Genome Evolution in Plants: Complex Thalloid Liverworts (Marchantiopsida).

Why do some genomes stay small and simple, while others become huge, and why are some genomes more stable? In contrast to angiosperms and gymnosperms, liverworts are characterized by small genomes with low variation in size and conserved chromosome numbers. We quantified genome evolution among five Marchantiophyta (liverworts), measuring gene characteristics, transposable element (TE) landscape, collinearity, and sex chromosome evolution that might explain the small size and limited variability of liverwort genomes. No genome duplications were identified among examined liverworts and levels of duplicated genes are low. Among the liverwort species, Lunularia cruciata stands out with a genome size almost twice that of the other liverwort species investigated here, and most of this increased size is due to bursts of Ty3/Gypsy retrotransposons. Intrachromosomal rearrangements between examined liverworts are abundant but occur at a slower rate compared with angiosperms. Most genes on L. cruciata scaffolds have their orthologs on homologous Marchantia polymorpha chromosomes, indicating a low degree of rearrangements between chromosomes. Still, translocation of a fragment of the female U chromosome to an autosome was predicted from our data, which might explain the uniquely small U chromosome in L. cruciata. Low levels of gene duplication, TE activity, and chromosomal rearrangements might contribute to the apparent slow rate of morphological evolution in liverworts.