Saturday 23 March 2013

The taxonomic implications of host preference in Large Blue Butterflies.

Butterflies of the genus Maculinea (Large Blues) are social parasites targeting Ants of the genus Myrmica. The Butterflies lay their eggs on Gentian Flowers (Gentiana spp.) where the caterpillars emerge and pass through a number of instars (growth phases separated by molting events), before reaching a final larval stage, which secretes chemicals attractive to the host Ants, which then cary them back to their nests and feed them as they would their own larvae. 

The life cycle of Large Blue Butterflies. (a) The adult female, living about 5 days, lays 50-100 white eggs on the leaves and flower buds of Gentiana spp. (b) The young caterpillar bores into the plant, where it feeds on the developing seeds for 10-15 days, reaching 2% of its adult biomass. (c) The caterpillar emerges and falls to the ground, where it secretes chemicals attractive to ants of the genus Myrmica, who carry the caterpillar back to their nest. (d) The caterpillar competes with the ant larvae in the nest. Each species is capable of secreting chemicals specific to one species of ant; if carried to the wrong sort of nest it stops secreting chemicals, relying on the general smell of the nest to provoke the ants to feed it, however this is much less reliable and frequently results in the death of the caterpillar. (e) The growing ant makes vocalizations similar to the ants' queen, causing it to be fed in preference to the ants' own larvae. It feeds this way for 10-22 months before emerging as an adult and repeating the life-cycle. Thomas et al. (2013).

The taxonomy of Large Blue Butterflies is notoriously hard to unravel. The genus is currently split into six morphospecies (species differentiated by their appearance) but it is thought likely that each of these is made up of a number of different cryptic species. In order to test this a team of scientists led by Jeremy Thomas of the Department of Zoology at the University of Oxford and the Centre for Ecology and Hydrology carried out an experiment in which Butterflies of the species Maculinea rebeli from Poland and Spain were raised under laboratory conditions, along with their respective hosts. The results of the study were published in a paper in the Proceedings of the Royal Society Series B on 22 January 2013.

Thomas et al. found that the Butterflies from Poland used Ants of the species Myrmica sabuleti, while those from Spain used Myrmica schencki. Neither group of Butterflies were capable of surviving with the host of the other, suggesting that they are reproductively isolated, and should be regarded as separate species. 

This has important conservation implications. All Large Blue Butterflies are considered to be threatened by habitat loss. If the known morphospecies are all made up of a number of different cryptic species, as seems likely, then the reproductive populations of each of these populations will be much lower than is currently thought, and the species will be at greater risk of extinction.

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