Colonist Butterflies Are Different from Sedentary Butterflies

Physiological differences have been found among female Glanville fritillary butterflies (Melitaea cinxia) (photo) that allow some to break away from their birth place and colonize new territories. These colonizing butterflies are better fliers and reproduce more quickly compared to the less mobile females. "The study is a window to how genetic differences influence behavior and how the environment influences genetic change by favoring individuals with certain traits," said lead author Howard W. Fescemyer.

This work could improve our knowledge about how animals could react to the man made habitat fragmentation. "We may be selecting for genes that enhance the dispersal or migratory capability of animals when we fragment the landscape," Fescemyer said.

"What we learn could apply to any organism that has to move to find food," he added.

The study was made on populations of the Glanville fritillary butterfly on the Åland Islands, located between Finland and Sweden in the Baltic Sea. This species lives in distinct habitats, rocky outcroppings containing plants that serve as food and provide a shelter for the butterfly larvae during the winter. On the Ålands about 4000 patches of suitable habitat were detected but only about 500 occupied every season. "Some of the patches are farther apart than most individual butterflies can migrate," Fescemyer said.

Annually, the configuration of the occupied areas changes because new populations appear in some patches while others vanish because of parasites, disease and lack of food plants. Only good flying females can reach new patches to establish new populations and flying quality must have been genetic somehow.

First, the scientists looked for physiological differences between females that established new colonies and the sedentary ones. "The study looked at seven patches which had not been colonized the year before and six old populations," Fescemyer said.

The scientists collected females from new colonies and from old colonies. They raised the butterfly strains in the laboratory during more generations, recording the time of development of their stages and the number of eggs. "The females from the new patches develop very quickly," Fescemyer said.

Their eggs matured faster (three days after emerging from the pupa compared to four in the case of the sedentary females), which enabled them to mate and lay eggs more quickly. In the butterflies' world (whose life span as adults is of two-three weeks) a day can make the difference concerning the reproductive exit. Blood analysis showed that the colonizing females had more total protein, vitellogenin and juvenile hormone than sedentary female. "Juvenile hormone plays an important role in regulating egg and larval development and also regulates reproduction, particularly egg maturation, in adults," Fescemyer said.

"Vitellogenin is the protein precursor to egg yolk, which the embryo uses for food. Total protein is the most important nutrient for egg maturation," he said.

Previous studies found that colonizing individuals mate sooner, lay more eggs and have a higher metabolic rate and a different form of an enzyme important to muscular function, phosphoglucose isomerase. "Future research will try to further uncover more physiological and molecular differences that account for the differences in reproduction and in flight," Fescemyer said. Also, the role of the host plant on these movements is unclear at the moment.