Preliminary investigations were carried out on cytomorphology, proximate chemical analysis and crossability relationships of three members of the genus
Solanum both classified as
S. incanum, with a view to establishing the genetic basis of their taxonomic relationships. Results from these investigations revealed variations in morphological characters, behaviour of chromosomes, proximate chemical composition and crossability relationships.
With basic chromosome numbers of n=24 in both thorny and non thorny
S. incanum (Plates 1 and 2) and preferential pairing of chromosomes, it is quite clear that these two wild types are not the same as the diploid West African
S. incanum whose genomic number was n=12 and in which pairing was allosyndetic.
Chemical composition of the two wild forms of
S. incanum showed 71.59 per cent for the thorny and 73.58 per cent moisture for the non thorny
S. incanum as against 85.56 per cent moisture content in the diploid cultivated
S. incanum. Also protein and lipid contents of the wild tetraploid species were 5.30, 5.25 (for protein) and 7.03 and 7.00 per cent respectively as against 0.83 per cent reported in the diploid
S. incanum.
There was high cross-compatibility between the wild tetraploid thorny and wild tetraploid non thorny
S. incanum but strong pistil pollen incompatibility between the two wild forms and the diploid cultivated
S. incanum. The similarity in some behaviour of chromosomes between the wild tetraploid forms and the cultivated diploid species was probably due to the fact that the wild tetraploid forms of
Solanum incanum were amphidiploids.
It is suggested that the non thorny wild tetraploid
S. incanum (Fig. 2) is a direct derivative of the thorny wild tetraploid
S. incanum (Fig. 1), since both are similar morphologically, in behaviour of their chromosomes and chemical composition of their berries. It is also suggested that the non thorny tetraploid
S. incanum is a botanical variety of the thorny tetraploid type and that the absence of thorns in the former was as a result of point mutation. If either the thorny wild tetraploid
S. incanum or its non thorny derivative was truly the ancestor of the diploid
S. incanum, then retrogressive evolution had taken place. But the trends in angiosperm evolution is for the lower genomic form to give rise to the higher genomic form.
It is therefore concluded that the wild tetraploid
S. incanum is not related to the cultivated diploid
S. incanum. The name
spinosum is suggested for the wild thorny
S. incanum while
incanum is reserved for the cultivated form of
Solanum incanum. The non thorny wild derivative is a botanical variety of the thorny wild tetraploid form.
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