Sex allocation variation in Daphnia pulex

Daphnia(Crustacea: Cladocera) reproduce by cyclical parthenogenesis in which the sex of offspring is environmentally determined. Although numerous studies have demonstrated that factors such as crowding and short‐day photoperiod stimulate male production, there is limited information on variation in allocation to male and female offspring for any species ofDaphnia. The present study assessed the presence or absence of male production in 96 isofemale lines (clones) from each of eight populations ofDaphnia pulex. An average of 37% (range 18–51%) of clones failed to produce males under crowded conditions in the laboratory. A subset of 14 of these non‐male‐producing clones also failed to produce males under short‐day photoperiod (8L:16D). Three male‐producing clones were within‐clone mated as well as crossed to three non‐male‐producing clones to study the inheritance of the failure to produce males. The average frequency of non‐male‐producing F1progeny was significantly higher (58%,N= 486) among the outcrossed progeny than the inbred progeny (5%,N= 86). In addition, when sixteen of the male‐producing outcrossed progeny were within‐clone mated, only 7% (N= 106) of the resulting F2progeny failed to produce males. These results are consistent with a genetic basis for the absence of male production. Average survival of the progeny from the nine outcrossed matings was more than twice (67%) that of the inbred progeny from the three within‐clone matings (30%), suggesting that within‐clone mating would result in significant inbreeding depression. We present a model that suggests that even low levels of inbreeding could allow non‐male‐producing females to be maintained in a population. The co‐occurrence of non‐male‐producing females and females that produce both males and females inDaphnia pulex bears a similarity to the gynodioecious breeding system found in some plant species.