The effect of larval density on an inversion polymorphism in the seaweed fly Coelopa frigida

Abstract

The seaweed fly, Coelopa frigida, is polymorphic for two gene arrangements on chromosome I. Inversion frequencies in natural populations are very stable, both geographically and temporally and there is a consistent excess of heterokary-otypes. Laboratory experiments are reported which demonstrate that the egg to adult viability of heterokaryotypes is higher than either homokaryotype. This advantage increases markedly with larval density. Field samples also show a strong correlation between larval density and relative viability, expressed as the excess of heterokaryotypes over Hardy-Weinberg expectations. Larvae of a closely related species, C. pilipes contribute significantly to this density effect in natural populations.