Host Expansion in a Specialist Herbivore is Facilitated by Whole‐genome Duplication in the Host Plant

Host plant shifts are central to diversification in insect herbivores. Many mechanisms can cause host shifts in insects, but one relatively unexplored mechanism is whole‐genome duplication (WGD) in the host plant. WGD, or polyploidy, is common in plants and causes spontaneous changes in physiology, morphology, and palatability that could impact the ability of herbivores to feed and develop on newly formed polyploids (neopolyploids). Here the authors tested if WGD affected the preference and performance of the specialist aphid, Acyrthosiphon pisum (pea aphids). Pea aphids seasonally form specialised lineages or ‘host forms’ on many host plant species including alfalfa and red clover. Aphid host forms on alfalfa and red clover naturally exist on different cytotypes of their respective hosts, with red clover aphids feeding on diploid clover and alfalfa aphids feeding on tetraploid alfalfa. Therefore, the authors predicted that these host forms would have a higher preference for and performance on their respective natal host cytotype. Neither host form exhibited a preference for a particular cytotype, but there were modest changes in aphid performance based on host cytotype. Specifically, aphids specialised to red clover had higher fecundity on diploid red clover than on neotetraploid red clover. Together, these results showed that both host forms were able to recognise and accept different cytotypes of the two host species, but only one host form experienced trade‐offs in performance when feeding on neotetraploids. These results suggest that WGD may act as a mechanism of host expansion in pea aphids as plants speciate via WGD.