Convincing evidence in support of a recently proposed model of crossover interference.

In most higher organisms, including humans, every cell carries two versions of each gene, which are referred to as alleles. Each parent passes on one allele to each offspring. As they are linked together on chromosomes, adjacent genes are usually inherited together. However, this is not always the case. Why?

The answer is recombination, a process that shuffles the allele content between homologous chromosomes during cell division. Mechanistically, recombination is achieved by crossovers, where homologous chromosomes contact each other, resulting in the exchange of genetic material.

Crossovers have long fascinated scientists and especially plant breeders because manipulating the crossover process offers the potential of increasing genetic diversity and of assembling desired combinations of alleles that boost crop productivity. Crossovers are subject to a "Goldilocks principle"; at least one is required per chromosome pair for successful sexual reproduction; indeed, a lack of crossovers is a major cause of human trisomy such as in the case of Down Syndrome.

Crossover numbers are also tightly regulated and generally do not exceed three. This limit on crossover number, and therefore, recombination, is achieved by crossover interference, a phenomenon through which crossovers inhibit additional crossovers in their vicinity. However, how this interference works has remained a mystery since it was first described some 120 years ago.

Now researchers have   found convincing evidence in support of a recently proposed model of crossover interference. They achieved these insights by manipulating the expression of proteins known to be involved in either promoting crossovers or in connecting chromosomes together in the model plant Arabidopsis thaliana, a species which some researchers use to gain fundamental insights into the mechanisms of heredity.

Boosting expression of the pro-crossover protein HEI10 resulted in a significant increase in crossovers, as did disrupting the expression of the protein ZYP1, a constituent of the synaptonemal complex, a protein structure that forms between homologous chromosomes.

When the scientists combined the two interventions, they were surprised to observe a massive increase in crossovers, showing that HE10 dosage and ZYP1 jointly control CO patterning. Importantly, massively increasing crossovers in this way barely affected cell division.

The considerable increase in crossovers upon increasing HEI10 levels chimes well with an emerging model for how crossover number is regulated. 

In the new model, HEI10 initially forms multiple small foci and is progressively consolidated into a small number of large foci that co-localize with sites of crossovers. In this simple model, increasing the levels of HEI10 will result in more foci and therefore more crossovers; thus, the formation of droplets along an axis appears to be the determinant of crossover sites.

Stéphanie Durand et al, Joint control of meiotic crossover patterning by the synaptonemal complex and HEI10 dosage, Nature Communications (2022). DOI: 10.1038/s41467-022-33472-w