Pepper is one of the most popular spice and vegetable crops in the world (Bosland and Votava, 2012). In their native habitats, peppers are grown as tender perennials, but in most parts of the world, they are grown as annuals. As with other crops, hybrid vigor can improve the yield, resistance, and quality of pepper. Cytoplasmic male sterility facilitates the production of hybrid seed. A major concern of hybrid seed production is prevention of self-pollination that can produce seeds that are not hybrid, and CMS greatly facilitates the production of F1 hybrid seeds without the need for flower emasculation (Chase, 2007; Hanson and Bentolila, 2004). The pepper CMS system was first reported by Peterson in 1958 from an Indian Capsicum annuum accession (PI164835). Since then, CMS and associated restorer-of-fertility genes have been used to produce F1 hybrids or hybrid cultivars (Kumar et al., 2009; Swamy et al., 2017).
Within the CMS system, the sterility (S) phenotype is controlled by mitochondrial genes. Two candidate genes for S are the mitochondrial loci, orf456 and atp6-2, identified and studied by Kim et al. (2007) and Kim and Kim (2006). Furthermore, two CMS-specific sequence-characterized amplified region (SCAR) markers, the coxII and the atp6 SCARs, have been developed from the sequences flanking orf456 and atp6-2, respectively (Kim and Kim, 2005). In addition, another molecular marker of S-cytoplam, SCAR130, was reported to be more reliable than previous markers (Ji et al., 2014).
The CMS phenotype can be restored by a nuclear Rf gene, which can suppress the expression of the sterility orf in the mitochondria (Janska et al., 1998). In pepper, orf456 expression was suppressed in lines containing a nuclear-encoded Rf gene (Kim et al., 2007). By taking advantage of Rf, the CMS/Rf system has been used to produce F1 hybrid seeds. The ability of restorer lines to reestablish fertility is one of the crucial components in the production of pepper F1 hybrids using the CMS/Rf system, and only restorer lines with very high restorer function and specificity can be used in the CMS/Rf system.
Although previous studies support the idea that restorer-of-fertility are controlled by one major gene (Gulyas et al., 2006; Peterson, 1958), several cases suggest that the inheritance of restorer-of-fertility is more complex. It has also been suggested that restorer-of-fertility in sweet pepper is controlled by two complementary genes (Novak et al., 1971), but it is still unclear how many Rf genes are present in pepper that contribute to restorer-of-fertility. Moreover, by mapping quantitative trait loci (QTLs), Wang et al. (2004) identified one major QTL, which was mapped to chromosome P6 and accounted for 20% to 69% of the phenotypic variation, and four minor QTLs related to restorer-of-fertility in pepper. In addition, other studies have shown that CMS can be temporarily broken down with a day/night temperature cycle of 25/17 °C, but a high temperature cycle of 35/22 °C leads to 100% sterility, indicating the presence of modifying genes that are affected by environmental factors such as temperature (Bückmann et al., 2014; Peterson, 1958; Shifriss, 1997). The genetic mechanism of restorer-of-fertility is still not clearly understood. Therefore, understanding the mechanism of inheritance for restorer-of-fertility will be beneficial for breeding restorer lines with higher expression ability.
An analysis of inheritance using the joint segregation analysis method provides a method to test for all genetic explanations. The joint segregation analysis method can analyze for the best genetic model, gene heritability, and gene effects that fit the data for the quantitative trait (Cao et al., 2013; Gai et al., 2007; Zhang et al., 2000a). As a useful and strongly recommended technique that can analyze the segregation of quantitative traits, the joint segregation analysis method provides beneficial information for breeding by assisting in the design of breeding approaches that improve quantitative trait loci and progeny and parent selection (Ullah et al., 2016). The joint segregation analysis method provides an analysis of multiple traits and has been used widely in inheritance studies on different traits in wheat [Triticum aestivum (Ullah et al., 2016)], soybean [Glycine max (Wang and Gai, 1997)], cotton [Gossypium hirsutum (Zhang et al., 2011)], cucumber [Cucumis sativus (Liang et al., 2015)], and pepper (Yao et al., 2013). In this study, two progenies, constructed from the hybridization of CMS line 8A with restorer lines R1 and R2, analyzed the inheritance mechanisms of fertility restoration for CMS in pepper by the joint segregation analysis method. The aims of this study were to understand the inheritance model of fertility restoration in pepper and the gene effects and inheritance, which will assist in choosing a breeding strategy for CMS hybrid.
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