Altering the binding properties of PRDM9 partially restores fertility across the species boundary

Meiosis through three centuries

Meiosis is the specialized cellular program that underlies gamete formation for sexual reproduction. It is therefore not only interesting but also a fundamentally important subject for investigation. An especially attractive feature of this program is that many of the processes of special interest involve organized chromosomes, thus providing the possibility to see chromosomes "in action". Analysis of meiosis has also proven to be useful in discovering and understanding processes that are universal to all chromosomal programs. Here we provide an overview of the different historical moments when the gap between observation and understanding of mechanisms and/or roles for the new discovered molecules was bridged. This review reflects also the synergy of thinking and discussion among our three laboratories during the past several decades.

Meiosis: Dances Between Homologs

The raison d'être of meiosis is shuffling of genetic information via Mendelian segregation and, within individual chromosomes, by DNA crossing-over. These outcomes are enabled by a complex cellular program in which interactions between homologous chromosomes play a central role. We first provide a background regarding the basic principles of this program. We then summarize the current understanding of the DNA events of recombination and of three processes that involve whole chromosomes: homolog pairing, crossover interference, and chiasma maturation. All of these processes are implemented by direct physical interaction of recombination complexes with underlying chromosome structures. Finally, we present convergent lines of evidence that the meiotic program may have evolved by coupling of this interaction to late-stage mitotic chromosome morphogenesis.

Different complex regulatory phenotypes underlie hybrid male sterility in divergent rodent crosses

Hybrid incompatibilities are a critical component of species barriers and may arise due to negative interactions between divergent regulatory elements in parental species. We used a comparative approach to identify common themes in the regulatory phenotypes associated with hybrid male sterility in two divergent rodent crosses, dwarf hamsters and house mice. We investigated three potential characteristic regulatory phenotypes in hybrids including the propensity towards over or underexpression relative to parental species, the influence of developmental stage on the extent of misexpression, and the role of the sex chromosomes on misexpression phenotypes. In contrast to near pervasive overexpression in hybrid house mice, we found that misexpression in hybrid dwarf hamsters was dependent on developmental stage. In both house mouse and dwarf hamster hybrids, however, misexpression increased with the progression of spermatogenesis, although to varying extents and with potentially different consequences. In both systems, we detected sex-chromosome specific overexpression in stages of spermatogenesis where inactivated X chromosome expression was expected, but the hybrid overexpression phenotypes were fundamentally different. Importantly, misexpression phenotypes support the presence of multiple histological blocks to spermatogenesis in dwarf hamster hybrids, including a potential role of meiotic stalling early in spermatogenesis. Collectively, we demonstrate that while there are some similarities in hybrid regulatory phenotypes of house mice and dwarf hamsters, there are also clear differences that point towards unique mechanisms underlying hybrid male sterility in each system. Our results highlight the potential of comparative approaches in helping to understand the importance of disrupted gene regulation in speciation.

Condensin dysfunction is a reproductive isolating barrier in mice

Reproductive isolation occurs when the genomes of two populations accumulate genetic incompatibilities that prevent interbreeding1,2. Understanding of hybrid incompatibility at the cell biology level is limited, particularly in the case of hybrid female sterility3. Here we find that species divergence in condensin regulation and centromere organization between two mouse species, Mus musculus domesticus and Mus spretus, drives chromosome decondensation and mis-segregation in their F1 hybrid oocytes, reducing female fertility. The decondensation in hybrid oocytes was especially prominent at pericentromeric major satellites, which are highly abundant at M. m. domesticus centromeres4-6, leading to species-specific chromosome mis-segregation and egg aneuploidy. Consistent with the condensation defects, a chromosome structure protein complex, condensin II7,8, was reduced on hybrid oocyte chromosomes. We find that the condensin II subunit NCAPG2 was specifically reduced in the nucleus in prophase and that overexpressing NCAPG2 rescued both the decondensation and egg aneuploidy phenotypes. In addition to the overall reduction in condensin II on chromosomes, major satellites further reduced condensin II levels locally, explaining why this region is particularly prone to decondensation. Together, this study provides cell biological insights into hybrid incompatibility in female meiosis and demonstrates that condensin misregulation and pericentromeric satellite expansion can establish a reproductive isolating barrier in mammals.

Down the Penrose stairs, or how selection for fewer recombination hotspots maintains their existence

In many species, meiotic recombination events tend to occur in narrow intervals of the genome, known as hotspots. In humans and mice, double strand break (DSB) hotspot locations are determined by the DNA-binding specificity of the zinc finger array of the PRDM9 protein, which is rapidly evolving at residues in contact with DNA. Previous models explained this rapid evolution in terms of the need to restore PRDM9 binding sites lost to gene conversion over time, under the assumption that more PRDM9 binding always leads to more DSBs. This assumption, however, does not align with current evidence. Recent experimental work indicates that PRDM9 binding on both homologs facilitates DSB repair, and that the absence of sufficient symmetric binding disrupts meiosis. We therefore consider an alternative hypothesis: that rapid PRDM9 evolution is driven by the need to restore symmetric binding because of its role in coupling DSB formation and efficient repair. To this end, we model the evolution of PRDM9 from first principles: from its binding dynamics to the population genetic processes that govern the evolution of the zinc finger array and its binding sites. We show that the loss of a small number of strong binding sites leads to the use of a greater number of weaker ones, resulting in a sharp reduction in symmetric binding and favoring new PRDM9 alleles that restore the use of a smaller set of strong binding sites. This decrease, in turn, drives rapid PRDM9 evolutionary turnover. Our results therefore suggest that the advantage of new PRDM9 alleles is in limiting the number of binding sites used effectively, rather than in increasing net PRDM9 binding. By extension, our model suggests that the evolutionary advantage of hotspots may have been to increase the efficiency of DSB repair and/or homolog pairing.

Genic and Chromosomal Components of Prdm9-Driven Hybrid Male Sterility in Mice (Mus musculus)

Hybrid sterility contributes to speciation by preventing gene flow between related taxa. Prdm9, the first and only hybrid male sterility (HMS) gene known in vertebrates, predetermines the sites of recombination between homologous chromosomes and their synapsis in early meiotic prophase. The asymmetric binding of PRDM9 to heterosubspecific homologs of Mus m. musculus x Mus m. domesticus F1 hybrids and increase of PRDM9-independent DNA double-strand break (DSB) hotspots results in difficult to repair DSBs, incomplete synapsis of homologous chromosomes and meiotic arrest at the first meiotic prophase. Here we show that Prdm9 behaves as a major HMS gene in mice outside the Mus m. musculus x Mus m. domesticus F1 hybrids, in the genomes composed of Mus m. castaneus and Mus m. musculus chromosomes segregating on the Mus m. domesticus background. The Prdm9cst/dom2 (castaneus/domesticus) allelic combination secures meiotic synapsis, testes weight and sperm count within physiological limits, while the Prdm9msc1/dom2 (musculus/domesticus) males show a range of fertility impairment. Out of five quantitative trait loci contributing to the Prdm9msc1/dom2-related infertility, four control either meiotic synapsis or fertility phenotypes and one controls both, synapsis and fertility. Whole-genome genotyping of individual chromosomes showed preferential involvement of nonrecombinant musculus chromosomes in asynapsis in accordance with the chromosomal character of HMS. Moreover, we show that the overall asynapsis rate can be estimated solely from the genotype of individual males by scoring the effect of nonrecombinant musculus chromosomes. Prdm9-controlled HMS represents an example of genetic architecture of HMS consisting of genic and chromosomal components.