Analysis of Biological Development (K. Kalthoff)

Updates to Topic 25: Experimental and Genetic Analysis of Caenorhabditis elegans Development

Answers to Questions in Text

Induction of Anterior Pharyngeal Muscle Cells (p. 673/674)

  1. The technique of Priess and Thompson for removing blastomeres is quite laborious. Killing unwanted cells with a laser microbeam is a much quicker and faster operation, but the dead cells stay within the egg shell. Can this present a problem for the interpretation of certain results? Answer: Yes, unfortunately. The dead cells might still send signals to the surviving ones. Therefore, if the surviving cells develop according to fate, it cannot be concluded safely that the development of the surviving cells is autonomous, i.e., independent of inductive interactions with other cells. See Section 1.3.
  2. In the experiments described here, blastomeres ABa and ABp were equally competent to form pharyngeal cells. What can be concluded from this result, given that only ABa produces anterior pharyngeal cells in normal development? Answer: Abp may be inhibited from forming pharyngeal cells by signals from P2, the only blastomere at the 4-cell stage that touches Abp but not Aba. Alternatively, or in addition, those descendants of Aba that form pharyngeal cells may receive positive inductive signals from neighboring EMS-descendants, since removal of EMS interfered with the formation of pharyngeal cells (see Table 25.1).

Polarization of EMS by a Signal from P2 (p. 678/679)

  1. What did the investigators have to measure in order to quantify the extent to which the mitotic spindle in an EMS cell was oriented by its interaction with P2? Answer: The angle between the mitotic spindle of EMS and a line perpendicular to the plane of contact between EMS and P2. A consistently small angle would indicate orientation of the EMS spindle by P2. An angle varying randomly between 0o and 90o would indicate a total lack of effect of P2 on spindle orientation. See Fig. 25.12d.
  2. What kind of internal control could the investigators have used (and they did) to convince themselves that their methods for inhibiting mRNA synthesis were indeed working? (Hint: It is feasible to introduce any transgene into C. elegans.) Answer: By introducing a transgene that is actively transcribed at the 4-cell stage and encodes a readily detectable protein. The investigators used a transgene encoding a fusion protein containing green fluorescent protein (GFP) as a reporter tag (see Section 15.4, p. 392).
  3. If an isolated P2 is recombined with Aba or ABp, no E cell is formed. Thus, it appears that only EMS is competent to respond to the inductive signal from P2 that generates the E blastomere. What gene product discussed earlier would be a good candidate for conferring this competence to EMS? How could this hypothesis be tested? Answer: skn-1 (see Fig. 25.8). To test this hypothesis, an EMS cell from an embryo derived from a homozygous mutant skn-1- hermaphrodite should be juxtaposed with a wild-type P2 cell. The prediction derived from the hypothesis would be that no E cell is formed.


Clarifications and Corrections

On p.677, Figure 25.12b, right panel: the sister cell of MS should be labeled "E".

New Review Articles

Banerjee D. and Slack F. (2002) Control of developmental timing by small temporal RNAs: a paradigm for RNA-mediated regulation of gene expression. BioEssays 24: 119-129

New Research Articles

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