Evolutionary implications of heterochromatin and rDNA in chromosome number and genome size changes during dysploidy: A case study in Reichardia genus

Abstract

In this study we showed that constitutive heterochromatin, GC-rich DNA and rDNA are implicated in chromosomal rearrangements during the basic chromosome number changing (dysploidy) in Reichardia genus. This small Mediterranean genus comprises 8–10 species and presents three basic chromosome numbers (x = 9, 8 and 7). To assess genome evolution and differentiation processes, studies were conducted in a dysploid series of six species: R. dichotoma, R. macrophylla and R. albanica (2n = 18), R. tingitana and R. gaditana (2n = 16), and R. picroides (2n = 14). The molecular phylogeny reconstruction comprised three additional species (R. crystallina and R. ligulata, 2n = 16 and R. intermedia, 2n = 14). Our results indicate that the way of dysploidy is descending. During this process, a positive correlation was observed between chromosome number and genome size, rDNA loci number and pollen size, although only the correlation between chromosome number and genome size is still recovered significant once considering the phylogenetic effect. Fluorescent in situ hybridisation also evidenced changes in number, position and organisation of two rDNA families (35S and 5S), including the reduction of loci number and, consequently, reduction in the number of secondary constrictions and nuclear organising regions from three to one per diploid genome. The potential mechanisms of chromosomal and genome evolution, strongly implicating heterochromatin, are proposed and discussed, with particular consideration for Reichardia genus.

Chromosome Evolution in Connection with Repetitive Sequences and Epigenetics in Plants

“…Increasing evidence suggests that major structural chromosomal repatterning is frequently associated with cytogenetically detectable heterochromatic regions composed of repetitive DNA sequences [ 70 , 80 , 81 , 82 ]. Repetitive sequences, especially TEs, are involved in various chromosomal rearrangements.…”

“…In fact, in the process of nested chromosome fusions that mostly occur in grasses, the concerned centromeric region and telomeric region are embedded with an abundance of repetitive sequences [ 77 , 93 ]. In has also been reported that repetitive sequence-abundant regions, such as constitutive heterochromatin, GC-rich DNA, and rDNA are implicated in chromosomal rearrangements when the basic chromosome number descends in the Reichardia genus [ 82 ].…”

Fluorescence In Situ Hybridization (FISH) Analysis of the Locations of the Oligonucleotides 5S rDNA, (AGGGTTT)3, and (TTG)6 in Three Genera of Oleaceae and Their Phylogenetic Framework

“…This (TTG) 6 probe has also shown signals at the pericentromeric chromosome regions of Hippophae rhamnoides L. (unpublished work). The ribosomal DNA (rDNA), such as 5S rDNA, have been physically mapped to the chromosome of plant species [44,45] and are useful for understanding the general patterns of chromosome evolution among related species and for cytotaxonomic approaches [46,47,48]. The physical chromosome maps for the Oleaceae genera Fraxinus , Syringa , and Ligustrum remain unknown because no repetitive sequence-based FISH studies have yet been performed.…”

Molecular Cytogenetics of Pisum sativum L. Grown under Spaceflight-Related Stress

“…The genome of P. sativum is rather large (1C = 4.45 Gbp) [47], and repetitive sequences make up 50-60 % of the total DNA [45, 48]. Repetitive DNA is an important component of the plant genome that is involved in genome reorganization during evolution and adaptation [34, 49]. It was shown that environmental stress factors could be responsible for quantitative changes in plant DNA (particularly, changes in satellite DNAs) [35].…”

“…These genes are organized into two distinct families (i.e., 45S and 5S rDNA) that occur as tandem repeat arrays at specific chromosomal regions. Due to high copy number, detection of rDNAs is highly reproducible and provides valuable information concerning chromosomal evolution [34]. In plant genomes, the copy numbers and chromosomal distribution of rDNAs can vary rapidly even within intraspecific taxa and can therefore provide chromosomal landmarks for genome plasticity [34–36].…”