Development of genome maps and genetic markers in coconut and mango using molecular marker technologies

Coconut (Cocos nucifera L.) and mango (Mangifera indica L.) are economically important and high-value crops of the Philippines. Efforts have been made to improve these fruit trees but several constraints including their long juvenile stage (i.e. days to reach fruiting stage) hamper conventional breeding strategies that have long been used. For this reason, molecular marker technology was utilized by researchers of the Institute of Plant Breeding in UPLB to hasten the delivery of benefits from plant breeding. The parental genotypes tested for coconut were Laguna Tall and Catigan Dwarf from PCA-DRC [Philippine Coconut Authority-Davao Research Center] and PCA-ZRC [Philippine Coconut Authority-Zamboanga Research Center] while for mango, 'Carabao' accessions from NPGRL [National Plant Genetic Resources Laboratory]. The microextraction method of Fulton et al. (1995) was found most suitable for extracting DNA from both species for use in molecular marker studies. Protocol for random amplified polymorphic DNA (RAPD) was optimized to yield reproducible results. Approximately 80.35% of random 10-mer oligonucleotide primers tested showed positive amplification and only 17.77% gave RAPDs that differ between the coconut parental genotypes. Polymorphisms were detected in coconut using SSR [simple sequence repeat] and AFLP [amplified fragment length polymorphism] markers. SSR markers revealed extensive allelic diversity in the tall coconut germplasm collections. A number of discriminant SSR markers were also identified useful of hybridity testing in coconut. Similarly in mango, polymorphic RAPD markers were identified and subsequently used in fingerprinting selected accessions. RAPD were able to differentiate the otherwise morphologically- and physiologically-similar 'Carabao' mango accessions. Data analyzed by Jaccard similarity coefficient and UPGMA clustering method from NTYSYS identified the collection as diverse. However, additional definitive RAPD markers are needed to make strong conclusive statements regarding the collections' diversity. Testing 5 representative 'Carabao' accessions, it was found that AFLP markers are more discriminating than RAPD in identifying polymorphisms, hence AFLPs were used for further fingerprinting of the remaining accessions. Furthermore, the protocol for isozyme analysis was optimized and polymorphic markers were identified for isocitrate dehydrogenase (IDH) in mango. On the other hand, shikimate dehydrogenase (SHD) was found to be more monomorphic as in previous studies. In coconut, no bands were resolved for both enzymes. The results of this study provided valuable insights for further molecular marker studies in coconut and mango.