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Chromosomal behaviour during meiosis in mosses Dill, Frederick John


The primary purpose of the present work was to examine meiosls in mosses, concentrating on the detail of prophase I, a meiotic stage which has been described in detail only in Pleurozium schreberi. Secondarily, an investigation was undertaken to study the effect of heat stress on meiosls. Hypnum circinale Hook, and Brachythecium frigidum (C.M.) Besch. were used to study meiosis. The procedure of preparing spore mother cell squashes was similar to that used by Steere et al. (1954), with modifications in handling the plants prior to fixation to ensure against production of heat induced anomalies. Except for late prophase I, meiosis in both species conformed to that found in P. schreberi. In H. circinale and B. frigidum diplotene was followed by chromosome elongation and resulted in a diffuse stage. This stage is morphologically analogous to the dictyotene stage of the growing oocytes of many animals, and appears to have been described, in plants, only in Balsamina hortensis. It is probable that the stage occurs in many moss families; at present its functional significance is unknown. In the heat stress experiments, plants of H. circinale were either maintained under laboratory temperatures while being studied or they were treated with a heat shock over a period of four or six hours with the maximum temperature in the general ranges of 25°C., 31°C., and 36°C. The maximum temperature was maintained for 4 hours in the 25°C. experiment and 1/2 hour in the remaining experiments. The heating and cooling gradients were almost equivalent (1°C./5 min.), and the starting and finishing temperature was 14°C. The temperature of the natural environment during the study ranged between 7-ll°C Severe anomalies, including chromosome clumping and multiple association, precocious disjunction, chromosome contraction, spindle breakdown and inhibition, premature meiotic induction and meiotic abortion were observed to some extent in spore mother cells from all treatments except the ones from the 25°C. heat shock experiment. Room temperature accelerates prophase I stages of H. circinale. The time available for these stages appears to be too brief for synthesis of necessary products leading to active stages, thus causing severe abnormalities which result in abortion of meiosis. On the basis of these results, it is apparent that cytologists working with moss material should take care in handling the plants prior to fixation to ensure against heat induced meiotic anomalies.

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