The impact of genetic structure on the dynamics of zooplankton populations

Abstract

Data on the genetic structure of parthenogenetic rotifer and Artemia populations are reviewed and analyzed to address the question: "How do the dynamics of natural populations of zooplankton reflect the 'packaging' of their genetic variation?'Two distinct patterns have been found in rotifers. When the clones comprising a population undergo a seasonal succession, low variances in both rates of increase and net reproduction are detected among individuals collected at the same time and high variances are found between collections made at different times. In contrast, high variances in life history characters occur both within and between collections when clonal succession does not occur. The latter condition has been found in only one rotifer population. In that case the population appeared to be present in the lake throughout the year and to lack sexual reproduction. Our research with parthenogenetic Artemia has focused on the effects of polyploidy on the dynamics of sympatric diploid and polyploid brine shrimp. Using life history characters to measure relative fitness, we show that diploids have higher fitness than polyploids under optimal conditions in the laboratory. Research in progress indicates that this situation may be reversed in stressful environments, where polyploids show a clear increase in relative fitness. These findings, although based on only three sympatric populations, correspond with the zoogeographic distribution of Artemia polyploidy in coastal populations. Polyploids tend to increase in relative frequency at the edges of the species' distribution in both high and low latitudes where thermal stress may be greater. In addition, our study reveals that sympatric diploids and polyploids have different responses to high salinity. These studies demonstrate that the genetic structure of a zooplankton population may have profound effects on its life history characteristics and dynamics. We argue that the dependency of life table parameters on genetic structure is likely to be a more important experimental variable in studies with obligate or cyclical parthenogens than in those with sexual species. Knowledge of the genetic structure is essential to correctly interpret ecological measures made on these populations. For this reason it is clear that researchers working with parthenogenetic species of zooplankton should include genetic as well as ecological parameters in their studies of population dynamics.