Segregation Distortion for Male Parents in High Density Genetic Maps from Reciprocal Crosses between Two Self-Incompatible Cultivars Confirms a Gametophytic System for Self-Incompatibility in Citrus

Self-incompatibility is an important evolutionary feature in angiosperms and has major implications for breeding strategies in horticultural crops. In citrus, when coupled with parthenocarpy, it enables the production of seedless fruits in a mono-varietal orchard. A gametophytic incompatibility system with one S locus was proposed for citrus, but its molecular mechanisms remain the subject of debate. The objective of this work was to locate the S locus by the analyzing segregation distortion in reciprocal crosses of two self-incompatible citrus sharing one self-incompatible allele and to compare this location with previously published models. High density genetic maps of ‘Fortune’ mandarin and ‘Ellendale tangor’ with, respectively, 2164 SNP and 1467 SNP markers, were constructed using genotyping by sequencing data. They are highly syntenic and collinear with the clementine genome. Complete rejection of one allele was only observed in male segregation in the two parents and in only one genomic area, at the beginning of chromosome 7 of the clementine reference genome. Haplotype data in the area surrounding the theoretical S locus were in agreement with previously proposed S genotypes. Overall, our results are in full agreement with the recently proposed gametophytic S-RNase system with the S locus at the beginning of chromosome 7 of the clementine reference genome.