Mps1p regulates meiotic spindle pole body duplication in addition to having novel roles during sporulation

Sporulation in yeast requires that a modified form of chromosome segregatio n be coupled to the development of a specialized cell type, a process akin to gametogenesis. Mps1p is a dual-specificity protein kinase essential for spindle pole body (SPB) duplication and required for the spindle assembly c heckpoint in mitotically dividing cells. Four conditional mutant alleles of MPS1 disrupt sporulation, producing two distinct phenotypic classes. Class I alleles of mps1 prevent SPB duplication at the restrictive temperature w ithout affecting premeiotic DNA synthesis and recombination. Class II MPS1 alleles progress through both meiotic divisions in 30-50% of the population , but the asci are incapable of forming mature spores. Although mutations i n many other genes block spore wall formation, the cells produce viable hap loid progeny, whereas mps1 class II spores are unable to germinate. We have used fluorescently marked chromosomes to demonstrate that mps1 mutant cell s have a dramatically increased frequency of chromosome missegregation, sug gesting that loss of viability is due to a defect in spindle function. Over all, our cytological data suggest that MPS1 is required for meiotic SPB dup lication, chromosome segregation, and spore wall formation.