Cells, Genes & Molecules

Predominance of L-S Type of rDNA Organization in Liverworts

The rDNA of Liverworts is unusually organised.

Unlike other nuclear genes in eukaryotes, rDNA genes (5S and 35S loci) are present in numerous copies per cell and, when stained, can therefore provide basic information about genome organization. In tracheophytes (vascular plants), they are usually located on separate chromosomes, the so-called S-type organization. An analysis of 1791 species of land plants suggested that S-type arrays might be ancestral in land plants, while linked (L-type) organization may be derived. However, no outgroup and only a handful of ferns and bryophytes were included.

Image: Canva.

Aretuza Sousa and colleagues analysed genome sizes and the distribution of telomere, 5S and 35S rDNA FISH signals in up to 12 monoicous or dioicous species of liverworts from throughout a phylogeny that includes 287 of the 386 currently recognized genera. We also used the phylogeny to plot chromosome numbers and the occurrence of visibly distinct sex chromosomes.

Chromosome numbers are newly reported for the monoicous Lejeunea cavifolia and for females of the dioicous Scapania aequiloba. We detected sex-related differences in the number of rDNA signals in the dioicous Plagiochila asplenioides and Frullania dilatata. In the latter, the presence of two UU chromosomes in females and additional 5S-35S rDNA loci result in a haploid genome 0.2082 pg larger than the male genome; sex-specific genome differences in the other dioicous species were small. Four species have S-type rDNA, while five species have mixed L-S rDNA organization, and transitions may have occurred multiple times, as suggested by rDNA loci not being conserved among closely related species of Pellia. All species shared an Arabidopsis-like telomere motif, and its detection allowed verification of the chromosome number of Radula complanata and chromosome rearrangements in Aneura pinguis and P. asplenioides, the latter also showing sex-specific interstitial telomere repeats.

“The two types of rDNA rearrangement (S and L) in liverworts appear to have evolved repeatedly, and the occurrence of linked and non-linked rDNA in the same species seems common (based on the few species so far investigated),” write Sousa and colleagues and colleagues. “The evidence for differential accumulation of rDNA between the sexes is limited, perhaps because in haploid-dominant organisms there is no heterogametic and homogametic sex (Bull, 1978, 1983), and instead either the U or the V chromosome may idiosyncratically accumulate repetitive DNA.”This paper is on an author’s ResearchGate page.