SEXUAL REPRODUCTION OF DAPHNIA-PULEX IN A TEMPORARY HABITAT

Species of Daphnia (Crustacea: Cladocera) typically reproduce by cycli cal parthenogenesis, in which a period of all-female parthenogenetic r eproduction is followed by sexual reproduction. Sex in Daphnia is dete rmined by the environment, with factors such as temperature, photoperi od and crowding stimulating the production of males and sexual females . Previous studies on Daphnia pulex from temporary pond habitats demon strated the coexistence of male-producing and non-male-producing (NMP) females, as determined under crowding in the laboratory. A strong gen etic component to this sex allocation variation suggested that sex exp ression in D. pulex is better described as a result of genotype-enviro nment interaction. The present study examined the switch from partheno genetic to sexual reproduction in two temporary-pond populations of D. pulex. Both populations showed a very early investment in sexual repr oduction, independent of population density, by producing males very s oon after the populations were reestablished from resting eggs in the early spring. Approximately 40% of the initial broods were male. Addit ional evidence for gender specialization was obtained by observing the sex of two or three successive broods for 85 individual females. Fift y-eight females produced successive broods of females, 13 females prod uced successive broods of males and 14 females produced successive bro ods which included both male and female broods. Females that produced successive female broods under natural conditions included a higher fr equency of NMP females compared to a random sample of females, confirm ing the existence of NMP females. Sexual females were observed in both populations after the first appearence of males, suggesting that the presence of males may stimulate the production of sexual females. For D. pulex populations in a temporary environment, there appears to be a n increased emphasis on sexual reproduction and a decreased influence of the environment on sex determination, compared to Daphnia populatio ns in more permanent habitats.