During the first and second cell divisions during early embryogenesis, some moll
ID: 216093 • Letter: D
Question
During the first and second cell divisions during early embryogenesis, some mollusk embryos produce a cytoplasmic protrusion called the polar lobe. This polar lobe is preferentially absorbed into the D blastomere lineage following mitosis. Removal of the polar lobe or the D blastomere results in the development of incomplete larvae that lack the intestine, meosodermal organs such as the heart and some muscles and also some ectodermal organs such as the eyes.
a. Propose a hypothesis to explain how the polar lobe is involved in patterning the early mollusk embryo.
b1. (2 pts.) Describe an experiment to determine if there is a correlation between the role you have hypothesized for the polar lobe and the normal patterning of the mollusk embryo (correlative, “show it”).
b2. (1 pts.) Describe a result that is consistent with your hypothesis.
b3. (1 pt.) Describe a result that is inconsistent with your hypothesis.
c1. (2 pts.) Describe an experiment to determine if the role that you have hypothesized for the polar lobe is necessary for the normal patterning of the mollusk embryo (loss-of-function, “block it”).
c2. (1 pt.) Describe a result that is consistent with your hypothesis.
c3. (1 pt.) Describe a result that is inconsistent with your hypothesis.
d1. (2 pts.) Describe an experiment to determine if the role that you have hypothesized for the polar lobe is sufficient to pattern the early mollusk embryo (gain-of-function, “move it”).
d2. (1 pt.) Describe a result that is consistent with your hypothesis.
d3. (1 pt.) Describe a result that is inconsistent with your hypothesis.
(A) Clear animal cytoplasm Granular equatorial cytoplasm AB Clear vegetal cytoplasm Polar lobe AB CD AB CD Polar lobe absorbed into CD blastomere Second polar lobe extrusion Polar lobe absorbed into D blastomereExplanation / Answer
a) Mollusks interestingly shows the mosaic development means where blastomere specify autonousmously and morphogenic determinants are located in specified region and no blastomere compensate the lack of others.
Hypothesis: Polar lobe involved in cell determination and axis formation and E. B. Wilson demonstrated that polar lobe cytoplasm contains the mesodermal determinants, and that these determinants give the D blastomere its mesoderm-forming capacity. In some spirally cleaving mollusk embryo this D polar lobe is critical to patterning and axis formation.
b1) Experiment: D blastomere having the content of polar lobe, when one removes the D blastomere or its first or second macromere derivatives (1D or 2D), one obtains an incomplete larva, lacking heart, intestine (the ciliated border of the larva), shell gland, eyes, and foot.
b2) Result is consistance to this hypothesis because D blastomere is attached to the polar lobe and if all the blastomere are intact we will get complete embryo but when D blastomere is removed complete embryo is not developed.
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