SEX DETERMINATION IN DAPHNIA-MAGNA

Fluctuating food levels had no effects on offspring sex when starvation was not severe. Water from crowded populations of different Daphnia species contained substances that induced up to 43% male broods. The chemically mediated crowding effect was detectable under conditions representing a low population density, implying that animals responded to their own metabolites. Daphnia species differed in their inducing effects, and in one case (water from D. pulex) strong male offspring induction was associated with growth inhibition. A short daylength also induced male offspring effectively. Evidence supported a mechanism of sex-ratio regulation that implies a regular alternation of brood sex, resulting in a sex ratio close to 1:2, the expected optimum ratio. Even individual mothers adjusted the sex ratio of their offspring within a sequence of 7 broods. Formation of male offspring and resting eggs are independently controlled, possibly by distinct sets of environmental cues. -from Authors