A report on new chromosome number of three Dioscorea species

Chromosomal study conducted in nine species of Dioscorea from different forest belts of Tripura revealed that their somatic chromosome number ranged from 2n=40 to 2n=60. The record of 2n=40 chromosome in the sexual phenotypes of Dioscorea hamiltonii, Dioscorea glabra and Dioscorea pubera are the first time report from Tripura, North East India. Moreover the somatic chromosome counts of 2n=60 in Dioscorea pentaphylla would be attributed as a new cytotype. However at the respective ploidy level no difference in somatic chromosome count was observed between their sexes.


Introduction
The genus Dioscorea commonly known as yam, a perennial climber belongs to the family Dioscoreaceae is highly economically important tuberous crop (1).Dioscorea tuber secure fourth rank as a tuber crop after potato, cassava and sweet potato (2).The taxon is mostly dioecious in nature although few monoecious species were also recorded (3)(4)(5)(6)(7).Interestingly, the male and female plants of Dioscorea species produced morphologically distinct tuber.Because of small chromosome sizes Dioscorea is considered as one of the most challenging genera with respects to cytotaxonomic and cytogenetic perspectives (8)(9)(10)(11).However, the occurrence of polyploidy in cultivated Dioscorea species is common possibly due to the usage of tubers or bulbils for multiplication.
Previous cytological investigation expounded D. alata somatic chromosome number ranged from 2n=30 to 2n=80 (12)(13)(14)(15).Lack of sex specific chromosomal information prompted us to evaluate the chromosome number of two different sexual phenotypes of nine Dioscorea species collected from different forest belts of Tripura.

Materials and Methods
For chromosomal study, healthy root tips were collected from tuber of male and female plants of all the Dioscorea species from the experimental garden of Tripura University campus.The collected healthy fresh root tips were carefully washed and pre-treated in saturated solution of Para-dichloro benzene (PDB) at 4°C for 5 min and finally at 12°C  for 3 h followed by fixation in acetic acid: ethyl alcohol (1: 3) for 24 h.Then root tips were treated with 45% acetic acid for 10 min and stained in 2% aceto-orcein and 1(N) HCl (9:1) mixture for overnight.After staining the root tips were squashed in 45% acetic acid and studied under a compound microscope (16).

Results and Discussion
In the present study the somatic chromosome count of both the sex forms of nine Dioscorea species was determined and it was found that their somatic chromosome count ranged from 2n=40 to 2n=60 (Table 1, Fig. 1 & 2).It was also observed that the male and female plant of Dioscorea bulbifera var.bulbifera, D. glabra, D. hamiltonii, D. hispida, D. oppositifolia, D. pubera and D. wallichii had a chromosome count of 2n=40.In comparison to these the diploid chromosome number of the sex forms of D. alata, and D. pentaphylla were recorded as 2n=60 chromosome.Curiously the diploid chromosome number of each sex form of three Dioscorea species viz.D. hamiltonii, D. pubera and D. glabra also had a chromosome count of 2n=40 and thus no difference was observed between the sexes at chromosome number level.This study also revealed that the most of the Dioscorea species found in Tripura have diploid chromosome number 2n=40 and two species viz.D. alata and D. pentaphylla have somatic chromosome count 2n=60 in their respective phenotypes as was reported by earlier researchers (8,(17)(18)(19)(20).The present study also depicts at the basic chromosome number, is indeed X=10 in all the dioecious species studied in this exploration.Similar such results were also recorded in previous findings (9,22).However, basic number X=8, 9 and 12 (8) reported by the earlier worker was not exposed in this investigation.The Ploidy level of Dioscorea species is an important trait for the utilization of these species in breeding programme as well as in the enhancement of ploidy manipulation in inter-and intra-specific crosses.

Conclusion
The present study embodies the first time chromosomal report (2n=40) in three dioecious species of Dioscorea viz.D. hamiltonii, D. glabra and D. pubera carried out from the forest belts of Tripura.In general no difference was observed in somatic chromosome number in relation to sex in all the species studied.The detail analysis of their karyotype is essential for further exploitation of germplasm conservation.