Ongoing Projects

Model organisms of speciation. (a) Apple maggot fly, Rhagoletis pomonella (photo credit: Hannes Schuler); (b) Three-spined stickback, Gasterosteus aceleatus (photo credit: Ingo Seiel), (c) annual phlox, Phlox drummondii (photo credit: Gertrud Kanu); buetterfly in the genus Heliconius (photo credit: Montse Poch), (e) cichlid Pundamilia nyererei (photo credit: Greg and Lee Ann Stevens), (f) Timema stick insect (photo credit: Mortize Muschik). Figure above taken from Modes and Mechanisms of Speciation by Schuler et al. 2016. 

1. Ecological Adaptation and Speciation: Speciation is the evolutionary process that generetes new biodiversity. Our lab looks at how ecological adaptation via divergent natural selection to different environments promotes population divergence. We are interested in (1) the ecological mechanisms that evolve to promote reproductive isolation between  populations, (2) how different evolutionary forces (mutation, drift, gene-flow, natural selection) facilitate divergence, and (3) the genetic/genomic signatures of population divergence and speciation. We take a multi-pronged approach to investigating speciation which includes laboratory and field-based experiments, observations of biogeography, natural history, behavior and life history, and genomics sequencing. Systems of interest include the apple maggot fly, Rhagoletis pomonella, a model for the study of rapid ecological speciation, and the community of parasitoid wasps that attack the fly; and gall forming insects and their hyper diverse communities of parasitic natural enemies, among others (see below for more information). 

2. Evolutionary Ecology of Multi-trophic Interactions: Our lab is broadly interested in the interaction between trophic levels primarily between plants, plant feeding insect and insect parasites. In particular, we are interested in c0-phylogenetic relationships between these interacting trophic levels and the role that phylogeography plays in shaping these interactions. 

One co-phylogenetic pattern of particular interest is the idea that "biodiversity begets biodiversity" in a term coined 'sequential divergence'. This is a co-evolutionary process whereby speciation at one trophic level induces parallel speciation event of interacting organisms at adjacent trophic levels. This process is centered on the premise that the same ecological mechanisms that reduce gene flow between diverging populations in one species cascade across trophic levels to similarly induce divergence of associated organisms (see below for more details).

See below for ongoing projects and in depth descriptions of the critters we research. (Photo credit: Andrew Forbes)

Rapid Ecological Speciation in the Apple Maggot Fly, Rhagoletis pomonella

The apple maggot fly, Rhagoletis pomonella, is a text-book example of rapid ecological speciation in real time in the eastern United States. The fly, native to laying eggs in the fruit of hawthorn, shifted introduced, domesticated apples ~170 ya. Today populations of the fly exist that differ in allele frequencies and are ecologically adapted to different environmental conditions, and it is these ecological conditions to reduce gene flow. between these diverging populations. 
Flies emerge after an overwinter to coincide with when fruit ripen. Apples ripen about 2-3 weeks earlier than hawthorns, and because the flies adult life span is short, differences in emergence time reduce chances for mating, thus reducing gene flow and increasing reproductive isolation between diverging populations. In addition, flies are attracted to the volatiles emitted from the surface of ripe natal host plant fruit and avoid those non-natal fruit odors. Given that flies mate on our near their natal host fruit, host choice leads directly to make choice, increasing reproductive isolation and also reducing gene flow between diverging fly populations. (Photo credit: Joseph Berger, www.bugwood.org)

