Genetic And Morphological Diversity In Monodora Myristica (Gaertn.) Dunal In Eastern Nigeria

Medical and Health Science Project and Seminar Material

Genetic And Morphological Diversity In Monodora Myristica (Gaertn.) Dunal In Eastern Nigeria


Chapter One


Introduction

1.1 Background Information

Monodoramyristica (Gaertn.)Dunal.,also known as African nutmeg or calabash nutmeg, is a tropical tree of the family Annonaceae (Custard-apple family). Its seeds are widely used as an inexpensive nutmeg substitute becauseof the similarity between the two in odour and taste. Nowadays, however, it is less common outside its region of production(Celtnet recipes, 2011)

The genus Monodora contains approximately 15 to 20 species includingMonodora borealis, Monodoraclaessensii andMonodoragrandiflora.Monodoramyristica is easily recognizable by its very long and pendulous pedicels, an undulate upper bract, a large globose fruit with a black and smooth but finely ribbed surface (Burkill, 1985).

The Calabash nutmeg tree is native to tropical West Africa, where it grows naturally in evergreen forests from Liberia to Nigeria and Cameroon. It is also native to Angola,Uganda and West Kenya (Weiss, 2002). Due to the slave trade in the 18th century, the tree was introduced to the Caribbean islands where it was established and became known as Jamaican nutmeg (Barwick 2004). In 1897, it was introduced to Bogor Botanical Garden, Indonesia, where the trees flowered on a regular basis but no fruit could yet be collected (Weiss, 2002).


1.2 Genetic Diversity in Plants

Genetic diversity refers to any variation in the nucleotides, genes, chromosomes, or whole genomes of organisms. At its most elementary level, it is represented by differences in the sequences of nucleotides (adenine, cytosine, guanine, and thymine) that form the DNA within the cells of the organism. Nucleotide variation is measured for discrete sections of the chromosomes, called genes. Thus, each gene comprises a hereditary section of DNA that occupies a specific place of the chromosome, and controls a particular characteristic of an organism (Harrison et al, 2004).

Diversity enhances the chances of populations’ adaptation to changing environments. With more variation, it is more likely that some individuals in a population will possess variations of alleles that are suited for the environment. Such individuals are more likely to survive to produce offspring bearing that allele. The population can thus continue for more generations because of the success of these individuals (NBII, 2011).

Most organisms are diploid, having two sets of chromosomes, and therefore two copies (called alleles) of each gene. However, some organisms can be haploid, triploid, tetraploid or more (having one, three, four or more sets of chromosomes respectively) (Harrison, et al, 2004). Within any single organism, there may be variation between the two (or more) alleles for each gene. This variation or polymorphism is introduced either through mutation of one of the alleles, or as a result of reproduction processes,especially if there has been migration or hybridization of organisms, so that the parents may come from different populations and gene pools. Harmless mutations and sexual recombination may allow the evolution of new characteristics which increases diversity(Andayani,et al.,2001).

Each allele codes for the production of amino acids that string together to form proteins. Thus differences in the nucleotide sequences of alleles result in the production of slightly different strings of amino acids or variant forms of the proteins.These proteins code for the development of the anatomical and physiological characteristics of the organism, which are also responsible for determining aspects of the behavior of the organism (Harrison, et al, 2004).

Plant diversity is part of the biological diversity and contributes towards achieving food security, poverty alleviation, environmental protection and sustainable development(Frankel 1984). It is being eroded rapidly in important spice plants and other crops mainly because of replacement of traditional landraces by modern, high yielding cultivars, natural catastrophes (droughts, floods, fire hazards, etc.), as well as large scale destruction and modification of natural habitats harboring wild species(Frankel 1984, Bramel-cox and Chritnick, 1998).M. myristica population is threatened by urbanization which damages its natural habitat, and leads to the cutting of most of the trees without replanting. Additionally, the plant is listed under Kew’s difficult seeds due to its inability to easily grow outside its natural habitat(Burkill, 1985). Genetic variation in traditional landraces and wild species is essential to combat pests and diseases and to produce cultivars better adapted to constantly changing environments(FAO, 1994).

Molecular tools such as have been found to be more useful and accurate in the study of inter-species and intra-species genetic diversity in several plants. Randomly amplified polymorphic DNA (RAPD) markers have been successfully employed for determination of intraspecies genetic diversity in several plants. These include Phaseolus vulgaris (Razviet al., 2013),Ocimumspp (Sairkaret al., 2012), Chrysanthemum (Martin et al., 2002), Annonacrassiflora( Cotaet al.,2011), Prosopis ( Goswami and Ranade, 1999), date palm (Corniquel and Mercier, 1994), papaya (Stiles et al., 1993), poplars (Bradshaw, et al., 1994) and amaranths (Ranade, et al. 1997). No such attempt has so far been reported for Monodoramyristica


1.3 Rationale

M. myristica is largely harvested from the wild and greatly affected by wild fires, urbanization, reckless and uncontrolled felling of trees for timber and firewood without replanting. There is need, therefore, to initiate breeding programs for this orphan crop by first documenting available genetic and phenotypic variations in this crop. The present report was done with this in mind, and should provide the much needed baseline for further studies.


