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What happens when stick insects must adapt to a new host plant?

A group of scientists including Spencer Johnston of Texas A&M University’s Department of Entomology recently captured the cover of Science magazine with a paper on the genetic changes driving the divergence of populations into new species, asking if this process can be predicted or repeated in stick insects.

Johnston said that these insects have been of interest for a long time as a model of host-plant-associated adaptation. Timema cristinae has adapted to thrive on two different host plants Adenostoma fasciculatum and Ceanothus spinosus. Insects adapted to the different hosts show distinct observed genomic and morphological differences such that the insect is easily recognized as associated with one or the other host plant.

Johnston and several co-authors looked at the genetic changes observed over one generation when populations of a species of stick insect (T. cristinae) were transplanted from their preferred host plants to alternative hosts.

The question the group wanted to answer is, “How predictable is the adaptive process?” The insect seemed an obvious choice for addressing this question. We could do a field experiment to ask, “What are the effects of switching hosts? Can we observe a repetition of genomic changes associated with adapting to the alternate host?”

“What we are really asking is, ‘Can the insect switch hosts and repeat the process of adaptation to the alternate host’?” Johnston said. “If so, which of the differences are adaptive and which are chance? What we expected to find was a subset of genes which act together in the adaptive response, and evolve in parallel with the changes seen in the original adaptation to the different host plants.”

“What we found in T. critinae was that parallel changes were more common than expected by chance,” he said. “Not surprisingly, we also found a large number of changes that did not parallel those that occurred in the original host-associated insects.”