Karyotype studies on ten genotypes of onion grown in Jos Plateau, Nigeria
The large number of inter- and intra-specific differences in the genus Allium has necessitated karyotype analysis of the cultivated species, in order to describe patterns and directions of chromosomal evolution, which affects morphological differences. In this study, ten genotypes of onion (Allium cepa L.) (Ares, Violet de Galmi, Red Creole, “Wase”, “Dan Zaria”, “Dan Garko”, “Dan Giyawa”, “Bahaushe”, “Bakana” and “Yar Aleiro”) were used for karyotype analysis in the Cytogenetics Laboratory of the University of Jos, Nigeria. Fresh root tips (1-1.5 cm long) obtained from each genotype were prepared and stained with aceto-orcein for 12 minutes. Each stained root tip was placed in a drop of water on a slide, covered with a coverslip and squashed. Each prepared slide was mounted on the microscope to observe the different stages of mitotic division. The results showed that the arm ratio, centromeric index, coefficient of variation, total form and disparity index varied with genotypes. Metacentric, sub-metacentric and sub-telocentric chromosomes were also observed. Based on a dendrogram of the phylogenetic relationships, the genotypes were grouped into four clusters. Cluster I comprised of genotypes Ares, “Wase” and “Bakana”. Cluster II comprised of genotypes “Dan Garko”, Violet de Galmi and “Dan Giyawa”. Cluster III consisted of the genotypes Red Creole, “Bahaushe” and “Dan Zaria”, while Cluster IV comprised of genotype “Yar Aliero”. The principal component analysis revealed that the centromere position and variation in complement length accounted for 92.71% of the total variations among the genotypes. These attributes could be responsible for the differences in fresh bulb size and dry matter yield of onion.
Key words: Allium cepa, chromosome, metacentric, phylogenetic