The interaction between diet breadth, geography and gene flow in herbivorous insects

Abstract

The geographical distribution and life history of insects and their host plants determines when, and if, they will interact. The vast majority of herbivorous insects are host-plant specialists and their narrow host range limits their distribution and may also restrict gene flow between their populations. Generalist insect herbivores, by contrast, might be expected to have much higher gene flow across populations because the distribution of multiple host plant species is likely to be much more contiguous. Species distributions shift continually, so with enough time the populations of a specialist insect species should experience more events that fracture their distribution because of the changing distribution of their relatively fewer hosts. The changing distribution of insects and their host plants is thus important for mediating host-plant interactions and will also influence how and when adaptation to a host plant, or subset of host plants, occurs. To investigate the influence of geography and host breadth on gene flow, and thus speciation, I explore the evolutionary history and population genetic structure of two herbivorous insects with different host plant relationships, the generalist bug Nezara viridula (Pentatomidae, Hemiptera), a global pest species, and the specialist thrips Cycadothrips chadwicki (Aeolothripidae, Thysanoptera), the pollinator in a brood-site mutualism with Macrozamia cycads. The results from these systems were then integrated with other similar data sets from phytophagous insects to understand better how gene flow and host specificity relate to one another, and to examine the role each has played (and may still play) in the evolution of insect-plant interactions more generally. The pest bug N. viridula has two globally distributed mitochondrial lineages, the Asian (mtDNA lineage A) and the European (mtDNA lineage E), and the species status of these lineages has not been resolved. In Australia, I found that both of these N. viridula mitochondrial lineages were present. Microsatellite data show two genetically and geographically distinct populations, but these did not correspond directly to the two mitochondrial lineages. Together, mtDNA and microsatellite data shown that secondary contact has occurred in eastern Australia between the two lineages and has resulted in introgression of mtDNA lineage A into predominantly mtDNA lineage E populations in a narrow area, but without widespread introgression across the nuclear genome. Past research on this insect suggested that these lineages represented a single species because these mtDNA haplotypes were found together in single populations. The evidence presented here indicates that mating between lineages can occur, but that an analysis of mtDNA alone is not enough to understand the evolutionary history of these insects. Further, spatial and temporal genetics analyses of eastern Australian populations of N. viridula show that they do not differ genetically across different host plants. Most host plants this insect feeds on in Australia have a narrower distribution than that of N. viridula and so gene flow is maintained between populations of N. viridula even in areas where most of its host plants are unavailable. Even substantial changes in the distribution of a number of potential host plants may not impact the population genetic structure of this insect significantly. The Australian endemic thrips C. chadwicki is shown here to be a complex of at least five species, with some having diverged from one another about six Mya and others less than one Mya. Each thrips species is associated with a different subset of Macrozamia cycad species, with each found only on one or two Macrozamia species. One of these Cycadothrips species, C. chadwicki sp. 4, is found in southern Queensland where it is associated with M. lucida and M. macleayi. Population genetics analysis of this thrips across its geographic distribution shows high amounts of gene flow between populations, but also some weak spatial genetic structure. For C. chadwicki, more generally, these results support the presence of a species complex as no gene flow occurs over even short geographic distances between putative species in the C. chadwicki complex, even though a high amount of gene flow is observed within the single species C. chadwicki sp. 4. Geography appears important for speciation in Cycadothrips and Macrozamia, and so the conditions that lead to speciation may be the product of rare events that isolate both thrips and cycad populations. How much gene flow occurs between populations of Macrozamia, relative to the amount of gene flow among the associated Cycadothrips populations, requires further resolution. The evolutionary history and population genetic structure of N. viridula and C. chadwicki fit with the general predictions of this study. An analysis of the population genetics literature involving phytophagous insects suggests that these results are consistent also for other herbivorous insect species, with relatively specialised phytophagous insects having lower amounts of gene flow between their populations. The approach developed in this thesis, as well as the insights from the study of N. viridula and C. chadwicki, will help understand the role that host breadth and geography play in the evolution of herbivorous insects.