The ability to count chromosomes is among the valuable tools used by plant breeders and cytogeneticists. This exercise is difficult with yams (Dioscorea spp.) since the chromosomes are small, dot-like, frequently having only a few cell division visible in a single root tip. The aim of this study was to investigate and confirm the chromosome numbers and raphide morphology of six yam genotypes which included two genotypes of Dioscorea alata (ER 07/030 and ER 07/036) and four genotypes of D. rotundata (NR 07/060, NR 07/071, SR 07/072 and TDr 95/18544) using the root tip method. A total of ten minisetts per genotype each weighing 20 g were grown in quarter liter pots containing vermiculite medium. Root tips were prepared using the acetocarmine staining technique. Genotype ER 07/030 had 20 chromosomes, while ER 07/036, NR 07/060, NR 07/071 and SR 07/072 had 40 chromosomes, and TDr 95/18544 had 60 chromosomes. These findings were in agreement with the ploidy results obtained by flow cytometry (FCM), which indicated that FCM was a reliable technique for rapid determination of ploidy level in yams. Results of the raphide study in D. alata showed the type III morphotype in ER 07/036 with six to eight sided crystals and symetrical pointed ends. The remaining genotypes exhibited no visible presence of raphides. Findings obtained in this study would provide guidance in a yam improvement programme both in terms of selection of initial breeding material and choice of breeding methods. The development of flowering genotypes and the exploration of chromosome size and content form part of future research.
Key words: Dioscorea, chromosome number, raphide morphology, genotypes.