REDUCED GENETIC DIVERSITY IN A MEIOBENTHIC COPEPOD EXPOSED TO A XENOBIOTIC

Populations that encounter a novel environment can lose genetic divers ity. Intense, directional selection can affect reproduction and lead t o a gene pool dominated by relatively few genotypes. A loss of haploty pe diversity (h) in the mitochondrial genome in populations of meioben thic, harpacticoid copepods around offshore oil platforms has been lin ked to lethal and sublethal responses to factors proximal to offshore oil platforms. However, it was not possible to determine if the effect s were related directly to increased concentrations of contaminants pr oduced by drilling at the platforms. An experiment was designed to tes t whether populations of Nitocra lacustris Schmankevitsch (Copepoda: H arpacticoida) would lose haplotype diversity over 3 generations follow ing exposure to sediment-associated hydrocarbons. Haplotype diversity was estimated by comparing restriction fragment length polymorphisms g enerated from mitochondrial DNA amplified from individual copepods. An additional component of the study was to determine the effects of pop ulation size on estimates of diversity. Haplotype diversity could be u nderestimated by using too few individuals, because random genetic dri ft in very small populations could cause lower diversity even in the a bsence of strong selection. The relationship between sample size and h was determined by randomly sub-sampling, at different sample sizes, a large data set collected for two copepod species. Cultures treated wi th phenanthrene, a polycyclic aromatic hydrocarbon, experienced lower adult survival, fewer surviving offspring per female, and lower haplot ype diversity relative to control cultures. After the first generation , mortality declined, but larval survival and haplotype diversity did not change in subsequent generations. Diversity was slightly underesti mated due to assaying too small a number of individuals, but could not account for all of the differences between treatments. Despite mortal ity, populations in culture were not reduced to the point where random drift seemed to have a significant effect. These results support the hypothesis that loss of mtDNA diversity in naturally occurring populat ions of Harpacticoida may serve as a marker for negative effects of ex posure to xenobiotic contaminants. (C) 1998 Elsevier Science B.V.