Normal Chromosome Number 2n Research Articles

CYTOLOGICAL CHARACTERIZATION OF SOME COWPEA (VIGNA UNGUICULATA L.) ACCESSIONS

Studying the cytological behavior of twenty seven cowpea (Vigna unguiculata L.) accessions to select new promising ones which could be used in cowpea breeding programs were carried out in the present investigation. All tested cowpea accessions exhibited the same normal chromosome number 2n = 22. The means of mitotic index (MI) were significantly different among most of studied accessions and some accessions exhibited high variability in the percentage values of mitotic stages. From all tested cowpea accessions, few of them showed high frequency of total mitotic chromosomal aberrations. Various types of mitotic abnormalities such as; lagging chromosomes, chromosomal bridges, outside chromosome, stickiness and micronuclei have been observed in all tested accessions. The cytological findings revealed that most of tested cowpea accessions had a potential cytological stability with normal chromosomal number 2n=22 and could be considered as good basic tools in cowpea breeding programs.Key wards: Cowpea, Mitotic index, Mitotic aberrations, Plant breeding

Cytogenetics and reproduction of sheep with multiple centric fusions (Robertsonian translocations).

The significance of centric fusions (Robertsonian translocations) in domestic animals, with special reference to sheep, is reviewed. The mating is described of a further 856 ewes with either a normal chromosome number 2n = 54 or carrying one or more of the three different translocations (centric fusions) t1, t2 and t3 in various heterozygous and homozygous arrangements. Rams which were used in the matings were homozygous for one of the translocation chromosomes (2n = 52), double heterozygotes (2n = 52), triple heterozygotes (2n = 51) or were carriers of 4 translocation chromosomes (2n = 50) and 5 translocation chromosomes (2n = 49). A remarkably even distribution of segregation products was recorded in the progeny of all combinations of translocation ewes x translocation rams in those groups in which sufficient animals were available for statistical analysis. Forty-eight chromosomally different groups of animals were mated. Further, the overall fertility of the translocation sheep, measured by conception rate to first service, lambing percentage and number of ewes which did not breed a lamb, was not significantly different from New Zealand national sheep breeding data. In some groups the poorer reproductive performance could be explained by the age structure of the flock and inbreeding depression, which probably affected the performance of some animals. Sheep with progressively decreasing chromosome numbers, due to centric fusion, 2n = 50, 2n = 49 and 2n = 48, are reported. The 2n = 48 category represents a triple homozygous ewe and a triple homozygous ram and is the first report of the viable evolution of such domestic animals. Less than 1% of phenotypically abnormal lambs were recorded in a total of 1995 progeny born over 10 years. It is now considered that there is little or no evidence to suggest that centric fusions in a variety of combinations affect the total productive fitness of domestic sheep. It is suggested that future research should be more actively directed to understanding their genetic significance.