Preferential Homologous Chromosome Pairing in a Tetraploid Intergeneric Somatic Hybrid (Citrus reticulata + Poncirus trifoliata) Revealed by Molecular Marker Inheritance

Abstract

The creation of intergeneric somatic hybrids between Citrus and Poncirus is an efficient approach for citrus rootstock breeding, offering the possibility of combining beneficial traits from both genera into novel rootstock lineages. These somatic hybrids are also used as parents for further tetraploid sexual breeding. In order to optimize these latter breeding schemes, it is essential to develop knowledge on the mode of inheritance in the intergeneric tetraploid hybrids. We assessed the meiotic behavior of an intergeneric tetraploid somatic hybrid resulting from symmetric protoplast fusion of diploid Citrus reticulata and diploid Poncirus trifoliata. The analysis was based on the segregation patterns of 16 SSR markers and 9 newly developed centromeric/pericentromeric SNP markers, representing all nine linkage groups of the Citrus genetic map. We found strong but incomplete preferential pairing between homologues of the same ancestral genome. The proportion of gametes that can be explained by random meiotic chromosome associations (τ) varied significantly between chromosomes, from 0.09 ± 0.02 to 0.47 ± 0.09, respectively, in chromosome 2 and 1. This intermediate inheritance between strict disomy and tetrasomy, with global preferential disomic tendency, resulted in a high level of intergeneric heterozygosity of the diploid gametes. Although limited, intergeneric recombinations occurred, whose observed rates, ranging from 0.09 to 0.29, respectively, in chromosome 2 and 1, were significantly correlated with τ. Such inheritance is of particular interest for rootstock breeding because a large part of the multi-trait value selected at the teraploid parent level is transmitted to the progeny, while the potential for some intergeneric recombination offers opportunities for generating plants with novel allelic combinations that can be targeted by selection.

Keywords: Citrus; SNP markers; SSR markers; disomic; intermediate inheritance; somatic hybrid; tetraploid; tetrasomic.

FIGURE 1

Example of deviance (G) for six observed SSR loci segregations in the “Flhorag1” somatic hybrid gametes with inheritance models ranging from τ = 0 (full disomic) to τ = 1 (full tetrasomic). Among the 16 analyzed SSRs markers Mest431 and mCrCIR02D09 display, respectively, the higher (0.58) and lower (0.07) τ values.

FIGURE 2

Correlation between the intergeneric heterozygosity transmission rate and the τ value for the 16 SSR and 9 SNP markers.

FIGURE 3

Distribution of genetic dissimilarities between diploid gametes of the Flhorag1 somatic hybrid based on intergeneric segregation (blue) and all allele segregation (red).