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STUDIES ON GENE FREQUENCIES OF POLYEMBRYONY AND KARYOTYPE IN FLUTED PUMPKIN (Telfairia occidentalis Hook. F.)

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( 124 pages: chapter 1:5 )

Topic Description

ABSTRACT

Five experiments were performed to investigate the polyembryonic expressions and karyomorphology of fluted pumpkin (Telfairia occidentalis Hook. F.). The results obtained revealed three morphotypes of polyembryony which included twin, triple and quadruple, irrespective of the shape of the pod. The three morphotypes had side shoots that varied in length. Some were unequal, a few of them were almost equal, and many had rudimentary side shoots, which subsequently degenerated with time. Chi-square test between the pod shape and the embryo type depicted independence (χ2= 1.02; P= 0.90) between pod shape and the embryo type. The triple and quadruple embryo types had the highest and lowest frequencies of occurrence, respectively. The gene frequencies in the first year were 0.64 and 0.36 for polyembryony (S allele) and monoembryony (S allele), respectively. The derived zygotic frequencies were 0.41 for the homozygous dominant (SS); 0.46 for the heterozygous (SS) and 0.13 for the homozygous recessive (SS). These summed up to 0.87 for the multiple embryonic traits and 0.13 for the single embryonic trait. In the second year, the zygotic frequencies for the homozygous dominant (SS), the heterozygous (SS) and the homozygous recessive (SS) were 0.33, 0.49 and 0.18, respectively. These corresponded with the 0.82 for the multiple embryonic traits and 0.18 for the single embryonic trait. Chi square analysis showed that the gene frequencies in both years did not differ significantly (χ2= 0.017; P= 0.96). The results obtained from the cytological studies of fluted pumpkin revealed the interphase and mitotic stages of the cell cycle, and a diploid chromosome number of 2n = 22. There were traces of polyploids namely; aneuploids (2n = 22 + 1), triploid (3n =

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3x = 33) and tetraploid (4n = 4x = 44) among the cells investigated. The karyotype consisted of 5 metacentric, 4 submetacentric, 1 subtelocentric and 1 telocentric chromosomes. The karyotypic analysis revealed that the chromosome pair number 5 is homomorphic in the female plant but heteromorphic in the male plant. The ideograms showed that the chromosome complement falls into 11 size class groups of one chromosome pair per class in the female and 12 size class groups with the 5th chromosome pair having two size groups arising from the non-identical lengths in the male. This would appear to implicate an XY system of sex chromosomes with homogametic XX female and heterogametic XY male in fluted pumpkin. In terms of the agronomic value of polyembryony, the triple (tri-embryony) and the single (mono-embryony) lines performed more impressively than the twin (bi-embryony) and quadruple (tetra-embryony) lines.

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INTRODUCTION

The fluted pumpkin (Telfairia occidentalis Hook. F.) belongs to the family, Cucurbitaceae which consists of 90 genera and 750 species (Purseglove, 1984). It is mainly cultivated in the southern parts of Nigeria, where it is recognized as an important crop for its palatable and nutritious leaves. The seeds are rich in protein and oil, and can be eaten whole, ground into powder for different kinds of soup, or made into a fermented porridge. Asiegbu (1987) reported protein and oil contents of 30.1% and 47%, in the seeds. The author also reported that the essential amino acid content compared favourably with those of important legumes such as soybean and groundnut. The plant also has some medicinal uses since the roots and the extracts from older leaves contain compounds like resins, alkaloids and saponins (Akubue et al., 1980). The high oil content of the seed makes the crop a potential raw material for the chemical and pharmaceutical industries (Okoli and Mgbeogwu, 1983).

Multiple seedlings do occasionally sprout from one seed (Esiaba, 1982), a phenomenon referred to as polyembryony. Different forms of polyembryony have been reported in T.occidentalis (Odiyi, 2003), and suggested that the occurrence of polyembryony is natural and that multiple seedlings were observed to develop in two areas of the seeds of the crop. These are the embryonic axis and the central groove in the

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lower region of the cotyledons. The emergence of multiple seedlings in the two regions of the cotyledons was reported to be independent, and so, many occur simultaneously as described in certain species (Maheshwari, 1950; Bhojwani and Bhatnagar, 1978; Batygina and Vinogradova, 2007).

The knowledge of the chromosomes of a crop is of fundamental importance in crop improvement (Haskell and Wills, 1986). Karyotype is a reliable taxonomic tool for characterizing plant species. It has served as a genetic marker in some crops for elucidating plants’ systematics and evolution (Robert and Foster, 1971; Doudrick et al., 1995). It is used for reference when screening and describing chromosome mutations, and for studying the relationship between different species (Dyer, 1979). The cytological features of fluted pumpkin, which are needed for its gene mapping and characterization of the morphotypes are yet to be comprehensively studied. It has been reported that fluted pumpkin has a chromosome number of 2n = 22 (Okoli and Mgbeogwu, 1983), but the report did not contain karyotypic details of the crop. There is therefore need for a detailed karyotyping of the chromosomes.

Some plant species are sexually polymorphic, including dieocious species with separate males and females. Fluted pumpkin is a dioecious species with female plants producing only pistillate flowers

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and male plants producing only staminate flowers (Asiegbu, 1985). This mechanism has made outbreeding mandatory and has, over the years, resulted in large genetic variability in fluted pumpkin. Some attempts have been made to explain sexual dimorphism in fluted pumpkin with morphological and molecular markers (Ndukwu et al., 2005). Based on phenotypic observation, the authors reported that the female plants were generally larger in size than the male plants. They were also able to provide evidence of polymorphism using DNA finger prints but could not provide any explanation for sexual dimorphism in fluted pumpkin. There is, therefore, need to address this lapse and provide convincing explanation for dioecy in fluted pumpkin which, as to date, is unavailable in the literature.

Polyploids do arise naturally, although many plant species are polyploidized by man through the creation of conditions, which disrupt mitosis and meiosis by preventing anaphase separation. Agarwal and Roy (1976) revealed that added advantage of polyploids in cucurbitaceae is their capacity to propagate vegetatively and a prolonged fruiting season, affecting uniformity of fruits, yield and availability of fruits during the off – season. In some plant species, polyploids have been associated with the production of larger plant parts, such as, leaves, flowers, fruits and seeds, and specialized characteristics such as seedlessness (Uguru, 1998). Fluted pumpkin

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