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Crossovers (CO) are necessary in breeding to introgress regions of interest in agronomic varieties from cultivated species such as bread wheat (Triticum aestivum L.). In order to evaluate the impact of factors influencing CO formation, we started the characterisation of their distribution on the largest wheat chromosome (chromosome 3B, 995 Mb) using the recently developed physical map as well as some sequenced regions of few megabases. Comparison between a dense genetic map (102 markers) and a physical deletion map shows that 77 % of CO are present in the distal regions covering 25 % of the chromosome. Comparison between different genetic maps indicates that this distribution is conserved despite local differences of CO ratio between different populations and between male and female meiosis. Positive interference impacts significantly CO distribution at distances less than 10 cM on chromosome 3B. However, factors involved in the inhibition of CO formation in the proximal regions remain unknown. Positioning proximal regions in distal region of the chromosome increase only weakly the frequency of CO, suggesting that the low frequency observed in the proximal regions is not due to their proximal position. Moreover, sequence divergence between the two homologous chromosomes is not involved in the low frequency observed in the proximal regions since homozygous regions do not exhibit more CO than heterozygous ones. On the contrary, analyses at the level of a 3.1 Mb sequenced region show that high CO frequencies correlate well with gene occurrence. Low CO frequency observed in the proximal regions could thus be explained by a low gene density.

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