Cytogenetic and SSR-marker evidence of mixed disomic, tetrasomic, and intermediate inheritance in a citrus allotetraploid somatic hybrid between 'Nova' tangelo and 'HB' pummelo

Citrus allotetraploid somatic hybrids obtained via protoplast fusion have been employed as pollen/seed parents in interploidy crosses to produce triploid plants aiming to develop seedless citrus. However, their meiotic behavior, which directs the parental heterozygosity transmission, has been rarely explored. In this study, the meiotic inheritance pattern was evaluated in a citrus allotetraploid somatic hybrid between `Nova' tangelo and `HB' (Hirado Buntan) pummelo through cytogenetic observation of meiosis and segregation analysis using 18 simple sequence repeat (SSR) markers. Meiotic analysis of pollen mother cells (PMCs) showed a high rate of quadrivalents (31.6 %) in this allotetraploid although bivalents were predominant (57.6 %). For segregation analysis, the meiotic chromosome-pairing behavior was analyzed by genotyping a triploid population derived from a cross between diploid `Nadorcott' tangor and this allotetraploid using the microsatellite DNA allele counting-peak ratios (MAC-PR) method, from which the genotypes of diploid male gametes were inferred. In agreement with the expectation of cytogenetic analysis, a mixture of diploid, tetraploid, and intermediated inheritance patterns was deduced due to the co-existence of preferential, non-preferential, and intermediate pairing models among all analyzed loci. Parental heterozygosity transmission was further analyzed among 14 loci where each chromosome in the allotetraploid could be clearly tracked. It showed that the mean value of 23.8% intra-parental and 76.2% inter-parental heterozygosity was transmitted. The preferential pairing pattern at the loci displaying an intermediate inheritance pattern was also discussed.