21–1 In the early cleavage stages, when the embryo 21–5 Changes in the coding regions of genes involved in cannot yet feed, the developmental program is driven and development are primarily responsible for the differences controlled entirely by the material deposited in the egg by between species. the mother.
21–6 The cell cycle is the ticking clock that sets the tempo of developmental processes, with maturational 21–2 Because of the many later developmental trans-changes in gene expression being dependent on cell-cycle formations that produce the elaborately structured organs, progression.
the body plan set up during gastrulation bears little resemblance to the body plan in the adult.
Discuss the following problems. 21–3 As development progresses, individual cells 21–7 Name the four processes that are fundamental to become more and more restricted in the range of cell types animal development, and describe each of them in a single they can give rise to.
sentence.
21–4 At different stages of embryonic development, 21–8 What are the three germ layers formed during gas-the same signals are used over and over again by different trulation, and what are the principal structures each gives cells, but with different biological outcomes. rise to in the adult?
21–9 In the early Drosophila embryo, there seems to be no requirement for the usual forms of cell–cell signaling; instead, transcriptional regulators and mRNA molecules move freely between nuclei. How can that be?
21–10 Morphogens play a key role in development, creating concentration gradients that inform cells of where they are and how to behave. Examine the simple patterns represented by the flags in Figure Q21–1. Which do you suppose could be created by a gradient of a single morphogen? Which would require gradients of two morphogens? Assuming that such patterns were present in a sheet of cells, explain how they could be created by morphogens.
Figure Q21–1 National flags from three countries (problem 21–10).
21–11 Two adjacent cells in the nematode worm normally differentiate into an anchor cell (AC) and a ventral uterine precursor (VU) cell, but which of the two becomes the AC and which becomes the VU cell is completely random: the cells have an equal chance of adopting either fate, but they always adopt different fates. Mutations of Lin12 alter these fates. In hyperactive Lin12 mutants, both cells become VU cells, while in inactive Lin12 mutants, both cells become ACs. Thus, Lin12 is central to the decision-making process. In genetic mosaics in which one precursor cell has the hyperactive Lin12 and the other precursor has the inactive Lin12, the cell with the hyperactive Lin12 always becomes the VU cell and the cell with inactive Lin12 always becomes the AC. Assuming that one cell sends a signal and the other cell receives it, explain how these results suggest that Lin12 encodes a protein required to receive the signal. Offer a suggestion for how the fates of these two precursor cells are normally decided in wild-type worms.
21–12 It was clear from the early days of studying development that certain “morphogenetic” substances were present in the egg and segregated asymmetrically into cells of the developing embryo. One such investigation in ascidian (sea squirt) embryos examined endodermal alkaline phosphatase, which could be visualized by a histochemical stain. Treatment of embryos with cytochalasin B stopped cell division, but did not block expression of alkaline phosphatase at the appropriate time. Treatment with actinomycin D, which blocks transcription, did not interfere with expression of alkaline phosphatase. Treatment with puromycin, which blocks translation, eliminated expression of alkaline phosphatase. What is the likely nature of the morphogenetic substance that gives rise to alkaline phosphatase?