Guild phylogeography of oak feeding gall wasps

There is considerable current research interest in the processes underlying the evolution of ecological communities. The role of history in this process remains unclear: do species assemble into communities independently of each other, or do component species show parallel phylogeographic histories, such that communities effectively give rise to daughter communities? The answer is central to appropriate comparative analysis of community properties: if the latter scenario is true, assemblages linked by phylogenetic processes do not represent statistically independent examples of community evolution, and analyses should incorporate phylogenetic history. Although there have been many studies on the phylogeographic histories of individual species, similar methods have yet to be applied to sets of interacting species.

This proposal concerns the evolution of Western Palaearctic oak gallwasp communities, a system that has been of major significance in the study of insect community structure. This application has been triggered by the demonstration of highly similar phylogeographic histories for several widespread gallwasp species. In each case, allele frequency and haplotype sequence data strongly support a Turkish origin, followed by westwards range expansion into the Balkans, Italy, and the Iberian Peninsular before the Pleistocene. Demonstration of concordant patterns in further species would suggest that oak gallwasp assemblages evolved outside Europe, in the far east of their current range, followed by parallel westwards expansion by component species - in effect, guild range expansion. We will use sequence and allele frequency data to assess the support for concordant centres of origin, and tempo and direction of range expansion, in 8 gallwasp species. Concordance of phylogeographic histories across this set of species would have major implications for the evolution of the rich communities of other organisms (particularly chalcid parasitoids and cynipid inquilines) centred on oak gallwasps. Many of these attack different gallwasp hosts in different generations, and guild range expansion is predicted to facilitate concordant range expansion by trophically dependent species. Later applications will extend the multispecies approach developed here to parasitoids and inquilines in oak gall communities.

We also aim to promote shifts in the geographic and directional emphases of phylogeographic research in the Western Palaearctic. Most work to date concerns latitudinal shifts in the distributions of single species and the resulting gradients in genetic diversity driven by recent glacial cycles. This work has identified important refugial centres of genetic diversity in southern Europe - for most species in one or more of Iberia, Italy and the Balkans. However, where a species occupies multiple refugia there has been little consideration of longitudinal relationships among refugia. Are refugia linked by a longitudinal colonisation history from an eastern or western origin, or are they unlinked fragments of a larger distribution? Analysis of longitudinal patterns is currently restricted by the fact that large parts of the distribution of many 'European' species to the east of the geopolitical bounds of Europe in Turkey, Iran and the Caucasus remain unsampled. The need to consider such eastern populations is underlined by the identification of multiple, ancient, eastern refugia and eastern origins for two oak gallwasps. Turkey and the Caucasus are supported as centres of diversification of many European plant and animal taxa, and these regions may be of general significance for intraspecific genetic diversity.

Specific Objectives (a) To identify Western Palaearctic centres of genetic diversity, and to establish the phylogenetic relationships among them, for 8 widespread oak gallwasp species using allozyme allele frequency and mitochondrial cytochrome b haplotype sequence data. (b) To assess the congruence in centres of diversity, and the direction and timescale of range expansion among species. Agreement between observed intraspecific phylogeny topologies and alternative origin and range expansion hypotheses will be tested using maximum likelihood.

1. Across a suite of species, most show genetic patterns consistent with an eastern origin in Asia Minor or Iran.
2. Application of molecular clock dating suggests that range expansion form the east is relatively recent, taking place in the second half of the Pleistocene.
3. Our results suggest that, in general, understanding patterns of genetic diversity and biodiversity in Europe requires appropriate consideration of genetic contributions and range expansion from populations outside Europe.