Population genetics, genital divergence and mate assessment in the common New Zealand stick insect genus Clitarchus

Abstract

The forever-changing environment directs taxa to undergo speciation, change their distributional range and become extinct. Current species distributions are a product of past evolutionary processes. During the Pliocene (~5-2 million years ago) current mainland Northland, New Zealand was an island archipelago. This geographic reduction forced populations into allopatry having a major influence on the diversity of modern species located there. Subsequent tolombo sand formations during the Pleistocene connecting major islands to the mainland allowed for species range expansion and provided opportunity for diverging taxa to make secondary contact. Secondary contact following allopatric speciation has great and varied influences on the taxa involved. If speciation is incomplete, gene flow will occur between diverging lineages. Whether gene flow occurs among populations ultimately depends upon the mating decisions of individuals. The mechanisms by which animals select who to mate with are greatly varied. Throughout the insects, male genitalia are commonly selected on and as a result display the greatest evolutionary divergence of any morphological traits. The objectives of this project were to identify the consequences of secondary contact between Northland Clitarchus populations using an inter-disciplinary approach and to provide a better understanding of how Clitarchus locate and assess potential mating partners. To clarify the distribution of the species Clitarchus hookeri and Clitarchus “tepaki”, populations were sampled throughout Northland. Divergent male and female genital structures were quantified using morphometrics. Mitochondrial DNA (COI and COII) was amplified from many individuals distributed across Northland to generate a species genealogy and microsatellite genotypes (14 loci) were used to decipher current patterns of gene flow and identify admixed populations. In combination these data provide complementary information to compare the effects of two secondary contact events between Clitarchus species. Behavioural crosses were additionally performed for populations covering the largest peninsula that secondary contact has formed along, to determine whether behavioural interactions between the recently diverged species, have resulted in differences in mating compatibility between conspecific and heterospecific pairs. This work leads onto an investigation of the mating behaviour of C. hookeri. To understand what influences mating decisions in C. hookeri a series of behavioural experiments are performed including: field observations to gain a general understanding of this system, Y choice tests to specifically identify how males locate females, manipulation experiments to understand what role genital structures play in mating. In addition Scanning Electron Microscopy is employed to look at the sensory ability of antennae and the female opercular organ. Results elucidated the dynamic consequences of secondary contact. Bayesian Structure cluster analysis revealed C. hookeri microsatellite markers have completely introgressed into morphological Clitarchus “tepaki” populations along the shorter Karikari Peninsula so that these two species cannot be distinguished using molecular markers along this peninsula. Across the longer Aupouri Peninsula, the genetic integrity of Clitarchus “tepaki” has been maintained with the formation of a hybrid swarm encompassing a distinct transition zone. Interspecific behavioural crosses revealed both species of Clitarchus to be highly promiscuous, and despite the lack of mate discrimination between heterospecifics differences in guarding duration indicate interspecific mates are recognised as suboptimal partners. Evidence supported that males detect females via airborne sex pheromones, and that Clitarchus are scramble competitors. Lastly an obvious female response to the manipulation of male claspers demonstrated a clear female role regarding mate choice.