Practice exam1_key-updated(Sp2024)

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Name____________________________________ Page 1 MCB 410 Developmental Biology Practice Exam #1 1. Earlier generations of embryologists debated whether embryonic development proceeds via preformation or epigenesis. What position would you take in this debate with respect to the development of the germ cell lineage? Please provide specific examples to support your answer. (5 points) Germ cell development exhibits characteristics of both preformation and epigenesis, depending upon the organism under consideration. Thus, Drosophila , C . elegans , and zebrafish all use maternally deposited determinants to specify their germ cells; these may be considered to be “pre-formed” in the egg. In contrast, mammals and many other organisms do not have a germ plasm and instead the germ cells are induced to form, depending upon their location in the embryo. This inductive signaling is more akin to epigenesis, the gradual unfolding of form. (Partial credit for choosing one mechanism and giving examples that support it) 2. Based on the Lonely Planet description of ascidian sashimi (“tastes like rubber dipped in ammonia”), you become inspired to study the development of the muscle cell lineage in this animal. Like other ascidians we have learned about, a pigmented cytoplasm segregates to the blastomeres that will give rise to muscle cells. You would like to figure out how this cytoplasm specifies muscle. You hypothesize that localization of a specific mRNA to the appropriate blastomere specifies muscle fate. To test your hypothesis, you isolate mRNAs from ascidian embryos and identify a candidate gene that you call macho-1 . What technique would you use to test if macho-1 is expressed in the appropriate embryonic cells? (2 points) In situ hybridization Assuming that it is expressed in the appropriate place, what experiment would you do to test if macho-1 function is necessary for muscle development (please include the predicted outcome of the experiment) (4 points) If macho-1 function is necessary for muscle development, then inhibiting its function (using some type of RNA interference, for example) should result in failure to develop muscles. What experiment would you do to test if macho-1 is sufficient for muscle development (please include the predicted outcome of the experiment)? (4 points) If macho-1 function is sufficient for muscle development, then expressing it in blastomeres in which it normally is not expressed (this can be done by injecting macho-1 mRNA into cells that normally do not form muscle) should lead to inappropriate production of muscle cells.

Name____________________________________ Page 2 3. You have just learned that in C. elegans , the sperm entry site determines the anterior- posterior (a/p) polarity of the embryo; so, you become interested in identifying the component of sperm that serves to trigger the cytoskeletal rearrangements that establish the a/p axis. You find a mutant that produces sperm that develop without nuclei, yet they crawl about normally and can fertilize eggs. What hypothesis could you test with these mutant sperm (4 points) and what experiment would you do to test this hypothesis? (4 points) You could address the hypothesis: does the sperm nucleus trigger the a/p polarity of the nematode embryo? By examining the initial stages of development in embryos that have been fertilized by these aberrant sperm, you can see whether the initial a/p polarity is established properly Based on your extensive knowledge of C. elegans biology, what result would you predict from this experiment? (4 points) Sperm lacking DNA should still be able to determine the a/p axis, since it is the sperm centrosome that establishes the a/p axis. 4. In Drosophila , oskar mRNA can be mislocalized to the anterior pole of the embryo when it is attached to the 3’untranslated region (UTR) of bicoid ( bcd ). This oskar-bcd 3’UTR construct results in a gain-of-function mutation, in which embryos from mutant mothers produce germ cells at both the anterior and posterior poles; these embryos also develop posterior (abdominal and telson) structures in place of anterior (head and thoracic) structures, so that they have two posterior abdomens in mirror image symmetry (see figure). You have identified a mutation in a new posterior group gene (that you call, not very creatively, post ). You wish to examine the role of this gene in posterior pole plasm function, so you generate a double mutant carrying the oskar mislocalization construct and your new mutant. The images below show the resulting segmentation phenotypes: Figure: Patterning of Drosophila first instar larvae from mothers with indicated genotypes. Anterior to the left in all images. Arrows indicate direction of anterior (A) to posterior polarity of segments; arrowheads indicate telson (posterior-most structure); (H) Head segments; (T) Thoracic segments. In post mutants, pole cells do not form; in post; osk-bcd-3’UTR mutants, pole cells only form anteriorly. Using Muller’s definitions, what is the nature of the osk-bcd-3’UTR mutation? Explain your reasoning (3 points) It is a neomorph. This construct expresses oskar mRNA in the wrong place (the anterior pole); the phenotype would be unaffected by changing wild-type oskar dosage.

Name____________________________________ Page 3 Does post interact genetically with this gain-of-function oskar mutation? Explain your reasoning (4 points) Yes. The post mutation alters the phenotype of the osk mislocalization construct, so there is a genetic interaction between these mutations. Based on these results, provide a plausible hypothesis for the role of wild-type post activity in the pole plasm (4 points) These results suggest that post is required for the proper posterior localization of the germ plasm. It is not required for germ plasm function, per se, because a functional germ plasm can still form anteriorly, when oskar is mislocalized. 5. Name three asymmetries that are observed in early C. elegans embryos. (6 points) AB/P1 division is asymmetric, with AB large, and P1 smaller; P granules are localized to the posterior blastomere; AB cell divides before P1; AB division plane is orthogonal to P1 division plane, asymmetric localization of PAR proteins (any other reasonable answer) 6. The machinery that carries out RNA interference seems unlikely to have evolved to help researchers figure out gene function in various and sundry creatures. Describe three roles that the RNAi machinery plays in cellular function. (6 points) The RNAi pathway plays roles in cellular defense against double-stranded RNA viruses, in silencing transposons, and in regulating endogenous genes via micro RNAs.

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