The evolution of soldier reproduction in social thrips

We estimated the degree of reproductive differentiation betweenfoundresses and soldiers in multiple populations of five speciesof haplodiploid Australian gall-forming thrips using microsatellitedata, ovarian dissections, and census data. Microsatellite-basedspecies estimates of average per capita reproductive outputof soldiers relative to the foundresses ranged from 0.005 to0.64, and dissection and census-based estimates ranged from0.17 to 1.1. Mapping of these estimates onto a phylogeny showedthat levels of soldier reproduction were apparently higherin three basal lineages than in two more derived lineages.We infer from this phylogenetic pattern that soldier morphologyand behavior of thrips evolved in the presence of substantiallevels of soldier reproduction. This pattern of evolutionarychange is similar to that proposed for the origin of soldiersin aphids and termites, but it differs from the scenario proposedfor the origin of workers in Hymenoptera, within which helpingand strong reproductive division of labor apparently evolvedbefore morphological differentiation. We suggest that this differencein evolutionary routes to eusociality between taxa with soldiersand taxa with foraging workers was driven by a weaker trade-offbetween helping and reproducing, and a greater ability of thehelpers to withstand reproductive domination, in taxa withsoldiers. This is the first study to analyze the social-evolutionarytrajectories of reproductive, behavioral, and morphologicaldifferentiation in the context of a species-level phylogeny.