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

Excision of Intron Sequences

Splicing  Removal of introns must be very precise.  Conserved sequences for removal of the introns of nuclear mRNA genes are minimal. –Dinucleotide sequences at the 5’ and 3’ ends of introns. –An A residue about 30 nucleotides upstream from the 3’ splice site is needed for lariat formation.

Types of Intron Excision  The introns of tRNA precursors are excised by precise endonucleolytic cleavage and ligation reactions catalyzed by special splicing endonuclease and ligase activities.  The introns of nuclear pre-mRNA (hnRNA) transcripts are spliced out in two-step reactions carried out by spliceosomes.

The Spliceosome  Five snRNAs: U1, U2, U4, U5, and U6  Some snRNAs associate with proteins to form snRNAs (small nuclear ribonucleoproteins)

What are Logo plots?

Logo for a) Splice acceptor b) Splice Donor c) Initiator Met

AG/GTCAG/NT exon 1 intron 1 exon 2

KEY SLIDE…Know it.

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. NOT CORRECT..from book

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.

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).