Yeast maybe homothallic or heterothallic. Sexual reproduction is a simple process of conjugation of (1) two somatic cells (ii) two haploid ascospores or (iii) an ascospore and a haploid vegetative cell.
During conjugation two haploid copulating cells called gametes unite by the development of short protuberances through beak-like conjugation tubes. The conjugation tubes widen and the nucleus of each gamete migrates into conjugation tube.
The separating walls of the conjugation tube dissolves and their two nuclei fuse to form diploid zygotic nucleus. The zygote develops into an ascus. It divides thrice, first one being meiotic division followed by two mitotic divisions resulting in 8 nuclei, all of which take part in ascospore formation. Ascospores are finally liberated by bursting the ascus wall. Each mature ascospore form yeast cells.