MICRO-EVOLUTION IN THE WEEVIL GENUS LARINUS - THE FORMATION OF HOST BIOTYPES AND SPECIATION

Data are presented on allozyme variation between 15 populations of the stenophagous capitulum weevil, Larinus cynarae, and three populations of its congener, L. latus, that had been collected throughout the nor thern mediterranean range of these species. A phenetic analysis of the se data revealed no direct relationship between genetic variation and host-plant association within L. cynarae, but there was a strong geogr aphical structuring of allozyme patterns. Most of the genetic variatio n was due to differences between geographical regions and variation wi thin these was small. Wright's F-ST values showed that Italian and Gre ek populations of L. cynarae were most distinct from L. latus, with so uthern Iberian, northern Spanish and French populations increasingly l ess so. This pattern was associated with a dine in the frequencies of certain alleles along this geographical are from France to Greece. A p henogram of Nei's genetic distances indicated the close genetic relati onship between the two species of Larinus and separated the population s of L. cynarae into three allopatric groups. These groups have differ ent host-plant spectra - dominated by Cynara cardunculus in Italy and Greece, Cynara humilis/Onopordum in southern Iberia and Onopordum spp. in France/Northern Spain - and can be considered to be host biotypes of L. cynarae. L. latus, which occurs in Greece and further east is al so an Onopordum specialist. An analysis of the phylogeny of this group of Larinus indicates a primary separation into eastern (L. latus) and western (L. cynarae) taxa, with further branching of the L. cynarae l ineage into the putative host-biotypes. An hypothesis for the evolutio n of these taxa is given, based on the evolutionary history of host-pl ant taxa and geographical constraints.