The evolution of eusociality in Australian gall-forming thrips

In this thesis I have focused on the origin of altruistic behaviour in the soldier caste of the gall-forming thrips in Australia. The origin of insect sociality has been a central question in evolutionary biology for decades. I have developed microsatellite genetic markers, specific to the gall-formers, in order to estimate traits thought to be important to the origin of sociality: inbreeding, high relatedness, and reproductive skew. Mapping these traits onto a phylogeny of the gall-formers has led to two inferences. First, high levels of inbreeding and genetic relatedness are inferred for the origin of the soldier caste; the most concrete example of an ancestral inbreeding population giving rise to eusocial lineages. Second, soldier behaviour and morphology likely evolved in the context of low reproductive skew. That is, early soldiers reproduced at or near the level of the foundress, and reduced reproduction by thrips soldiers evolved more recently. This is a novel origin of a helping caste, distinct from the scenario for the evolution of helping in the Hymenoptera. I have developed an inclusive fitness model for the origin of helping incorporating these new insights. Proto-helper production of male and female offspring via sib-mating is predicted to increase the ease by which helping evolves. Application of the model to gall-forming thrips in Australia reveals a phylogenetic pattern consistent with helper production of male and female offspring facilitating the origin of soldiering.