The role of sexual isolation during rapid ecological divergence: evidence for a new dimension of isolation in Rhagoletis pomonella

The pace of divergence and likelihood of complete speciation may depend how and when different types of reproductive barriers evolve. After initial reproductive barriers evolve, questions remain about how subsequently evolving barriers may facilitate additional divergence and potential speciation. We tested for the presence of sexual isolation (reduced mating between populations due to divergent mating preferences and traits) in Rhagoletis pomonella flies, a model system for incipient ecological speciation. We measured the strength of sexual isolation between two very recently diverged (~170 years) sympatric populations, adapted to different host fruits. We found that sexual isolation was significantly stronger than expectations of random mating. Thus, sexual isolation may play an important role in reducing gene flow allowed by earlier-acting ecological barriers. We also found that sexual isolation was markedly asymmetric between the sexes of each population. Lastly, we tested how warmer temperatures predicted under climate change could alter sexual isolation and found that mating interactions were sensitive to temperature experienced during development. Our findings provide a window into the early divergence process and the role of sexual isolation after initial ecological divergence, in addition to examining multiple factors that could shape the likelihood of further divergence.

Connecting neuromodulation, brain development, and life history timing associated with ecological speciation in Rhagoletis pomonella

Host shifts are considered a key generator of insect biodiversity, yet how insects synchronize life history timing and preference for new hosts is not well understood. Neurochemicals such as biogenic amines play key roles in both development and behavior, and therefore provide a potential source for such synchronization. Here, we correlated life history timing, brain development, and corresponding levels of 14 neurochemicals in Rhagoletis pomonella (Diptera: Tephritidae), a species undergoing ecological speciation through an ongoing host shift from hawthorn to apple fruit. We found that adult brain differentiation in both host races was initiated only after pupal diapause ended, and at this stage the titers of precursors for several neurotransmitter pathways also increased significantly. Conversely, neurotransmitter product titres generally increased at later stages up to adult fly sexual maturation. Further analysis showed that the apple host race exhibited adult brain differentiation three weeks earlier than the hawthorn host race, which correlated with significantly lower titers of several neurochemicals, particularly biogenic amines, in the earlier developing apple race as compared to the hawthorn race. We thus propose that these neurotransmitter levels could influence the differences in life history timing and host preference observed in these two races, promoting their ongoing speciation.

Tolerances of Apple Maggot (Diptera: Tephritidae) Larvae and Different Age Puparia to Water Flotation and Immersion

Tolerance of terrestrial insects in temperate regions to water immersion and hypoxia has rarely been studied but can be an important adaptation to moist environments, with implications for insect dispersal through waterways. In the Pacific Northwest of the United States, apple maggot fly, Rhagoletis pomonella (Walsh) (Diptera: Tephritidae), can be found in riparian habitats subject to flooding. Here, survival of R. pomonella larvae and different age puparia after flotation or immersion in 13.3°C or 21.1°C water for 1–12 d was determined. Larvae sank in water and when submerged for 1 or 2 d suffered greater mortality than control larvae. Fewer young (1–2 d old) than older puparia (13–15 d old) floated in water. When immersed in water for 1–12 d, young puparia suffered greater mortality than older puparia, which were not affected by water immersion. Consequently, fewer adult flies eclosed from puparia that had been water treated when young than older. Adult flies from pre-chill and post-chill puparia that had been water treated eclosed later than control flies, but treatment flies survived about 60 d and reproduced. Although newly-formed puparia are susceptible to hypoxic water conditions, increased buoyancy and water tolerance occur rapidly after formation, perhaps making survival possible and allowing water-borne dispersal of older puparia.

Diferencias en las propiedades ópticas de pupas de machos y hembras de Anastrepha fraterculus (Diptera: Tephritidae)

Una alternativa al uso de inseticidas sintéticos en el control de especies importantes de moscas de la fruta (Tephritidae) es la técnica del insecto estéril (TIE). La TIE se basa en liberaciones masivas de machos estériles (sometidos a radiación) para controlar una población de plagas. Para especies de importancia como Rhagoletis pomonella, Bractocera oleae y Anastrepha fraterculus, es un desafio diferenciar los machos de las hembras en el estadio pupal. Aunque para algunas especies se liberan hembras estériles en campo, esto no es deseable en Anastrepha spp., ya que puede ocasionar daño por punción. Por lo tanto, hemos investigado imágenes hiperespectrales y RGB para classificar el sexo en pupas de A. fraterculus. Nuestro objetivo fue discriminar, a través de la descripción de espectros RGB y de perfiles hiperespectrales, machos y hembras en diferentes estadios de desarrollo pupal. Se obtuvieron imágenes de pupas de diferentes tiempos de desarrollo de grados-día (GD). Los perfiles hiperespectrales de las pupas de 288, 345 y 403 GD mostraron diferencias, indicando que el tono de la pupa se altera durante el desarrollo. En el espectro NIR (800 a 1.700 nm) se observó una mayor reflectancia entre 1.100 y 1.400 nm. En estas bandas ocurren diferencias en la desviación estándar de la reflectancia entre machos y hembras. En las imágenes RGB se observaron diferencias en el ángulo de variación radiométrica, desde la parte superior hasta la parte basal de las pupas. Para las hembras, el ángulo de variación radiométrica fue mayor (26°) que para los machos (15°). Por lo tanto, es posible determinar características discriminantes utilizando los perfiles espectrales, técnicas de imagen y análisis de datos para separar los sexos.

