Chromosome pairing in X-irradiated Pteris longifolia L.

Abstract

Effects of X-irradiation on chromosomal pairing is known in a good number of fl owering plants ( Lesley and Lesley 1956, Bora et al. 1961). To our knowledge such informations in ferns are not fully available. This group of plants is important in having two independent phases of gametophyte and sporophyte generations in their life cycle. Selfing of gametophytes provides genetically and cytologically homozygous sporophytes in the very first generation. The sporophyte establishes a perennial rhizome where ample time is available for the repair of any damage caused by irradiation in the first generation. The species Pteris longifolia grows wild here and is a sexual tetraploid having regular bivalent formation during meiosis. For the study of effects of X-irradiation on pairing of chromosomes during meiosis in ferns, therefore, this plant was selected. The results thus obtained from the R, generation, after a few years of irradiation, are presented in this paper. Materials and methodsSpores of Pteris longifolia were obtained from wild plants. Gametophytes were raised from such spores on sterile agar media (Knop's media). These were then selfed using the culture techniques known for ferns (Lovis 1968). Sporo phytes at one leaf stage, on agar media were placed for X-irradiation under a cathode tube. The doses of exposures varied from 2000 r to 10,000 r at intervals of 2000 r each. The tube used was a 220Kv. 15mA current 0.2 copper filter at focal distance of 150cms from the target.After five weeks of treatment, these sporophytes were thoroughly washed to remove the media and were transferred to pots filled with sterile mixture of sand, peat and loam. These plants were left to grow in controlled conditions of humidity and temperature. Fixings were taken for meiotic analysis at intervals soon after the fertile fronds started coming up. Procedure for cytological preparation was the same as given by Manton (1950). ObservationsPeriod for the juvenile plants either control or irradiated, to attain maturity and to produce fertile fronds is mentioned in a tabular form in Table 1. While the control plants start producing fertile leaves after 9 months of selfing, those irradiated take 10-24 months to produce first fertile leaf.