After winning a 21st Century Research Grant last year, PhD student Simon recently gave an update on his research in the latest newsletter for the NZ Entomological Society:
New Zealand is home to four native species of fishing spider (Dolomedes), including two sister species: D. minor and D. aquaticus which are the focus of my PhD research. There is genetic evidence to show that these two species are undergoing introgression. Introgression is the movement of genes from one species to another, caused by the backcrossing of a hybrid specimen with its parent species. In the case of these spiders, the introgression is one-way (with hybrids only produced by the mating of D. aquaticus females and D. minor males) and geographically restricted to the lower South Island, despite the species co-occurring throughout the range of D. aquaticus.
The reasons for these limitations are currently unknown, but my work is investigating several hypotheses: 1) habitat use differs in the introgression zone, facilitating encounters between the two species; 2) variation in genital morphology (specifically variation in the retrolateral tibial apophysis (RTA)) limits their sperm transfer; 3) variation in courtship behaviour limits mating; 4) timings of reproductive maturity limit mating opportunities.
The preliminary results of my experiments suggest that the two species possess different mating behaviours and systems. For example, unmated D. aquaticus females rarely attack males, whereas D. minor females often attempt to cannibalise males who try and escape during extremely brief copulations. This divergence in mating behaviours could also have an impact on the introgression, as these could limit sperm transfer between the two species.
Hence, my aims are to investigate the morphological and/or behavioural barriers that limit the geographic range of the introgression, and restrict it to one-way (i.e.: what prevents D. aquaticus males mating with D. minor females).
I am grateful to have received funding from the New Zealand Entomological Society, via the 21st Anniversary Research Grant. These funds allowed me to travel to Southland to collect specimens of D. minor and D. aquaticus from within the introgression zone. Working in these locations was a key part of my research, not to mention a highly enjoyable part of my work so far.
These specimens were brought back live to Waikato (despite some concerns from aviation security), where they were used in extensive mating experiments. I am currently in the process of analysing the results of these experiments and how they fit into my wider PhD findings.
Additionally, I am also working to scan the male genitals of these spiders using micro-CT, to analyse the morphology of the RTA and the impact this could have on introgression.
I cannot wait until these analyses are complete so I can share my full finding with The Society and the scientific community.