THE EVOLUTION OF AGRICULTURE IN BEETLES (CURCULIONIDAE: SCOLYTINAE AND PLATYPODINAE)

Ultrastructure of the mesonotal mycangium of Xylosandrus mutilatus (Coleoptera: Curculionidae)

Mycangial structures of female Xylosandrus mutilatus (Blandford, 1894), an Asian ambrosia beetle recently introduced and now established in the southeastern United States, were examined. In addition to the glandular (secretory) mesonotal mycangium located below the scutellum, nonglandular pit mycangia containing fungal propagules were discovered to occur externally on the scutellum. The ultrastructure of the glandular mesonotal mycangium, which consisted of an outer secretory layer covering an inner paired cuticular sac, is described and illustrated. Both secretory and cuticular layers of the mesonotal mycangium were highly tracheolated. Tracheae and tracheoles in association with a Xyleborine mycangium are reported here for the first time. Type 1 secretory cells were identified and observed passing electron-dense material into the glandular mycangium via efferent cuticular ductules, although type 2 secretory cells were not observed. Secretory cells were not observed in association with the nonglandular pit mycangia.

Evolutionary rates in the adaptive radiation of beetles on plants

Herbivorous insects and other small consumers are often specialized both in use of particular host taxa and in use of particular host tissues. Such consumers also often seem to show consistent differences in the rates of evolution of these two dimensions of host use, implying common processes, but this has been little studied. Here we quantify these rates of change in host use evolution in a major radiation of herbivorous insects, the Chrysomeloidea, whose diversity has been attributed to their use of flowering plants. We find a significant difference in the rates of evolutionary change in these two dimensions of host use, with host taxon associations most labile. There are apparently similar differences in rates of host use evolution in other parasite groups, suggesting the generality of this pattern. Divergences in parasite form associated with use of different host tissues may facilitate resource partitioning among successive adaptive radiations on particular host taxa.

The evolution of alternative genetic systems in insects

There are three major classes of insect genetic systems: those with diploid males (diplodiploidy), those with effectively haploid males (haplodiploidy), and those without males (thelytoky). Mixed systems, involving cyclic or facultative switching between thelytoky and either of the other systems, also occur. I present a classification of the genetic systems of insects and estimate the number of evolutionary transitions between them that have occurred. Obligate thelytoky has arisen from each of the other systems, and there is evidence that over 900 such origins have occurred. The number of origins of facultative thelytoky and the number of reversions from obligate thelytoky to facultative and cyclic thelytoky are difficult to estimate. The other transitions are few in number: five origins of cyclic thelytoky, eight origins of obligate haplodiploidy (including paternal genome elimination), the strange case of Micromalthus, and the two reversions from haplodiploidy to diplodiploidy in scale insects. Available evidence tends to support W.D. Hamilton's hypothesis that maternally transmitted endosymbionts have been involved in the origins of haplodiploidy. Bizarre systems of extrazygotic inheritance in Sternorrhyncha are not easily accommodated into any existing classification of genetic systems.