Differential mating success in populations of seaweed flies (Coelopa frigida)

Abstract

The reproductive success of seaweed flies (Coelopa frigida) was studied in natural populations. The sizes of females, both mothers and those not producing eggs, as well as of the males available to be fathers, were estimated from their wing lengths. The animals were also classified for their genotype at the alcohol dehydrogenase (Adh) locus—a polymorphic locus tightly associated with a large chromosomal inversion. The fathers' genotypes were inferred from the genotypes of the mother and her offspring. Males of different Adh genotypes had different reproductive success. Reproductive success of genotypes was negatively correlated with the mean sizes of males. Differences in the reproductive success of females could be accounted for by their differential egg laying capacity, and is probably not a consequence of their mating behaviour. The incidence of progeny with mixed paternity varied from 20 per cent initially, to about 65 per cent when the animals had been mating for 6 days. It appears from previous work that large males are more successful in laboratory trials, yet under natural conditions genotypes associated with large male size have low mating success. Ways of reconciling these results and their relevance to understanding the inversion polymorphisms are discussed.