Meiotic Recombination in Holocentric Plants

The first ever meiotic recombination landscape reported for a holocentric plant and a deep exploration of meiotic recombination dynamics and their conservation using cutting-edge pollen sequencing and microscopy
Meiotic Recombination in Holocentric Plants

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!

a, In zygotene, synapsis is visualized as the loading of ZYP1 while the ASY1 signal disappears. b, In early zygotene when synapsis starts, HEI10 is immediately loaded as many closely spaced foci, forming an irregular and patchy signal and co-localizing with ZYP1. c, In pachytene, HEI10 is visible as lines that co-localize with ZYP1. d, In late pachytene, the linear signal of HEI10 co-localizes with ZYP1 but becomes weaker, except for a few highly intense foci. e, During the diplotene and diakinesis stages, HEI10 appears as foci only on bivalents, with no linear signal. f, MLH1 appears in diplotene and diakinesis as foci on bivalents, representing chiasmata. g, Box plots of HEI10 and MLH1 foci count at late prophase I. Scale bar, 5 µm.

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.

a, Recombination landscape of the five chromosomes in R. breviuscula.  b, Genetic linkage map computed from COs in pollen nucleic, Marey map calculated from the linkage map in b. Marey maps for each chromosome (colour lines) show genetic position as a function of physical position. d, CO number derived by counting CO events from the genetic analysis in pollen nuclei compared with the one extrapolated from the F1 offspring. e, Distribution of CO number for each single chromatid in gametes. Note the higher incidence of double COs on chr3. f, CoC curve in pollen nuclei.

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.

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Go to the profile of Marco Castellani
2 months ago

Full article at the above link!

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Life Sciences > Biological Sciences > Cell Biology > Cell Division > Meiosis
DNA Recombination
Life Sciences > Biological Sciences > Molecular Biology > DNA Recombination
Life Sciences > Biological Sciences > Cell Biology > Chromosomes
Plant Science
Life Sciences > Biological Sciences > Plant Science