The Sigel Lab

Unraveling Polyploid and Reticulation Species Complexes               

An estimated 31% of fern speciation events are accompanied by genome duplication (polyploidization) and/or hybridization (reticulation), making fern excellent systems for investigating the role of these forces in the diversification, genome evolution, and geographic distribution of land plants. However, to utilize ferns for understanding these processes, we must first infer their phylogenetic relationships and evolutionary histories. In the Sigel lab we approach this challenge by focusing on the phylogenomic inference of multiple fern genera to serve as “model” polyploid and reticulation species complexes. These phylogenetic investigations provide the foundation for all other research directions in the lab.

Model Fern Species Complexes

POLYPODIUM. Over the past decade, the Sigel lab has worked on resolving the phylogenetic relationships within the Polypodium vulgare (the polypod ferns) complex, a temperate group of approximately 17 diploid and allopolyploid species. Once thought to be a single circumboreal species, the members of the P. vulgare complex have rampantly and repeatedly hybridized with one another and exhibit a gradation of subtle morphological characters. Using a combination of molecular and morphological data we are: (1) resolving the diploid backbone of the P. vulgare complex and assessing the placement of the complex within Polypodium s.s. (Sigel et al. 2014, Systematic Botany); (2) working to establish stable morphological characters to delineate allopolyploid taxa from their progenitor species (Sigel et al. 2014, Brittonia); (3) identifying multiple origins of allopolyploid taxa (Sigel et al. 2014, American Journal of Botany). Most recently, Jonas Mendez Reneau applied 408 target capture nuclear loci and his novel bioinformatics pipeline, SORTER, to resolving all relationships among the diploid taxa, confirm the progenitors of five allopolyploid species, infer the prevalence of incomplete lineage sorting in the group, and reconstruct relationships in the Mesoamerican sister group of the P. vulgare complex (Mendez Reneau, in press, Systematic Botany).

SALVINIA. Over the past several years, the Sigel Lab has been collaborating with Dr. James Beck’s lab at the Wichita State University and Dr. Pedro Swartsburd at the Universidade Federal de Viçosa, Brazil to infer the evolutionary history of the heterosporous aquatic fern genus Salvinia (water spangles). Native to Brazil, many species of Salvinia are aggressive invasives, including the clonally-reproducing pentaploid Salvinia molesta in the southeastern United States. Salvinia molesta is likely a hybrid species, but almost nothing else is known about its evolutionary origins. Using a combination of whole plastome sequencing (Holt et al., in review, Invasion Biology) and a suite of target capture nuclear genes we are working towards developing a comprehensive phylogeny for the genus, infering the parent species and the number of independent origins of S. molesta, and comparing the distribution of S. molesta genotypes in its native and invasive ranges.

ARGYROCHOSMA. In ferns, polyploidy and hybridization often occur with apomixis, a form of asexual reproduction whereby ferns produce unreduced spores. Common is xeric-adapted ferns, apomixis has been touted both as a mechanism for maintaining diversity and an evolutionary dead-end. To better understand the distribution of apomixis in cheilanthoid ferns, we worked Dr. Michael Windham at Duke University to complete systematic survey of the American genus Argyrochosma (false cloak ferns) using plastid markers (Sigel et al. 2011, Systematic Botany). By incorporating spore size measurements, data on the number of spores per sporangium, and chromosome counts, we were able to evaluate the distribution of polyploid and apomictic species across the genus. Remarkably, we identified some individuals producing both sexual and apomictic spores, suggesting frequent, repeated origins of asexuality in the genus. Moving forward, we would like to apply target capture phylogenomic methods to resolving polyploid parentage in this group and develop it as a system for assessing genomic change in sexual vs. apomictic lineages.