Читать книгу Biotechnology of Fruit and Nut Crops - Группа авторов - Страница 91

Genetic resources can be conserved in situ or ex situ. The study of crop genetic resources in their centres of origin and natural distribution (in situ) requires a sustainable development strategy (Larranaga and Hormaza, 2016) but due to the rapid loss of plant diversity, ex situ based studies are also very valuable and complementary (Hawkes et al., 2000). In the Annonaceae, ex situ germplasm collections are limited in the number and quality of the accessions. Except for cherimoya, germplasm collections of Annona species are rare. The largest ex situ germplasm collection of cherimoya cultivars with 350 accessions, in addition to different accessions of ten additional species in the Annonaceae, is conserved at the IHSM ‘La Mayora’ (Spain). Another important cherimoya germplasm collection is maintained by INIA (Instituto Nacional de Innovación Agraria) of Peru in Ayacucho. Other small collections of this and other species in the genus have been reported but not much information is available. Regarding pawpaw, Kentucky State University (USA) holds the USDA (United States Department of Agriculture) National Clonal Germplasm Repository with around 2000 Asimina accessions (Pomper and Layne, 2010). Although protocols for micropropagation of cherimoya and other species of the family have been developed (Encina et al., 2014), this has not been utilized for germplasm conservation.

Cultivars of species of the Annonaceae have been identified by morphological traits (Pérez de Oteyza et al., 1999; Andrés-Agustín et al., 2006), but variations influenced by the environment are very common and can cause misidentification.

The first molecular studies of genetic diversity in cherimoya were performed using isozymes (Ellstrand and Lee, 1987; Pascual et al., 1993; Perfectti and Pascual, 1998a,b, 2004, 2005). Ronning et al. (1995) used randomly amplified polymorphic DNAs (RAPDs) for identifying cherimoya cultivars and Mahbubur Rahman et al. (1998) demonstrated that amplified fragment length polymorphisms (AFLPs) can also be used for cultivar identification. RAPDs and AFLPs are dominant markers. Microsatellites or simple sequence repeats (SSRs) are codominant markers that can be used to identify heterozygous genotypes and also provide high polymorphism and reproducibility (Wunsch and Hormaza, 2002). They have been more widely used in the diversity analysis and are the marker of choice for genotype identification in plants and animals (Larranaga and Hormaza, 2016). Escribano et al. (2004, 2008a) reported a first set of microsatellite loci in cherimoya. Those markers have been used to analyse the germplasm collection of IHSM ‘La Mayora’ and material collected in situ in South and Central America, allowing significant advances in assessing genetic diversity and phylogeny (Escribano et al., 2004, 2007, 2008b; Chatrou et al., 2009; van Zonneveld et al., 2012), construction of a core cherimoya collection (Escribano et al., 2008a) and establishing Mesoamerica as the centre of origin of cherimoya (Larranaga et al., 2017). Two SSR markers were used in a phylogenetic study of the Annona genus (Chatrou et al., 2009). The interspecific transferability of microsatellite markers has allowed further studies in other species of the genus, e.g. Annona senegalensis (Kwapata et al., 2002, 2007).

Another set of microsatellites was developed with Annona crassiflora (Pereira et al., 2008) and used to assess its diversity in the Brazilian Cerrado (Collevatti et al., 2014); they were efficiently transferred to Annona coriacea (Ribeiro et al., 2014). A. crassiflora has also been analysed with AFLPs (Egydio-Brandão et al., 2016) and RAPD markers (Cota et al., 2011); A. muricata diversity was measured using RAPD markers (Brown et al., 2003). A. squamosa genetic diversity was initially evaluated using isozymes and RAPDs (Bharad et al., 2009) and later using AFLP (Zhichang et al., 2011) and RAPD markers (Guimarães et al., 2013).

Initially, the geographical diversity of pawpaw in North America was evaluated by the M13 probe that revealed variation of minisatellite DNA, repetitive DNA composed of tandem repeats of a consensus sequence of c.10–60 bp (Rogstad et al., 1991). Isozymes were also used in these first molecular identifications (Huang et al., 1997, 1998). A different microsatellite-based method, which does not require previous genome information, inter-simple sequence repeats (ISSRs), was used to characterize 19 pawpaw cultivars (Pomper et al., 2003) and 34 pawpaw cultivars were analysed by RAPD markers (Huang et al., 2003). Two ISSR-PCR primers were also used to study the clonality of pawpaw patches, a characteristic trait of pawpaw; the results showed the expected clonality, but also the presence of several non-clonal patches (Pomper et al., 2009). An interspecific cross between Asimina triloba and Asimina reticulata was also studied by RAPD markers (Huang et al., 2000). AFLP markers have discriminated the diversity of pawpaw cultivars (Wang et al., 2005), and SSR markers have been used to study the diversity of 41 genotypes (Pomper et al., 2010).

Plastid sequences have also been used in phylogenetic studies in the Annonaceae at the family level and in specific groups of the family (Mols et al., 2004; Richardson et al., 2004; Pirie et al., 2005; Su et al., 2010; Chatrou et al., 2012; Erkens et al., 2012; Thomas et al., 2012; Chaowasku et al., 2014; Guo et al., 2017; Hoekstra et al., 2017; Larranaga et al., 2019) or in barcode analysis (Larranaga and Hormaza, 2015; Larranaga et al., 2019). Blazier et al. (2016) sequenced, assembled and annotated the cherimoya plastid genome.