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Blastopore dorsal lip organizer3/21/2024 ![]() ![]() Nodal converts the adjacent Nodal- cells into mesoderm (red), resulting in a stripe of mesoderm just above the vegetal pole. ![]() Vegetal pole proteins activate the expression of TGFb (BMP) and Nodal. After fertilization, cortical rotation brings the Vegetal pole outer cortex protein Dishevelled (Dsh, green) into contact with the Animal inner cortex, driving the accumulation of b-catenin in dorsal cells. Maternally derived proteins set up an Animal/Vegetal axis in the unfertilized egg, including VegT and Vg1 in the Vegetal pole inner cortex (orange). Now we have Vg1 and VegT at the vegetal pole and Dsh and b-catenin at the future dorsal end.įigure 8: Genetics of the Spemann-Mangold organizer. Dsh helps localize and stabilize b-catenin, the transcription factor of the canonical Wnt pathway. The displacement of Dsh creates a new zone - a part of the embryo with Dsh, but no Vg1 or VegT (Figure 8) 10. The outer cortex has vegetally localized Dishevelled (Dsh) protein, a component of the Wnt pathway, which is transported towards the animal pole during cortical rotation 9. In the inner mass this includes VegT and Vg1 7,8. The vegetal pole has several localized proteins and mRNAs tethered to its cytoskeleton. In cortical rotation, microtubules rotate the outer cortex of the fertilized egg relative to the inner mass. Using genetics, we can explain two huge questions: 1) What causes the organizer to develop? and 2) How does it induce the formation of dorsal structures? The first step in organizer formation is cortical rotation itself, as shown in Figure 1 of Cortical Rotation. Amphibians with an extra Spemann-Mangold organizer grow a second A/P axis (conjoined twin tadpoles) and amphibians missing a Spemann-Mangold organizer develop into a "belly piece." This organizer, as it develops into the notochord, induces the formation of dorsal structures like the central nervous system and spine. Briefly, the Spemann-Mangold organizer is mesoderm found at the position of the grey crescent, the dorsal pole of the frog (or newt) embryo. If you need a quick refresher on the Spemann-Mangold organizer, check out Cleavage and Gastrulation. The genetics of the Spemann-Mangold organizer and the notochord Now that you have a firm grasp of gene regulatory networks, we can take a closer look at a famous organizer we have already considered - the Spemann-Mangold organizer of the frog gastrula. Organizer outputs are not only affected by the Hox code, they can also themselves set up the Hox code (as in the case of Nanos and Bicoid) or can set up a different axis (like the dorsoventral axis of a frog or the anteroposterior axis of a limb). The fate of a serial homolog often depends on the Hox genes that are expressed in the region of the serial homolog organizer. Serial homologs are incredibly important in our understanding of evolution because they are genetically "cheap," since they use the same core gene regulatory network, but they can provide novel functions -like tool use plus locomotion. They express the same core limb genes (as we will soon consider) and have the same bones, albeit in slightly different shapes. Developmentally, they are nearly identical for the first part of their growth, they only grow substantial differences later. One easy to understand example of serial homologs are arms and legs. Serial homologs are a special type of homolog wherein the same type of tissue expressing a core set of genetic regulators (an organizer) is found in multiple spots along a body axis. We will be coming back to this concept later, but it is a conceptually important idea in Evo-Devo. This type of programming is known as serial homology. For example, dragonflies have two flying wings, while the first wing in a beetle is a protective elytron, and the second wing in a fly is a proprioceptive organ. is also dependent on the position of the segment in the body and the species it is expressed in. The type of wing - elytra, haltere, flying wing, etc. For example, we will look later on at wing master control genes -these genes build wings in thoracic segments and different structures, like gin traps, in other segments. Depending on the Hox gene expressed in that segment, they will activate different body parts. These are locally expressed genes that are often expressed in many segments in similar spots. Much of this differentiation is ruled by local organizers and master control genes. For example, a thoracic segment in a fruit fly might express a single Hox gene across a segment, yet parts of this segment take on many different forms. We see intriguing diversity within bodies as well. For example, even though the Hox gene patterning in fruit flies and mice is very similar, the end results (an adult mouse or an adult fly) are extremely different. One of the most interesting things about building animal bodies is the diversity we see across and within bodies.
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