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.

Preferential Disomic Segregation and C. micrantha/C. medica Interspecific Recombination in Tetraploid ‘Giant Key’ Lime; Outlook for Triploid Lime Breeding

“…LGs (LG1, LG3, LG4, LG6, and LG9), and for intermediate segregation for LG5 (PP = 0.5). A similar tendency to disomic segregation was also observed for an intergeneric allotetraploid somatic hybrid between 'Willow Leaf' mandarin and Poncirus trifoliata cv 'Pomeroy' with PP values ranging from 0.53 to 0.91 (Kamiri et al, 2018). Conversely, the PP results reported by Kamiri et al (2011) for C. deliciosa + C. limon allotetraploid somatic hybrids were lower, ranging between 0.05 and 0.71, as were those reported by Aleza et al (2016) on the meiosis of the doubled-diploid clementine where significant PP was observed for only four out of nine LGs and did not exceed 0.55, thus largely exhibiting LGS, linkage group size; GS, genome size; RR, recombination rate.…”

Varietal variation and chromosome behaviour during meiosis in Solanum tuberosum

“…However, in autopolyploids, chromosomes may have more than one possible pairing partner in the formation of bivalents. Pairing may occur randomly, leading to polysomic inheritance, or involves preferential pairing between particular chromosomes (Stift et al 2008 ; Bourke et al 2017 ; Kamiri et al 2018 ). Furthermore, an individual chromosome may pair with one or more homologous chromosomes at the same time, leading to the formation of multivalents (Sybenga 1975 ; Lloyd and Bomblies 2016 ).…”

“…The particular mode of inheritance involving multivalent and/or bivalent chromosome pairing has been investigated in many autotetraploid species using a combination of molecular marker data and cytogenetic analyses (Soltis et al 1993 ; Stift et al 2008 ; Stift et al 2010 ; Kamiri et al 2018 ). In some autotetraploids including bird’s-foot trefoil (Fjellstrom et al 2001 ), blueberry (Qu et al 1998 ) and kiwi (Wu et al 2014 ), chromosomes pair and recombine exclusively or almost exclusively as bivalents, with quadrivalent formation occurring rarely (<10%).…”

Male and female inheritance patterns in tetraploid ‘Moncada’ mandarin

“…In cases where parents are divergent, but have retained enough homology to prevent exclusive preferential pairing, inheritance patterns intermediate between di-and tetrasomic can be expected (Jeridi et al 2012;Stebbins 1947;Stift et al 2008;Sybenga 1996). Intermediate inheritance patterns have been revealed in citrus allotetraploid somatic hybrids (Kamiri et al , 2018 and for the tetraploid 'Clemenules' clementine ). Stift et al (2008) developed a likelihood-based approach to evaluate whether disomic, intermediate, or tetrasomic inheritances best fitted the segregation of genetic markers and to estimate preferential pairing and double reduction (DR) rates.…”

“…The results showed that tetraploid 'Mexican' lime has intermediary inheritance with a preferential disomic trend. In addition, Kamiri et al (2018) assessed the meiotic behavior of an intergeneric tetraploid somatic hybrid resulting from symmetric protoplast fusion of diploid C. reticulata and diploid Poncirus trifoliata, and observed an intermediate inheritance with a preferential disomic trend. On the other hand, the genotyping of the triploid progeny derived from a cross between diploid pummelo (C. maxima) and an allotetraploid intergeneric somatic hybrid between C. reticulata and C. limon showed a tetrasomic and intermediate inheritance for this citrus interspecific somatic hybrid .…”