Beetles in the weevil subfamilies Scolytinae and Platypodinae are unusual in that they burrow as adults inside trees for feeding and oviposition. Some of these beetles are known as ambrosia beetles for their obligate mutualisms with asexual fungi-known as ambrosia fungi-that are derived from plant pathogens in the ascomycete group known as the ophiostomatoid fungi. Other beetles in these subfamilies are known as bark beetles and are associated with free-living, pathogenic ophiostomatoid fungi that facilitate beetle attack of phloem of trees with resin defenses. Using DNA sequences from six genes, including both copies of the nuclear gene encoding enolase, we performed a molecular phylogenetic study of bark and ambrosia beetles across these two subfamilies to establish the rate and direction of changes in life histories and their consequences for diversification. The ambrosia beetle habits have evolved repeatedly and are unreversed. The subfamily Platypodinae is derived from within the Scolytinae, near the tribe Scolytini. Comparison of the molecular branch lengths of ambrosia beetles and ambrosia fungi reveals a strong correlation, which a fungal molecular clock suggests spans 60 to 21 million years. Bark beetles have shifted from ancestral association with conifers to angiosperms and back again several times. Each shift to angiosperms is associated with elevated diversity, whereas the reverse shifts to conifers are associated with lowered diversity. The unusual habit of adult burrowing likely facilitated the diversification of these beetle-fungus associations, enabling them to use the biomass-rich resource that trees represent and set the stage for at least one origin of eusociality.

The insects that feed on the related plant families Apocynaceae and Asclepiadaceae (here collectively termed "milkweeds") comprise a "component community" of highly specialized, distinctive lineages of species that frequently sequester toxic cardiac glycosides from their host plants for defense against predators and are thus often aposematic, advertising their consequent unpalatability. Such sets of specialized lineages provide opportunities for comparative studies of the rate of adaptation, diversification, and habitat-related effects on molecular evolution. The cerambycid genus Tetraopes is the most diverse of the new world milkweed herbivores and the species are generally host specific, being restricted to single, different species of Asclepias, more often so than most other milkweed insects. Previous work revealed correspondence between the phylogeny of these beetles and that of their hosts. The present study provides analyses of near-complete DNA sequences for Tetraopes and relatives that are used to establish a molecular clock and temporal framework for Tetraopes evolution with their milkweed hosts. (C) 2001 Academic Press.

Studies of a variety of phenomena, ranging from rates of molecular substitution to rates of diversification, draw on estimates of geological age. Studies incorporating estimates of timing from fossils or other geological evidence are largely of relatively young, Tertiary divergences, to which older systems may provide useful comparisons. One apparently old assemblage comprises the beetle groups associated with the ancient genus Araucaria that share comparable, ostensibly Gondwanan distributions with their host. Our previous studies suggested a possibly Cretaceous age for Araucaria associations in bark beetles. However, the absence of confirmed bark beetle fossils earlier than the Tertiary has been taken as evidence of Cretaceous absence, and their confirmed phylogenetic position within the primitively angiosperm-feeding weevil family rules out pre-angiosperm, Jurassic origins. Nevertheless, an early shift from angiosperms to Araucaria seemed plausible in the light of Araucaria fossil history which spans the Mesozoic since the Jurassic. To resolve the phylogenetic affinities and to estimate divergence times of the Australian and South American bark beetle genera affiliated with Araucaria we analysed DNA sequences of nuclear and mitochondrial genes: protein coding elongation factor alpha, enolase and cytochrome oxidase I. The most parsimonious reconstruction of the host relationships of Tomicini from the combined dataset corroborates the ancestral association with the genus Araucaria of both South American and Australian Tomicini. Bayesian estimation of divergence times indicates that the divergence between the Australian and the South American Araucaria-feeding taxa occurred at the very latest in the Cretaceous/Paleocene border and that the age of the first Scolytinae-Araucaria association would then be during the later stages of the Late Cretaceous, while other known beetle/Araucaria associations are Jurassic. (C) 2001 The Linnean Society of London.