Nuclear fusion in multinucleated giant cells during the sexual development of Dictyostelium discoideum

Macrocyst development in Dictyostelium discoideum, is generally considered a sexual phase. This development is initiated by the formation of a giant cell, the result of the fusion of two different mating type haploid cells, such as NC4 and HM1. The giant cell engulfs unfused surrounding cells to develop into a macrocyst. Therefore, if the macrocyst is a sexual structure, the giant cell must be a diploid zygote. However, under certain conditions, a very large multinucleated giant cell containing several dozens of nuclei is formed, followed by normal development into a macrocyst. In such a multinucleated giant cell, it was found that only two nuclei fuse together to produce a diploid zygote and all others disappear at the early stage of development. The diploid nucleus undergoes meiosis and subsequently subdivides into a number of haploid progeny cells later released from the macrocyst to initiate new life cycles.