Abstract
Cocoa self-compatibility is an important yield factor and has been described as being controlled by a late gameto-sporophytic system expressed only at the level of the embryo sac. It results in gametic non-fusion and involves several loci. In this work, we identified two loci, located on chromosomes 1 and 4 (CH1 and CH4), involved in cocoa self-incompatibility by two different processes. Both loci are responsible for gametic selection, but only one (the CH4 locus) is involved in the main fruit drop. The CH1 locus acts prior to the gamete fusion step and independently of the CH4 locus. Using fine-mapping and genome-wide association studies, we focused analyses on restricted regions and identified candidate genes. Some of them showed a differential expression between incompatible and compatible reactions. Immunolocalization experiments provided evidence of CH1 candidate genes expressed in ovule and style tissues. Highly polymorphic simple sequence repeat (SSR) diagnostic markers were designed in the CH4 region that had been identified by fine-mapping. They are characterized by a strong linkage disequilibrium with incompatibility alleles, thus allowing the development of efficient diagnostic markers predicting self-compatibility and fruit setting according to the presence of specific alleles or genotypes. SSR alleles specific to self-compatible Amelonado and Criollo varieties were also identified, thus allowing screening for self-compatible plants in cocoa populations.
Highlights
Theobroma cacao L. is a tropical tree originating from the humid forests of the northern part of South America, where large genetically diverse populations still exist in situ (Cheesman, 1944; Lanaud, 1987; Laurent et al, 1993, 1994; Figueira et al, 1994; Motamayor et al, 2002, 2008; Loor Solorzano et al, 2012)
They are characterized by a strong linkage disequilibrium with incompatibility alleles, allowing the development of efficient diagnostic markers predicting self-compatibility and fruit setting according to the presence of specific alleles or genotypes
The alleles of TSH516 inherited from ICS 1 in both the chromosome 1 (CH1) and chromosome 4 (CH4) regions are ‘Amelonado alleles’
Summary
Theobroma cacao L. is a tropical tree originating from the humid forests of the northern part of South America, where large genetically diverse populations still exist in situ (Cheesman, 1944; Lanaud, 1987; Laurent et al, 1993, 1994; Figueira et al, 1994; Motamayor et al, 2002, 2008; Loor Solorzano et al, 2012). Various mechanisms of self-incompatibility (SI), preventing the self-fertilisation of plants and favouring plant heterozygosity, have been described in angiosperms (de Nettancourt, 1997; Takayama and Isogai, 2005; Rea and Nasrallah, 2008). These mechanisms act at the level of inhibition of pollen growth for most species, or at the ovary level, leading to a failure of embryo development. Several mechanisms of genetic control of SI have been described in plants, as follows
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