Differences in performance and transcriptome-wide gene expression associated with Rhagoletis (Diptera: Tephritidae) larvae feeding in alternate host fruit environments.

Host race formation, the establishment of new populations using novel resources, is a major hypothesized mechanism of ecological speciation, especially in plant-feeding insects. The initial stages of host race formation will often involve phenotypic plasticity on the novel resource, with subsequent genetically based adaptations enhancing host-associated fitness differences. Several studies have explored the physiology of the plastic responses of insects to novel host environments. However, the mechanisms underlying evolved differences among host races and species remain poorly understood. Here, we demonstrate a reciprocal larval performance difference between two closely related species of Rhagoletis flies, R. pomonella and R. zephyria, specialized for feeding in apple and snowberry fruit, respectively. Microarray analysis of fly larvae feeding in apples versus snowberries revealed patterns of transcriptome-wide differential gene expression consistent with both plastic and evolved responses to the different fruit resources, most notably for detoxification-related genes such as cytochrome p450s. Transcripts exhibiting evolved expression differences between species tended to also demonstrate plastic responses to fruit environment. The observed pattern suggests that Rhagoletis larvae exhibit extensive plasticity in gene expression in response to novel fruit that may potentiate shifts to new hosts. Subsequent selection, particularly selection to suppress initially costly plastic responses, could account for the evolved expression differences observed between R. pomonella and R. zephyria, creating specialized races and new fly species. Thus, genetically based ecological adaptations generating new biodiversity may often evolve from initial plastic responses in gene expression to the challenges posed by novel environments.

Molecular species identification of cryptic apple and snowberry maggots (Diptera: Tephritidae) in western and central Washington.

In Washington state, identification of the quarantine apple pest Rhagoletis pomonella (Walsh) is complicated by the presence of the cryptic species Rhagoletis zephyria Snow (Diptera: Tephritidae). Distinguishing the two flies is important because there is a zero tolerance policy for R. pomonella in apple production for export. Here, we attempt to distinguish the two species by scoring R. pomonella and R. zephyria populations from western and south-central Washington for a set of 11 nuclear markers, including four single nucleotide polymorphisms (SNPs) developed for rapid and inexpensive genotyping using Taqman real-time quantitative-polymerase chain reaction. We show that the four SNPs may be adequate in most cases for distinguishing whether a fly originated from apple or black hawthorn (the two major host plants for R. pomonella representing an economic risk) versus snowberry (the major host for R. zephyria, and not a commercial threat). However, directional introgression of R. zephyria alleles into R. pomonella can complicate the identification of flies of mixed ancestry based only on the four SNPs. Moreover, this problem is more acute in the sensitive apple-growing regions of central Washington where our results imply hybridization is common. Consequently, application of the four SNP quantitative-polymerase chain reaction assay can immediately assist ongoing apple maggot monitoring, while the development of additional genetic markers through next-generation sequencing would be valuable for increasing confidence in species identification and for assessing the threat posed by hybridization as R. pomonella further spreads into the more arid apple-growing regions of central Washington.