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).