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[(3)H]RNA with a high specific activity was prepared from larvae of Drosophila melanogaster grown 4 days in contact with [(3)H]uridine. Purified tritiated 5S RNA was annealed to the DNA of polytene chromosomes, which had been denatured in formamide. The 5S RNA genes are placed within the region 56E-F of the right arm of chromosome 2. This localization was determined from autoradiographs, where the radioactivity from hybrids of [(3)H]RNA and DNA was confined to the 56E-F segment.

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rasshoppcrs \rere given an acute dose of X-rays, injected with Ht b n~i d i n e and thereafter lnaintained at 42°C. Autoradiographic analysis of testicular tubules taken at daily intervals after injection gave meiotic stage durations. T h e mean chiasll~a frequency n.as reduced 4 days after the hoppers \vere placed at +lo, in both X-rayed and non-irradiated aninials, and remained low in the non-irradiated animals throughout the experiment. In the X-rayed animals the chiasma frequency rose to control levels on the 6th day and dropped again thereafter. Analysis of the data indicates a period ending approxi~nately at zygotene during 15-hich chiasma frequc~lcy can be reduced by high temperature. T h e chiasma frequency can be increased by irradiating leptotene-zygotene. T h e end of the heat and X-ray sensitive stages are ternporally d~s t~n c t . Univalents \ \ w e never produced by the hear treatment. Changes in chiasma frequency are restrictetl to the four pairs of larger chromoson~es where multiple chiasniata are com~nonly found. Hence, the presence o r abscncc of chiasniata sho\vs a con~ples interaction \vith the physical environment.Little is known about thc mechanism of gcnctic crossing-over in higher organisms. In fact, thcre is e\.en disagrecmcnt as to thc time during ~nciosis when crossing-over occurs (Grcll and Chandlcy, 1965;Henderson, 1966). Since chiasma formation and gcnetic recombination are usually considered to be manifestations of the same cvcnt (Brov n and %ohar\r, 1955), an understanding of chiasma formation should clucidatc the problcn; of crossing-otrcr.Production of chiasmata is affected b y both y-:111d X-radiation (La~vrcnce, 196 la, 1961 b; i\lathcr, 1934; Westerman, 1967). At lcast thrcc radioscnsitive periods affecting chiasma formation occur during meiosis and one during premeiotic mitoses. 1,awrence (1961) demonstrnted a decrcasc in chiasma frequency whcn ./-radiation was applied during the prc-inciotic D N A synthetic pcriod and an increase when the treatment was given during latc zytotcne or early pachytene in both Liliz~7/1 1 0 1 1 g i f l o~~1~~1 and Tmdescn/lti;l pnl[~dosn. Similar results werc obtained by Westerman ( 1 9 6 7 ) using X-radiation on thc dcscrt locust, Schistoce~ca greynr-in, with the exception that the increase occurrcd when the trcatmcnt nlas given during leptotcnc-zygotenc. ,\/lather (1934) and Wcsterman (1967) also notcd a significant increasc in chi:islna frcqucncy \vhen X-radiation was applied during pre-mciotic mitoses.Heat shock also affccts chiasma production in S. ,yrega~in (tlenderson, 1962, 1963, 1966) and in a grasshopper, Go?linen nzlstrnlinsiae (Peacock, 1968).A drastic decreasc in chiasma frequency occurs when cither S. gre,qar-in or G. azcstrnliasiae is subjected to a temperat;rc of 4 0°C . 'The heat-sensitive period appears to be latc ygotenc-early pachvtenc.