Limitations of gamete sequencing for crossover analysis

[...]along bivalents with more versus fewer total crossovers, those crossovers will tend to be more versus less closely spaced (in megabases), as a simple crowding effect (Fig. 1b). [...]this megabase spacing difference will propagate directly to the crossovers on the analysed two-crossover gametic chromatids (Fig. 1c, e), because of the biological fact that crossover interference spaces crossovers evenly along each bivalent, and the mathematical fact that the average distance (in megabases) between two crossovers sampled at random from n evenly spaced crossovers along a bivalent is increasing in n. In addition, because of even spacing along bivalents with more versus fewer total crossovers, those crossovers will tend to be more versus less centrally located (Fig. 1a, b). In summary, the crowding effect that Bell et al. wished to exclude with their analysis is still present. [...]the analysis fails as a control, and therefore cannot be used to support their overall interpretations that a nucleus-wide factor jointly coordinates the number, spacing and positions of crossovers, and heritable variation of such a factor could underlie individual-to-individual variation in these crossover phenotypes. [...]the inter-crossover distance, when measured in megabases (or, equivalently, fraction of total chromosome length), was smaller in the individual with a high number of crossovers. In that study it was found that chromosome 8 bivalents typically showed two crossovers in the individual with a low number of crossovers, but two or three crossovers in the individual with a high number of crossovers (Fig. 1g). [...]crossover positions were bimodally distributed along the chromosome in the individual with a low number of crossovers, but showed a more crowded trimodal distribution, with a central peak, in the individual with a high number of crossovers (Fig. 1g).