1.4 Objectives

The general aim of the project was to characterize accessions of African nutmeg inSouth eastern Nigeria and estimate the range and distribution of genetic diversity.

The major objectives of this work were:

  1. To determine the level of genetic diversity among 21 accessions of Monodoramyristica using RAPD technique
  2. To compare morphological and yield related traits among the accessions using analysis of variance tests
  3. To confirm the efficiency of RAPD technique in genetic diversity studies of this important plant.
  4. To identify traits contributing significantly to variation in this species.

Chapter Five


5.0 Discussion

Genetic and morphological analysis on M .myristica was an effort to understand the genomic structure of African nutmeg population in the eastern region of Nigeria as availability and assessment of genetic variation are central to the improvement of any plant species. This study was helpful in identifying the level of diversity among the population and understanding their relatedness, which will help in the breeding of this crop, considering that the spice is endangered and difficult to propagate.

According to Ferreira and Grattapaglia (1995), RAPD markers are sensitive and may generate different quantities of amplified fragments depending on the quality and quantity of the DNA used, as well as the amplification conditions. The number of fragments observed in this study was satisfactory for interpretation and conversion into molecular data for the populations studied, Dendrograms from genetic and morphological data gave 4 and 3 clusters respectively, suggesting some environmental effect. The difference could also stem from the limited number of morphological and yield traits studied. However, some relationships between genetic and morphological traits were still evident. For instance, many members of cluster 2 in the genetic data were also grouped together(cluster 1) based on their morphological data. Similarly, all cluster 2 members in the morphological data except AGL 05 belong to cluster 1 in the genetic data analysis.

The study showed that M.myristica population studied had a significant level of genetic diversity comparable to those obtained with RAPD by Moura (2005) in Eremanthus erythropappus populations, with index values between 0.498 and 0.539. This index varies between zero and one, with values closer to one indicating greater genetic diversity (Estopaet al., 2006). The high level of diversity in the present study may be ascribed to the fact that the genotypes were collected from their original habitat and in their natural state, and there has been little deliberate attempt to propagate this plant outside its natural habitat. This also was in conformity with that expected for tree species, which generally present greater genetic variation within populations (Zimbacket al., 2004).

The correlation between the genetic and geographical distances of the populations did not indicate much tendency towards isolation by distance.From the morphological data it was discovered that accessions can vary greatly in appearance and yield irrespective of the geographical distances.

The distributions of the accessions in different clusters indicates that even though samples were selected from different geographical areas, the genetic makeup, genetic drift and natural selection pressure remain as the major cause of diversity among the accessions; although, the effect of geographical location should not be totally overlooked. On the other hand, the possibility that the tree grows naturally only in selected areas indicates that the tree may only germinate in those areas with very similar climatic and environmental factors.

The RAPD-PCR technique has not been used previously in diversity studies on M.myristica but has been successfully used severally for diversity studies in various other species including in Changiumsymrymioedes, (Fu et al, 2003), Eremanthus erythropappus, (Estopa, et al, 2006) Ocimumspp, (Saikar, 2012) and Annonacrassiflora, (Cotaet al., 2011) among othersThe accessions showed a high level of polymorphism with almost unique fingerprints.

The RAPD pattern observed in this study is able to distinguish the minute differences within the accessions. The quantitative estimate of genetic similarity obtained showed a high level of genetic diversity.

The results show clearly that the biodiversity of this plant is much, however, the conservation and diversity of the plant also shows from literature that a great amount of genetic and morphologic diversity may have been lost due to urbanization (Frankham, 2005), hence the urgent need for conservation of the spice plant.

This investigation has thus shown clearly that the RAPD-PCR is a reliable tool that could be used for precise and reliable analysis of genetic variability in M.myristica and probably other related species.


Summary and Conclusion

The dendogram based onboth the morphological data and the molecular data, even though apparently different shows great diversity among the species.

The observations from this study reflects the utility of RAPD in the analysis of genetic diversity within this medicinally important tree. On the basis of the similarity values computed, we conclude that there is high interspecies variability and also high intraspecies variability. Further, the diversity study in M .myristica has implications for studying the biodiversity of the genus, it is possible now to undertake longterm breeding and conservation program of this specie. In practice,better understanding of distribution of genetic and morphologic variation at the intra and interspecific level would help to identify the superior genotypes for cultivar upgrade as well as to evolve strategies for the establishment of effective conservation program for M. myrstica.


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