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Lecture 23: "Developmental Genetics II"

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Biology 2581B
Jim Karagiannis

Genetics Lecture No. 23: Developmental Genetics II - Drosophila rd Wednesday April 3 , 2013 Determining A Pathway: -In order to find how one gene affects the expression of another (determining a pathway), we can follow expression patterns. For example, in the imaginal discs (area which develops into wings) of Drosophila, the expression of two proteins, Wingless protein and Vestigial protein, determines either a wild-type or mutant phenotype. The expression of Vestigial protein, which is not produced in many cells in the wild- type, is affected by Wingless protein, which must be present in neighbouring cells for Vestigial protein to be expressed. We can also thus, determine pathways by analyzing double mutants for genes that could be in epistasis (the ability of the activity of one gene to mask the activity of another gene). Double Mutants: -Given mutant phenotypes for two genes A and B (aaBB & AAbb), there are different possibilities as to what the phenotype of an aabb double mutant will be? If gene A and B act independently, the double mutant could have mutant phenotypes for both genes A and B. If mutations in one gene are counteracted by mutations in a second suppressor gene, the double mutants would have a phenotype closer to the wild-type. If mutations in one gene are simultaneously present with mutations in another enhancer gene, the double mutant would have a more severe phenotype than either of the single mutants. If gene A is epistatic to gene B, then we would expect the double mutant to show only the phenotype for gene A, while gene B’s phenotype is obscured or masked. Epistasis In The Secretion Pathway: -The product of gene A helps lead red molecules into small, round vesicles and mutants of gene A do not load vesicles, leading to accumulation of molecules in the ER. The product of gene B allows vesicles to fuse with the cell membrane, causing secretion of the vesicles’ contents and mutants of gene B do not fuse their loaded vesicles with the cell membrane, leading to accumulation of molecules in vesicles. Since the double mutant cell shows a phenotype with unloaded vesicles and ER accumulation of molecules, we can say that mutations in gene A are epistatic to those in gene B (gene A acts before gene B). Much like in the signal transduction pathway for vulva formation, any mutation in the gene controlling a later step is epistatic to a mutation in a gene whose product acts earlier. Even though epistasis defines the order of gene interaction and is best observed in double mutants, it is necessary to know the type of mutation in the gene. Drosophila Development: -The first rounds of mitosis in the Drosophila embryo produce a syncytial blastoderm. Cell membranes then grow around the thousands of nuclei under the embryonic surface, forming the cellular blastoderm. Some of these cells invaginate toward the middle of the embryo to make a gastrula. Although segmentation is first visible only after gastrulation, the genes responsible for segmentation function even earlier in development. During the blastoderm stage, individual cells are visible at the periphery of the embryo, and the pole cells at the posterior end can be distinguished. During the gastrulation stage, some furrows start to form and by the segmentation stage it is clear that the embryo is subdivided into segments. The identities of embryonic segments are preserved through the larval stages and are also retained through metamorphosis into the adult. Early Development & Maternal Effect Genes: -In the embryo, mutations that affect early embryogenesis are mutations derived from the mother in maternal effect genes. There are two important groups of maternal effect genes: Genes required for normal anterior development and genes required for normal posterior development. Maternal effect genes have bizarre effects on the resultant phenotype of the offspring because they are so important to normal embryogenesis. If an embryo receives normal maternal effect proteins from a normal mother, it will grow into a normal phenotype even if it carries a homozygous genotype. If an embryo receives altered or no maternal effect proteins from a mutant mother, it will grow into a mutant phenotype even if it carries a normal heterozygous genotype. Anterior & Posterior Morphogens (Bicoid & Nanos): -Embryos from mothers homozygous for null alleles of the bicoid (bcd) gene lack all head and thoracic structures. The protein product of bicoid itsel
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