Mechanism of 2n gametes formation and centromere mapping in citrus

Citrus triploid hybrids can be recovered by 2x X 2x hybridisations as a consequence of 2n gamete formation. Two main meiotic processes are at the origin of such gametes, the first division restitution (FDR) and thesecond division restitution (SDR). These two mechanisms lead to very different gamete genetic structures. Particularly, the opposite feature of parental heterozygosity restitution (PHR) is observed in relation with thedistance to the centromere. Without previous knowledge on centromere location, the determination of therestitution mechanism requires an analysis of the frequency of PHR within a large population with numerouscodominant markers. With this method we determined that SDR is the mechanism involved in clementinesand ‘Fortune’ mandarin. With SDR, PHR is a direct function of the distance of the considered locus to thecentromere. Therefore the analysis of the evolution of PHR, within a linkage group (LG) allows mapping of thecentromere position. Centromere has been located in the 9 LGs of the clementine genetic map by genotypingtriploid hybrids with SSR and SNP markers. Analysis of the 2n gamete allelic configuration with loci close tothe centromere can shed light on the restitution mechanism at the individual level and open the way forsimplified study of this mechanism in a large range of genotypes. We are applying this approach to analysethe 2n gamete origin in several genotypes of mandarin and one genotype of sweet orange.