three previously described sex -linked loci (Fum, Idh-1,and Idh-2) are all tightly linked to each other, w ith crossing over on the order of 1% per generation. Although selfing rates are clearly high, we present evidence that early inbreeding depression (ha tching rates, juvenile survival, and age at sexual maturity) exists in all four populations. For all of these factors, inbreeding depression was infer red by the positive correlation of multilocus heterozygosity and fitness. C umulative inbreeding depression (delta) is between 0.41 and 0.47 across all populations, which appears to be too low to limit the effects of purging v ia identity disequilibrium. Instead, we suggest that the maintenance of inb reeding depression in these populations is due to the observed linkage grou p, which we suggest contains a large number of genes including many related to fitness. Segregation of such a large linkage group would explain our ob servations of the predominance of amphigenic hermaphrodites in our field sa mples and of survival differences between monogenics and amphigenics within selfed clutches. We propose that a modified form of the overdominance mode l for inbreeding depression operating at the level of linkage groups mainta ins the observed levels of inbreeding depression in these populations even in the face of high rates of selfing.