Chromosome evolution in asymmetrical meiosis

Despite their abundant odd-ploidy (2n = 5x = 35), dog-roses (Rosa sect. Caninae) are capable of sexual reproduction due to their unique meiosis. During canina meiosis, two sets of chromosomes form bivalents and are transmitted by male and female gametes, whereas the remaining chromosomes form univalents and are exclusively transmitted by the egg cells. Thus, the evolution of chromosomes is expected to be driven by their behaviour during meiosis.

To gain insight into differential chromosome evolution, Jana Lunerová and colleagues conducted fluorescence in situ hybridization for mitotic and meiotic chromosomes in four dog-roses (two subsections) using satellite and ribosomal DNA probes. By exploiting high-throughput sequencing data, we determined the abundance and diversity of the satellite repeats in the genus Rosa by analysing 20 pentaploid, tetraploid and diploid species in total.

A pericentromeric satellite repeat, CANR4, was found in all members of the genus Rosa, including the basal subgenera Hulthemia and Hesperhodos. The satellite was distributed across multiple chromosomes (5–20 sites per mitotic cell), and its genomic abundance was higher in pentaploid dog-roses (2.3 %) than in non-dog-rose species (1.3 %). In dog-rose meiosis, univalent chromosomes were markedly enriched in CANR4 repeats based on both the number and the intensity of the signals compared to bivalent-forming chromosomes. Single-nucleotide polymorphisms and cluster analysis revealed high intragenomic homogeneity of the satellite in dog-rose genomes.

“According to Pele et al., polyploids tend to have more relaxed meiosis mechanisms and a higher rate of crossovers than diploids,” write Lunerová and colleagues. “Dogroses apparently escape this global trend, maintaining no or reduced recombination in univalent genomes over thousands of generations. There is increasing evidence that satellite DNA may have a function in chromosome pairing and segregation; thus, CANR4 (or the lack of large CANR4 loci) might be important for chromosome recognition.”