Diversification of Cellulose Synthase (CESA) Genes in Mosses Suggests Both Ancient and Recent Gene duplications

Cellulose is an important polysaccharide that constitutes all plant cell walls, giving them strength and stability. The plant cellulose synthase (CESA) gene family, which encodes the catalytic subunits of cellulose synthesis complexes (CSCs), has diversified independently in several plant lineages, providing an interesting model for understanding selection for gene duplication. Here we quantified the presence of CESA genes across mosses to understand how the process of gene family diversification occurred in this group and how it parallels diversification in other groups. We first examined the CESA gene family in eight species of mosses across seven families for which whole genome assemblies were available. We then identified CESA genes from additional species, for which only short-read sequence data was available, by using BLAST searches and targeted gene assemblies. We validated this approach by comparing the assembled paralogs from the short-read data to the genes identified from whole genome assemblies in the eight reference species. This approach allowed us to identify paralogs directly from short-read data and greatly expand our sample set. Results from the combined empirical data support the hypothesis that CESA genes diversified within the moss lineage at least as early as the mesozoic period, during or possibly even prior to the onset of moss diversification, but also continue to diversify within modern species. In addition, we found evidence for purifying selection as the dominant force shaping these genes and observed that different lineages experienced different levels of evolutionary constraint. Lastly, our approach to assemble paralogs has the potential to allow researchers to improve analyses of gene duplication events.