Dioscoreophyllum cumminsii is a tropical dioecious rainforest vine. It grows in humid and heavily shaded understorey vegetation of closed forest, from May to October. The fruit, serendipity berry, contains a protein sweetener called monellin that could replace sugar in foods for diabetics and dieters. Other parts of the plant, for example, tubers, stems and leaves have medicinal values. This work was set up to study the mitotic chromosomes of root tips of D. cumminsii and establish the number of chromosomes in the genome. This is necessary since it appears that there is no published work on the chromosome number and structure of this important plant. The study was carried out at the Cytology Laboratory of the Department of Botany, University of Nigeria Nsukka. Root tips for the chromosome work were obtained from three sources: runners originating from main vine; sprouting pieces of tubers and redifferentiated root tips from callus. The results showed that normal metaphase plates of root tips of D. cumminsii contained sixteen chromosomes, 2n = 16. These chromosomes showed variations in size based on centromeric positions giving rise to metacentric and submetacentric chromosomes. Metaphase cells with more than 16 chromosomes were encountered from root tips regenerated in vitro from callus cells. This cytological knowledge is useful for future micropropagation or breeding activities involving D. cumminsii.
A summary of studies made with Brassica napocampestris (B. nc.), 2n=58, obtained by colchicine treatment of the F1 hybrid between B. napus and B. campestris is given. Cytological studies of the primary hybrids reveal that the single genome of B. oleracea in B. napus—“c1”– may be utilized for elucidating the basic genomic number of the genus Brassica. Based on the mode of chromosome pairing at MI in the primary hybrids, the conclusion was drawn of a basic genomic number of 6. From the backcrosses of these F1 hybrids to B. campestris types, the same conclusion was indirectly arrived at.
The backcross experiments with the F1 hybrids indicated the possibility of extracting not only a whole genome “a1” from B. napus but also, the supposed basic genome.
For the hexaploid plants (2n=58), meiotic studies revealed that besides many bivalents there were some multivalents. Anaphase movements were slightly disturbed by bridges, presumed to result from inversions. On the whole, secondary balance accompanied by chromosomal alterations, may be responsible for the present serial polyploidy in the tribe Brassiceae.
Aneuploid plants were not encountered in the hexaploid populations used. However, by continuous backcrosses of F1 hybrids (2n=29) to the hexaploids, plants with chromosome numbers close to 58 were obtained which showed fairly good seed fertility. It is suggested that selection against aneuploidy occurs at the gametic stage and this would account for the relatively low pollen fertilities found in the hexaploid B. nc. types.
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