In a new study published online in the open-access journal PLoS Biology, Ilkka Hanski and Ilik Saccheri present their analysis of the Glanville fritillary butterfly on the Åland Islands in Finland, where its population dynamics are well studied in relation to its habitat--patches of meadows spread across the landscape. They provide evidence that variants of one gene influence population growth in a species of butterfly in a complex and habitat-dependent manner.
The authors investigated the gene phosphoglucose isomerase (Pgi). The Pgi gene has several forms, or alleles; The f and d alleles are most common--previously, butterflies with either ff or fd genotype were seen to have a higher flight metabolic rate and to be more fecund than those with a dd genotype--making the gene a good candidate for a population effect.
Applying a simultaneous analysis of genotype, population growth, and habitat among >130 small butterfly populations, the authors showed that, in small meadows, growth was highest when the ff or fd genotypes predominated, but in larger meadows, dd was favored the opposite was true--these genotypes predicted a decline in numbers instead of a rise, while dd was favored. This effect was specific to Pgi, as there was no correlation for six other genes analyzed. The authors suggest this might be related to differences in maturation and egg laying. Females with f alleles mature quickly and lay more eggs early on, just the strategy for exploiting a small patch, from which many butterflies risk drifting away rather quickly in their life. Females with d alleles mature later but also die later, allowing them to exploit a larger habitat more thoroughly. There are likely other reasons for the genotype-habitat area effect, since Pgi is likely to influence many different aspects of life history.
This study confirms that intraspecific genetic variation can influence population growth. It also brings home the point that there is not one specific "best" genotype--the favorable genotype will alter with environment according to the selective pressures in play.