PHENOTYPIC VARIATION IN A ZOOPLANKTON EGG BANK

Dormant propagule pools may store potentially significant genetic vari ation 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 po ol and fitness characters expressed in the active population. The thir d process can interact with either of the first two to produce distinc t effects. Each process can lead to a different distribution of genoty pes and phenotypes between active and dormant subpopulations. We compa red 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 immedi ately hatching eggs to diapausing eggs, determines the relative repres entation of copepods with different switch dates in future generations and is subject to fluctuating selection due to year-to-year changes i n the timing and intensity of the seasonal onset of fish predation. Th e mean timing of diapause is significantly later in the season for cop epods reared eggs than it is for copepods reared from individuals coll ected from the water column. Phenotypic variance for diapause timing d oes not differ between the two subpopulations. Within the sediment sub population, the distribution gf diapause timing depends upon two featu res of the diapausing eggs: (1) individuals originating from eggs near the sediment surface exhibit a slightly earlier switch date with grea ter phenotypic variance than individuals from deep in the sediments, a nd (2) individuals from eggs that hatched shortly after they were coll ected from sediments have a later seasonal switch to diapause than tho se that hatched later in time. We hypothesize that our results are exp lained bg adaptive covariance between traits that influences how long an egg spends in the sediments before hatching and traits that influen ce the seasonal timing of diapause. The covariance may result from eit her phenotypic plasticity or from genetic covariance between diapause timing and hatching probability.