We seek to understand the circumstances under which hybridization is beneficial or detrimental, whether genetic or ecological.
Toward this goal, our current research focuses on the following questions:
1) What phenotypic traits are altered by hybridization, and how do these changes impact fitness?
2) What environmental factors exert selection on hybrids?
3) Do the benefits and drawbacks of hybridization vary across the genome?
4) Which ecological and evolutionary processes drive hybrid/parent coexistence in hybrid zones?
5) What are the impacts of hybridization on insect visitation and insect community diversity?
6) How might hybridization perturb the relationship between growth and defense?
Our study system is Boechera, an unassuming North American wildflower known for widespread hybridization, as well asexual reproduction and polyploidy.
My previous research focused on Boechera's incredible diversity of reproductive strategies: asexuality, self-fertilization, and outcrossing (whether within a population, across populations, or hybridization between species). Reproduction is vital for evolution, shaping population-level processes and ultimately, the biodiversity we see around us every day.
Toward this goal, our current research focuses on the following questions:
1) What phenotypic traits are altered by hybridization, and how do these changes impact fitness?
2) What environmental factors exert selection on hybrids?
3) Do the benefits and drawbacks of hybridization vary across the genome?
4) Which ecological and evolutionary processes drive hybrid/parent coexistence in hybrid zones?
5) What are the impacts of hybridization on insect visitation and insect community diversity?
6) How might hybridization perturb the relationship between growth and defense?
Our study system is Boechera, an unassuming North American wildflower known for widespread hybridization, as well asexual reproduction and polyploidy.
My previous research focused on Boechera's incredible diversity of reproductive strategies: asexuality, self-fertilization, and outcrossing (whether within a population, across populations, or hybridization between species). Reproduction is vital for evolution, shaping population-level processes and ultimately, the biodiversity we see around us every day.
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Boechera biology primer
Evidence suggests all 80 species of Boechera hybridize with one another Outcrossing between divergent populations also produces within-species hybrids But they usually use their main form of reproduction: self-fertilization
The vast majority of wild-collected hybrid Boechera reproduce asexually
To keep things simple, we focus on B. stricta & B. retro two ecologically and genetically divergent species that hybridize in the wild |
B. stricta x B. retro hybrid
with characteristic intermediate fruit & flower orientation |
Boechera stricta
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Boechera retrofracta or "retro"
Rosette (above) Fashionable fringe of fruits (right) |
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Hybrid fitness in the wild
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Hybridization and asexuality have unique effects on fitness. Whether interspecific or intraspecific, hybrids are more likely to survive the winter. Non-hybrid lines with the lowest genetic diversity have the poorest survival.
Later in the season, hybrids experience more insect herbivory than non-hybrids, and insect reduces survival to subsequent years (Rushworth et al. 2020). These results show that both biotic and abiotic factors contribute to the evolution and persistence of hybrids in the wild. |
Ongoing and future projects
- Identify the trait(s) and loci underlying increased survival
- Link patterns of insect herbivory with chemical defense variation
- Quantify the effects of heterozygosity on trait values and functions
- Assess herbivore preference and performance on hybrids and parental species (PhD candidate Mia Stevens)
- Characterize hybridization-associated variation in herbivorous insect community (PhD candidate Mia Stevens)
Heterosis in natural populations
A and B) Survival is predicted by parental divergence in B. retro F2s. C and D) Some between-population F2s exhibit midparent heterosis at one garden site (asterisks). From Sandstedt and Rushworth 2024.
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What factors underlie the hybrid survival advantage? The observation that the most homozygous lineages suffer the most mortality could suggest that heterozygosity confers a fitness advantage in wild populations.
