Abstract
Self-incompatibility is an important evolutionary feature in angiosperms and has major implications for breeding strategies in horticultural crops. In citrus, when coupled with parthenocarpy, it enables the production of seedless fruits in a mono-varietal orchard. A gametophytic incompatibility system with one S locus was proposed for citrus, but its molecular mechanisms remain the subject of debate. The objective of this work was to locate the S locus by the analyzing segregation distortion in reciprocal crosses of two self-incompatible citrus sharing one self-incompatible allele and to compare this location with previously published models. High density genetic maps of ‘Fortune’ mandarin and ‘Ellendale tangor’ with, respectively, 2164 SNP and 1467 SNP markers, were constructed using genotyping by sequencing data. They are highly syntenic and collinear with the clementine genome. Complete rejection of one allele was only observed in male segregation in the two parents and in only one genomic area, at the beginning of chromosome 7 of the clementine reference genome. Haplotype data in the area surrounding the theoretical S locus were in agreement with previously proposed S genotypes. Overall, our results are in full agreement with the recently proposed gametophytic S-RNase system with the S locus at the beginning of chromosome 7 of the clementine reference genome.
Highlights
IntroductionStrategies for breeding seedless varieties are based on the association of parthenocarpy and mechanisms that prevent fertilization of the ovules by pollen or results in embryo degeneration
Seedlessness is a major citrus breeding objective for the fresh fruit market
The SNP matrix containing 167 individuals was filtered for markers heterozygous for ‘Fortune’ mandarin and homozygous for ‘Ellendale’ tangor that had less than 15% of missing data and segregations in agreement with the parental genotypes
Summary
Strategies for breeding seedless varieties are based on the association of parthenocarpy and mechanisms that prevent fertilization of the ovules by pollen or results in embryo degeneration. SI was first described in pummelos [4], but the seedlessness of some small citrus cultivars such as ‘Ellendale’, ‘Fortune’, ‘Nadorcott’, ‘Nova’, and most clementine varieties, if grown in solid blocks, results from the association of parthenocarpy and SI [5,6,7,8]. It is an important evolutive reproductive biological feature, favoring cross pollination, preventing inbreeding, and conserving high diversity within natural populations [9]. It is found in many horticultural crops. SI has been described in most species including the family Rosaceae (apple, pear, plum, and apricot [13,14,15,16]) and other families such as Malvaceae (cocoa [17]), Oleaceae (olive tree [18]) and Rutaceae (citrus species; Zhang et al [19] for a review)
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