Chapter 22: Genetic Control of Animal Development

Early development is controlled by maternal-effect genes that load the egg with crucial RNAs and proteins, establishing the fundamental body axes. For instance, the dorsal–ventral axis is determined by the Dorsal transcription factor, whose activation relies on the interaction between the Toll receptor and the Spätzle ligand on the ventral surface, leading to the differentiation of mesoderm. Simultaneously, the anterior–posterior axis is defined by opposing concentration gradients of morphogens like Bicoid (anterior determination) and Nanos (posterior determination), which regulate the translation of transcripts like caudal and hunchback. Subsequently, zygotic gene activity takes over, initiating a segmentation cascade beginning with gap genes, followed by pair-rule genes (like fushi tarazu that define 14 parasegments), and finally, segment-polarity genes. The ultimate identity of these segments is determined by homeotic genes (selector genes) found in complexes such as ANT-C and BX-C. Organ formation is discussed using the example of the master regulatory gene eyeless, which is highly homologous to the mammalian Pax6 gene, demonstrating conserved pathways across phyla. Cellular differentiation, such as the specification of the R7 photoreceptor in the Drosophila eye, highlights the necessity of cell-cell interactions where a ligand (BOSS protein) signals a receptor (SEV protein) to initiate a specialized genetic program. The principles derived from invertebrates are extended to vertebrates through the identification of homologous Hox genes and the use of genetic techniques like knockout mutations in mice. Finally, the chapter addresses the role and ethics of stem cells (embryonic and adult) in regenerative medicine and contrasts therapeutic cloning with reproductive cloning. It concludes by detailing a special case of differentiation in the immune system, where a vast array of antibodies is generated not by possessing millions of genes, but by the physical rearrangement and somatic recombination of gene segments (LκVκ, Jκ, Cκ) within differentiating B cells.