رویان‌زایی مگس سرکه
الگوی آسه پیشین - پسین در مگس سرکه > Additional segmentation genes

Two additional classes of segmentation genes are expressed after the gap gene products. The pair-rule genes are expressed in striped patterns of seven bands perpendicular to the anterior-posterior axis. These patterns of expression are established within the syncytial blastoderm. After these initial patterning events, cell membranes form around the nuclei of the syncytial blastoderm converting it to a cellular blastoderm.
The expression patterns of the final class of segmentation genes, the segment polarity genes, are then fine-tuned by interactions between the cells of adjacent parasegments with genes such as engrailed. The Engrailed protein is a transcription factor that is expressed in one row of cells at the edge of each parasegment. This expression pattern is initiated by the pair-rule genes (like even-skipped) that code for transcription factors that regulate the engrailed gene's transcription in the syncytial blastoderm.
Cells that make Engrailed can make the cell-to-cell signaling protein Hedgehog. The motion of Hedgehog is limited by its lipid modification, and so Hedgehog activates a thin stripe of cells anterior to the Engrailed-expressing cells. Only cells to one side of the Engrailed-expressing cells are competent to respond to Hedgehog because they express the receptor protein Patched. Cells with activated Patched receptor make the Wingless protein. Wingless is a secreted protein that acts on the adjacent rows of cells by activating its cell surface receptor, Frizzled.
Wingless acts on Engrailed-expressing cells to stabilize Engrailed expression after the cellular blastoderm forms. The Naked cuticle protein is induced by Wingless to limit the number of rows of cells that express Engrailed. The short-range, reciprocal signaling by Hedgehog and Wingless, held in check by the Patched and Naked proteins, stabilizes the boundary between each segment. The Wingless protein is called "wingless" because of the phenotype of some wingless mutants. Wingless and Hedgehog also function in multiple tissues later in embryogenesis and also during metamorphosis.
The transcription factors that are coded for by segmentation genes regulate yet another family of developmental control genes, the homeotic selector genes. These genes exist in two ordered groups on Drosophila chromosome 3. The order of the genes on the chromosome reflects the order that they are expressed along the anterior-posterior axis of the developing embryo. The Antennapedia group of homeotic selector genes includes labial, antennapedia, sex combs reduced, deformed, and proboscipedia. Labial and Deformed proteins are expressed in head segments where they activate the genes that define head features. Sex-combs-reduced and Antennapedia specify the properties of thoracic segments. The bithorax group of homeotic selector genes control the specializations of the third thoracic segment and the abdominal segments. Mutations in some homeotic genes can often be lethal and the cycle of life will end at embryogenesis.
In 1995, the Nobel Prize for Physiology or Medicine was awarded for studies concerning the genetic control of early embryonic development to Christiane Nüsslein-Volhard, Edward B. Lewis and Eric Wieschaus. Their research on genetic screening for embryo patterning mutants revealed the role played in early embryologic development by homeobox genes like bicoid. An example of a homeotic mutation is the so-called Antennapedia mutation. In Drosophila, antennae and legs are created by the same basic "program", they only differ in a single transcription factor. If this transcription factor is damaged, the fly grows legs on its head instead of antennae. See images of this "antennapedia" mutant and others, at FlyBase. Another example is in the bithorax complex. If nonlethal mutations occur in this complex, it can cause the fly to have two sets of wings, instead of one pair of wings and one pair of halteres, which aid in balance in flight.