Recurrent nuclear DNA introgression accompanies chloroplast DNA exchange between two eucalypt species

Abstract

Numerous studies within plant genera have found geographically structured sharing of chloroplast (cp) DNA among sympatric species, consistent with introgressive hybridization. Current research is aimed at understanding the extent, direction and significance of nuclear (nr) DNA exchange that accompanies putative cpDNA exchange. Eucalyptus is a complex tree genus for which cpDNA sharing has been established between multiple species. Prior phylogeographic analysis has indicated cpDNA introgression into the widespread forest species Eucalyptus globulus from its rare congener E. cordata. In this study, we use AFLP markers to characterize corresponding nrDNA introgression, on both a broad and fine spatial scale. Using 388 samples we examine (i) the fine-scale spatial structure of cp and nrDNA introgression from E. cordata into E. globulus at a site in natural forest and (ii) broad-scale patterns of AFLP marker introgression at six additional mixed populations. We show that while E. globulus and E. cordata retain strongly differentiated nuclear gene pools overall, leakage of nrDNA occurs at mixed populations, with some AFLP markers being transferred to E. globulus recurrently at different sites. On the fine scale, different AFLP fragments show varying distances of introgression into E. globulus, while introgression of cpDNA is extensive. The frequency of E. cordata markers in E. globulus is correlated with spatial proximity to E. cordata, but departs from expectations based on AFLP marker frequency in E. cordata, indicating that selection may be governing the persistence of introgressed fragments in E. globulus.