Evidence for phylogeographic structure in Lolium species related to the spread of agriculture in Europe. A cpDNA study

In order to explain the present distribution area of natural populations of two forage grasses species (Lolium perenne and L. rigidum), we studied gen etic variation for maternally inherited chloroplast DNA (cpDNA) in 447 indi vidual plants from 51 natural populations sampled throughout Europe and the Middle East. The detection of polymorphism by restriction analysis of PCR- amplified cpDNA fragments resulted in the identification of 15 haplotypes. Hierarchical analysis of chloroplastic diversity showed a high level of wit hin-population diversity while, for both species, we found that about 40% o f the total diversity still remains among populations. The use of previous isozymes data enabled us to estimate the pollen to seed flow ratio: pollen flow appears to be 3.5 times greater than seed flow for L. perenne and 2.2 times higher for L. rigidum. A stepwise weighted genetic distance between p airs of populations was calculated using the haplotypes frequencies of popu lations. A hierarchical clustering of populations clearly divides the two s pecies, while two main clusters of L. perenne populations show a strong geo graphical structure. Different scenario are proposed for explaining the dis tribution area of the two species. Finally, evidence attesting that these g eographical structures are related to the spread of agriculture in Europe a re presented and discussed.