Seasonal phenotypes of Daphnia: post-diapause and directly developing offspring

Life-history and physiology of post-diapause and directly developing offspring in Daphnia were compared to determine differences in life-history pattern between offspring origins, identify the basic traits effecting pattern distinction, assess the consequencies of differing life-history for relative fitness, and qualify this seasonal phenomenon. Although the size of ex-ephippial hatchlings was positively correlated with maternal size, ephippia produced by differently sized females of D. magna yielded neonates which body size and it’s variation were similar to those for the first-clutch parthenogenetic neonates produced under high food conditions. Otherwise, post-diapause neonates of D. pulex were larger than first-clutch parthenogenetic neonates and varied in size close to that for neonates from the later clutches. Ex-ephippial offspring showed elevated metabolic activity. Under rich food, they grew faster during juvenile development and were larger at maturation than parthenogenetic females. Post-diapause offspring of D. magna showed higher juvenile growth rates also at limited food availability. Female origins during early maturity differed in resource allocation strategy. A greater allocation to progeny in offspring exiting diapause, however, resulted in increased progeny numbers and higher relative fitness only under rich food environments. Whereas, parthenogenetic females of D. magna exhibited a trend for higher fitness across limited food concentrations. For D. pulex, variability of fitness across differing nutritional environments was lower in directly developing females than that for ex-ephippial offspring. Higher metabolic activity in combination with small initial size resulted in lower resistance to starvation of post-diapause offspring of D. magna. Descendants of female origins in this species showed similar fitness. We conclude that although some traits of ex-ephippial offspring in comparison to parthenogenetic ones may differ across Daphnia species, the life-history strategy of post-diapause females in Daphnia is the same. Ex-ephippial females are adapted to predictable optimal conditions, whereas parthenogenetic females are adapted to unpredictably varying environment. Daphnia have evolved two alternative seasonal phenotypes matching environmental conditions in which they occur, and perform in accordance with predictions of seasonal polyphenism. Due to higher metabolic activity, which must lead to increased resource acquisition and probably is associated to larger allocation to progeny, post-diapause females are superior to directly developing offspring under high food conditions, which are expected during early season, but inferior under limiting food environments, which frequently occur later in the season. In seasonal climate, the adjustment of resource acquisition with respect to resource availability may be a general evolutionary trend for multivoltine organisms resulting in a seasonal polyphenism.