Sequential Divergence in the Community of Parasitoids attacking R. pomonella

The community of host-specific endoparasitoid wasps that attack fruit flies in the Rhagoletis pomonella sibling species complex. From left to right: Diachasma alloeum, Diachasmimorpha mellea, Utetes canaliculatus. 
Understanding how new life forms originate is a central question in biology. Population divergence is usually studied with respect to how single lineages diverge into daughter taxa. However, populations may not always differentiate in isolation; divergence of one taxon could create new niche opportunities in higher trophic levels, leading to the sequential origin of many new taxa. Here, we show that this may be occurring for three species of parasitoid wasps attacking Rhagoletis fruit flies. As flies shift and adapt to new host plants, wasps follow suit and diverge in kind, resulting in a multiplicative increase of diversity as the effects of ecologically based divergent selection cascade through the ecosystem. Biodiversity therefore may potentially beget increasing levels of biodiversity in certain scenarios.
Interestingly, the same host-related ecological selection pressures that differentially adapt and reproductively isolate Rhagoletis to their respective host plants (host-associated differences in the timing of adult emergence, host fruit odor preference and avoidance behaviors, and mating site fidelity) cascade through the ecosystem and induce host-associated genetic divergence for each of the three members of the parasitoid community. (Photo modified from Hood et al. 2016, PNAS)

Sequential Divergence in a more Temporally Proximate Context

The apple maggot fly was recently introduced ~~60 years ago into the Pacific Northwest (PNW) via larval infested apples. It is hypothesized that R. pomonella was introduced via larval infested apples from the east in the 1950’s and the fly subsequently shifted onto black hawthorn and ornamental hawthorn (pictured above) forming host races in the region. Thus, the recent introduction and radiation of R. pomonella in the west provides a new niche for parasitoids to exploit, potentially catalyzing a chain reaction of speciation events in the parasitoid community in the last 60 yrs, even more rapidly that in eastern populations (~170 years). We are currently using genetic, behavioral, life history and natural history data to examine if the community of parasitoids attacking R. pomonella in the PNW has also undergone a sequential divergence.  

Gall Forming Cynipids

Upon laying eggs into specific host plant tissues (stems, leaves, flowers), gall-inducing manipulate the plant genome to produce outgrows of hyper-nutrient-rich plant tissues referred to as "galls" wherein the immature insect feeds and develop, and eventually emerges from as an adult. the gall itself is hypothesized to serve as protection from species-rich natural enemy communities (see photo right). Cynipid wasps in the Order Hymenoptera represent one of the most species-rich and phenotypically variable groups of galling insects comprising ~1400 described species. For a look at the phenotypic variation in gall morphologies, click here! The majority of these species (~70%) form galls on oaks in the genus Quercus. Another interesting aspect of gall biology is that many species display heterogonic (icyclical parthenogenesis) where sexual and asexual generations that form distinct gall morphologies on different plant tissues alternate to complete a bivoltine life cycle (see the leaf and root galls above, induced by the sexual and asexual generations of the gall wasp, Belonocnema treatae on live oak, Quercus fusiformis). We are interested in all things related to the evolutionary ecology of interactions between gall formers, their host plants, and their natural enemy communities. 

Natural Enemies of Gall Wasps

Gall forming insects are often associated with hyper diverse communities of natural enemies. Our lab is currently involved in a collaborative project to explore the origins of biodiversity of these natural enemies and their insect hosts. Specifically, we want to understand the processes that underlie the patterns of diversification at different scales across time and space. The objective of this research is to test where population-level phenomenon (i.e., micro-evolutionary processes) predict macro-evolutionary patterns of co-diversification in species rich insect communities. To accomplish this goal, we are using population genomics and phylo-genomics to link evolutionary dynamics within local populations and geographic mosiacs of co-evolving local communities to broad-scale tests of phylogenetic (in)congruence at the continental scale.
Pictured above is an example of the diverse community of natural enemies that attack a single species of gall wasp, Belonocnema treatae, that lives on live oaks in the southern U.S. This community includes parasitoids, hyperparasioids (parasitoids of parasitoids) and inquilines (an organism that exploits the living space of another organism). This community consists of several other species of hymenopteran wasps, flies, beetles, and moths. We are currently processing population genomics data from natural enemies collected from different locations and host associations across the southern to better understand the population genomics, biogeography, and co-evolution of natural enemy communities and their gall forming hosts. 
My lab is also interested in exploring the evolutionary ecology (and particularly host plant adaptation and speciation) of a number of gall wasp systems that are pests for important agricultural crops, including (but not limited to) gall-inducers of blueberries in Michigan. Click here for information about cynipid wasps galling the stems of blueberries.