Influence of Temperature on Crossing Over in an Inversion Heterozygote in Barley

T HE paracentric )nversion has been used widely in studies of crossover variation and of the mechanism of crossing over. However, it has been used very little for measuring the influence of various agents on crossing over. The present paper describes the effect of different growing temperatures on crossing over in barley as measured by the frequency of meiotic bridges and fragments resulting from crossing over within and adjacent to an inversion heretozygote. The inversion heterozygote was chosen even though criteria such as frequencies of chiasmata and of genetic recombination for measuring the influence of various agents on crossing over have been used extensively in a wide range of organisms ’a, 4 (2, 5, 15, 16, 18). For experiments with barley, the latter two criteria have at least two shortcomings. First, diakinesis is not distinct enough for obtaining reliable frequencies of chiasmata. Second, recombination experiments based on backcross or F.~ progeny are of limited usefulness because they require hybridization, the evaluation of 2 generations, the production of large numbers of plants, and laborious classifications. On the other hand, the use of the inversion heterozygote as a criterion for rapidly obtaining exact data on the variation of crossing over offers several advantages. For example, a single spike from a barley plant may yield several hundred microsporocytes at the proper stage for scoring the presence or absence of bridges and fragments. Thus, from a single well-tillered plant that can be grown under accurately controlled conditions, several thousand cells can be evaluated. Furthermore, a single generation is required and very little growing space is needed for an experiment.