Summary

1. The evolution of divergent species is driven primarily by changes in later development, so phylogenetically closely related species appear very similar to one another as embryos. Thus the embryos of all vertebrates appear rather similar to one another and share many stages of development and genetic mechanisms to guide development. See Figure 2.1
2. In Drosophila, maternal factors result in the establishment of a basic anterior-posterior axis, with bicoid concentrated at the head end of the syncytium, and nanos concentrated at the tail end. See Figures 2.3 and 2.4
3. Placing bicoid mRNA in various parts of the embryo will result in the formation of head structures at those positions. See Figure 2.5
4. Maternal polarity genes are transcription factors that organize anterior-posterior gradients in several other transcription factors within the syncytium. See Figure 2.6
5. Maternal polarity genes regulate the expression of another set of transcription factors, the gap genes, such as hunchback. Because maternal polarity genes often antagonize each other’s effects, the resulting spatial distribution of gap gene products tends to be more sharply defined than the distribution of maternal polarity genes. See Figures 2.7 and 2.8
6. Gap genes in turn regulate the expression of transcription factors called pair-rule genes. Each of the pair-rule genes is expressed in alternating stripes along the anterior-posterior axis. See Figure 2.9
7. Pair-rule genes regulate the expression the segment polarity genes, which encode cell-cell signaling systems to regulate gene expression as cellularization of the embryo takes place. Because each of the segment polarity genes is expressed in a stripe about one segment in width, each of the newly separated cells has been exposed to a sequence of transcription factors that is unique for the particular segment where it resides. See Figure 2.10
8. Gap genes, pair-rule genes, and segment polarity genes all regulate the expression of homeotic selector genes, called Hox genes, which all contain a homeobox. See Figures 2.11 and 2.12
9. Hox gene mutant flies may have whole body parts transformed, such as a leg constructed where an antenna should be. See Figures 2.13 and 2.14
10. Hox genes in Drosophila show colinearity: their sequence on the chromosome aligns with their expression in the anterior-posterior axis. See Figure 2.16
11. Hox genes in mammals also display colinearity, despite forming complexes on four different chromosomes, with the homologous genes on the various chromosomes diverging in function. See Figures 2.17 and 2.18
12. As in insects, Hox genes and other homeobox genes play a role in segmentation in mammals, including in the nervous system. For example, the homeobox gene Otx2 guides the fate of the developing midbrain and forebrain, while other homeobox genes are expressed only in the spinal cord. Engrailed is expressed only in the posterior midbrain, which seems to make the midbrain/hindbrain junction a “local organizer” capable of using FGF to induce a wide range of neural tissue to form a midbrain and cerebellum. See Figures 2.19–2.21
13. Homeobox genes Emx2 and Pax6 are expressed in opposite gradients in the developing cortex, and together they regulate the size of various cortical regions. See Figure 2.22
14. The hindbrain forms eight rhombomeres, each of which takes on a separate fate, under the direction of homeobox genes and ephrins that sharpen the demarcation between rhombomeres. See Figures 2.23 and 2.24
15. There are four factors concentrated at the posterior end of the embryo to direct differentiation along the anterior-posterior axis: BMP proteins, FGF, Wnt, and retinoic acid (RA). Each regulates Hox genes and other homeobox genes to guide differentiation appropriate to a particular segment of the nervous system. See Figures 2.26 and 2.27
16. RA is a steroid hormone and teratogen, so exogenous RA causes embryos to develop more caudal structures, for example, “posteriorizing” the development of rhombomeres, or preventing the formation of the brain. See Figures 2.28–2.30
17. The dorsal-ventral axis is formed by secretion of BMP from ectodermal cells above the neural tube and Sonic hedgehog (Shh) from the notochord below. See Figures 2.31 and 2.32
18. BMP stimulation induces the dorsal neural tube to form a roof plate, while Shh induces the ventral tube to form a floor plate and, just dorsal to the floor plate on each side, motor neurons. See Figure 2.33