Phenotypic Variation in a Zooplankton Egg Bank

Dormant propagule pools may store potentially significant genetic variation that can influence the rate and direction of microevolution via directional selection, temporally fluctuating selection, and evolution of trait covariance between timing of emergence from the propagule pool and fitness characters expressed in the active population. The third process can interact with either of the first two to produce distinct effects. Each process can lead to a different distribution of genotypes and phenotypes between active and dormant subpopulations. We compared the phenotypic distributions of an important fitness character for individuals collected from active and diapausing subpopulations of a freshwater copepod, Diaptomus sanguineus, with a long—lived egg bank. The character, seasonal timing of the switch from production of immediately hatching eggs to diapausing eggs, determines the relative representation of copepods with different switch dates in future generations and is subject to fluctuating selection due to year—to—year changes in the timing and intensity of the seasonal onset of fish predation. The mean timing of diapause is significantly later in the season for copepods reared from long—lived diapausing eggs than it is for copepods reared from individuals collected from the water column. Phenotypic variance for diapause timing does not differ between the two subpopulations. Within the sediment subpopulation, the distribution of diapause timing depends upon two features of the diapausing eggs: (1) individuals originating from eggs near the sediment surface exhibit a slightly earlier switch date with greater phenotypic variance than individuals from deep in the sediments, and (2) individuals from eggs that hatched shortly after they were collected from sediments have a later seasonal switch to diapause than those that hatched later in time. We hypothesize that our results are explained by adaptive covariance between traits that influences how long an egg spends in the sediments before hatching and traits that influence the seasonal timing of diapause. The covariance may result from either phenotypic plasticity or from genetic covariance between diapause timing and hatching probability.