Grapes, galls, and geography: The distribution of nuclear and mitochondrial DNA variation across host-plant species and regions in a specialist herbivore

Studies of patterns of molecular variation in natural populations can provi de important insights into a number of evolutionary problems. Among these, the question of whether geographic factors are more important than ecologic al factors in promoting population differentiation and ultimately speciatio n has been an important and contentious area in evolutionary biology. Syste ms involving herbivorous insects have played a leading role in this discuss ion. This study examined the distribution of molecular variation in a highl y specialized gall-forming insect, grape phylloxera (Daktulosphaira vitifol iae Fitch), that is found on both sympatric and allopatric host-plant speci es of the genus Vitis. In addition, the relationship of insects in the intr oduced range in the United States to ancestral populations in the native ra nge was examined. Evidence for differentiation along host-plant lines from both nuclear (RAPD) and mitochondrial (COI) DNA was confounded with the eff ect of geography. Differentiation was found where hosts were allopatric or parapatric, but no evidence was found for such differentiation on two hosts , V. vulpina and V. aestivalis, that are broadly sympatric. The question of population differentiation onto these sympatric hosts can be considered to be resolved-it has not occurred in spite of a long history of association. Evidence was equivocal, but suggestive of a period of divergence in allopa try prior to reestablishment of contact, for insects associated with anothe r host plant species, V. cinerea, found in both sympatric and parapatric po pulations. A low level of diversity and placement of samples collected from the grape species V. riparia at the tip of a phylogenetic tree supports th e hypothesis that this host has been recently colonized from populations fr om the Mississippi Valley. A polyphyletic origin for biotype B grape phyllo xera was supported: Although most samples collected from vineyards in the i ntroduced range in California had similar haplotypes, they were closely rel ated to natives on V. vulpina from the Atlantic Coast-Piedmont region. All samples collected from vineyards in Oregon and Washington were closely rela ted to natives on V. riparia in the northern United States.