STUDIES ON GENE FREQUENCIES OF POLYEMBRYONY AND KARYOTYPE IN FLUTED PUMPKIN (TELFAIRIA OCCIDENTALIS HOOK. F.)

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 =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.

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

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

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

farmers  prefer  plants  with  broad  succulent  leaves  as  they  attract higher premium in both rural and urban markets. The generation of polyploids with such potentials would be a  virtue and would lead to increased  yield  and  enhance  rural  income  in  Tropical  West  Africa. There is therefore need to investigate the existing ploidy levels of the Nigerian fluted pumpkin landraces. To date, there is paucity of information  on  the  polyembryony  and  karyotype  of  fluted  pumpkin, and breeding efforts to improve the crop have received very limited attention. This study was therefore initiated to;

(1) study the transmission pattern of polyembryony from one generation  to  another  and    determine  the  gene  frequencies  of the trait in relation  to the Hardy-Weinberg   equilibrium.

(2) investigate the benefits or otherwise of plants raised from the different  embryonic  types  through  their  performance  in  the field.

(3)  determine the karyotypic status of fluted pumpkin and, (4)   establish the genetic basis of sexual dimorphism in the crop.

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