Crossing-over and interference in a multiply marked chromosome arm of Neurospora.

HE experiments reported here employ Neurospora tetrads for a type of invesT i g a t ion earlier carried out with multiply marked attached-X half-tetrads of Drosophila by EMERSON and BEADLE (1933), BEADLE and EMERSON (1935), BONNIER and NORDENSKIOLD (1937) and WELSHONS (1955), within the theoretical framework developed by such workers as E. G. ANDERSON, C. B. BRIDGES, and A. WEINSTEIN (see WEINSTEIN 1936, 1958). Combinations of six or more markers have been used that enable the detection and characterization of nearly all exchanges occurring within a segment 74 units long in one chromosome arm. The results provide information on the main features of normal meiotic crossingover and demonstrate a close resemblance between crossing-over in Neurospora and in Drosophila. The previous major studies of crossing-over with Neurospora tetrads have employed as many as four linked markers distributed over a shorter portion of the genome, either in regions spanning the centromere ( LINDEGREN and LINDEGREN 1942; HOWE 1956; STADLER 1956) or within arms (STRICKLAND 1961). These have agreed in showing that crossing-over normally occurs reciprocally between chromatids at a 4-strand stage, that 4-strand double exchanges are rare within short intervals, and that all four chromatids are involved in successive exchanges. They have been inconsistent with regard to chromatid interference, which is apparently mariifested in some experiments but not in others (STRICKLAND 1961; EMERSON 1962 review). Inasmuch as double crossing-over in Neurospora is rare within intervals as long as 20 units (except across a centromere), extensive data on multiple exchanges in short adjacent intervals have been difficult to obtain. STRICKLAND, for example, had to analyze over 10,000 tetrads from his four-point crosses in order to obtain 140 double exchanges between intervals totalling 20 units. One way in which efficiency can be increased is by using intervals across the centromere, where chiasma interference does not obtain (HOWE 1956; STADLER 1956). If this is done, and the centromere itself is used as a marker by collecting asci as ordered tetrads, errors may arise from such causes as spindle overlap. These errors can be controlled to some extent by employing markers close to unlinked centromeres, as shown by HOWE and by STADLER. Additional information can be obtained, how-