Abstract
This study addressed three aspects of the tolerance to temperature using Daphnia as a model system. First, a controversial assumption about the genetic trade-off of fitness between a generalist and a specialist was experimentally tested, also often described by a proverb "the jack-of-all-trades is a master of none". Three approaches were used to examine the trade-offs: investigation of a genetic trade-off between height and breadth of the tolerance curve, analysis of variance of the area under the tolerance curve among genotypes, and estimation of genetic correlations of fitness on pairs of temperatures. In general, the study observed "winner" and "loser" genotypes of D. pulicaria in response to temperature, and it was not consistent with optimality and antagonistic pleiotropy predictions. Second, three questions were examined regarding the evolution of clutch size of Daphnia: (1) the existence of the trade-off between clutch size and juvenile survivorship; (2) the prediction that the most common is the most productive clutch size; and (3) the effect of optimal/stressful temperatures on juvenile survivorship, the most common and the most productive clutch sizes. Results indicated no trade-off between clutch size and juvenile survivorship and that the most productive clutch size in D. pulicaria was significantly higher than the most common clutch size, contrary to Lack's predictions originally formulated for altricial birds. The observed significant correlation between D. pulicaria's unrealized clutch and temperature suggests a possible increase of reproduction cost at higher temperatures. Third, the study examined genetic variation and heritability of thermal tolerance of three D. pulicaria populations. Although among population variation was low, significant variation was observed in intrinsic rate of increase within each lake population at all examined temperatures. In addition, broad-sense heritability increased at marginal temperatures which may influence zooplankton's ability to survive and adapt to stressful conditions, including the rise in temperature due to global warming.