Kirsten Bomblies, Francesca Cole, Neil Hunter, Susan Johnston, Gareth Jones, Scott Keeney, Nancy Klekner, Thomas Lenormand, Michael Lichten, Olivier Martin, Bernard de Massy, Raphaël Mercier (Organisateur), Christine Mézard, Simon Myers, Matthew Neale, Molly Przeworski, Anne Villeneuve, Liangran Zhang, Denis Zickler
and Héloïse Dufour (Cercle FSER) and Claude Tran (Educavox) for the “Scientific meeting open to the public”
par Raphaël Mercier
15 – 20 mai 2017
Meiotic crossovers shuffle genetic information at each generation, and their randomness makes each offspring unique. Crossover are thus at the heart of heredity and evolution of eukaryotes. Despite the stochastic occurrence of crossovers, their distribution in the genome is under strong constraints, whose mechanisms and function are obscure. First, the number of crossover is almost always very limited, typically 1-3 per chromosome. This is intriguing as the capacity of the meiotic machinery to make crossovers is many folds higher. Second, the density of crossovers is extremely inhomogeneous at both large (chromosomal domains) and small scales (hotspots). Third, as noted by the very first geneticists, crossovers tend to repel one another along chromosomes, a fascinating phenomenon called crossover interference. In this symposium « meiotic crossover regulation: how and why regulating crossover number and distribution? », we brought together people focusing on the mechanism of crossover formation with people addressing the evolutionary consequences of genetic recombination. It gathered 19 scientists (including 8 women and 2 junior participants). The format of the meeting triggered fruitful interactions and passionate discussions around the mechanisms and the raison d’être of crossover regulation.
Keywords: recombination, crossover, meiosis, evolution
Proceedings of the conference are summarized below in the form of a brief outline of the individual presentations.
Gareth Jones presented a cytological perspective on crossover control. Meiotic crossover control can be regarded as the imposition of constraints on an otherwise random distribution of events. Using examples drawn mainly from classical cytogenetics, three major identifiable constraints were presented and discussed, namely crossover assurance (obligate crossover), crossover interference and crossover localization
Young men have an increased risk of fathering a child with Down syndrome, but why this occurs is not understood. Francesca Cole provided evidence for why juvenile mouse spermatocytes also have increased chromosome mis-segregation and propose a model that altered activity of DNA repair pathways leads to reduced crossing over, which is required for accurate chromosome segregation. Further, we show that juvenile human spermatocytes are similarly reduced for markers of crossing over suggesting this phenomenon is evolutionarily conserved.
Thomas Lenormand presented an overview of the evolutionary mysteries in meiosis. The mechanistic details of meiosis have been uncovered in several model organisms, and most of its essential features have received various and often contradictory evolutionary interpretations. In this perspective, he presented and evaluated the various evolutionary scenarios and selective pressures that have been proposed to account for the origin of meiosis, its secondary modifications, its importance in punctuating life cycles, and features associated with recombination.
Bernard de Massy raised the question about the mechanism and consequences of the localization of meiotic recombination in the genome. He presented the two main pathways that have been identified, one where recombination initiates preferentially in nucleosome depleted regions of the chromatin (such as in yeast, bird, plants…), the other one being driven by the DNA binding specificity of PRDM9 (in humans, mice, and other vertebrates). The evolutionary consequences of these two very distinct pathways have been discussed.
Kleckner presented her recent evidence that spatial patterning along mitotic prophase chromosomes arises via accumulation and relief of mechanical stress and proposed that the same molecular processes could analogously underlie spatial patterning of crossover recombination sites during meiotic prophase.
Molly Przeworski presented her work on the evolution of recombination mechanisms in vertebrates. She showed that recombination directed by PRDM9 likely arose before the origin of vertebrates and is conserved across the clade, but was also lost many times. She also showed how different mechanisms for directing recombination can impact patterns of hybridization between species
Scott Keeney presented studies on the mechanisms that control the number and distribution of the DNA double-strand breaks that initiate recombination. New work was discussed establishing a previously undescribed mode of break regulation in which the smallest chromosomes in the yeast Saccharomyces cerevisiae are ensured of getting sufficient breaks to support their pairing, recombination, and segregation at the first meiotic division. The evolutionary conservation of this and other modes of break regulation was also discussed.
Zhang presented his recent discovery of “meiotic crossover maturation inefficiency” in human female meiosis and how this inefficiency sets up the baseline for aneuploidy. This inefficiency also works with age-dependent factors to promote a higher level of segregation errors in older women. And he also discussed the general effects of this inefficiency on the number and distribution of meiotic crossovers and how to distinguish this from other factors caused crossover number reduction.
