| Papaya(Carica papaya L.)sometimes known as pawpaw is one of the major tropical fruit crops cultivated and consumed in most parts of the world today.It is said to be a fruit with lots of potentials for the world as it is highly productive and has far reaching benefits nutritionally and medically.Therefore breeding of this crop to obtain better quality and yield is essential.In the past,papaya breeding was greatly hampered by the difficulty of determining papaya sex type in its juvenile developmental stage,papaya diseases and reported low genetic variety among accessions.In recent times with the increase and development of new research technologies,these challenges are gradually being tackled successfully.Nevertheless,information on genetic diversity is still few and insufficient.The increasing adoption of molecular markers in addressing challenges in papaya studies has paved a way for great improvements in papaya breeding and production.So far,simple sequence repeats(SSRs)have proven to be the most employed marker for papaya genetic diversity studies because of its high polymorphic information contents(PICs),and simplicity of technique.Sequence related amplified polymorphism(SRAP),on the other hand is a new technique rarely applied in papaya studies yet.As it concerns papaya genetic studies in china and Nigeria,research efforts have been geared mainly towards developing resistant varieties,identifying sex traits markers and morphological diversity.There is a dearth of information on genetic diversity of papaya accessions in China and Nigeria.This study hereby seeks to explore the gaps and add to the genetic resources and tools available in the papaya community.The relationship between the papaya accessions collected from both geographic regions are studied and compared to morphological data available.This study investigated the genetic diversity and relationship amongst sixty(60)accessions(44 from china and 16 from Nigeria)mostly obtained from the wild using two PCR-based markers,SSR and SRAP,54 in total.DNA was extracted from fresh leaves and amplified according to each marker protocol.To visualize the bands,amplified products were separated on SDS page.PIC was determined and genetic diversity was calculated for three(3)categories-Nigeria accessions,China accessions and all accessions together using the NTSYSpc version 2.10e.SSR assay generated a total of 34,29 and 31 alleles with an average of 3.78,3.22 and 3.44 alleles per locus and PIC average of 0.76,0.73 and 0.61 for all,Nigeria and China accessions respectively ranging from one to six alleles in all categories.SSR02 and SSR21 were the markers with the highest PIC and are therefore found to be most suitable among the markers studied for accessing polymorphism in China and Nigerian papaya accessions whereas SSR49 was the least in all categories.SRAP assay detected 88,25 and 67 with an average of 1.96,1.47 and 1.76 alleles per locus and PIC average of 0.49,0.37 and 0.47 for all,Nigerian and Chinese accessions respectively.M9E1 and M2E16 with the highest polymorphic content indicated that they are the most informative markers for conducting similar analyses.SSR-SRAP combined assay generated a total of 121,54,and 98 alleles with average of 4.4,2.07 and 2 allele per locus and a PIC average of 0.52,0.5 and 0.5 for all,Nigerian and Chinese accessions respectively.The combined markers and SSR markers efficiently distinguished all accessions with all primers amplifying at least one allele per locus whereasSRAP markers varied between categories.SSR produced the highest PIC average and therefore indicates a higher degree of allelic diversity than the combined primers or SRAP alone.This explains why SSR may be the best choice of marker for papaya studies.Cluster analysis of SRAP-SSR,SSR and SRAP data revealed a considerable genetic diversity among all accessions with a GS coefficient of 0.65,0.59 and 0.66 respectively each generating 2 clusters separated based on geographical origins and thus suggested that Chinese and Nigerian accessions are highly genetically distinct.Furthermore,group I containing the Nigerian accessions were sub-clustered clearly based on regions within demonstrating that the accessions from the different regions are genetically diverse.The findings generally indicated that SSR was more detailed in discriminating and elucidating relationships between the accessions. |
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