Some Peculiarities of MLH1 Foci Distribution in Common Degu (Octodon degus, Rodentia: Octodontidae) Chromosomes: Presence in Pericentromeric Regions and Absence in XY.

Abstract

Introduction: Meiotic recombination is one of the major sources of genetic diversity. Understanding the cytogenetic basis for recombination rate alterations is essential to explain the patterns of variation observed between different groups of species. Common degu (Octodon degus) is a South American rodent of the speciose and highly chromosomal variable Ctenohystrica clade, on which relatively few cytogenetic studies have been carried out. It has a mostly bi-armed karyotype, making it an interesting model for cytogenetic research.

Methods: Using immunolocalization of key meiotic proteins and electron microscopy in pachytene spermatocytes, we determined the frequency and distribution of recombination events along a number of chromosome bivalents and the characteristics of sex chromosome synapsis.

Results: Recombination rate of common degu was the highest among the Hystricognathi species studied. In contrast to most mammals, no pronounced recombination peaks near the telomeres were observed in degu. We detected late recombination nodules in the pericentromeric regions of some bivalents, which is a highly extraordinary pattern due to the centromere effect. Within the heterochromatic blocks located on the chromosome arms and marked by H3K9me3, one of the major constitutive heterochromatin marks, we observed a significant decrease in recombination frequency. We describe for the first time the bridge between X and Y in the late pachytene stage in common degu and the absence of late MLH1-dependent recombination nodules in the sex bivalent.

Conclusion: We can assume that the absence of H3K9me3 signaling at centromeres is unrelated to the presence of MLH1 near the centromere. Findings on potential achiasmatic meiosis in common degu were discussed in relation to sex chromosome evolution.