Experimental evolution of parasitoid performance on two aphid hosts

An insect’s host range is determined by a number of traits, including its acceptance of and performance on different host species. Tradeoffs in fitness-related performance measures on different host species are thought to limit the host range of many species, and theory predicts that those species in highly variable environments will evolve broader host ranges than species in more homogeneous environments.  We designed an experiment to assess the potential for, and constraints on, the evolution of performance in the parasitoid wasp, Lysiphlebus testaceipes, on two different aphid hosts.  Baseline populations exhibited lower performance on Aphis nerii, a species known to sequester chemical defenses from its host plant, than on Rhopalosiphum maidis.  We maintained replicate lines of four different selection regimes for 11 generations and assayed performance on both hosts at generations 5,6, 10, and 11.  The four selection regimes were applied by rearing populations on: 1) A. nerii only, 2) R. maidis only, 3) A. nerii alternating with R. maidis each generation, and 4) A. nerii, but each generation the parasitoid population was subject to gene flow from a population reared on R. maidis.  We observed no change in the number of aphids mummified in any selection regime over time; however, all selection regimes resulted in higher emergence rates on A. nerii over time.  At generations 5 and 6, we find that parasitoids from selection lines on A. nerii have higher emergence rates than those selection lines on R. maidis when assayed on A. nerii, but we find no evidence of a tradeoff in performance on the two hosts.  The alternating host and gene flow selection lines emerged on A. nerii at rates intermediate to those of single host selection lines, consistent with the intermediate level of exposure to A. nerii.   However, these trends all disappeared by generations 10 and 11, perhaps due to effects of inbreeding.  Together, these results suggest selection may increase the performance of this parasitoid without costs in the short term; however, the potential for inbreeding to influence the response to selection is worthy of further investigation.