Studying segmentation mutants in balanced stocks
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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.

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Studying Segmentation Mutants in Balanced Stocks

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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 21The 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).


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