Heterosis is the increase in hybrid fitness relative to either the best performing parent or the parental average. Often used as a breeding approach in agriculture and conservation, few studies address it in wild systems. Former postdoc Gabrielle Sandstedt led a project quantifying heterosis in the field, finding that parental divergence predicts survival in B. retro F2s (left, A and B). Some F2 lines exhibited mid-parent survival heterosis, but only in certain environments (left, C and D). Collectively, our results show that heterosis is environmentally-variable and dependent on parental background (Sandstedt & Rushworth 2024). |
Ongoing and future projects
- Characterize the complex genetic basis of heterosis in Boechera
- Identify the environmental factors that contribute to heterosis
- Investigate the roles for adaptation, plasticity, and dosage effects in heterosis
- Test the hypothesis that survival and resistance trade-offs mediate both heterosis and reproductive isolation
(incoming PhD student Theo Addo)
Characterizing reproductive isolation in Boechera
A) Evidence of a unidirectional crossing barrier (crosses fail if B. retro is the ovule parent.) B) The crossing barrier is also present in the wild. In a phylogeny of chloroplast marker trnL, wild-collected hybrids (gray) more often cluster with B. stricta (black) than B. retro (white). From Rushworth and Mitchell-Olds 2020.
Aphid swarm on hybrid
(Custer County, Idaho)
Although wild hybrids generally have high fitness, B. stricta and B. retro exhibit multiple reproductive isolating barriers:
Ongoing and future projects
- Ecological isolation: B. stricta occupies high elevation mesic forests and meadows; B. retro is an arid to semi-arid species.
Where they co-occur, fine-scale ecological differentiation may enable coexistence (Rushworth et al. 2018) - Unidirectional cross incompatibility: Crosses only succeed if B. stricta is the ovule parent
- Hybrid inviability and sterility: Although germination is equivalent in hybrids and parents, hybrid seedlings often fail to survive
- Hybrid weakness: Lab-generated hybrids have poor fruit and seed set; high herbivore and pathogen susceptibility in wild hybrids
Ongoing and future projects
- Model long-term hybrids and parent coexistence with genetic and ecological interactions (MS student Kallol Mozumdar)
- Characterize postmating prezygotic reproductive isolating barrier (pollen-pistil interactions; Undergraduate student Cole Keele)
- Pinpoint loci underlying pollen-pistil barrier
- Identify potential inviability traits (work on seedling radicle extension is ongoing)
Past research
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Conflict over fertilization underlies the transient evolution of reinforcement
A) Reinforcement dynamics from a model of postmating prezygotic barrier loci called "gametophyte factors". B) Reinforcement is transient unless selection is exceptionally strong. From Rushworth et al. 2022.
We modeled the evolution of transient reinforcement via a compatibility allele expressed in pollen and a choosiness allele expressed in stigmas.
Rushworth, C.A., A. Wardlaw, J. Ross-Ibarra, and Y. Brandvain. 2022. Conflict over fertilization underlies the transient evolution of reinforcement. PLOS Biology 20(10): e3001814. https://doi.org/10.1371/journal.pbio.3001814.
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The Boechera model system for evolutionary ecology
Boechera is a model system for understanding climate adaptation, herbivory, microbial community, and reproductive diversity.
Rushworth, C.A., M.R. Wagner, T. Mitchell-Olds, and J.T. Anderson. 2022. The Boechera model system for evolutionary ecology. American Journal of Botany 109(11):1939-1961. https://doi.org/10.1002/ajb2.16090.
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Who are we now? A demographic assessment of the evolution societies
The first demographic study of three major evolutionary biology professional societies found that some groups are overrepresented in our field compared to others.
Rushworth, C.A., R.S. Baucom, B.K. Blackman, M. Neiman, M.E. Orive, A. Sethuraman, J. Ware, and D.R. Matute. 2021. Who are we now? A demographic assessment of the evolution societies. Evolution 75(2): 208-218 https://onlinelibrary.wiley.com/doi/10.1111/evo.14168.
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Boechera Microsatellite Website: an online portal for species identification and determination of hybrid parentage
This database of >4400 genotyped herbarium specimens permits identification of Boechera species and hybrids, and allows users to investigate genus-level patterns. For example, most sexually reproducing specimens were largely self-fertilizing, with characteristic low heterozygosity.
Li, F.-W., C.A. Rushworth, J.B. Beck, and M.D. Windham. 2017. Boechera Microsatellite Website: an online portal for species identification and determination of hybrid parentage. Database 1: baw169
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