CHAPTER 1
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
1.3RATIONALE
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:
v To determine the level of genetic diversity among 21 accessions of Monodoramyristica using RAPD technique
v To compare morphological and yield related traits among the accessions using analysis of variance tests
v To confirm the efficiency of RAPD technique in genetic diversity studies of this important plant.
v To identify traits contributing significantly to variation in this species.