The inheritance of autosomal and sex-linked cuticular pigmentation patterns in the marine isopod, Paracerceis sculpta Holmes, 1904 (Isopoda: Sphaeromatidae)

Cuticular pigmentation is highly variable in Paracerceis sculpta Holmes, 1904, a Gulf of California isopod. Individuals expressing L2r (= Laterals-2 red) have red pigmentation on the lateral margins of their 6-7th body segments. Individuals expressing 3rs (= Three red stripes) have three red pigmentation zones running the length of their bodies. Individuals expressing Cd (= Cephalon dark) have black head capsules. In 12 years of field collections, L2r and 3rs represented less than 0.3% of all individuals (N = 5491) with the frequency of each sex proportional to population frequencies. Of all individuals scored as Cd (N = 178), 98% were females. We crossed marked and unmarked parents in all possible combinations. Progeny ratios for L2r and 3rs met Mendelian expectations within families as did adult expression of 3rs despite observed biases in family sex ratio. In three generations, Cd females crossed to unmarked males produced families with 1:1 sex ratios, 100% Cd daughters and no Cd sons. Sons from these families never produced Cd daughters. Our results suggest that L2r and 3rs are controlled by dominant, autosomal alleles. The sex-limited expression of Cd corroborates previous results suggesting female heterogamety in P. sculpta and in other flabelliferan isopods. The appearance of rare phenotypes controlled by dominant alleles is paradoxical given the hypothesis that allelic dominance evolves in response to positive selection. However, this combination might persist if apostatic selection imposed by visual predators occurs in this species' structured populations, thereby favoring dominance modifiers that suppress fitness losses by heterozygotes. This species with its large number of cuticular markers could provide tests of this hypothesis.