Gall inducers take a leap: host-range differences explain speciation opportunity (Thysanoptera: Phlaeothripidae)

Phytophagous insects that specialise on broadly distributed plant groups are exposed to host-species diversity gradients. The gall-inducing thrips genus Kladothrips (Froggatt) that specialise on Australian Acacia Mill. (Mimosoideae: Leguminosae, subgenus Phyllodineae DC.) is expected to exhibit variation in host range that is dependent on host ecology. Host Acacia species distributions show structuring between the arid Eremean and non-arid biomes of the monsoonal tropics and temperate south-western and south-eastern Australia. We investigate two aspects of host use in: (1) the Kladothrips rugosus species complex that specialises on hosts whose distributions overlap among sibling lineages on different Acacia species; and (2) Kladothrips nicolsoni that specialises on a species that is relatively isolated from hosts of sibling lineages. First, several approaches that use DNA sequence data are combined to infer putative species among K. rugosus lineages collected from multiple Acacia species using: phylogenetic inference; statistical parsimony; amova; maximum likelihood genetic distance relationships; and generalised mixed Yule coalescent likelihood test of lineage delimitation. Second, haplotype network analysis is used to estimate population structuring of K. nicolsoni that specialises on a geographically isolated Acacia host species. Analyses indicated below species-level relationships among lineages that each induces galls on different Acacia host species. In contrast, haplotype network analysis for a gall-thrips species that is largely allopatric with hosts of sibling species indicates isolation by distance and range expansion effects. Greater host-species richness enhances gall-thrips opportunity to host shift resulting in founder effects that lead to disruptive selection. Restricted gene flow among gall-thrips populations specialising on a relatively isolated host species implies genetic drift via allopatric mechanisms. The results suggest gall-thrips speciation is driven by the combined influence of ecologically based selection with genetic drift that is largely determined by variation in host-species richness between arid and non-arid Australia.