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MCDB 138 Lecture 5


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LECTURE 5: OOPLASMIC (CYTOPLASMIC) DETERMINANTS 1. OVERVIEW As a result of cleavage in many organisms, different maternally provided gene products become localized to different blastomeres. These localized mRNAs (and possibly also proteins) play roles in establishing the axes of the embryo and in the fate of individual cells 2. BLASTOMERE SEPARATION EXPERIMENTS: REGULATIVE VERSUS MOSAIC DEVELOPMENT There is a long history of experiments in which blastomeres or parts of embryos were isolated and allowed to develop independently of other portions of the embryo. We have already considered how these experiments demonstrated genomic equivalence. Another purpose of these experiments was to determine whether the fate of cells was determined very early during cleavage (due to the segregation of different determinants in the cytoplasm among different blastomeres). Basically, the results of these experiments fell into two classes. Either the separated portions of the embryo developed only according to its prospective fate (the fate they would have had, if they had been left together in the embryo), or the separated portions developed into a whole embryo, a fate greater than their prospective fate. The former result was taken to demonstrate the presence of cytoplasmic determinants (mosaic development) (each part of the embryo is specified very early, the potency of each blastomere is the same as its prospective fate); the latter result was taken to demonstrate regulative developmen t (the potency, or developmental potential, of each blastomere is greater than its prospective fate). You will recall that this latter result was also taken as evidence that the genome is not irreversibly modified. Although each of the separated first two and four blastomeres of sea urchins, frogs, and mammals can give rise to a whole organism (regulative development), for certain other organisms, separated blastomeres only give rise to the part of the embryo they would have become if left in contact with the other blastomeres (thus suggesting the segregation of cytoplasmic determinants). Are mosaic and regulative development really fundamentally different types of development? We think not. As explained by E.B. Wilson, the apparent difference between regulative and mosaic development is a function of whether localized cytoplasmic determinants are divided differentially among the early blastomeres by the early cleavage planes. The result is the same—activation of different patterns of gene expression in different cells in the embryo, allowing the establishment of different cell types in the embryo. 3. GERM CELL DETERMINANTS Many organisms display specialized germ plasm with polar granules, cytoplasmic organelles that appear to play a role in germ cell determination ( determination = commitment, or decision, of a cell or group of cells to take on a certain differentiated state, at a future time. Recall that cells can become determined long before they are differentiated). Polar granules appear to contain RNA and protein and are unique to germ cells and to oocytes. Examples of some organisms in which germ cell
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