Avoiding the cost of males in obligately asexual Daphnia pulex (Leydig)

Asexual organisms are thought to gain an advantage by avoiding the cost of producing males. In the cladoceran Daphnia pulex (Leydig), male production is determined by the environment and is independent of the origin of the as exual obligate parthenogens from the sexual cyclical parthenogens. If there is a cost to producing males, successful obligate parthenogens should have reduced or eliminated male production. Field and laboratory observations s howed that obligate parthenogens have much-reduced male production compared to cyclical parthenogens. Although the reduction or elimination of males i n the obligate parthenogens suggests that the cost of males is avoided, the coexistence of sexual and asexual forms of D.pulex may be partially explai ned by cyclical parthenogens compensating for the cost of males by having g reater fecundity. In addition, the absence of a mating constraint for the o bligate parthenogens may favour an increased allocation to asexual diapausi ng eggs earlier in the season compared to the cyclical parthenogens which r equire mating with males to produce sexual diapausing eggs. No difference i n the production of diapausing eggs was observed, probably because males we re abundant in populations of cyclical parthenogens and do not appear to li mit the production of sexual diapausing eggs. D.pulex is a useful system fo r determining the ecological consequences of abandoning sexual reproduction and explaining the coexistence of sexual and asexual forms of a species.