Selection alters the pattern of emergence from the host cadaver in the entomopathogenic nematode, Steinernema glaseri

Abstract

SUMMARYWe used selection to test for a genetic component to the pattern of emergence of infective juveniles from the host cadaver in the entomopathogenic nematode, Steinernema glaseri (Steiner), and whether other traits would respond to selection on this major and complex life-history character. We selected for early (‘ fast’) and late (‘ slow’) emerging lines by perpetuating nematodes that emerged on the first and after the seventh day of emergence respectively. After 12 cycles of selection, the pattern for the slow line but not the fast line differed significantly from the base population. Cumulative emergence for the slow line was less than the base population from Days 4 to 14 of the 18-day emergence period. The maximum difference occurred on the fourth day when 72·6% of emergence was complete for the base population but only 55·4% for the slow line. Decreases in infective juvenile size over the emergence period were consistent with the change in emergence pattern, but variation in sex ratios was not. No differences in infectivity were found. These results indicate that the emergence pattern has a genetic component, and that genetic variability for this trait occurs in natural populations. Furthermore, the asymmetric response to selection suggests that our field population is under strong selection for a highly skewed early emergence.