SELECTION ALTERS THE PATTERN OF EMERGENCE FROM THE HOST CADAVER IN THE ENTOMOPATHOGENIC NEMATODE, STEINERNEMA-GLASERI

We used selection to test for a genetic component to the pattern of em ergence of infective juveniles from the host cadaver in the entomopath ogenic nematode, Steinernema glaseri (Steiner), and whether other trai ts would respond to selection on this major and complex life-history c haracter. We selected for early ('fast') and late ('slow') emerging li nes by perpetuating nematodes that emerged on the first and after the seventh day of emergence respectively. After 12 cycles of selection, t he pattern for the slow line but not the fast line differed significan tly from the base population. Cumulative emergence for the slow line w as less than the base population from Days 4 to 14 of the 18-day emerg ence 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 emerg ence period were consistent with the change in emergence pattern, but variation in sex ratios was not. No differences in infectivity were fo und. These results indicate that the emergence pattern has a genetic c omponent, and that genetic variability for this trait occurs in natura l populations. Furthermore, the asymmetric response to selection sugge sts that our field population is under strong selection for a highly s kewed early emergence.