When a species with many varieties are considered, the morphological data cannot be sufficient to obtain a clear representation of the phylogenetic relationships between species. In these cases cytotaxonomy can be an effective tool and it can allow a more accurate knowledge of the relationships (Dewey, 1984; Gianfranco et al., 2008). Cytotaxonomy is a branch of cytogenetics, devoted to the comparative study of karyological features for systematic and evolutionary purposes (Siljak-Yakovlev and Peruzzi, 2012). Today, a number of information can be obtained by chromosome studies; among which include, chromosome number, size and structure and chromosome behaviour at meiosis. These are good taxonomic characters because hereditary materials are housed in the chromosome. Karyotypes can provide information about taxonomic relationships, genetic aberrations, and the evolutionary origins of species (Young et al., 2012).
The chromosomes have been considered as the physical bases of heredity because they have a specific organization, individuality, functions and capable of self reproduction. Their main chemical constituent is DNA, a universally accepted genetic or heredity material, found to carry genetic information from one generation to next generation. They occur in all living beings in a specific number and organization and usually fall into different categories: 1. Prokaryotic chromosome: Most contain single main DNA molecule. 2. Eukaryotic chromosome: In contrast to the single main DNA.
Molecule of most prokaryotic, the chromosome genome is partitioned into several molecules of DNA
42 or chromosomes. The chromosomes occur in the nucleus of a eukaryotic cell. In a chromosome, a single, linear and double – stranded DNA molecule is associated with almost equal amount of histone proteins. There is also certain non-histone or acidic proteins in the chromosomes which act as structural, enzymatic and regulatory proteins (Verma and Agarwal, 2009).
Chromosome identification is critical for cytological analyses, as well as subsequent studies in genomics, taxonomy, and the evolution of polyploidy, enabling an understanding of the relationship between visible landmarks and genetic or physical map features (Harper and Cande, 2000).
Chromosome size and morphology may help indicate evolutionary relationships among plant species. In plant taxonomy, breeding and genetic studies, information about chromosome karyotypes can be useful in species identification and analysis of hybrid populations (Qu et al., 2004). Somatic cells of higher plants usually have chromosome in pairs; that is, two of each kind of chromosome present in each cell (Verma and Agarwal, 2009). Species in which there are two sets of chromosomes are referred to as diploid, given the symbol ‘2n’. About half of plant species are diploid, with two sets of chromosome per nucleus or cell while ‘n’ normally signifies the gametic or haploid number.
There is variation in the chromosome number of C. esculenta. Chromosome numbers reported include 2n = 21, 22, 26, 28, 38, and 42 (Coates et al., 1988; Parvin et al., 2008). Moreover, the basic chromosome numbers of x=12 and x=14 for C. esculenta were reported by earlier workers. The somatic chromosome numbers 28 and 42 are considered to be diploid and triploid respectively (Kuruvilla and Singh, 1981; Coates et al., 1988; Lebot and Aradhya, 1991). Lee (1999) reported that the greatest variation in chromosome number occurs is India. The "Polynesian taros" primarily all have twenty-eight chromosomes, while generally there is a greater concentration of 42-chromosome types in East Asia. It has been speculated that the 28-chromosome cultivars preceded the 42-chromosome types into the
43 Pacific islands. The disparity in numbers may be due to the fact that taro chromosomes are liable to unpredictable behaviour during cell divisions. The most commonly reported results are 2n = 28 or 42;
however, Ekanem and Osuji (2006) reported chromosome count of 2n = 24 for four cultivars of C.
esculenta cultivated in Southeastern Nigeria namely: ‘Coco India’, ‘Ede Ukpong’, ‘Ede Ghana’ and
‘Ede Ofe’. Chromosome number of 2n=28 was reported for three species of Colocasia (Yang et al., 2003). They include Colocasia gaoligongensi, C. gigantea and C. gongii.
Having gone through the literature, it became clear that local names of C. esculenta varieties have their weaknesses because they originated from different localities based on their dialects and languages. Consequently, the names are restricted only to the people of the dialects and languages. In addition, they are not regulated by any constituted authority. Obviously therefore, there is confusion in the taxonomy of C. esculenta. Moreover, there is scanty of information on the taxonomic investigation of the varieties cultivated in Nigeria. It therefore, became crucially important to study varieties of C.
esculenta cultivated in Anambra State with a view to taxonomically elucidating them.
