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Chapter13_SSM - 65781_CH13_259_274.qxd 1:19 PM Page 259...

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259 Chapter 13: Genetic Control of Development Chapter Summary The genotype determines the developmental potential of the embryo. By means of a developmental program that results in different sets of genes being expressed in different types of cells, the genotype controls the developmental events that take place and their temporal order. Mutations that interrupt developmental processes identify genetic factors that control development. The early development of the animal embryo establishes the basic developmental plan for the whole organ- ism. The earliest events in embryonic development depend on the correct spatial organization of numerous constituents present in the oocyte. Developmental genes that are needed in the mother for proper oocyte formation and zygotic development are maternal-effect genes. Genes that are required in the zygote nucleus are zygotic genes. Fertilization of the oocyte initiates a series of mitotic cleavage divisions that form the “hollow ball” blastula, which rapidly undergoes a restructuring into the gastrula. Accompanying these early morphological events is a series of molecular events that determine the developmental fates that cells undergo. Execution of a developmental state may be autonomous (genetically programmed), or it may require positional information supplied by neighboring cells or the local concentration of one or more morphogens. The soil nematode Caenorhabditis elegans is used widely in studies of cell lineages because many lineages in the organism undergo virtually autonomous development and the developmental program is identical from one organism to the next. Most lineages are affected by many genes, including genes that control the sublineages into which the lineage can differentiate. Mutations that affect cell lineages define several types of developmental mutants including apoptosis (programmed cell death) mutants, in which cells fail to undergo a normal developmental program that normally leads to their death. Genes that control key points in development can often be identified by the unusual feature that loss-of- function mutations (usually recessive) and gain-of-function mutations (usually dominant) have opposite effects on phenotype. For example, if loss of function results in failure to execute a developmental program
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260 in a particular anatomical position, then gain of function should result in execution of the program in an abnormal location. The order of gene function in a linear switch-regulation pathway can often be deter- mined from double mutants using the principle that the epistatic gene in a double mutant acts downstream of the hypostatic gene. Early development in Drosophila includes the formation of a syncytial blastoderm by early cleavage divi- sions without cytoplasmic division, the setting apart of pole cells that form the germ line at the posterior of the embryo, the migration of most nuclei to the periphery of the syncytial blastoderm, cellularization to
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