Cytogenetics of Crotalaria VI. Chiasma frequency and position, and univalent behaviour in a (partially) asynaptic mutant of C. juncea R. C. Verma & S. N. Raina Cytogenetics Laboratoo', Department of Botany, University o f Jodhpur, Jodhpur-342001, India Abstract In Crotalaria.juncea (n = 8) a plant exhibiting partial asynapsis was isolated in the M~ of a combined treatment of 50 kR gamma rays ÷ 0.2% EMS. The majority (48.14%) of PMCs at diplotene, diakinesis and metaphase I had 16 univalents. The bivalents in the asynaptic mutant were always rod-shaped with one terminal chiasma. In comparison, controls had on the average 7.08 ring bivalents. The asynapsis is genetically controlled, monofactorially recessive, and it is concluded that chromosome pairing is interrupted at a very early stage, There is a possible correlation between the number of bivalents and the arrangement of the univalents at metaphase I. When there were less than four bivalents, the univalents tended to be polar, and when there were more than four, the univalents were more equatorial in arrangement. The arrangement of univalents was random and apparently not influenced by the bivalent's, when their number (4) was exactly half the zygotic number. Introduction The normal behaviour of male and female meiosis is dependent on coordinated systems governed by genes and their interaction with external factors like temperature, water, nutrition. Mutation in the former or drastic change in the latter may lead the course of meiosis to deviate in various ways. The homologous chromosomes may fail to pair at zygotene (asynapsis), or else subsequent chiasma formation may fail (des.vnap- sis). Desynapsis/asynapsis, besides being found spontaneously, has been induced by gamma and X rays (Martini & Bozzini, 1966; Bozzini & Martini, 1971; Gottschalk & Baquar, 1971; Gottschalk, 1978; Katiyar, 1977; Singh et al., 1977; Srivastava, 1974), chemical mutagens (Bozzini & Martini, 1971; Sree Ramulu, 1973; Seetharam et at., 1975; Singh et al., 1977; Sharma & Reinbergs, 1974; Tyagi & Dasl 1975) and by combined treatments of physical and chemical mutagens (Singh et al., 1977). The cytogenetics of desynapsis/asynapsis has helped towards a better understanding of the intricate mechanisms involved in meiosis, as in Pisum sativum where Gottschalk (1978) has isolated 58 genes influencing meiotic behaviour. Experiments were conducted to investigate the nature and extent of mutation induction by treat- ing seeds of Crotalaria juncea, commercially known as Sunnhemp, with EMS, MMS, NMU and gamma rays separately and in combination, and their implications for meiotic behaviour. The present communication deals with the meiosis of a (partially) asynaptic mutant, which has some bearing on chromosomal pairing, exchange and orientation of univalents. Material and methods In a mutation experiment on C.juncea (2n = 16) several radiation doses and chemical mutagens in several concentrations, separately and in combina- Genetica 58, 65-70 (1982). 0016 6707.'82.581 0065..SI.20. © Dr W. Junk Publishers, The Hague. Pript,'d ;n The Netherlands.