There and Back Again: Reticulate Evolution in Ceratopteris

Examples of reticulate evolution are known from throughout the tree of life, but are particularly common in ferns due to their unique reproductive biology and few prezygotic reproductive barriers. However, untangling the complex evolution history of these groups can be challenging. Often, several different types of data are needed to understand the full story of reticulate evolution; for example, chloroplast markers trace maternal inheritance, while nuclear markers complete the picture by suggesting paternal inheritance. Next generation sequencing can provide thousands of nuclear loci, which are informative for estimating reticulate evolutionary histories. The model fern genus Ceratopteris is known to have cryptic allotetraploid taxa, and hybridization is common between many species in the genus. To better understand the patterns of hybridization and reticulate evolution in the genus, we constructed a split network analysis using thousands of single nucleotide polymorphisms from samples collected throughout the pan-tropical range of Ceratopteris. Our split network organizes taxa based on genomic similarity, revealing potential introgression between lineages. Combining this analysis with Patterson's D to measure gene flow, as well as contributions by previous authors, we show extensive hybridization and reticulate evolution in Old World Ceratopteris, and also provide evidence for natural hybridization events involving the model species C. richardii.