(l to r) Shunxin Wang, Nancy Kleckner, Liangran Zhang, and Shen Yin
Gamete formation for sexual reproduction involves the specialized cellular program of meiosis. As part of this program, the maternal and paternal versions of each chromosome become organized and pair with one another along their lengths. Concomitantly, at the DNA level, local biochemical reactions occur in which the left and right “arms” of the two chromosomes switch partners, giving a “crossover recombination product”. Crossovers increase genetic diversity for evolution. They also play essential roles in chromosome mechanics during the meiotic program itself. Correspondingly, defects in crossover patterning underlie genetic diseases (notably Down’s Syndrome), miscarriages and infertility.
Interestingly, despite the fact that crossovers occur stochastically at different specific positions in different meiotic cells, along each given chromosome, they always tend to be evenly spaced. Existence of this spatial pattern implies the existence of communication along the chromosomes. Existence of such patterning was recognized a century ago as the genetic phenomenon of crossover interference: occurrence of a crossover at one position disfavors the occurrence of another crossover nearby.
Spatial patterning is a fundamental feature of many biological systems at length scales from atomic to organismic. Mechanisms are known in very few cases. Meiotic crossover patterning is an especially remarkable case because, among different organisms, the same type of communication occurs over distances ranging from 300nm to tens of microns in relation to differences in chromosome length. What type of process can be tuned in this way?
Our recent study has investigated the molecular basis for this process by analysis of crossover patterns in budding yeast. The positions of crossover sites along individual, marked chromosomes were defined, on a per-cell basis, by fluorescence microscopic visualization of spread nuclei. Crossover sites are marked by prominent foci of a recombination/structure protein (Zip3-MYC). Chromosome paths were defined by the synaptonemal complex (SC) which links maternal and paternal chromosome axes along their lengths. Analysis of mutations affecting crossover patterning revealed a coherent genetic pathway. The central player of the pathway is the catalytic activity Topoisomerase II (TopoII). SUMOylation is also required, with TopoII among its targets. SUMO-targeted ubiquitin ligase (STUbL) activity, which targets SUMOylated molecules for removal, is also required, presumably acting in concert with SUMOylation. Additional findings imply that communication for interference is transmitted along the chromosome axes (but not the SC) and, correspondingly, that the metric for communication is physical length along the chromosomes (?m) rather than genomic length (kb).
Our previous work, in collaboration with John Hutchinson (SEAS), described a model for crossover patterning in which communication occurs by redistribution of mechanical stress. We now find that crossover patterns are quantitatively well-described by this model in both wild-type and mutant cases, encouraging further exploration. Oppositely, certain other classes of models are excluded. We can thus now propose specifically that: (i) the chromosomes’ axial protein/DNA meshwork is under stress because of a constrained conformation of the DNA; (ii) crossover interference involves reduction of this stress via readjustment of the meshwork; and (iii) TopoII is required for such readjustments via its ability to pass one duplex through another.