Microevolution of Insect–Bacterial Mutualists: A Population Genomics Perspective

Abstract

The advent of high-throughput sequencing has ushered in a new era with enormous promise for study of short-timescale evolution. Insect–bacterial mutualists typically have tiny genomes allowing deep coverage for large populations. The importance of mutualists in ecological phenomena, such as invasions, leave much room for study. Nutritional mutualists of Order Hemiptera are of interest for being long coevolved with their hosts, thus degenerated in gene content, yet able to support their hosts on varying diets and during invasions. Determining the symbiont’s role and how adaptation might be achieved with a limited genetic repertoire is a challenge. One fruitful approach is highlighted in a recent population genomic study using a natural experiment: the 2009—present spread of the U.S. soybean pest Megacopta cribraria (Hemiptera: Plataspidae). This invasive species’ symbiont Ishikawaella capsulata (Gamma-proteobacteria) was previously shown to determine the pest-status of its host. Deep sequencing revealed allele frequency change since arrival that matched predictions, showing signatures of purifying and positive selection, with differences in “symbiont role” genes associated with different host plants. In the near future, this approach applied to other systems may illuminate the important role and dynamic potential of microbial symbionts in ecosystems.