Nuclear orientation in the ascospores of Neurospora tetrasperma.

OUR successive nuclear divisions occur during ascus development in Neurospora tetrasperma. The first two divisions are meiotic; the last two, mitotic. Previous cytological studies of ascus development have emphasized primarily the importance of the second and third divisions, during which rather elaborate mechanisms of nuclear movement provide that descendants of each of the two nuclei separated at the first division will be present in each of the four ascospores of the ascus (DODGE 1927; COLSON 1934; DODGE, SINGLETON and ROLNICK 1950; PINCHEIRA 1963). These mechanisms are important in providing the basis for the secondarily homothallic ascospores (i.e., heterokaryotic for mating type) which are characteristic of this species (SANSOME 1946). The present investigation concerns the fourth division and its consequences. It will be shown that this division, the only one of the four occurring after ascospore delimitation, may accomplish more than merely doubling the number of ascospore nuclei from two to four. Our studies of the nuclear composition of hyphal tips excised from both ends of germinating ascospores indicate that nuclei of opposite mating type consistently emerge into each tip. Our cytological observations suggest that a mechanism of nuclear movement occurs at the fourth division which may account for this high frequency of heterokaryotic hyphal tips, although our data cannot exclude certain alternative mechanisms. Nevertheless, it is clear that heterokaryosis for mating type in N . tetrasperma is present in ascospore germlings, rather than being dependent upon initiation by postgerminational hyphal fusions.