Cytological Studies of Ascus Development in Cochliobolus Sativus

Conjugate divisions occurred in the binucleate crozier of Cochliobolus sativus (Ito & Kurib. ex Kurib.) Drechs. ex Dast. Two cross walls were laid down in such a way that the penultimate cell was binucleate. A lateral bulge developed in this cell and it quickly assumed the clavate shape of the young ascus. The two nuclei in the clavate ascus fused and this was accompanied by a fusion of their nucleoli. In meiosis two divisions occurred with pairing between highly contracted chromosomes early in the first division. A third division (mitotic) followed and resulted in the formation of eight haploid nuclei. Ascospore delimitation began at this stage and each spore was initially uninucleate. Evidence of structural hybridity was seen in all divisions in the ascus. The haploid chromosome complement, as determined from pachytene and metaphase stages, consisted of seven or eight chromosomes.