Differences in the Genetic Structure of Citrus Triploid Hybrids Recovered from 2x x 2x and 4x x 2x Sexual Hybridizations
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Citrus triploid hybrids can be recovered by 2x x 2x hybridisations as a consequence of the formation of 2n gametes or by interploid hybridisation. Most of the tetraploid parents used are doubled-diploid (DD). The genetic structure of diploid gametes and particularly the rate of parental heterozygosity restitution (PHR) depend on the meiotic process by which they were originated. Second-division restitution (SDR) is the 2n gamete formation mechanism involved in 'Fortune' and clementines. With SDR, PHR is positively linked with the distance of each locus to the centromere. With DD parents, PHR depends of the rate of preferential pairing and thus the proportion of disomic versus tetrasomic segregations. We have compared the genetic structure of two populations of diploid gametes of clementine (one population of 2n gametes and one of 2x gametes produced by DD) with SNPs and SSRs markers covering the 9 citrus chromosomes. The DD displays mostly tetrasomic segregation; however three linkage groups present intermediate segregation and one display a tendency for disomy. The PHR in 2n gametes is 2/3 of the one obtained in 2x gametes produced by DD. The two methods of triploid production appear complementary in terms of genotypic variability. 4x x 2x hybridizations are more efficient than 2x x 2x for developing new cultivars phenotypically closer to the diploid parent of the DD. Conversely, 2x x 2x hybridisations provides the opportunity to select more innovative products.