Life-history variation among lines isolated from a laboratory population of Heligmosomoides polygyrus bakeri

The extent of variation in several life-history traits within a laboratory population of the parasitic nematode, Heligmosomoides polygyrus bakeri, was studied in 10 relatively inbred parasite lines isolated from a stock population and characterized in BALB/c mice after 4, 8 and 11 generations of isolation. As expected, within-line variation for most traits at generation 11 compared to generation 4 significantly decreased (P < 0.01). At each generation of characterization, variation was observed among lines for parasite establishment, rate of development in the host, rate of early egg production, per capita fecundity, short-term and long-term survival and profiles of egg production, rate of decline in egg production, life-long reproductive effort and in vitro egg hatchability. Measures of all traits, except establishment, were highly repeatable. The rate of development was higher at generation 8 compared to the stock (P < 0.0001), and regression analysis revealed that early egg production of lines increased over 11 generations of isolation (P = 0.003). These results, together with the observed decrease (P < 0.01) in total variation of most of the traits over all lines during the process of isolation, suggested an evolutionary response of the traits, probably to the rapid passage of lines every month. The rate of development subsequently decreased between generations 8 and 11 in all lines (P < 0.0001), suggesting that the random genetic drift procedure used to isolate the lines eventually exerted detrimental inbreeding effects on this trait. The evolutionary responses of life-history traits to rapid passage and inbreeding suggest a genetic basis for variation in these life-history traits.