Puzzles in Moss Genome Evolution

Some plants have pairs of chromosomes, they’re diploid organisms like us. Other plants have more than two copies of chromosomes, they’re polyploid. Strawberries can even be octoploid. Mosses can be polyploid too, but a new study by Jillian Bainard and colleagues shows that mosses do things differently.

Mosses tend to have small genomes. Bainard and colleagues note that mosses vary in genome size from 170 Mbp in Holomitrium arboreum to 2004 Mbp in Mnium marginatum. For comparison, the largest known genome is 148 852 Mbp in Paris japonica. But while moss genomes are small, they can be polyploid. Notably, mosses can be endopolyploid. Endopolyploidy is when an individual plant has multiple levels of polyploidy with different cells at different levels. “Endopolyploidy is the result of endoreduplication, which occurs when DNA replication is not followed by mitotic division, and is largely due to modification of cyclin-dependent kinase activity,” write the authors. “The prevalence of endopolyploidy varies widely across plant lineages. It is common in angiosperms and mosses, appears to be rare in both gymnosperms and ferns, and is entirely lacking in liverworts…”

Bainard and colleagues conducted the first analysis of moss genome evolution over a broad taxonomic sampling using phylogenetic comparative methods. They aimed to determine whether genome size evolution is unidirectional as well as examine whether genome size and endopolyploidy are correlated in mosses.

The results were largely negative, which is a much more helpful result than inconclusive. “These data do not support the genetic obesity hypothesis for mosses, which postulates that genome size evolution is unidirectional, resulting in species with larger genomes occupying derived positions within the phylogeny. We determined that there is phylogenetic signal for genome size across mosses, which is the tendency of closely related species to resemble each other more than a random set of species from the same tree; however, no phylogenetic signal was detected for endopolyploidy. We also did not find a significant correlation between endopolyploidy and genome size across mosses…”

The results mean that there is definitely something interesting happening inside moss cells that needs to be examined further, say the authors. “The highly ubiquitous nature of endopolyploid nuclei in mosses, which is absent in many other early-diverging plant lineages, provides an impetus to study this group in more detail. Targeted approaches with high levels of sampling within particular lineages, such as the Bryales and Hookeriales, will enable us to test explicit hypotheses about the evolution of relatively larger genome sizes in these lineages.”