Abstract
Cyperaceae is a family of Monocotyledons comprised of species with holocentric chromosomes that are associated with intense dysploidy and polyploidy events. Within this family the genus Rhynchospora has recently become the focus of several studies that characterize the organization of the holocentric karyotype and genome structures. To broaden our understanding of genome evolution in this genus, representatives of Rhynchospora were studied to contrast chromosome features, C-CMA/DAPI band distribution and genome sizes. Here, we carried out a comparative analysis for 35 taxa of Rhynchospora, and generated new genome size estimates for 20 taxa. The DNA 2C-values varied up to 22-fold, from 2C = 0.51 pg to 11.32 pg, and chromosome numbers ranged from 2n = 4 to 61. At least 37% of our sampling exhibited 2n different from the basic number x = 5, and chromosome rearrangements were also observed. A large variation in C-CMA/DAPI band accumulation and distribution was observed as well. We show that genome variation in Rhynchospora is much larger than previously reported. Phylogenetic analysis showed that most taxa were grouped in clades corresponding to previously described taxonomic sections. Basic chromosome numbers are the same within every section, however, changes appeared in all the clades. Ancestral chromosome number reconstruction revealed n = 5 as the most likely ancestral complements, but n = 10 appears as a new possibility. Chromosome evolution models point to polyploidy as the major driver of chromosome evolution in Rhynchospora, followed by dysploidy. A negative correlation between chromosome size and diploid number open the discussion for holokinetic drive-based genome evolution. This study explores relationships between karyotype differentiation and genome size variation in Rhynchospora, and contrasts it against the phylogeny of this holocentric group.
Highlights
The size, morphology and composition of chromosomes have been useful parameters for comparing karyotypes of phylogenetically related species, and to resolve some taxonomic conflicts (Guerra, 2000)
When we evaluated the hypothetical monoploid 1Cx-value distribution, a difference of about 15× appeared, from 0.06 pg in the polyploid R. pilosa (2n = 10x = 50) to 0.94 pg in the polyploid/dysploid R. globosa with 2n = 12x = 61, and about 27× in relation to the diploid R. pubera with 2n = 10 (Figure 1A and Supplementary data 1)
The study of DNA content in holocentrics can lead to interesting results, as genome sizes can be maintained, or not, in clades with regular or rearranged karyotypes (Roalson, 2008), such as observed in R. tenuis with 2n = 4 (0.80 pg) and 2n = 6 (0.83 pg)
Summary
The size, morphology and composition of chromosomes have been useful parameters for comparing karyotypes of phylogenetically related species, and to resolve some taxonomic conflicts (Guerra, 2000). These features have been widely regarded as drivers of evolutionary processes (Comai, 2005; Doyle et al, 2008), as they are the result of duplication or deletion of entire chromosomes, polyploidy, fission, fusion, and/or chromosome translocation (Greilhuber, 1995; Luceño and Guerra, 1996; Schubert and Lysak, 2011). More detailed work in this area is needed to provide insights into the evolution of holocentric chromosomes in Cyperales, such as whether it arose independently in each of the major clades
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