Chromosomal rearrangements can drive speciation by reshaping the genome architecture and facilitating reproductive isolation between divergent populations. Yet, the molecular mechanisms behind this pattern are not fully understood. In this context, natural populations of house mice carrying Robertsonian (Rb) fusions serve as a valuable natural model for examining these evolutionary processes. In particular, the Barcelona Rb system (BRbS) features polymorphic Rb fusions, in both homozygous and heterozygous states, resulting in mice with diploid numbers ranging from 28 to 40 chromosomes. In this thesis, we investigated the spermatogenesis in mice from the BRbS, paying special attention to meiotic recombination. Specifically, we addressed two interconnected objectives: (i) to evaluate fertility costs and meiotic functional constraints imposed by Rb fusions; and (ii) to analyze changes in the recombination landscape associated with Rb fusions. To archive these objectives, we employed an integrative methodological framework by combining cytogenetics, population structure analysis, highthroughput sequencing and morphological evaluation of sperm. High-throughput sequencing approaches included transcriptomic profiling to assess gene expression changes and single-strand DNA sequencing to map genome-wide double-strand break initiation sites.
Regarding to the first objective, our findings revealed a complex interplay between conserved meiotic mechanisms and chromosomal reorganizations. While Rb fusions induced mild delays during prophase I, they did not prevent successful meiotic completion, highlighting the resilience to large-scale structural variation. Specifically, Rb mice exhibit an increased frequency of heterologous chromosomal interactions relative to wild mice without Rb fusions. This reorganization also extends to the sex chromosomes which exhibited frequent associations with autosomes. Moreover, genome-wide transcriptomic data indicated that Rb fusions can lead to widespread gene expression changes, not limited to fused chromosomes. In testicular tissue, we detected a pro-inflammatory profile, up-regulation of ribosomal genes, and down-regulation of genes involved in cell-to-cell adhesion. Additionally, these molecular alterations were accompanied by sperm head morphological changes, including shorter hook and thinner head.
As for the second objective, we detected key signatures of meiotic recombination dynamics associated with Rb fusions. Population-level analyses revealed significant genetic differentiation patterns of Rb fusions, suggesting an incipient role in limiting genetic exchange between populations. The diversity of Prdm9 alleles further modulated these effects, as distinct variants generate population-specific recombination signatures. Fine-scale mapping of DSB formation uncovered an increased use of PRDM9-independent recombination hotspots in Rb carriers, indicating adaptive recombination initiation mechanisms.
Additionally, crossover (CO) formation was systematically reduced in Rb mice and redistributed toward telomeric regions, likely due to a high centromeric effect and increased CO interference. Lastly, we detected that the assembly of the centromeric protein CENP-A was reduced in Rb mice, particularly in alternative configurations of Rb fusions. Despite the observed alterations, overall fertility remained preserved in Rb mice. These results highlighted the balancing mechanisms that enable natural populations to accommodate large-scale genome reorganizations while preserving essential cellular functions. Overall, the observed chromatin remodeling, transcriptomic responses, and recombination landscape shifts provided critical insights into the mechanisms by which chromosomal rearrangements can drive speciation.
Structural and functional implications of Robertsonian fusions in the mouse germline
Marín García, C. (Author). 17 Oct 2025
Student thesis: Doctoral thesis
Student thesis: Doctoral thesis