Chiasma F'requency and Distribution in a Maize Family Segregating for Kio and Trisomy

Chiasma frequency was determined from total chiasma counts, and the distribution of these exchanges was determined by the ratio of proximal to distal chiasmata. No effect of trisomy IO could be demonstrated. Confirmation was obtained of earlier work showing more proximal and fewer distal chiasmata in K10 plants than in controls. However, diakinesis data failed to confirm the ability of KIO to increase total chiasmata as suggested from metaphase I results. AMONG factors affecting crossover frequency in Zea mays L. are the supernumerary elements, abnormal chromosome IO (K10, to differentiate it from the normal MO), and B chromosomes. Although sensitivity to the elements varies, elevation of crossing over by both these factors has been reported for proximal regions of chromosome 3 (RHOADES and DEMPSEY 1966; HANSQN 1969), chromosome 5 (NEL 1973), and chromosome 9 (CHANG and KIKUDQME 1974; but see also NEL 1973). Considerable enhancement of recombination by both elements has also been noted for a chromosome 9 transposition (RHOADES 1968; WARD 1973, 1975). Analysis of crossover frequency is a most desirable approach to determining the effects of various factors, and we have made a beginning with K10 and B chromosomes at mapping responsive regions of the genome. Although some progress has been made in deciphering effects of largely heterochromatic accessory elements, little is known about the consequences of extra euchromatic material on crossover frequency. GHIDONI (1975) reported increased crossing over in the sh-wx region of chromosome 9 as a response to trisomy 6. No effect could be demonstrated for a more distal region. Similar data are needed for various regions of the genome and for the different trisomies. Linkage analysis of the total genome is most important for high-resolution mapping of genome responses, but an estimate of reaction to extra material may also be obtained from examination of chiasma frequency and distribution. Both K10 and B chromosomes have been found to have higher chiasma frequency (AYONOADU and REES 1968; WARD 1976), and a greater proportion of chiasmata in proximal positions than do appropriate controls. (WARD 1976). Information Contribution No. 5%. Department of Plant Science, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2. Genetics 92: 223-230 May, 1979.