Evolution of Sexual Systems

Evolution of Sex Chromosomes

Plant-Animal Interactions

  Pollination in Biodiversity Hotspots

  Polyploidy as a Driver of Biodiversity

Evolution of Sexual Systems:

The most important evolutionary route from combined sexes (hermaphroditism) to separate sexes (dioecy) is via gynodioecy, yet our understanding of the 'second step', i.e. the evolution of males in subdioecious species is only in its infancy. The Ashman lab has been using Fragaria as a model system to test several mechanistic hypotheses for how resource and herbivore contexts can affect (either facilitate or retard) sexual system evolution, with special emphasis on the evolution of males from hermaphrodites. We combined phylogenetic and observational approaches with manipulative experiments and use DNA markers to track paternity. Our work seems to ultimately expand our current understanding, not only by testing classic (but long untested) theoretical models of dioecy, but also by testing contemporary ideas for the role of ecological factors in male fitness gain curves, selfing, inbreeding depression, and their relative consequences for this key sexual system transition.

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Evolution of Sex Chromosomes:

The evolution of sex chromosomes from autosomes is thought to be such a universal feature of separate-sexed organisms that the steps involved in the transition are believed to have been similarly traversed by animals, plants, and fungi. However, the very earliest steps in sex chromosome evolution -- from two linked but recombining sex-function loci to suppression of recombination between them (and thus the transition from subdioecy to dioecy)--remain virtually unexplored. We are merging genetic, genomic and bioinformatic approaches with ecology and phylogeny to test hypotheses regarding recombination suppression, sequence divergence and selection for sexually antagonistic genes linked to sex. We are utilizing several species within the sexually variable clade of Fragaria to gain transformative insight into early sex chromosome evolution, as well as the unique roles of hybridization and polyploidy in the transition from hermaphroditism to separate sexes.

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Plant-Animal Interactions and the Evolution of Floral Phenotypes:

Floral phenotypes are all traditionally thought to have evolved in response to pollinator-mediated selection. However, recent work has shown the whole suite of interacting agents that can shape the evolution of these traits. Work in the Ashman lab strives for a holistic perspective on floral evolution. In doing so, we have explored the interface of plant phenotype and many types of selective agents, including pollinators, herbviores, florivores, mycorrhizae, and the abiotic environment. We are especially interested in traits that have traditionally been overlooked--but mediate these important interactions and evolve in response to them-- such as floral longevity, floral scent, and ultraviolet floral patterning. We are also interested in how these interactions affect mating system evolution, and thus study how they directly or indirectly affect mating system expression, as well as components of mating system evolution (e.g., inbreeding depression, autonomous selfing).

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Pollination in Biodiversity Hotspots:

Understanding the nature of plant-plant-pollinator interactions (i.e., whether plants interfere or facilitate each other's pollination), how this translates into pollination sufficiency and ultimately plant population persistence is central to our understanding of the generation and maintenance of diverse plant communities.  The Ashman lab has had a long standing interest in the causes and consequences of pollen limitation of plant reproduction, and work in this area is even more relevant now that native pollinator-plant interactions are at risk from habitat destruction, habitat invasion, and climate change. Our current focus to determine if there is a universal pollination response to increasing plant community diversity. Specifically, we are assessing the effects on pollinator diversity and abundance, conspecific and heterospecific pollen transfer, post-pollination interactions, and thus both quantity and quality aspects of pollen limitation, across wide range of plant community diversities. We are using phylogenetically informed meta-analysis and an internationally coordinated research effort (in three biodiversity hotspots: Spain, California and the Yucatan; see collaborators) to gain universally relevant knowledge of pattern and process that will inform conservation strategies of wild flora in the face of modern threats.

Our work across three regions is profiled in greater detail at this webpage

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Polyploidy as a Driver of Biodiversity:

All flowering plants are ancient and/or recent polyploids (possessing two or more copies of every chromosome as a result of whole genome duplication). This phenomenon is thought to be a main driver of plant speciation, biological adaptation, expansion and sexual dimorphism. Yet exactly how genome doubling contributes to biodiversity, and which genomic mechanisms or functional traits underlie the success of polyploids, remain unanswered questions.

The Ashman Lab is leading an international collaboration to address these questions using wild strawberry (Fragaria) that has centers of diversity in China and North America. Strawberries possess numerous features (small genome size, amenity to replicated experiments through clonal propagation, availability of synthetic neopolyploids, susceptibility to climate change due to early-spring flowering and northern latitude or high elevation distribution) that make them an outstanding model system to resolve uncertainty concerning the manner in which genome doubling contributes to biodiversity. By targeting seven triplets - a polyploid and its diploid progenitors - and using phylogenetic, population genomic and transcriptomic approaches we will determine whether similarities in functional diversity and ecological amplitude of polyploid species are the result of the same patterns ('rules') of genetic diversity, chromosome structure or gene expression in a polyploid genome or whether multiple genetic and genomic pathways could lead to successful responses to environmental change. More information can be found on our project website:

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