Morpho-molecular characterization and determination of resistance to lethal yellowing disease in elite and local coconut (cocos nucifera l.) Varieties.

Abstract

Breeding for resistance to LYD in coconut using the conventional method has not been very effective as it takes a very long time to make meaningful progress and the financial requirement is huge. A more effective and rapid method of detection of resistance to LYD is vital for sustainable coconut production. There is limited research work to develop durable coconut varieties using morphological markers and SSR markers (microsatellites). The aim of the study was to obtain an early, fast and effective method of detection of resistance using morphological traits and Simple Sequence Repeats (SSR) markers (microsatellites). The specific objectives of the study were to; (I) Determine the cumulative LYD incidence in coconut genotypes, (II) Identify agronomic traits associated with resistance to lethal yellowing, (III) Identify SSR markers which are polymorphic for resistance and SSR markers which discriminate between resistance and susceptibility and (IV) Select coconut lines which are resistant to lethal yellowing at the seedlings stage. Materials used were 1871 first filial generation (F1) palms on field trials sited in disease prevalent locations of Nigeria by Nigerian Institute for Oil Palm Research (NIFOR). The open pollinated varieties and dwarf varieties were planted one row of West African tall to two rows dwarf varieties in ten hectares. The hybrid varieties were planted in two hectares in a randomised complete block design. Varieties were replicated twice with eight palms per plot. Spacing was 7.5 m triangular for the open pollinated and hybrid varieties. The varieties characterized were the West African Tall (WAT), Malayan Green Dwarf (MGD), Malayan Orange Dwarf (MOD), Malayan Yellow Dwarf (MYD), Sri-Lankan Brown Dwarf (SBD), Chowghat Green Dwarf (CGD); MGD x WAT, CGD x WAT, MYD x WAT, MYD x VTT (Vanuatu Tall), and Sri-Lankan Green Dwarf (SGD) x VTT hybrids. Data were collected on 14 agronomic traits. The extraction of total genomic DNA from leaf samples, Polymerase Chain Reaction (PCR) Amplification and SSR marker analysis were done at the Bioscience Centre, International Institute of Tropical Agriculture (IITA), Ibadan. SSR marker data was analysed using PowerMarker version 3.25 software. Twenty six genomic DNA samples were analysed. Based on phenotypic assessment, the coconut varieties were clustered in four disease severity classes namely; highly resistant, moderately resistant, susceptible and highly susceptible. The SBD cultivar was highly resistant with 0% cumulative loss. The MGD and CGD were moderately resistant with 8.9 and 9.1% cumulative losses, respectively. The MGD x WAT and CGD x WAT hybrids were susceptible with 31.3% cumulative loss. The WAT, MYD and MOD genotypes and the SGD x VTT, MYD x WAT, MYD x VTT hybrids were highly susceptible with 98.5, 83.7 and 66.7, 62.5, 75.0, 84.4% losses, respectively. The susceptible genotypes differed significantly from the resistant genotypes at P ≤ 0.05 in height of trunk, girth of trunk at 20 cm and at 1.5 m above soil level, length of leaf, length of petiole and crown size. Six out of the 20 SSR markers were polymorphic. Thirty-five amplified alleles were generated with a mean number of 6.0 alleles per locus. The primer CAC68 amplified the highest number of alleles (8) while CAC8 amplified the least number of alleles (4). The frequency of the major alleles per locus ranged from 0.3846 to 0.7308 with a mean of 0.6218. The mean genetic diversity for all loci was 0.5459 with a range of 0.4497 – 0.7101. The SSR markers had mean polymorphism information content (PIC) of 0.5109 with a range of 0.4103 to 0.6625. Primer CAC8 and CAC68 distinguished between resistant and susceptible coconut genotypes to LYD by showing resistance SSR markers in the resistant control and in the resistant genotypes at 130 and 180, and at 140 base pairs levels, respectively. Rogers’ dissimilarity coefficient matrix revealed two main clusters. Principal Co-ordinates Analysis (PCoA) clustered the genotypes in two groups using the agronomic data based matrix and the SSR marker based matrix. The SSR genetic distance matrix ranged from 0.1667 to 1.000. The PCoA validated the earlier groupings in this study in clustering the genotypes in two main groups of resistance and susceptibility to LYD based on agronomic data and in two distinct clusters using SSR marker data. The SSR marker based PCoA clustered the genotypes more convincingly in two groups comprising ten resistant samples and eleven susceptible palms to LYD in each group. The SSR marker data based PCoA resistant group includes the SBD1, SBD2, SBD3, MGD1, MGD2, MGD3, MGD4, MGD5, CGD1, CGD2, and CGD3. The susceptible group comprises the CGD4, Susceptible bulk, MYD x VTT hybrid, SGD x VTT hybrid, WAT, CGD x WAT hybrid, MYD x WAT hybrid, MOD, MYD, MGD x WAT hybrid, and EWAT2. Resistant lines were SBD x WAT hybrid and WAT x SBD hybrid. It is evident that the genotypes studied constitute a genepool for resistance and susceptibility to lethal yellowing disease. The markers which discriminated between resistance and susceptibility would be used routinely to select LYD resistant lines at the seedling stage and to select LYD resistant parents for resistance breeding taking cognizance of the genetically distant genotypes for maximum heterosis. This would enhance fast production of durable LYD-resistant coconut hybrids to boost coconut production and productivity in Nigeria besides saving time, space and cost of breeding programmes.