Differences in the genetic structure of citrus triploid hybrids recovered from 2x X 2x and 4x X 2x s

Citrus triploid hybrids can be recovered by 2x X 2x hybridisations as a consequence of the formation of 2ngametes or by interploid hybridisation. Most of the 4x parents used in our program are doubled-diploid (DD),also known as autotetraploid. The genetic structure of diploid gametes and particularly the rate of parentalheterozygosity restitution (PHR) depend on the meiotic process by which they were originated. Second-divisionrestitution (SDR) is the 2n gamete formation mechanism involved in ‘Fortune’ and clementines. With SDR, PHRis positively linked with the distance of each locus to the centromere. With DD parents, PHR depends on the rateof preferential pairing and thus the proportion of disomic versus tetrasomic segregations. We have comparedthe genetic structure of two populations of diploid gametes of clementine (one population of 2n gametes andone of 2x gametes produced by DD) with SNP and SSR markers covering the 9 citrus chromosomes. The DDdisplays mostly tetrasomic segregation; however three linkage groups present intermediate segregation andone displays a tendency for disomy. The PHR in the 2n gametes is 2/3 of that obtained in 2x gametes producedby DD. The two methods of triploid production appear complementary in terms of genotypic variability. 4x X 2xhybridizations are more efficient than 2x X 2x for developing new cultivars phenotypically closer to the diploidparent of the DD. Conversely, 2x X 2x hybridisations provides a greater opportunity to select innovative products.