Tetraploid citrus progenies arising from FDR and SDR unreduced pollen in 4x X 2x hybridizations

Polyploid citrus arise by somatic and sexual polyploidization, and both have been used in triploid breeding programs. Sexual polyploidization ismainly achieved by firstdivision restitution (FDR) or second-division restitution (SDR) meiotic restitution mechanisms. In citrus, mostly SDR producing 2n ovule has been described. However, we obtained 72 tetraploid hybrids from 4x X 2x sexual hybridizations using two doubled-diploid mandarins as female parents ('Moncada' mandarin and 'Fina' clementine) and a diploid hybrid tangor as male parent (clementine X sweet orange- CSO') suggesting 2n pollen formation. This material was used to confirm the existence of 2n pollen in Citrus and to analyze its origin. SSR and SNP molecular marker analyses revealed that 64 out of the 72 recovered tetraploid plants resulted from the fertilization of a reduced diploid female gamete by unreduced (diploid) pollen from 'CSO', whereas eight tetraploid plants arose from self-pollination of the tetraploid parent. The maximum-likelihood method based on parental heterozygosity restitution (PHR) of centromeric loci identified both FDR and SDR as the mechanisms leading to unreduced male gamete formation. From the 64 unreduced gametes produced by diploid 'CSO' tangor, 41 (64.1%) were originated by FDR, whereas 12 (18.8%) were significant for SDR. Non-conclusive results were obtained for 11gametes (17.2%). The pattern of PHR variation of markers located along the linkage group 2 confirmed our results at population level. To our knowledge, this is the first report of tetraploid citrus progenies arising from unreduced pollen and the first description of the coexistence of two meiotic restitution mechanisms (SDR and FDR) producing unreduced pollen in citrus.