Natural selection in the tropical treehopper Alchisme grossa (Hemiptera: Membracidae) on two sympatric host-plants

Abstract

The pronotum is the most distinctive and representative structure in treehoppers. Although several functions have been proposed for this structure, its involvement in fitness has not been formally evaluated. Given the high degree of maternal investment exhibited by the subsocial membracid Alchisme grossa (Hoplophorionini), the pronotum has been suggested as a shield for offspring protection. We performed selection gradient analyses on A. grossa in order to evaluate how natural selection is acting upon two traits associated with this structure considering three different fitness proxies: (1) number of eggs, (2) the ratio between the number of first-stage nymphs and the number of eggs (early survival), and (3) the number of third-stage nymphs (late survival). Since A. grossa feeds, oviposits and mates on two alternative host-plants (Brugmansia suaveolens and Solanum ursinum, both Solanaceae), we also evaluated and compared selection gradients between these host-plants. We found positive linear selection acting upon pronotum length considering the number of eggs and the late survival fitness proxies and positive linear selection acting on the distance between the suprahumeral horns considering the early survival proxy on females ovipositing on both host-plants. These results highlight the importance of the pronotum in treehoppers females' fitness and suggest that maternal care appears to have a greater importance in the way that natural selection is operating than the host-plant where oviposition occurs.