Cytological Studies on Different Species of Mentha with Special Reference to the Occurrence of Chromosomal Biotypes

抄録

1. A cytological investigation of five different species and varieties of the genus Mentha have been carried out. Their chromosome numbers noted here are as follows:
i) Mentha viridis Linn. var. I 2n=32
ii) Mentha viridis Linn. var. II 2n=48
iii) M. piperita Linn. var. I 2n=72
iv) M. piperita Linn. var. II n=66
v) M. arvensis Linn. 2n=90
2. Karyotype analysis in detail has been performed in four species and varieties. Meiosis has been worked out in M. viridis var. I and M. piperita var. II.
3. It has been shown that each species and variety has got a karyotype, characteristic of its own. This fact indicates that evolution has been associated with a considerable degree of structural changes of chromosomes in them.
4. Role of polyploidy in speciation in this genus has been emphasized in view of the present and the previous data available on this aspect in the different species. Six has been considered as the possible basic number of chromosomes, from which all the euploid and aneuploid series have evolved.
5. Early literature as well as the present records show that in species of this genus, a large number of chromosomal biotypes occur. The importance of analysis of these biotypes from a cyto-ecological standpoint has been emphasized.
6. Variation in chromosome number in the same somatic tissue and the ineffective method of sexual reproduction have been pointed out as responsible for the origin of chromosomal biotypes or individuals with different chromosome numbers. Their role in speciation as an additional means is obvious.
In conclusion, the authors wish to express their sincere thanks to Dr. K. P. Biswas, Director in charge, Medicinal Plants Scheme, Government of West Bengal, for giving us all facilities to collect different wild and cultivated species of Mentha at Rongo, Darjeeling.