Meiosis is the developmental program by which sexually reproducing diploid
organisms generate haploid gametes, In yeast, meiosis is followed by spore
morphogenesis. These two events are normally coordinated in such a way that
spore formation is dependent upon completion of the meiotic nuclear divisi
ons. Here we describe a meiosis-specific protein, mfr1, that is involved in
this coordination. mfr1 is an activator of the anaphase-promoting complex
(APC), which is necessary for the rapid degradation of the cdc13 cyclin at
the end of meiosis II, prior to the formation of spores. An mfr1 null mutan
t completes meiosis II but remains with high levels of cdc13 and cdc2 kinas
e activity and has considerably delayed spore formation. By analogy with th
e mitotic cell cycle, where proteolysis and inactivation of cdc2 kinase are
necessary to trigger mitotic exit and cytokinesis, we propose that at the
end of meiosis rapid and timely proteolysis of cyclins is required to switc
h on the differentiation program that eventually leads to the formation of
haploid gametes.