Studying Segmentation Mutants in Balanced Stocks

1. Studying Segmentation Mutants in Balanced Stocks

2. 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.

5. 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.

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

7. 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.

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

9. 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

10. 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.

11. 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.

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

13. 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.

15. Segmentation Gene Mutants

16. Chapter 21The Genetic Control of Animal Development

17. 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.

18. 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.

19. 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.

21. 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.

23. 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.

25. 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.

26. 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.

27. 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.

29. 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).