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 Program Rationale:
Meiosis plays a central role in the sexual reproduction
of nearly all eukaryotes. The major genetic events that occur during its
two cell divisions are critical for generating genetic diversity and
producing offspring with normal chromosome numbers. The long-range
objective of our research program is to understand the genetic mechanisms
that govern meiotic development and coordinate a complex series of events
into a successful developmental pathway. Our approach is to genetically
identify and determine the structure, function, and regulation of selected
meiosis-specific genes required for recombination and chromosome
segregation, and to use these genes to uncover critical regulatory
functions that specify the orderly progression of meiotic events. Of
particular interest are the relationship between meiotic and mitotic cell
division controls, and the extent to which they interact. The unicellular
eukaryote, Saccharomyces cerevisiae, is utilized as a model
system. Budding yeast was initially chosen for this analysis because
features of its life cycle facilitate the isolation of meiotic mutants,
and the process can be simply triggered and readily examined by change of
nutritional conditions. In addition, its well-developed genetics and
molecular manipulability offer powerful means to investigate specific gene
functions. In our earlier efforts, genetic strategies were developed that
led to the detection of ~25 SPO genes among ~200 genes estimated to be
essential for proper meiotic development but not for mitotic
growth. Specific genes required for recombination (SPO11), chromosome
segregation (SPO1, SPO12 and SPO13), commitment to the meiotic divisions
and spore formation (SPO3, SPO14 and SPO73), and regulators of the mitotic
repression and meiotic induction of these loci (UME1-7), were subsequently
cloned and characterized. Analyses of their gene products and functions in
meiosis constitute the primary focus of our research program.Aims: Our current experimental plan uses these genes to address three broad questions regarding gametogenesis in yeast. First, how is the meiotic expression program regulated? Second, how are the stages of the meiotic nuclear divisions controlled? Third, how are meiotic events coordinated with packaging of meiotic products into spores? The experimental focus of the research is on 1) continuing the identification of essential meiosis-specific genes through genetic screens, DNA array detection of meiosis-specific gene expression, and related deletion studies, 2) analysis of the specific functions controlling MI chromosome segregation, particularly how SPO12 and SPO13 regulate centromere cohesion and sister chromatid separation at MI, 3) molecular and genetic characterization of key positive and negative regulators controlling meiosis-specific transcription, primarily how UME6 is converted from a repressor to an activator of meiotic gene expression, and how the transition from early to middle gene expression occurs, 4) the pathways by which SPO1 and other meiosis-specific phospholipases (SPO14) act to regulate SPB morphogenesis and coordinate the nuclear divisions with spore formation, and 5) the genetic basis of irreversible commitment to meiosis. Significance: Meiosis may be thought of as a process of cellular development in which a change of cell type occurs by alteration of the pattern of cell division. The stages and transition periods in the two successive meiotic divisions are exquisitely regulated and differ significantly in timing and chromosome behavior from the corresponding stages of mitosis. The first division deviates most dramatically from mitotic division in its extended DNA S phase and prolonged prophase. The coordinated events of homologous pairing, relatively high levels of genetic recombination and separation of homologs, which uniquely occur in this division, are of fundamental importance in generating diversity and in accurate transmission of genetic material during sexual reproduction. In contrast to the first meiotic division, the second division, in which sister chromatids separate, has a shortened G1, lacks an S phase, and is otherwise similar to mitosis, utilizing many of the same gene products. These dramatic changes in chromosome behavior during meiosis offer a fascinating look at the regulatory mechanisms that control both cell division and cell differentiation. Analysis of these mechanisms, and the factors essential for distribution of chromosomes in meiosis, should contribute significantly to understanding of cell division control, malignancy, and reproductive diseases associated with genomic instability and abnormal chromosome transmission. |