Evolution of gall morphology and host-plant relationships in willow-feeding sawflies (Hymenoptera : Tenthredinidae)

There are over 200 species of nematine sawflies that induce galls on willow s (Salix spp.). Most of the species are mono- or oligophagous, and they can be separated into seven or eight different groups based on the type of gal l that they induct. We studied the evolution of different gall types and ho st plant associations by reconstructing the phylogeny of five outgroup and 31 ingroup species using DNA sequence data from the mitochondrial cytochrom e b gene. Maximum-parsimony and maximum-likelihood analyses resulted in ess entially the same phylogeny with high support for important branches. The r esults show that: (1) the galling species probably form a monophyletic grou p; (2) true closed galls evolved only once, via leaf folders; (3) with the possible exception of leaf rollers, all gall type groups are mono- or parap hyletic; (4) similar gall types are closer on the phylogeny than would be e xpected by a random process; (5) there is an apparent evolutionary trend in galling site from the leaf edge towards the more central parts of the host plant; and (6) many willow species have been colonized several times, whic h excludes the possibility of parallel cladogenesis between willows and the gallers; however, there are signs of restrictions in the evolution of host use. Many of the patterns in the evolutionary history of nematine gallers have also been observed in earlier studies on other insect gallers, indicat ing convergent evolution between the independent radiations.