2. Many sea urchin embryos exhibit clear differences in blastomere size along th

Question

2. Many sea urchin embryos exhibit clear differences in blastomere size along the animal-v large macromeres in the other vegetal tiers above them, and intermediate-sized mesomeres in the animal hemisphere. Micromeres and macromeres form predominantly mesodermal and endodermal derivatives (red and yellow structures in (A), below). Removal of the micromeres, or culturing the animal hemisphere in isolation, results in an "animalized" embryo consisting only of ectoderm (as shown in (B), below). Given the apparent importance of the micromeres, you conducted a subtraction hybridization screen for genes expressed in the vegetal hemisphere during early development, and have identified a gene you've called UV1 (Urchin Vegetal 1). (Subtraction hybridization screens are a method for "subtracting" genes expressed in one tissue from genes expressed in another. So for example, you could subtract genes expressed in the animal half from those expressed in the vegetal half, which would leave following questions: egetal axis during cleavage stages, with small micromeres at the vegetal pole, you with the vegetal-specific genes.) With this information in mind, answer the (A) Normal development Animal hemi- sphere vegz Micromeres Pluteus larva (B) Animal hemishere alone Dauerblastula Complete animalization a. Propose a hypothesis about a process (the cause) that might be responsible for some aspect of the phenomenon (the effect) described above.

Explanation / Answer

Ans a) It is show that on isolating the animal hemisphere, the embryo only forms the ectoderm. The upper hemisphere consists of the egg cell with yolk free cytoplasm and nucleus which helps in formation of the complete nervous system. On treatment of such embryos, it leads to formation of oversized animal body parts through an expanded gastrointestinal tract which is formed by the ectoderm.

Ans b) An experiment to prove the process of complete animalisation and the related phenomenon is to isolate the hemisphere from the complete part. It should be given proper growth conditions and situation so that it shows growth and development. Gradually it will only form the large body parts and bypassing a lot of development phase, it undergoes complete animalisation.

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