Female mate choice for large males in several species of seaweed fly (Diptera: Coelopidae)

The mating system of the seaweed fly Coelopa frigida involves a vigorous premating struggle during which females attempt to dislodge mounted males by kicking and shaking from side to side. Additionally females can prevent engagement of genitalia by curling their abdomens downwards. Large males gain a mating advantage. Male size is partially determined by a chromosomal inversion polymorphism which is maintained by strong heterosis. Thus female mate choice on the basis of size will affect offspring fitness. We report the occurrence of premating struggles and mate choice for large males in five additional species of seaweed fly, namely, C.nebularum, C.vanduzeei, C.pilipes, Gluma musgravei andG.nitida . Four of these species appear to lack the inversion system, suggesting that mate choice for large males can be maintained in its absence and also evolved before its establishment. Gluma females had stronger preferences thanCoelopa females and showed an additional response to mounting, namely, curling their abdomens upwards into the male. This may allow assessment of male size.