Dosage effect of the Ph1 locus on homologous crossovers in a segment of chromosome 1B of bread wheat, Triticum aestivum L.

Introduction: The Ph1 locus in polyploid wheat enforces strictly bivalent behavior in meiotic metaphase I, by preventing homoeologues from crossing over. It has always been considered as completely dominant as no homoeologous metaphase I pairing has ever been detected with its single dose present. However, Ph1 also affects pairing and crossing over of homologous chromosomes. Methods: Crossover frequencies with Ph1 in two, one and null doses were scored cytologically, as exchanges between a ca. 9.5 – 9.9 Mbp terminal wheat segment of a wheat-rye translocation T-9 and corresponding segments in chromosome arms 1BS originating from three different wheat cultivars. Results: In all cases, the crossover rates with a single dose of Ph1 present were intermediate between two and null doses. Averaging across all three chromosomes, the crossover rate with a single dose of Ph1 present was 124% higher from that with two doses, and 54% of that with a zero dosage. Conclusion: The Ph1 locus in wheat is not dominant, but operates in a dosage dependent manner.