44 CHAPTER 3
MATERIALS AND METHODS 3.1 Study Area
Anambra State is one of the five states that make up the Southeastern states of Nigeria. Geographically, Southeastern states of Nigeria extend from latitudes 4° 40’ to 7° 20’ north latitude, and 6° 00’ to 8° 20’
east longitude (Okeke et al., 2009). The states cover an area of about 50 000 km2 of Nigeria’s total area of 923 768 km2. Anambra State extends from latitude 6o 20’ 00’’ N, longitude 7o 00’ 00’’ E (NGA, 2012). Two seasons occur in a year, rainy season with annual rainfall, ranging from 1,400 mm in the north to 2,500 mm in the south; and dry season lasting for about four months, November to February. As a result, the natural vegetation of Anambra State is mainly rainforest.
Anambra State is divided into three senatorial zones namely; Anambra North, Anambra Central and Anambra South. Three towns, each from these three senatorial zones were selected as three geographical locations for this study. They include: Umuikwu – Anam is situated in Anambra West Local Government Area. It is bounded by famous River Niger on the south. Agulu is in Aniocha Local Government Area. Its geographical coordinates are 6o 07’ 0’’ North, 7o 4’ 0’’ East. The famous Agulu Lake is located along Awka road in Agulu. Uga is situated in Aguata Local Government Area. Its geographical coordinates are 5o 56’ 0’’ North, 7o 5’ 0’’ East. The famous ‘Obizi’ spring is located in this town.
45 3.2.0 Sources of Materials
Cormels of Colocasia esculenta varieties were obtained from the local farmers in three geographical locations of Anambra State, namely: Umuikwu-Anam in Anambra West LGA (Anambra North); Agulu in Anaocha LGA (Anambra Central) and Uga in Aguata LGA (Anambra South).
3.2.1 Umuikwu-Anam in Anambra West LGA (Anambra North) Collection Numbers Varieties
ACE-41 Colocasia esculenta var. antiquorum (Eddoe, ‘Ugwuta’, ‘Ede ofe’, ‘Ede ofe green’)
ACE-42 C. esculenta var. esculenta (Dasheen, ‘Nkashi’, ‘Nachi’,’Ede ofe purple’) ACE-43 ‘Kochuo’ (‘Coco India’)
ACE-44 ‘Nwine’ (‘Akonoke’, ‘Nkpong’)
3.2.2 Agulu in Anaocha LGA (Anambra Central) Collection Numbers Varieties
ACE- 45 Colocasia esculenta var. antiquorum (Eddoe, ‘Ugwuta’, ‘Ede ofe’, ‘Ede ofe green’)
ACE-46 C. esculenta var. esculenta (Dasheen, ‘Nkashi’, ‘Nachi’, ‘Ede ofe purple’) ACE-47 ‘Kochuo’ (‘Coco India’)
46 ACE-48 ‘Nwine’ (‘Akonoke’, ‘Nkpong’)
ACE-49 ‘Akpa – ahuri’ (‘Ogeriobosi’, ‘Nworoko’)
3.2.3 Uga in Aguata LGA (Anambra South) Collection Numbers Varieties
ACE-50 Colocasia esculenta var. antiquorum (Eddoe, ‘Ugwuta’, ‘Ede ofe’, ‘Ede ofe green’) ACE-51 Colocasia esculenta var. esculenta (Dasheen, ‘Nkashi’, ‘Nachi’, ‘Ede ofe purple’) ACE-52 ‘Kochuo’ (‘Coco India’)
ACE-53 ‘Nwine’ (‘Akonoke’, ‘Nkpong’) ACE-54 ‘Ogeriobosi’ (‘Nworoko’) ACE-55 ‘Opa’
Five varieties, namely, Colocasia esculenta var. antiquorum (eddoe) traditionally known as ‘akikara’
in Aguata area of Anambra State, ‘Ugwuta’ in Nsukka area of Enugu State and ‘Ede ofe green’ in National Root Crops Research Institute, Umudike; Colocasia esculenta var. esculenta (dasheen) traditionally known as ‘mgbagwurike’ in Amesi, Anambra State, ‘Nkashi’ and ‘Nachi’ in Nsukka area of Enugu State and ‘Ede ofe purple’ in National Root Crops Research Institute (NRCRI), Umudike;
‘Kochuo’ (‘Coco India’), ‘Nwine’ (‘Akonoke’, ‘Nkpong’) and ‘Ogeriobosi’ (‘Nworoko’) were identified and the morphological features documented and compared with other cultivars from Nsukka
47 in Enugu State and National Root Crops Research Institute, Umudike in Abia State. They were then grown in a common environment in Uga, Anambra State. The specimens’ identification and the voucher specimens’ authentication were performed by C.A. Ezeabara in liaison with Prof. C.U. Okeke, Plant Taxonomists, and deposited in the herbarium of Department of Botany, Nnamdi Azikiwe University, Awka, Anambra State.