genes G C A T T A C G G C A T Article Meiotic Behavior of Achiasmate Sex Chromosomes in the African Pygmy Mouse Mus mattheyi Offers New Insights into the Evolution of Sex Chromosome Pairing and Segregation in Mammals Ana Gil-Fernández 1 , Marta Ribagorda 1 , Marta Martín-Ruiz 1 , Pablo López-Jiménez 1 , Tamara Laguna 1 , RocíoGómez 1 , María Teresa Parra 1 , Alberto Viera 1 , Frederic Veyrunes 2 and Jesús Page 1, *   Citation: Gil-Fernández, A.; Ribagorda, M.; Martín-Ruiz, M.; López-Jiménez, P.; Laguna, T.; Gómez, R.; Parra, M.T.; Viera, A.; Veyrunes, F.; Page, J. Meiotic Behavior of Achiasmate Sex Chromosomes in the African Pygmy Mouse Mus mattheyi Offers New Insights into the Evolution of Sex Chromosome Pairing and Segregation in Mammals. Genes 2021, 12, 1434. https://doi.org/10.3390/ genes12091434 Academic Editor: Juan Del Coso Received: 24 August 2021 Accepted: 15 September 2021 Published: 17 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Departamento de Biología, Facultad de Ciencias, Universidad Autónoma de Madrid, 28049 Madrid, Spain; anula.ulula@gmail.com (A.G.-F.); martariba2@gmail.com (M.R.); ruizm5@upmc.edu (M.M.-R.); pablo.lopezj@uam.es (P.L.-J.); tamara.laguna@estudiante.uam.es (T.L.); rocio.gomez@uam.es (R.G.); mayte.parra@uam.es(M.T.P.); alberto.viera@uam.es (A.V.) 2 Institut des Sciences de l’Evolution, ISEM UMR 5554 (CNRS/Université Montpellier/IRD/EPHE), 34090 Montpellier, France; frederic.veyrunes@umontpellier.fr * Correspondence: jesus.page@uam.es Abstract: X and Y chromosomes in mammals are different in size and gene content due to an evolutionary process of differentiation and degeneration of the Y chromosome. Nevertheless, these chromosomes usually share a small region of homology, the pseudoautosomal region (PAR), which allows them to perform a partial synapsis and undergo reciprocal recombination during meiosis, which ensures their segregation. However, in some mammalian species the PAR has been lost, which challenges the pairing and segregation of sex chromosomes in meiosis. The African pygmy mouse Mus mattheyi shows completely differentiated sex chromosomes, representing an uncommon evolutionary situation among mouse species. We have performed a detailed analysis of the location of proteins involved in synaptonemal complex assembly (SYCP3), recombination (RPA, RAD51 and MLH1) and sex chromosome inactivation (γH2AX) in this species. We found that neither synapsis nor chiasmata are found between sex chromosomes and their pairing is notably delayed compared to autosomes. Interestingly, the Y chromosome only incorporates RPA and RAD51 in a reduced fraction of spermatocytes, indicating a particular DNA repair dynamic on this chromosome. The analysis of segregation revealed that sex chromosomes are associated until metaphase-I just by a chromatin contact. Unexpectedly, both sex chromosomes remain labelled with γH2AX during first meiotic division. This chromatin contact is probably enough to maintain sex chromosome association up to anaphase-I and, therefore, could be relevant to ensure their reductional segregation. The results presented suggest that the regulation of both DNA repair and epigenetic modifications in the sex chromosomes can have a great impact on the divergence of sex chromosomes and their proper transmission, widening our understanding on the relationship between meiosis and the evolution of sex chromosomes in mammals. Keywords: sex chromosomes; meiosis; evolution; pygmy mouse; Mus mattheyi 1. Introduction Meiosis is a specialized type of cell division essential for the transmission of chro- mosomes across generations [1]. During prophase-I, homologous chromosomes pair and associate all along their length owing to the assembly of a specific structure, the synaptone- mal complex (SC), that holds the two homologs together [2,3]. Concomitantly, homologous chromosomes undergo recombination. This is a DNA repair process that initiates at the beginning of meiosis with the endogenous production and processing of hundreds of DNA double strand breaks (DSBs) by SPO11 (a topoisomerase-like protein) and a number of Genes 2021, 12, 1434. https://doi.org/10.3390/genes12091434 https://www.mdpi.com/journal/genes