Polarized recombination and fine structure within the me-2 gene of Neurospora crassa.

HIS study of interallelic recombination was originally initiated to compare Tthe recombination and complementation maps of the me-2 gene of Neurospora crassa (MURRAY 1960b). Methionine-independent progeny from crosses between me-2 alleles were classified with respect to the markers which were present on both sides of the me-2 gene. One of the two classes of methionine prototrophs having markers recombined occurred in excess of the other, and when the markers entered the cross in the opposite phase, a similar excess was found in the reciprocal class. In this respect the results resembled those obtained for the pan-2 gene (CASE and GILES 1958), and on the basis of these asymmetries the me-2 alleles were ordered linearly. The order determined agreed with that based on additive prototroph frequencies. In another respect, however, the results differed strikingly from most studies of intragenic recombination such as those of CASE and GILES (1958), FREESE (1957) , and ST. LAWRENCE (1956) ; pronounced differences were observed in the numbers of the two parentally marked classes of prototrophs. These asymmetries were reversed when the markers entered the cross in the opposite phase. On the assumption that recombination was the result of both reciprocal crossing over and conversion, the asymmetries between the two classes of parentally marked prototrophs were at first described in terms of possible differences in conversion frequency between the me-2 alleles (MURRAY 1960b); this interpretation is disproved by the present data. The low frequency of recombination between me-2 alleles has prevented any investigation involving tetrads, and therefore no distinction has been possible between reciprocal and nonreciprocal recombinations. Other studies have shown intragenic recombination in Neurospora to be primarily nonreciprocal ( MITCHELL 1955; CASE and GILES 1958; STADLER 1959a; SUYAMA, MUNKRES and WOODWARD 1959). In spite of the lack of information from tetrads, further investigation of recombination within the me-2 gene seemed desirable, because the data suggested a polarization of intragenic recombination (MURRAY 1961 ) . Polarized recombination has been reported for Aspergillus nidulans ( SIDDIQI 961, 1962) and, earlier, on the basis of a different criterion, it was demonstrated for the ASCOmycete Ascobolus immersus ( LISSOUBA and RIZET 1960) .