Host shifts, Reproductive Isolation, and Speciation
We maintain a wiki page that collects and evlauates studies dealing the the relationships between insect host use, reproductive isolation, and speciation. This dataset is an outgrowth of a manuscript the lab worked on in 2014-2015, and that is now published in the journal Evolution. The wiki-dataset can be found here: https://wiki.uiowa.edu/display/InsectHostShifts/Insect+Host+Shifts+and+Speciation) and anyone is invited to collaborate on its maintenance.
Forbes AA, Devine SN, Hippee AC, Tvedte ES, Ward AKJ, Widmayer HA, Wilson CJ. 2017. Revisiting the particular role of host shifts in initiating insect speciation. Evolution. Online early
Hidden niches and tropical insect diversity across three trophic levels
We are collaborating with Marty Condon at Cornell College, Brian Wiegmann at NC State, Gaelen Burke at U. Georgia, Sonja Scheffer at USDA-ARS, and Nina Theis at Elms College to understand patterns of extreme diversity among neotropical flies and parasitoid wasps. Blepharoneura flies are morphologically cryptic, highly diverse, and highly host-specific, yet many species overlap in the same host plant niche. Likewise, their Bellopius parasitoid wasps are also speciose, cryptic, and host specific (to both flies and plants). We are using molecular markers, natural history collections, and patterns of lethal interactions between flies and wasps to ask what explains patterns of diversity in both trophic levels. An NSF Dimensions of Biodiversity grant currently supports this research. Our recent published work on this topic includes:
Condon MA, Scheffer S.J., Lewis M., Wharton R.A., Adams D.C., Forbes AA. 2014. Lethal interactions between parasites and prey increase niche diversity in a tropical community. Science. 343:1240-1244.
Adaptation and long-term persistence of an asexual insect species
In a collaboration with John Logsdon (University of Iowa), we are studying patterns of genomic evolution in Diachasma muliebre, an asexual Braconid wasp found in the Northwestern United States. Females of this wasp lay diploid eggs which develop into females and do not make males. Our work so far (Forbes et al. 2013) suggests that all present-day lineages have evolved from a single ancestral wasp, and that lineages have since diverged phenotypically, now inhabiting significantly different slices of the species's overall temporal niche. Current work investigates rates of molecular evolution across genus Diachasma, with a focus on changes that may occcur across a transition to aexuality. Our recent published work on this topic includes:
Forbes AA, Rice LA, Stewart NB, Yee WL, Neiman M. 2013. Niche differentiation and colonization of a novel environment by an asexual parasitic wasp. Journal of Evolutionary Biology. 26:1330-1340.
The origin and evolution of reproductive barriers in Strauzia (sunflower maggot) flies.
Flies in genus Strauzia span the so-called "speciation continuum" from long-diverged species to more closely related species, to lineages that continue to experience gene flow. As with many specialist phytophagous insects, their host associations and phylogeny suggest that divergent ecological selection has played a role in their speciation. Our lab is using this system to study the origin and evolution of different reproductive barriers across the continuum of ecological speciation. We are also working on assembling a robust phylogeny for the genus, relating morphological variation to genetic variation among closely related lineages, and trying to understand the phylogeographic history of these flies in the context of the historical distribution of their host plants.Our recent published work on this topic includes:
Hippee AC, Elnes ME, Armenta JS, Condon MA, Forbes AA. 2016. Divergence before the host shift? Prezygotic reproductive isolation among three varieties of a specialist fly on a single host plant. Ecological Entomology 41:389-399.
Forbes AA, Kelly PH, Middleton KA, Condon MA. 2013. Genetically differentiated races and speciation-with-gene-flow in the sunflower maggot, Strauzia longipennis. Evolutionary Ecology. 27:1017-1032.
Parasitoids of Rhagoletis flies - Starbursts of Speciation?
The apple maggot fly, Rhagoletis pomonella, is one of the best-known examples of ecological speciation in action. Over the last 400 years, populations of this native North American fly have moved from their native hosts, fruits of hawthorn trees (Crataegus) into introduced European apples. These two populations, though not isolated by geographic barriers, nevertheless appear to have formed incipient species in just this short period of time.
Diachasma alloeum (figure at right) is a parasitoid wasp that lays its eggs in the developing larvae of R. pomonella. Wasps in this genus attack several different Rhagoletis fly species, including those that infest apples, hawthorns, blueberries and snowberries. We asked the following question: has the divergence of these wasps' fly hosts driven a sequential, or "cascading" host race formation in D. alloeum? Several pieces of evidence suggest that the answer is yes: First, eclosion of wasps in each host-associated population closely tracks eclosion of their respective fly hosts, meaning that the wasps are at least partially allochronically isolated from one another. Second, the two populations of wasps respond differently to fruit odors: they prefer their natal fruit odor and (in some tend to avoid nonnatal odors. Third, significant microsatellite allele frequency differences define host-associated populations (Forbes et al. 2009).
With funding from the National Science Foundation (DEB-1145666), we asked whether other egg and larval parasitoids of Rhagoletis are also undergoing host-associated divergence. A second larval parasitoid (Diachasmimorpha mellea) and an egg parasitoid (Utetes canaliculatus) are also commonly found attacking the new apple race of R. pomonella. Our research (Hood et al. 2015) showed that these parasitoids are also forming new species (a 'starburst' of speciation). This project was a collaboration between the Forbes lab and researchers at the University of Notre Dame (Jeff Feder and Glen Hood) and Cornell University (Charlie Linn). Our recent published work on this system includes:
Hamerlinck G, Hulbert D, Hood GR, Smith JJ, Forbes AA. 2016. Histories of host shifts and cospeciation among free-living parasitoids of Rhagoletis flies. Journal of Evolutionary Biology 29:1766-1779
Hamerlinck G, Lemoine NP, Hood G R, Abbott KC, Forbes AA. 2016. Meek mothers with powerful daughters: effects of novel host environments and small trait differences on parasitoid competition. Oikos. 125: 1516-1527.
Hood GR, Forbes AA, Powell THQ, Egan SP, Hamerlinck G, Smith JJ, Feder JL. 2015. Sequential Divergence and the Multiplicative Origin of Community Diversity. Proceedings of the National Academy of Sciences, U.S.A. 112:E5980–E5989.