Parallel diversification of Australian gall-thrips on Acacia.

The diversification of gall-inducing Australian Kladothrips (Insecta: Thysanoptera) on Acacia has produced a pair of sister-clades, each of which includes a suite of lineages that utilize virtually the same set of 15 closely related host plant species. This pattern of parallel insect-host plant radiation may be driven by cospeciation, host-shifting to the same set of host plants, or some combination of these processes. We used molecular-phylogenetic data on the two gall-thrips clades to analyze the degree of concordance between their phylogenies, which is indicative of parallel divergence. Analyses of phylogenetic concordance indicate statistically-significant similarity between the two clades. Their topologies also fit with a hypothesis of some degree of host-plant tracking. Based on phylogenetic and taxonomic information regarding the phylogeny of the Acacia host plants in each clade, one or more species has apparently shifted to more-divergent Acacia host-plant species, and in each case these shifts have resulted in notable divergence in aspects of the phenotype including morphology, life history and behaviour. Our analyses indicate that gall-thrips on Australian Acacia have undergone parallel diversification as a result of some combination of cospeciation, highly restricted host-plant shifting, or both processes, but that the evolution of novel phenotypic diversity in this group is a function of relatively few shifts to divergent host plants. This combination of ecologically restricted and divergent radiation may represent a microcosm for the macroevolution of host plant relationships and phenotypic diversity among other phytophagous insects.

Brood insurance via protogyny: a source of female-biased sex allocation.

Sex allocation patterns and colony productivity are examined in Exoneura nigrescens, a social allodapine bee. As for previous studies on Australian allodapine bees, numerical sex ratios were strongly female biased in the smallest broods, but neared equality in larger broods. Local fitness enhancement has been suggested previously to explain female-biased allocation in allodapine bees. Here, we propose an alternative model, the 'insurance model', which predicts protogyny and, as a consequence, female-biased sex allocation in small broods with decreasing bias in larger broods. Because allodapine broods are reared progressively in an open burrow, broods require that adult females be present throughout their development in order to survive to maturity. If mothers invest in daughters (alloparents) first, these daughters can rear younger, dependent brood in cases in which orphaning occurs. If such daughters behave as surrogate mothers, then investment in them by mothers should not be regarded as investment in female sex allocation per se, giving rise to apparently female-biased broods. The model predicts a pattern of sex ratios as a function of total brood size that very closely match empirical data from E. nigrescens.

Reproductive division of labour coevolves with gall size in Australian thrips with soldiers.

An analysis of multiple species of Australian gall-inducing thrips with soldiers reveals a significant negative correlation between the size of gall produced and the reproductive division of labour. This correlation suggests that the evolution of smaller galls limited the available space and feeding sites for the offspring of female soldiers, and was a major factor that led to the evolution of an altruistic caste in the gall-inducers. We argue that high levels of inbreeding by singly mated foundresses and incestuous mating by her soldier offspring are key to this evolutionary relationship because they make the relatedness of a female soldier to her daughters and sisters approximately equal. Evidence that relatedness plays an important role is strengthened by the observation of outbred multiply mated foundresses and unbiased sex ratio of dispersers in Oncothrips waterhousei, and the inference that both gall volume and skew decreased along this lineage.

High relatedness and inbreeding at the origin of eusociality in gall-inducing thrips.

Within the haplodiploid eusocial gall-inducing thrips, a species-level phylogeny combined with genetic data for five eusocial species enables an inference of levels of relatedness and inbreeding values for lineages at the origin of eusociality. Character optimization using data from five eusocial species indicates that the lineage or lineages where eusociality is inferred to have originated exhibit relatedness of 0.64-0.92, and F(IS) of 0.33-0.64. The high inbreeding coefficients found in these eusocial thrips have increased relatedness among and within both sexes and have reduced the haplodiploidy-induced relatedness asymmetries [Hamilton, W. D. (1964) J. Theor. Biol. 7, 1-52]. These results indicate that unusually high relatedness is associated with the origin of eusociality, and they suggest a role for inbreeding in the evolution of bisexual helping.

Molecular phylogeny of the large carpenter bees, genus Xylocopa (Hymenoptera: apidae), based on mitochondrial DNA sequences.

Carpenter bees, genus Xylocopa Latreille, a group of bees found on all continents, are of particular interest to behavioral ecologists because of their utility for studies of the evolution of mating strategies and sociality. This paper presents phylogenetic analyses based on sequences of two mitochondrial genes cytochrome oxidase 1 and cytochrome b for 22 subgenera of Xylocopa. Maximum-parsimony and maximum-likelihood methods were used to infer phylogenetic relationships. The analyses resulted in three resolved clades of subgenera: a South American group (including the subgenera Stenoxylocopa, Megaxylocopa, and Neoxylocopa), a group including the subgenera Xylocopa s.s. and Ctenoxylocopa, and an Ethiopean group (including the subgenera Afroxylocopa, Mesotrichia, Alloxylocopa, Platynopoda, Hoploxylocopa, and Koptortosoma). The relationships between the 11 other subgenera and the resolved clades are unclear. Within the Ethiopian group we found a clear separation of the African and the Oriental taxa and apparent polyphyly of the subgenus Koptortosoma. Using an evolutionary rate for ants, we investigated whether Gondwana vicariance or more recent dispersal events could best explain the present-day distribution of subgenera. Although some taxa show divergences that approach Gondwanan breakup times, most divergences between geographic groups are too recent to support a vicariance hypothesis.

SPATIAL DISTRIBUTION OF A PRIMITIVELY SOCIAL BEE: DOES GENETIC POPULATION STRUCTURE FACILITATE ALTRUISM?

Exoneura bicolor is a univoltine, facultatively social bee exhibiting a solitary/quasisocial/semisocial colony polymorphism (Schwarz, 1986, 1987). Intracolony relatedness in semisocial colonies has been previously estimated at 0.49 ± 0.06 (Schwarz, 1987), although the crucial relatedness between altruists and the brood that they rear will be about half this value. This value is unlikely to be increased by the preferential rearing of only close relatives (Schwarz, 1988a) and no known morphological specializations preclude workers from reproducing in this species. Hamilton (1972, 1975) suggested that relatedness may be increased through population subdivision, if this leads to significant inbreeding and increased between-colony genetic variance. The same process may also operate at higher levels of population structure (e.g., Wade, 1978). Population structure and intracolony relatedness in E. bicolor were investigated in seven localities in southern Victoria, Australia, to determine if inbreeding at any level of population structure was contributing to relatedness between altruists and beneficiaries within these colonies. Population structure was described using hierarchical F-statistics and an identity by descent measure, developed by Queller and Goodnight (1989), was used to estimate intracolony relatedness. It was found that inbreeding was not contributing to between-group genetic variance, at any level, in a consistent manner across localities. Therefore relatedness, considered in isolation, does not seem sufficient to account for the presence of worker behavior. It is suggested that large benefits for group living may be responsible for maintaining altruistic behavior, in part, in this species. Significant heterogeneity among localities for all F-statistics estimated in our analysis was found and this may be attributable to stochastic elements such as cofounding behavior and the low percentage of males in the brood. The possible consequences of such heterogeneity in population structure for the maintenance of altruism in E. bicolor are discussed.