A reversal in sensory processing accompanies ongoing ecological divergence and speciation in Rhagoletis pomonella

Abstract

Changes in behaviour often drive rapid adaptive evolution and speciation. However, the mechanistic basis for behavioural shifts is largely unknown. The tephritid fruit fly Rhagoletis pomonella is an example of ecological specialization and speciation in action via a recent host plant shift from hawthorn to apple. These flies primarily use specific odours to locate fruit, and because they mate only on or near host fruit, changes in odour preference for apples versus hawthorns translate directly to prezygotic reproductive isolation, initiating speciation. Using a variety of techniques, we found a reversal between apple and hawthorn flies in the sensory processing of key odours associated with host fruit preference at the first olfactory synapse, linking changes in the antennal lobe of the brain with ongoing ecological divergence. Indeed, changes to specific neural pathways of any sensory modality may be a broad mechanism for changes in animal behaviour, catalysing the genesis of new biodiversity.

Rapid brain development and reduced neuromodulator titres correlate with host shifts in Rhagoletis pomonella

“…Furthermore, a recent study of a similar neurological change in the European corn borer has implicated a cis-regularly element (Unbehend et al, 2021), several of which also influence biogenic amine pathways (Monastirioti, 1999). As previously suggested, connecting host preference and survival through relatively simple changes could be a widespread mechanism for generating biodiversity across phytophagous insects, contributing to the origin of the large number of species observed (Tait et al, 2021).…”

“…To characterize brain morphology, brain samples were dissected in 1X phosphate buffer solution, PBS (PH: 7.4) on ice with fine forceps, and then underwent immunohistochemical staining using a protocol adapted from those used in Drosophila (Ibba et al, 2010;Singh et al, 2013;Tait et al, 2021). Briefly, dissected brains were transferred into plastic cell culture plates containing 4% paraformaldehyde (PFA), and plates were covered with aluminium foil.…”

“…Optical sections (512 x 512 pixels) of each brain sample with 1 µm step size, were imaged with a confocal scanning microscope (Olympus FV3000, DSS Imagetech, Bangalore) under a 10x, 20x and 40x DIC objective. Using the 3d reconstruction of R. pomonella brain as a guide (Tait et al, 2021) different brain regions were visualized and identified in FIJI open-source software (Schindelin et al, 2012).…”

“…Further, pupae were individually photographed using an infinity HD camera (lumenera, model number N9033210) attached to a stereomicroscope. After that, the brain was individually dissected using 50ul of 1X phosphate buffered saline solution, PBS (PH: 7.4) on ice with fine forceps, and stored at - To characterize brain morphology, brain samples were dissected as mentioned above and then underwent immunohistochemical staining using a protocol adapted from those used in Drosophila and Rhagoletis (24,39,40). Further details can be found in Supplementary Materials, Extended Methods.…”

“…Recent studies have shown that the change in host fruit olfactory preference between the two host races does not occur at the chemoreception stage in the antenna (21)(22)(23), but rather at the first synapse of the olfactory system in the brain, the antennal lobe (24). The coupled difference in pupal diapause timing, adult preference behaviour, and adult brain physiology in R. pomonella provides a unique opportunity to examine if there are corresponding differences in brain development between the races that could simultaneously impact both developmental rate and adult olfactory host choice.…”