Genetics Lecture No. 23: Developmental Genetics II - Drosophila
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).
-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.
-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