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In this study we used 2 reciprocal transference experiments to estimate the mode and magnitude of selection on life history traits of the aphid Brevicoryne brassicae L. on 2 sympatric host plants, Brassica campestris L. and Brassica oleraceae L. In the first experiment, we recorded success in establishment, age at first reproduction, duration of reproduction, number of nymphs laid, reproductive rate, and rate of increase (r i) on each host. In the second experiment, the offspring of females collected on Br. campestris or Br. oleraceae were reared on hosts through 3 generations. Age at first reproduction, duration of reproduction, fecundity, and rate of increase (r i) were measured during the third generation in order to determine presence of maternal effects. Within each recipient host, we estimated selection gradients by performing a multiple linear regression of relative number of nymphs in relation to age at first reproduction and duration of reproduction. Results of the first experiment indicate that successful establishment was higher on the source host on which the mother was collected. A significant effect associated with the recipient host was detected for most life history traits. Positive directional selection on duration of reproduction was detected on both hosts whereas selection on the age at first reproduction was detected only in Br. oleraeae. In the second experiment we did not find evidence of maternal effects since the recipient host exerted a significant effect on fecundity. Again, the selection on duration of reproduction was detected on both hosts. Despite these results suggesting incipient ecological specialization in B. brassicae, we recognize factors that may prevent the evolution of host plant specialization. Translation by the authors.

Natural Selection and Maternal Effects in Life History Traits of Brevicoryne brassicae (Homoptera: Aphididae) on Two Sympatric Closely Related Hosts