ISOZYME VARIATION AND SPECIES RELATIONSHIPS IN THE GENUS LOLIUM L (RYEGRASSES, GRAMINACEAE)

Thirty-two natural populations belonging to the eight species of the g enus Lolium (ryegrass) or to Festuca pratensis (meadow fescue) were re corded for allelic frequencies at 13 isozyme loci. Cultivated rye-gras s (L. perenne and L. multiflorum), meadow fescue, and the annual L. ri gidum, are true outbreeders. The other species are true inbreeders, ex cept for L. canariense, which shows a moderate level of cross fertilis ation (20%). Hierarchical clustering from Nei's unbiased distance lead s to four groups. The three self-pollinating, weed species, L. temulen tum, L. remotum and L. persicum, belong to the first cluster, which is the most differentiated one. The second cluster comprises L. multiflo rum, L. subulatum and most populations of L. rigidum. All L. perenne p opulations belong to the third cluster, as do two of L. rigidum. The a verage genetic distance within the L. perenne group is very low. Surpr isingly, the fourth cluster groups together L. canariense and Festuca pratensis. The data suggest that L. rigidum is the species with the gr eatest diversity, and could be a common ancestor of the genus. Knowled ge of historical processes of domestication could help to calibrate th e molecular clock.