Genome size and endopolyploidy evolution across the moss phylogeny

Abstract

Background and aims: Compared with other plant lineages, bryophytes have very small genomes with little variation across species, and high levels of endopolyploid nuclei. This study is the first analysis of moss genome evolution over a broad taxonomic sampling using phylogenetic comparative methods. We aim to determine whether genome size evolution is unidirectional as well as examine whether genome size and endopolyploidy are correlated in mosses.

Methods: Genome size and endoreduplication index (EI) estimates were newly generated using flow cytometry from moss samples collected in Canada. Phylogenetic relationships between moss species were reconstructed using GenBank sequence data and maximum likelihood methods. Additional 1C-values were compiled from the literature and genome size and EI were mapped onto the phylogeny to reconstruct ancestral character states, test for phylogenetic signal and perform phylogenetic independent contrasts.

Key results: Genome size and EI were obtained for over 50 moss taxa. New genome size estimates are reported for 33 moss species and new EIs are reported for 20 species. In combination with data from the literature, genome sizes were mapped onto a phylogeny for 173 moss species with this analysis, indicating that genome size evolution in mosses does not appear to be unidirectional. Significant phylogenetic signal was detected for genome size when evaluated across the phylogeny, whereas phylogenetic signal was not detected for EI. Genome size and EI were not found to be significantly correlated when using phylogenetically corrected values.

Conclusions: Significant phylogenetic signal indicates closely related mosses have more similar genome sizes and EI values. This study supports that DNA content in mosses is defined by small genomes that are highly endopolyploid, suggesting strong selective pressure to maintain these features. Further research is needed to understand the functional significance of DNA content evolution in mosses.

Keywords: Bryophytes; endopolyploidy; endoreduplication; flow cytometry; genome size; mosses; phylogenetic independent contrasts (PIC); phylogenetic signal.

Fig. 1.

Pruned phylogram of 173 species with the average 1C-values for each species plotted and the moss orders indicated with vertical bars. For species with multiple 1C-values, the smallest value was used as a conservative estimate of genome size.

Fig. 2.

Continuous character state mapping of the average 1C-values, ranging from 0.17 pg (red) to 2.05 pg (blue), onto a pruned ultrametric tree that includes 173 taxa using the ContMap function with default settings from the phytools package in R. For species with multiple 1C-values, the smallest value was used as a conservative estimate of genome size. Species with more than one 1C-value that was approximately double the smallest value are indicated on the phylogeny with a blue asterisk as a potential within-species polyploidization event.

Fig. 3.

Continuous character state mapping of the endoreduplication index (EI) ranging from 0 (red) to 1.71 (blue) on a pruned ultrametric tree that includes 48 taxa using the ContMap function with default settings from the phytools package in R (Revell, 2012; R Core Team, 2017).