The genetic structure of populations of an invading pest fruit fly, Bactrocera tryoni, at the species climatic range limit

Abstract

Previous population genetic studies of the Queensland fruit fly, Bactrocera tryoni Froggatt (Diptera: Tephritidae), in its central range have shown barely detectable genetic differentiation across distances of almost 3000 km (F st ¼ 0.003). In this study, we investigated the genetic structuring of southern border populations of B. tryoni, in the region extending from the central population to the recently colonized southern range limit. The expectation was that marginal populations would be small, fragmented population sinks, with local adaptation limited by gene flow or drift. Unexpectedly, we found that the population at the southern extreme of the range was a source population, rather than a sink, for the surrounding region. This was shown by assignment testing of recent outbreaks in an adjoining quarantine area and by indirect migration estimates. Furthermore, populations in the region had formed a latitudinal cline in microsatellite allele frequencies, spanning the region between the central population and the southern range limit. The cline has formed within 250 generations of the initial invasion and appears stable between years. We show that there is restricted gene flow in the region and that effective population sizes are of the order of 10 2 -10 3 . Although the cline may result from natural selection, neutral evolutionary processes may also explain our findings.

Genetic and cytogenetic analysis of the American cherry fruit fly, Rhagoletis cingulata (Diptera: Tephritidae)

“…In the case of agricultural pests, they can be used to address a variety of important biological questions and can help to improve planning of environmental control methods, such as the Sterile Insect Technique (SIT). Especially in Tephritidae, microsatellites have been used to study recent invasion phenomena (Bonizzoni et al 2001;Khamis et al 2009;Zygouridis et al 2009), species expansion routes (Augustinos et al 2005;Nardi et al 2005;Aketarawong et al 2007), adaptation and ongoing speciation (Michel et al 2007;Cameron et al 2010), mating patterns in nature (Kraaijeveld et al 2005;Song et al 2007) and the structure of populations of cosmopolitan and invading species (Gilchrist and Meats 2010;Virgilio et al 2010). One major problem is the difficulty and cost of developing de novo microsatellite markers, but cross-amplification of microsatellite markers from closely related species may provide an alternative.…”

Screening mitochondrial DNA sequence variation as an alternative method for tracking established and outbreak populations of Queensland fruit fly at the species southern range limit

“…Major genetic groups (from Figure 4) are indicated by circles of different colors (yellow =  NSW /Gippsland, red =  NSW /Eastern Fruit Fly Exclusion Zone ( FFEZ ), green = Central FFEZ , blue = Southern FFEZ ), triangles represent sites that are not closely related, with each being genetically distinct from both each other and the major groups. (b) Inset showing B. tryoni populations found previously using microsatellite genotyping—modified from figure 2 in Gilchrist and Meats (2010)…”

“…Compared with previous multilocus nuclear microsatellite marker studies of B. tryoni populations (Chen et al., 2016; Gilchrist & Meats, 2010; Gilchrist et al., 2004, 2006), mtDNA provides information from only a single locus, which has a clonal–maternal mode of inheritance. Differences between these two sets of markers could result in divergent patterns of population genetic variation being detected.…”

“…Similarities with previous studies include the following: (1) the large geographic scale of widespread established southeastern Australian populations (Figure 5a; Gilchrist et al., 2006); (2) the presence of multiple populations along the border of the FFEZ and NSW, including a similar pattern across the region boundary (Figure 5a) to the population cline previously detected (Figure 5b; Chen et al., 2016; Gilchrist & Meats, 2010; Gilchrist et al., 2006); (3) multiple populations detected within the FFEZ, which cross the border between the FFEZ and the RRZ (Figure 5a; Chen et al., 2016; Gilchrist & Meats, 2010; Gilchrist et al., 2006); (4) outbreak populations appear to have multiple origins, with greatly reduced gene flow in the control regions compared with established regions (Figure 2; Gilchrist & Meats, 2010; Gilchrist et al., 2004, 2006); (5) many of the southeastern Australian sites sampled in both sets of studies appear genetically distinct; for example, the Sydney region was found to be dissimilar to inland NSW for mtDNA (our study) and microsatellites (Gilchrist et al., 2006). …”

“…Previous microsatellite studies have indicated the presence of a number of genetic populations in southeastern Australia, including a genetic cline between two populations in the RRZ along the border of the established and managed zones (Gilchrist & Meats, 2010). Recently, the use of microsatellites for management of B. tryoni outbreaks has been significantly improved through the optimization of nine loci to work together in a single multiplex PCR (Chen et al., 2016), greatly reducing the labor required to genetically screen specimens.…”