Host-associated allozyme variation in tree cambium miners, Phytobia spp. (Diptera: Agromyzidae)

The larvae of the agromyzid flies that belong to the genus Phytobia Lioy feed by mining in the differentiating xylem just below the cambium of growing forest trees. The genus, which is apparently one of the most primitive groups in the Agromyzidae, comprises over 50 currently recognized species. Most of the species are mono- or oligophagous, and the host plants belong to numerous genera in about 60 families. Thus, Phytobia is an attractive candidate for studies on the evolution of insect-plant relationships. In spite of this, the taxonomy of Phytobia is currently poorly understood, mainly because the morphological differences between species are small. We used allozyme electrophoresis to investigate whether molecular markers could be used to separate and identify species in Phytobia, and to study the patterns of host use in the group. For this, we collected Phytobia larvae from eight host tree species occurring in southern Finland. An analysis of 10 variable allozyme loci showed that there are probably five species of Phytobia that feed on the hosts included in our study: one occurs on birches (Betula pubescens Ehrh. and B. pendula Roth) and alders (Alnus incana (L.) Moench and A. glutinosa (L.) Gaertn.), one on rowan (Sorbus aucuparia L.), and three species with overlapping feeding ranges on aspen (Populus tremula L.) and two willow species (Salix phylicifolia L. and S. caprea L.). Because birches and alders belong to the plant family Betulaceae, rowan to Rosaceae, and aspen and willows to Salicaceae, the host associations of the individual fly species can be explained by the taxonomic affinities of the hosts. However, our results also show that on a larger scale the evolution of host-plant associations in Phytobia cannot be explained by strict parallel cladogenesis (cospeciation) between the flies and their hosts.