Hybridization rates of many species are likely to increase as climate change causes range and phenology shifts. Our study examined the coexistence of two native tree-hole dwelling mosquito species in the greater Boston area: Aedes triseriatus (Say, 1823) and Ae. hendersoni (Cockerell, 1918). In previous studies of coexisting populations of these species, hybridization rates are likely underestimated or even ignored due to hybrid individuals’ cryptic character. Identifying rates of hybridization in these species is important because of the competence of Ae. triseriatus as a vector of LaCrosse Virus, which is not shared by Ae. hendersoni.
Our surveys and genetic analysis focused on three questions:
- What is the species composition of these tree hole communities?
- How similar are the microhabitats and phenologies of the two species?
- What is the rate of hybridization between them?
We sampled 59 artificial oviposition sites (“ovitraps”), along with natural tree holes, at five sites in the greater Boston area. Over six months, we collected 533 egg batches and identified approximately 1200 individuals morphologically. We verified hybrid identity genetically using DNA barcoding of the mitochondrial gene cytochrome c oxidase, subunit I (COI), as well as a duplex PCR reaction with a ribosomal ITS (Internal Transcribed Spacer) region—the first application of this protocol to field-caught hybrids.
Results/Conclusions
We found that Ae. triseriatus is the dominant species in treehole habitats around Boston, but frequently shares breeding sites with Ae. hendersoni. Collection sites with only Ae. triseriatus eggs accounted for 87.8% of egg batches collected, Ae. hendersoni traps accounted for 9.9%, and both species’ eggs were found together in 2.3% of traps.
The species’ coexistence could be explained by spatial or temporal niche partitioning; for example, the two species seem to prefer tree holes at different heights, and we observed anecdotally that Ae. hendersoni egg batches took longer to hatch. Contingency table analysis of our ovitrap positivity rates indicates non-random oviposition by females (𝜒 2 = 16.178, df=2, P=0.003), which could be explained if gravid Ae. triseriatus females avoid sharing oviposition sites with the opposite species, and vice versa.
Our results suggest significant rates of hybridization in this population (5.12% of egg batches). Rates of introgression are of interest for both the ecology and genetic differentiation of these two species, and carry implications for public health, inviting research into the vector competence of hybrid individuals.