The Idea

Meiosis is a specialized cell division at the base of evolution. Meiotic recombination specifically involves controlled DNA breaks, later repaired with homologous template, creating new allelic combinations. Strikingly, meiotic recombination is highly conserved among most eukaryotes!

One of the prominent features of meiotic recombination is its suppression at centromeres. However, not all eukaryotes share the same centromeric configuration. Some of them, called holocentric, harbor centromeres along the whole length of their chromosomes!

So, if recombination is supposed to be suppressed at centromeres, what if these are everywhere along the genome?

The Approach

Our model is Rhynchospora breviuscula, a holocentric grass. It´s an obligated outcrosser with a highly heterozygous diploid genome (2n=10). Enough to distinguish the two haplotypes.

We combined the visualization potential of microscopy with the quantification analyses high-throughput single-cell pollen sequencing. With the first, we can use specific antibodies to study how recombination proteins behave and localize during meiosis. With the second, we can map crossovers (the outcome of recombination) directly in thousands of pollen nuclei, by detecting genotype conversion using haplotype-specific polymorphisms.

The Discovery

We discovered that meiotic recombination is highly conserved in R. breviuscula. Most of the recombination proteins behave the same way as in other monocentric models. However, in contrast with its diffused and equally distributed (epi)genomic features (typical of Rhynchospora genomes), recombination is more frequent at chromosome ends. In monocentric models this is usually explained by the presence of the centromere in the middle of the chromosomes, but here we have (holo)centromeres everywhere!

We then studied the behaviour of the telomeres. They assume a clustered configuration and display a prioritized loading of HEI10, a key regulator of recombination. We propose that this drives crossover maturation, increasing their frequencies at chromosome ends compared to central genomic regions.

The Relevance

Our work points the spotlight at the mysterious world of holocentric plants and how meiotic recombination might have evolved specific adaptations with them. Interestingly, most of it is conserved, but with a deeper look, we can see detailed interesting new features. Not only this constitutes novelty, but it can help better understand features usually taken for granted in monocentric species, like the effect of centromeres on recombination.

Looking Onward...

We pave the way for holocentric grasses to take over as model species to study meiosis in organisms with alternative chromosome configurations. We push cutting-edge techniques like  pollen sequencing to establish new standards to study recombination landscapes. Next, functional studies and the exploration of new intriguing Rhynchospora species will consolidate our knowledge and open new doors into the unknown world of holocentricity.