Evidence from Sordaria that recombination and conversion frequencies are partly determined before meiosis, and for a general model of the control of recombination frequencies.

TUDIES of the control of recombination frequencies (abbreviated “RFs” ) have several aims: to determine how genetic and environmental controls of RFs operate, to provide information on the mechanism of recombination, and to find way of manipulating RFs to suit breeding programmes. The present study concerns details of the control system and the time, or times, in the meiotic cycle at which an environmental factor, temperature, affects RFs. The meiotic stages at which temperature is reported to affect recombination or chiasma frequencies lare: leptotene or later (Sphaerocarpus-hEL 1965) , zygotene to pachytene (Chlamydomonas-LAWRENCE 1965) and zygotene (SchistoCerca-HENDERsoN 1966). These stages are after meiotic DNA duplication and are consistent with the time of crossing over proposed in hybrid-DNA models ( WHITEHOUSE 1963; HOLLIDAY 1964). In contrast, GRELL and CHANDLEY (Drosophila-1965) and LEVINE (Chlamydomonas-1956) reported that the time of temperature effect on recombination was close to that of meiotic DNA duplication. The latter findings, which are consistent with copy-choice models of recombination (LEDERBERG 1955), involve the assumption that the time at which an organism is sensitive to temperature treatments affecting RFs is actually the time at which recombination occurs. This assumption has been tested here in Sordaria fimicola.