Chromosomal and genomic data in Neotropical Dorstenia species (Moraceae) and their relationships with African species

Abstract

Dorstenia spp. are characterized by a herbaceous habit, the presence of rhizomes and inflorescences being coenanthia. They occur in different forest formations, and it is believed that there are two distribution centres for Dorstenia, one Palaeotropical (Africa) and one Neotropical (Central and South America). The present study aimed to undertake a comparative analysis of the karyotype diversity of Dorstenia, based on chromosome data and DNA C values of 13 Neotropical species, and an analysis of low-coverage genomic sequencing of three African and one Neotropical species. The most frequent chromosome number is 2n = 32, with variations for 2n = 28 and 30, suggesting an important role of polyploidy in the karyotype differentiation of Dorstenia. Karyotypes varied in the number of metacentric, submetacentric and acrocentric chromosomes, with evidence of rearrangements. Genome sizes (C values) varied almost two-fold, from 2C = 2.50 to 5.47 pg. There was no clear relationship between DNA C value variations and karyotype features, such as chromosome numbers and distribution of CMA/DAPI bands or rDNA sites. Proximal heterochromatin was most frequent, but CMA+ and DAPI+ bands predominated in relation to DAPI+ bands. 5S and 35S ribosomal DNA (rDNA) sites were also observed in the proximal regions in almost all species. Based on a chromosome modelling approach, our data suggest that dysploidy events may have been responsible for the chromosome number variations, whereas rearrangements involving repetitive fractions may have been associated with changes in genome size. There was great variability in the amount of rDNA between species, including a remarkable 5S rDNA amplification in D. bahiensis, and a diversity of transposable elements in the four genomes with genomic sequence data. The combination of data from karyotypes, genomes and phylogeny suggests that dysploidy has played a predominant role in the evolution of Neotropical Dorstenia spp.