A key decision in the life of a diploid yeast cell is whether to undergo meiosis and form spores. Sporulation is induced by starvation. Glucose and nitrogen are inhibitors of sporulation, and acetate (a “poor” carbon source) activates the program (65
). Commitment to meiosis was first studied using return-to-growth (RTG) protocols in which cells that had been transferred to sporulation medium (acetate) were transferred back to rich growth medium (yeast extract-peptone-dextrose [YPD]) at various times. Rather than continue through meiosis, these cells reentered the mitotic cell cycle. These studies demonstrated that cells must be exposed to the inducing signals for sporulation for a defined interval before completion of the program will take place (35
). Cells that had entered the program and completed meiotic DNA replication could efficiently return to growth. Even more surprising, cells that were undergoing meiotic recombination could return to growth. The ability to return to growth declines dramatically around the time that cells exit meiotic prophase and enter the first meiotic division (MI). More recent microscopic studies of live cells showed that cells that had completed MI prior to transfer to rich medium formed spores, while cells that had not yet completed MI exited the meiotic program and resumed vegetative growth (120
Meiotically induced cells that were transferred to water at time points when rich medium caused RTG efficiently completed meiosis and spore formation (161
). These studies led to the concept of “readiness,” which occurs prior to meiotic S phase. In addition, a stage at which transfer to rich medium blocked sporulation but did not permit the resumption of mitotic growth (termed partial commitment) was identified. Thus, inhibitory signals (glucose and nitrogen) and activating signals (acetate) differentially control passage through meiotic development, and cells at different stages of meiotic development respond to these signals differently (reviewed in reference 160
). These studies led to a model in which cells transit through a series of steps: first “readiness,” then “partial commitment,” and finally “full commitment”. In this article, the point in meiotic development after which cells complete meiosis and form spores even when transferred to rich medium (full commitment) will be referred to as the meiotic commitment point.
The RTG studies described above demonstrate that commitment to meiotic development takes place in prophase. Meiotic prophase has been divided into stages based on the microscopic appearance of chromosomes (199
). In leptotene, lateral elements of the synaptonemal complex (SC) are observed. During leptotene, homolog coalignment takes place, the Spo11 endonuclease initiates recombination by introducing double-strand breaks (DSBs) into the genome (81
), and these DSBs are further processed into single-stranded nucleoprotein filaments that contain the Rad51 and Dmc1 strand exchange proteins (11
). The next cytological stage of prophase is zygotene, when central regions of the SC, which connect homologs, appear. During this stage, DSBs are processed into either nonrecombinants or joint-molecule (JM) intermediates. Pachytene is defined as the stage when homologous chromosomes are fully connected by continuous tripartite SCs. At this stage, JMs that contain double Holliday junctions have formed but have not yet been resolved, and spindle pole bodies (SPBs) (the yeast equivalent of centrosomes) have duplicated but not yet separated (18
). SC disassembly is the cytological feature that defines exit from pachytene. In yeast, pachytene exit is when JMs are resolved as crossovers (4
) and when the duplicated SPBs separate to form the MI spindle (18
). In organisms with large genomes, SCs can be gradually disassembled during diplotene, and further condensation of chromatids occurs during diakinesis. However, in Saccharomyces cerevisiae
, SC disassembly is rapid, diplotene is not apparent, and the later changes in condensation are subtle (33
). While diplotene can be a significant stage of regulation in other organisms (in mammals, primary oocytes are held in the hormonally regulated diplotene stage [dictyate arrest] for decades), pathways that specifically delay prophase progression after pachytene have not been identified in yeast. As described below, exit from pachytene is the key regulatory transition that regulates progression from meiotic prophase in yeast.
Molecular/genetic studies of meiotic mutants demonstrate that cells blocked at pachytene can efficiently return to growth (159
). Ndt80 is the transcription factor that drives exit from pachytene (discussed in more detail below), and the RTG response of the ndt80
Δ mutant has been well studied (31
). In pachytene-arrested ndt80
Δ cells that have been exposed to rich medium, the SC rapidly disappears and chromatids are segregated in a mitosis-like division (31
). While JMs are resolved in the meiotic pathway mainly as crossovers, a distinct pathway that minimizes crossover formation processes JMs during the RTG response, thus maximizing heterozygosity in RTG diploids. Cells in pachytene are therefore able to modify the meiotic recombination pathway in response to nutritional signals to generate outcomes that are beneficial to the vegetative cell. The ability of cells in pachytene to mount a highly regulated RTG response and the inability of cells that have entered the meiotic divisions to return to growth indicate that pachytene exit is closely associated with meiotic commitment.
A temperature-sensitive mutation in CDK1
), which encodes the cell cycle-regulatory cyclin-dependent kinase (CDK), blocks meiosis in pachytene (159
). More recent studies with a mutant form of Cdk1 that is sensitive to cell-permeative ATP analogs (Cdk1-as1) show that exit from pachytene is especially sensitive to Cdk1 inhibition (10
). Exit from pachytene is prevented by a checkpoint pathway that is activated in response to persistent recombination intermediates (100
). This pathway, termed the recombination checkpoint or pachytene checkpoint (see below), inhibits Cdk1 through the Swe1 protein kinase (which downregulates Cdk1 by phosphorylating a residue near the Cdk1 ATP-binding pocket) (90
). These observations indicate that commitment is tightly connected to cyclin-dependent kinase.
While wild-type cells that have entered MI do not normally return to growth, there are certain circumstances in which postmeiotic RTG can take place. This was first described in studies of SPO14
, which regulates formation of the prospore membrane, a double membranous structure that envelopes haploids following the completion of MII (121
). In early studies it was shown that spo14
Δ-blocked cells could return to growth at postmeiotic stages in meiotic development (62
). While this observation led to the suggestion that SPO14
is a specific regulator of commitment, it was subsequently shown that blocking prospore membrane closure using an SSP1
) mutant or by temperature upshift of wild-type cells also allowed postmeiotic cells to return to growth (63
). Notably, the RTG that occurs in postmeiotic spo14
, or temperature-upshifted cells takes substantially longer than RTG in precommitment cells. These studies suggest that postmeiotic cells do not normally return to growth because a cellular state has been established in these cells that kinetically favors completion of sporulation and not because mitotic growth has been irreversibly inhibited.