Fitness consequences of population bottlenecks in an invasive blowfly.

Invasive species often undergo demographic bottlenecks that cause a decrease in genetic diversity and associated reductions in population fitness. Despite this, they manage to thrive in novel environments. Investigating the effects of inbreeding and genetic bottlenecks on population fitness for invasive species is, therefore, key to understanding how they may survive in new environments. We used the blowfly Calliphora vicina (Sciences, Mathématiques et Physique, 1830, 2, 1), which is native to Europe and was introduced to Australia and New Zealand, to examine the effects of genetic diversity on population fitness. We first collected 59 samples from 15 populations across New Zealand and one in Australia, and used 20,501 biallelic SNPs to investigate population genomic diversity, structure and admixture. We then explored the impacts of repeated experimental bottlenecks on population fitness by creating inbred and outbred lines of C. vicina and measuring a variety of fitness traits. In wild-caught samples, we found low overall genetic diversity, signals of genetic admixture and limited (<3%) genetic differentiation between North and South Island populations, with genetic links between the South Island and Australia. Following experimental bottlenecks, we found significant reductions in fitness for inbred lines. However, fitness effects were not felt equally across all phenotypic traits. Moreover, they were not enough to cause population collapse in any experimental line, suggesting that C. vicina (when under relaxed selection, as in laboratory settings) may be able to compensate for population bottlenecks even when highly inbred. Our results demonstrate the value of a tractable experimental system for investigating processes that may facilitate or hamper biological invasion.

Population structure and interspecific hybridisation of two invasive blowflies (Diptera: Calliphoridae) following replicated incursions into New Zealand.

Rates of biological invasion are increasing globally, with associated negative effects on native biodiversity and ecosystem services. Among other genetic processes, hybridisation can facilitate invasion by producing new combinations of genetic variation that increase adaptive potential and associated population fitness. Yet the role of hybridisation (and resulting gene flow) in biological invasion in invertebrate species is under-studied. Calliphora hilli and Calliphora stygia are blowflies proposed to have invaded New Zealand separately from Australia between 1779 and 1841, and are now widespread throughout the country. Here, we analysed genome-wide single nucleotide polymorphisms (SNPs), generating genotyping-by-sequencing data for 154 individuals collected from 24 populations across New Zealand and Australia to assess the extent of gene flow and hybridisation occurring within and between these blowflies and to better understand their overall population structure. We found that New Zealand populations of both species had weak genetic structure, suggesting high gene flow and an absence of dispersal limitations across the country. We also found evidence that interspecific hybridisation is occurring in the wild between C. hilli and C. stygia in both the native and invasive ranges, and that intraspecific admixture is occurring among populations at appreciable rates. Collectively, these findings provide new insights into the population structure of these two invasive invertebrates and highlight the potential importance of hybridisation and gene flow in biological invasion.