Professor Studied Stick Insects to Test Prediction of Population Divergence

A male stick insect (Timema knulli) sitting on its host redwood (Sequoia sempervirens) in the Landels-Hill Big Creek Reserve, Big Sur, California. The process of Timema populations adapting to different hosts has repeatedly initiated parallel genetic changes in this group of insects. In turn, these replicate instances of differentiation have resulted in predictable and repeatable patterns of genome divergence by natural selection. See page 738. Photo: © Moritz Muschick

A male stick insect (Timema knulli) sitting on its host redwood (Sequoia sempervirens) in the Landels-Hill Big Creek Reserve, Big Sur, California. The process of Timema populations adapting to different hosts has repeatedly initiated parallel genetic changes in this group of insects. In turn, these replicate instances of differentiation have resulted in predictable and repeatable patterns of genome divergence by natural selection. See page 738. Photo: © Moritz Muschick

Who would have thought that stick insects would be so helpful in studying the predictability of the adaptive process that occurs when an insect adapts to a new host plant?

It is a question of more than academic interest. Many agricultural insect pests are the consequence of just such a host switch.

A group of scientists including Dr. Spencer Johnston of Texas A&M Department of Entomology recently captured the May 16 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.

Led by Dr. Victor Soria Carrasco, 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 (the species of insect being studied) 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.”

Johnston also said that natural selection is both predictable and opportunistic and that the observed changes were made in the context of the genetic variation and genome architecture available in the insects when the host plant switch occurred.

“What makes the paper exciting is that we were able to use next generation sequencing technology to produce a massive data set, and specifically identify the changes that confirm the reality of this very intuitive concept,” he said.

The article can be found at: http://www.sciencemag.org/content/344/6185/738.full

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