For whatever reason, there was a spate of papers on the coevolution of plants and their pests last week. Or at least I got to hear about them last week.
The one that got the most attention by the popular press — well, actually, the only one that got any attention by the popular press — was a study comparing changes in glucosinolates in the brassica family with speciation in the Pieridae butterflies, whose caterpillars feed on these plants. Glucosinolates give wasabi and mustard their zing (hence the press interest), but are deadly to insects, which is why they evolved in the first place. Each major innovation in the chemistry of glucosinolates since they first arose in the brassicas at the K-T boundary is correlated, the authors found, with a burst of diversification in detoxification mechanisms among the insects at which they were aimed.
The other two studies don’t delve quite so deeply back into evolutionary time, focusing on the role of domestication. The first looked at populations of the green peach aphid (Myzus persicae) growing on a diversity of both crops and their wild relatives. The authors found that aphid populations on domesticated species were more genetically diverse, but evolved more slowly, because selection was less strong and populations larger, weakening the effect of genetic drift. Applying this result to the brassica-butterfly model would suggest that the strength of the association between glucosinolate and butterfly diversity should decrease for the domesticated brassicas compared to the wild ones, but I’m not sure this was looked at in that study.
The third paper investigated the apple’s fungal pathogen Venturia inaequalis. The dispersal characteristics of dozens of strains collected on both domesticated and wild apples in Kazakhstan were compared. The authors found that apple domestication has led to enhanced colonization capacity by the pathogen: strains from orchards have more, bigger spores. Seems to me that’s somewhat contradictory to the aphid example.
The relationship between plants and their pests is, well, complicated.