The Eclosion of Rhagoletis pomonella (Diptera: Tephritidae) Under Different Chill Durations and Simulated Temperate and Tropical Conditions

The apple maggot fly, Rhagoletis pomonella (Walsh) (Diptera: Tephritidae), is a serious pest of apple in North America that is subject to quarantine measures to prevent its spread to currently pest-free regions, including the tropics. How the fly may survive in warmer climates is unclear. Here, we studied the effects of exposing postchill puparia to simulated temperate and tropical environmental conditions on eclosion of R. pomonella from Washington State, U.S.A. Puparia were chilled for 0–30 wk at 3°C and then held under four postchill conditions: A = 23°C, 16:8 L:D, 40% RH; B = 26°C, 12:12 L:D, 80% RH; C = 26°C, 16:8 L:D, 80% RH; and D = 23°C, 12:12 L:D, 40% RH, with B and D representing tropical conditions and A and C temperate conditions. Within each chill duration, total numbers of flies eclosed were equally high in tropical treatment B and temperate treatment C, while they were lower in treatments A and D. Mean weeks of the first eclosion in treatments B and C were earlier than in treatment D; mean week of peak eclosion and 50% eclosion in treatments A, B, and C were earlier than in treatment D. Eclosion spans in treatments A, B, and D were generally shorter than in treatment C. Results suggest that if introduced into a humid tropical country, R. pomonella puparia from Washington State could produce adult flies, regardless of chill duration or lack of chilling during the pupal stage, but whether flies could establish there would require further study.

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

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.

Distinct Adult Eclosion Traits of Sibling Species Rhagoletis pomonella and Rhagoletis zephyria (Diptera: Tephritidae) Under Laboratory Conditions

Closely related phytophagous insects that specialize on different host plants may have divergent responses to environmental factors. Rhagoletis pomonella (Walsh) and Rhagoletis zephyria Snow (Diptera: Tephritidae) are sibling, sympatric fly species found in western North America that attack and mate on plants of Rosaceae (~60 taxa) and Caprifoliaceae (three taxa), respectively, likely contributing to partial reproductive isolation. Rhagoletis zephyria evolved from R. pomonella and is native to western North America, whereas R. pomonella was introduced there. Given that key features of the flies’ ecology, breeding compatibility, and evolution differ, we predicted that adult eclosion patterns of the two flies from Washington State, USA are also distinct. When puparia were chilled, eclosion of apple- and black hawthorn-origin R. pomonella was significantly more dispersed, with less pronounced peaks, than of snowberry-origin R. zephyria within sympatric and nonsympatric site comparisons. Percentages of chilled puparia that produced adults were ≥67% for both species. However, when puparia were not chilled, from 13.5 to 21.9% of apple-origin R. pomonella versus only 1.2% to 1.9% of R. zephyria eclosed. The distinct differences in eclosion traits of R. pomonella and R. zephyria could be due to greater genetic variation in R. pomonella, associated with its use of a wider range of host plants than R. zephyria.

Virulence of Entomopathogenic Fungi to Rhagoletis pomonella (Diptera: Tephritidae) and Interactions With Entomopathogenic Nematodes

The objectives of this study were to quantify the virulence of four entomopathogenic fungal species to pupae of Rhagoletis pomonella (Walsh) (Diptera: Tephritidae) and to determine the potential to combine entomopathogenic fungi (EPFs) and entomopathogenic nematodes (EPNs) for biological control of this pest. The four species of EPFs included Beauveria bassiana (strain GHA), Metarhizium brunneum (strain F52), Isaria javanica (wf GA17), and Isaria fumosorosea (Apopka 97 strain). In laboratory assays, all fungi reduced adult emergence but there were no differences between fungal species. Isaria javanica and M. brunneum were examined further in a EPFs and EPNs bioassay that also included the EPNs Steinernema carpocapsae (ALL strain) and S. riobrave (355 strain). All nematodes and fungi were applied either alone or in combination (fungus + nematode). There were no differences between species within the same entomopathogen group (fungi and nematodes). However, the treatment with S. riobrave resulted in lower R. pomonella emergence than either fungal species. The combination of S. riobrave and I. javanica resulted in the lowest R. pomonella emergence (3%) at fourth-week interval, which was significantly lower than any of the single-agent applications, yet virulence of the other three combination treatments was not different from their respective nematode treatments applied alone. Additive interactions were detected for all fungus–nematode combinations. This study suggests that application of entomopathogenic nematodes and fungi could be an effective option to suppress R. pomonella populations.