Genetic bias in repeated evolution of pigment loss in cave populations of the Asellus aquaticus species complex

Similar phenotypes can evolve repeatedly under the same evolutionary pressures. A compelling example is the evolution of pigment loss and eye loss in cave-dwelling animals. While specific genomic regions or genes associated with these phenotypes have been identified in model species, it remains uncertain whether a bias towards particular genetic mechanisms exists. An isopod crustacean, Asellus aquaticus, is an ideal model organism to investigate this phenomenon. It inhabits surface freshwaters throughout Europe but has colonized groundwater on multiple independent occasions and evolved several cave populations with distinct ecomorphology. Previous studies have demonstrated that three different cave populations utilized common genetic regions, potentially the same genes, in the evolution of pigment and eye loss. Expanding on this, we conducted analysis on two additional cave populations, distinct either phylogenetically or biogeographically from those previously examined. We generated F2 hybrids from cave × surface crosses and tested phenotype-genotype associations, as well as conducted complementation tests by crossing individuals from different cave populations. Our findings revealed that pigment loss and orange eye pigment in additional cave populations were associated with the same genomic regions as observed in the three previously tested cave populations. Moreover, the lack of complementation across all cross combinations suggests that the same gene likely drives pigment loss. These results substantiate a genetic bias in the recurrent evolution of pigment loss in this model system. Future investigations should focus on the cause behind this bias, possibly arising from allele recruitment from ancestral surface populations' genetic variation or advantageous allele effects via pleiotropy.