Ploidy variation in fungi: Polyploidy, aneuploidy, and genome evolution

Cellular ploidy is the number of complete sets of chromosomes in a cell. Many eukaryotic species have two (diploid) or more than two (polyploid) sets of chromosomes (1). These diploid and polyploid states are often the result of ancient whole-genome duplication (WGD) or hybridization events that occurred throughout the evolution of plants, animals, and fungi (2 - 4). Ploidy changes also occur during the development of many organisms and can vary within different tissues of the same organism and between individuals of the same species. For example, ploidy changes occur during the sexual cycle of eukaryotes, from haploid gametes to diploid somatic cells. Additionally, some cells continue to increase in ploidy during development, resulting in somatic tissues that have a mixture of diploid and polyploid cells, including human hepatocytes and megakaryocytes (5 - 7). These ongoing, developmentally programmed changes in ploidy are important for viability and are beneficial to many organisms (8), but the mechanisms controlling ploidy and the physiological significance of each ploidy level are not well characterized.