Anne Villeneuve discussed her research investigating the mechanisms that promote, ensure and limit the formation of crossovers during meiosis in C. elegans. She reviewed evidence for multiple “engineering design features”, such as positive and negative feedback and self-limiting properties, that collaborate to ensure a robust outcome of meiosis in this system. Further, she discussed the interrelationships between crossovers and meiotic chromosome structures, highlighting 1) recent work using structured illumination microscopy to visualize the dynamic architecture of the DNA repair complexes that assemble at meiotic recombination sites, and 2) experiments demonstrating that nascent recombination events alter the structure and dynamic properties of the synaptonemal complex.
Martin presented ways to quantify interference strength, using either model-independent or model-dependent measures. Analyses of the two pathways of CO formation in tomato show that they are not independent. Furthermore, interference signals decay with cytological distance rather than with genetic distance.
Susan Johnston summarized her work on the genetic basis of variation in crossover rates in wild populations of sheep and deer. She showed that crossover rates are heritable and that most variation can be explain by the genes RNF212, RNF212B and REC8. She showed that high recombination rates in female sheep may have a detrimental effect on the reproductive success of their daughters. She also presented preliminary work on how broad-scale recombination patterns differ between the sexes in a number of mammalian species, and how high peri-centromeric recombination in females may be an evolutionary adaptation to counter centromeric drive
Hunter presented new investigations into the interplay between meiosis-specific chromosome structures and recombination proteins. Together, these factors stabilize homolog synapsis and protect crossover-designated recombination intermediates from aberrant resolution. A second topic described how the instability of an essential recombination factor is counteracted by meiosis-specific phosphorylation.
Raphael Mercier presented recent work from his group on the mechanisms that limits meiotic crossovers in Arabidopsis. He showed that three pathways prevent crossovers in parallel, and that the concomitant disruption of these pathways led to unprecedented 7.8-fold increase in crossover frequency genome wide. This analysis showed that the distribution and number of the two known class of crossovers are differently regulated.
Simon Myers explored the factors that affect the binding of PRDM9 to hotspots and subsequent recombination outcomes.
Denize Zickler. Defined by Sturtevant and Muller in 1913 and 1916, interference is the spatial patterning phenomenon that ensures that crossovers are regularly spaced along meiotic chromosomes. Interference implies the existence of communication along the homologous chromosomes and is likely imposed early in the crossover process. Timely analysis of both pairing and crossover events in three Sordaria mutants suggests that there are at least two important factors involved in crossover patterning. (i) Changes in chromosome axis lengths (e.g. in absence of the STUbl protein Slx5 and the Sirtuin Sir2). (2) Partial and/or asynchronous homologous recognition and pairing which “forces” the crossover designation events to occur in a limited array of paired/synapsed segments. However the evolutionary rationale for crossover interference remains unsolved.
Christine Mezard showed that in the axr1 Arabidopsis mutant, the crossover distribution is dramatically disorganized whereas their number is not altered. Large central regions devoid of crossovers are heavily hypermethylated at the DNA level suggesting a relationship between the crossover distribution and the structure of the chromatin. She also presented preliminary data obtained on single meiosis showing a large increase of the number of crossovers when the crossover controlling pathways are inactivated together with abnormal molecular repair events associated to crossovers.
Michael Lichten described recent work from his group on control of the choice between recombination partners and pathways during meiotic recombination. This work suggests that early recombination events are subjected to multiple rounds of formation and disassembly. Their unpublished analyses of parental DNA strand contributions to meiotic recombinants, performed at an unprecedented high level of resolution, reveal that most recombinants are highly mosaic, consistent with this model, and suggesting that the initial steps of recombination involve frequent switches between DNA molecules.
Matt Neale, from the Genome Damage and Stability Centre in Brighton, UK, presented new observations about the molecular regulation of meiotic recombination. A key and unexpected finding is that, rather than forming in isolation, the DNA breaks that initiate recombination are able to form coincidently within meiotic recombination hotspots, creating short chromosomal gaps that must be repaired. These results compel us to look again, with new understanding, at the original mechanism of meiotic recombination as a form of double-strand gap repair
Kirsten Bomblies explained that polyploids can stabilize the segregation of their multiple chromosome copies by evolving a lower crossover frequency. They found that an evolved tetraploid lineage of Arabidopsis arenosa shows evidence of natural selection having acted on genes that encode core structural meiosis proteins. We show that the naturally evolved variants of these genes affect crossover number and placement.
In addition to the scientific activities, an outreach event was organized by the Cercle FSER. The scientist discussed with high school students in the form of a « speed dating » event. This led to enthusiastic discussions about science and fundamental research.
The diversity of the participant expertise and the format of the meeting, with long presentations and large time for discussion, in addition to the magnificence of the Treilles domain and the kindness of the staff, made this event particularly fruitful and memorable.