Studying Segmentation Mutants in Balanced Stocks

Drosophila Development  Each egg is surrounded by a chorion.  The anterior end has two filaments to allow oxygen to enter the cell.  Sperm enter through the micropyle at the anterior end.

Early Drosophila Development  It takes 1 day for the embryo to develop into a larva.  The larva hatches, feeds, and sheds its skin twice.  After 5 days, the larva becomes immobile and forms a pupa.  During the pupal stage, cells in the imaginal discs differentiate into adult structures.

Maternal Gene Activity in Development Materials transported into the egg during oogenesis play a major role in embryonic development.

Maternal-Effect Genes  Maternal-effect genes contribute to the formation of healthy eggs; effects of mutations in these genes may not affect the phenotype of the female making the eggs but may be seen in the next generation.  A maternal-effect mutation causes a mutant phenotype in the offspring of a female with a mutant genotype.

The dorsal Gene: Offspring of dl/dl Females are Dorsalized and Inviable

Segmentation Genes  Segmentation genes are required for segmentation along the anterior-posterior axis.  They are classified into three groups based on embryonic mutant phenotypes. –Gap genes –Pair-rule genes –Segment-polarity genes

Gap Genes  Gap genes define segmental regions in the embryo.  Mutations in the gap genes cause a set of contiguous body segments to be missing.  Four gap genes have been well characterized: Krüppel, giant, hunchback, and knirps.  Gap gene expression is controlled by bicoid and nanos.  The gap genes encode transcription factors.

Pair-Rule Genes  Pair-rule genes define a pattern of segments within the embryo.  Pair-rule genes are regulated by the gap genes and are expressed in seven alternating bands, dividing the embryo into 14 parasegments along the anterior- posterior axis.  In pair-rule mutants, every other parasegment is missing.  The pair-rule genes encode transcription factors.

Expression of fushi tarazu (ftz) in a Drosophila Blastoderm Embryo

Segment-Polarity Genes  Segment-polarity genes define the anterior and posterior compartments of individual segments.  Mutations in segment-polarity genes cause part of each segment to be replaced by a mirror-image copy of an adjoining half-segment.  Segment-polarity genes refine the segmental pattern established by the pair-rule genes.  These genes encode transcription factors and signaling molecules.

Segmentation Gene Mutants

Chapter 21 The Genetic Control of Animal Development

Sex Determination in Drosophila and C. elegans  The sex determination signal in both animals is the ratio of X chromosomes to autosomes. If the ratio is 1.0 or greater, the animal is a female; if the ratio is 0.5 or less, the animal is a male.CLASSIC Definition  But wrong  In Drosophila, the key genes in sex determination encode proteins that regulate RNA processing.

Sex Determination in Drosophila  Components of the sex-determination pathway include –A system to ascertain the X:A ratio, –A system to covert this ratio into a developmental signal, and –A system to respond to this signal by producing either male or female structures.

Ascertaining the X:A Ratio  The system that ascertains the X:A ratio involves interactions between maternally synthesized proteins in the egg cytoplasm and embryonically synthesized proteins encoded by several X-linked genes.  The X-linked gene products are called numerator elements and are twice as abundant in XX embryos as in XY embryos.  The autosomal gene products are called denominator elements and antagonize the products of the numerator elements.

The Sex-lethal (Sxl) Gene  Sxl is the mater regular of the sex determination pathway in Drosophila.  The X:A ratio is converted into a molecular signal that controls the expression of the X-linked Sxl gene.

Function of SXL  SXL regulates splicing of its own transcript to maintain SXL protein expression in XX embryos.  SXL also regulates splicing of the transformer (tra) gene.

Differentiating in Response to the Signal  TRA, along with TRA2, regulate splicing of doublesex (dsx) and fruitless (fru).  In XX embryos, where TRA is present, dsx transcripts are processed to encode a DSX protein that represses the genes for male development.  In XY embryos, where TRA is absent, dsx transcripts are processed to encode a DSX protein that represses the genes for female development.

Fruitless (fru)  Males homozygous for the fru mutation court other males.  The fru gene encodes a zinc- finger transcription factor that regulates the genes for male sexual behavior.

Loss-of-Function Mutations in Sex- Determination Genes in Drosophila  Mutations in Sxl prevent SXL protein from being made in males; homozygous mutants would develop into males but die as embryos.  Mutations in transformer and transformer2 cause both XX and XY animals to develop into males.  Mutations in dsx cause both XX and XY embryos to develop into intersexes.

Key Points  In Drosophila the pathway that controls sexual differentiation involves some genes that ascertain the X:A ratio, some that convert this ratio into a developmental signal, and others that respond to the signal by producing either male or female structures.  The Sex-lethal (Sxl) gene plays a key role in Drosophila sexual development by regulating the splicing of its own transcript and that of another gene (tra).