INDIFFERANT · Individual differences of a social parasite of ants

Individual differences of a social parasite of ants

Project context and objectives The globally threatened butterflies in the genus Maculinea are social parasites of Myrmica ant colonies, gaining access to the colonies by mimicking the specific cuticular hydrocarbons (CHCs) of their host ants. Maculinea butterflies have been heavily impacted by changes in land use over the last century, and now exist primarily in fragmented, isolated populations with little gene flow, and hence are thought to be in danger of extinction not only due to small population sizes, but also due to reduced genetic variation, which could reduce their ability to adapt to new environments and to climate change. Maculinea alcon exploits different host Myrmica species in different parts of Europe, and has been shown to be in a co-evolutionary 'arms race' with at least one of its hosts, involving evolution of the CHC profiles of both parties. Locally, Maculinea alcon is rather host-specific, showing local adaptation to one or two host Myrmica, often the most common species. However, there is large variation across Europe in the hosts with which Maculinea alcon associates. This study therefore set out to investigate the geographical variation in host specificity, CHCs and genetic variation in Maculinea alcon, comparing populations from the core of the species distribution with more peripheral populations. Populations of Maculinea alcon that either specialised on a single Myrmica host species, or exploited two hosts simultaneously were identified in a core area of the species range (Hungary and Romania) and a peripheral area (Denmark and Sweden), and samples were collected to test two main hypotheses: that populations using multiple host species are more variable than populations using a single host species; and that populations at the core of the species range are more variable than those at the periphery. Variation in CHCs was tested using gas chromatography of cuticular extracts, and genetic variation was tested using a number of variable microsatellite markers. In addition, host-use data was collected from a large number of studies of Maculinea populations across Europe to test the hypothesis that peripheral populations were more likely to specialise on a single host Myrmica species. Project results Although all data has been collected, the analysis of genetic variation is still under way, so only the preliminary analysis of CHCs is reported here. The hydrocarbon profiles of Maculinea alcon were significantly different between the core and the peripheral. They were also significantly more variable in populations from the core area, but not more variable in those populations which exploited two hosts rather than one. In fact, those that exploited a single host were significantly more variable, due primarily to the low variation in the population from the island of Læsø. Overall, there was no pattern as to whether core or peripheral populations exploited one or more host species, but two groups of populations were found, one in the north west, which exploited hosts in the Myrmica rubra species group, and a southern set that exploited hosts in the Myrmica scabrinodis species group. Within the latter, all populations that exploited more than one host species were found in the core, eastern area of the range of Maculinea alcon. The predicted pattern of increased variation in the populations form the core area of Maculinea alcon's distribution was found for the cuticular hydrocarbons of this species. However, the predicted increase in variation in CHCs in populations using two hosts was not found, suggesting that the caterpillars produce a compromise profile rather than individuals producing profiles that may match one host of the other. Host-use data confirmed the existence of two sets of populations, a north-western group that exploit hosts in the Myrmica rubra species group, and a larger, southern group that exploit hosts in the Myrmica scabrinodis species group. Within the latter, multiple host ant use is only found in the core of the range of Maculinea alcon, supporting predictions that the lower genetic variation in peripheral populations may reduce their ability to mimic multiple hosts.

Data: CORDIS, © European Union