Читать книгу Biotechnology of Fruit and Nut Crops - Группа авторов - Страница 156

Several studies have focused on development and analysis of molecular markers (Johnson et al., 2009; Cullis, 2011). In principle, any polymorphic marker can be used to characterize different accessions, individual plants or cultivars. Molecular markers have been extensively used to compare different cultivars and identify characteristic band patterns. Early methods were based on isozymes (Baaziz and Saaidi, 1988; Bendiab et al., 1998). Later work included restriction fragment length polymorphism (RFLP) (Corniquel and Mercier, 1994, 1997), random amplified polymorphic DNA (RAPD) (Corniquel and Mercier, 1994; Sedra et al., 1998; Saker and Moursi, 1999), amplified fragment length polymorphism (AFLP) (Cao and Chao, 2002; El-Assar et al., 2005; Gurevich et al., 2005; Jubrael et al., 2005; Rhouma et al., 2007; Othmani et al., 2010), simple sequence repeats (SSRs) (Hamama et al., 2003; Zehdi et al., 2005; Zhao et al., 2012; Zehdi-Azouzi et al., 2015) and representational difference analysis (RDA) markers (Vorster et al., 2002; Johnson et al., 2009). These studies have enabled the genetic characterization of cultivars and the assessment of genetic distances between them as well as the assessment of clonal fidelity and true-to-typeness of micropropagated trees. Some variation exists between trees of the same cultivar (Devanand and Chao, 2003; Gurevich et al., 2005; Elhoumaizi et al., 2006). An especially high variation detected in ‘Medjool’ indicates that it is not a single cultivar but rather a landrace of closely related genotypes (Elhoumaizi et al., 2006). Genomic DNA sequences and transcriptomes of several cultivars have demonstrated genetic variation among cultivars. Hundreds of thousands of single-nucleotide polymorphisms (SNPs) as well as other markers can now be used for date palm germplasm analysis (Al-Dous et al., 2011; Hazzouri et al., 2015).

Until recently, it was impossible to determine the sex of seedlings in the juvenile phase. Early identification of sex type is important for breeding. The sequencing of the date palm genome enabled identification of a sex-related region (Al-Dous et al., 2011) enabling development of specific markers for sex using PCR-RFLP, PCR-based markers (Al-Mahmoud et al., 2012) and SSR markers (Cherif et al., 2013). Other protocols using different PCR-based techniques have been reported for sex identification (Mohamed and Sami, 2015; Atia et al., 2017; Awan et al., 2017; Kharb and Mitra, 2017). A candidate gene approach has detected a date palm homologue to a sex-linked Tormozembryo defective gene from aspen. The date palm transducin beta-like protein 3 (TBL3) gene has male- and female-specific haplotypes that can be identified using SNPs. While female date palms are either homozygous or heterozygous, male date palms are hemizygous (Ali et al., 2018). These SNPs can serve as efficient sex-specific markers for date palms.

Regeneration of cultivars resistant to, or at least tolerant of, Bayoud disease is very important (El Modafar, 2010; Sedra, 2011b). For many years, screening for resistant clones was performed by inoculation with the pathogen in infected plots, and with young plantlets in the laboratory (Sedra and Lazrek, 2011; Sedra, 2011a). An alternative method involves screening leaves with the pathogen toxin. Both techniques are laborious and inefficient. Two mitochondrial plasmid-like DNA molecules associated with susceptibility or resistance to Bayoud disease were identified, and a marker was proposed (Quenzar et al., 2001). However, some reports indicate that correlation of this marker with resistance is not always supported and complex inheritance of the resistance trait is possible (El Modafar, 2010; Sedra, 2011a). A large-scale evaluation of Saudi germplasm has detected conservation in the sequence of the resistant cultivars but variation in the sequence of sensitive cultivars. A detailed protocol for the use of the mitochondrial markers to identify resistant date palms has been published (Saleh et al., 2017).