Discovery Talk Questions and Answers: Finding Genes that Control Development

Eric Wieschaus
Assessments created by Dr. Gabrielle Miller-Messner

Questions

Drs. Wieschaus and Nüsslein-Volhard set out to identify “all” of the genes involved in embryonic development. Using the 1984 paper, evaluate the evidence that Drs. Nüsslein-Volhard and Wieschaus did indeed identify nearly all of the genes that result in lethal phenotypes visible in embryos. [1984 paper]
In the list below, select three statements that best describe the three “levels of spatial organization” in embryonic patterning in Drosophila discussed by Drs. Wieschaus and Nüsslein-Volhard in their 1980 Nature paper. [1980 paper]
1. Gooseberry, hedgehog, and patch mutants
2. The entire egg, the segment pairs, and the individual segments
3. Organization reflected in gap, pair-rule, and segment polarity mutants
4. Mutations on chromosomes 1, 2, and 3
5. None of the above
Screening 40,000 lines of flies is an enormous undertaking. Which of the following strategies did Drs. Wieschaus and Nüsslein-Volhard use to streamline the screening process? (Select all that apply) [1984 paper]
1. Temperature-sensitive inbred lines
2. Crosses that employed eye color to mark mutant genotypes
3. A vacuum cleaner
4. None of the above
In science, the importance of experiments which are “hypothesis-driven” is often emphasized. Evaluate whether or not the work of Drs. Wieschaus and Nüsslein-Volhard is “hypothesis-driven”?
You encounter a larva that has 5 evenly spaced denticle bands instead of 11. At what spatial scale is this mutation likely to act? [1980 paper]
1. An individual segment
2. A repeating unit comprised of two segments
3. The entire embryo
4. The mesothorax
5. None of the above
The Drosophila melanogaster genome contains about 15,000 genes; in their screen, Drs. Wieschaus and Nüsslein-Volhard found about 18,000 lethal mutations. Of those thousands of genes, only 139 ended up exhibiting lethal mutations in embryonic development. In his talk, Dr. Wieschaus says that he and his colleagues were very “lucky” that such a small number of genes were involved in embryonic development.
Is it surprising that so few mutations result in lethal mutations in embryos? Suggest an explanation for this phenomenon.
Drs. Nüsslein-Volhard and Wieschaus (1980) were surprised to find Drosophila embryos patterned two segments at a time, as seen in the paired-rule class of mutants. What explanation do they propose for this level of spatial organization? [1980 paper]
How did Nüsslein-Volhard and Wieschaus know that all of the lethal mutations they identified were distinct? How did they ensure that they didn’t have duplicates, multiple lines representing the same mutation? [1984 paper]
1. Temperature-sensitive inbred lines were used to select for mutagenized chromosomes
2. The embryos were viewed simultaneously by two observers to ensure accurate classification
3. Mutants with similar phenotypes were crossed in complementation studies
4. None of the above
Beyond “What are the genes that control embryonic development?” what is the ultimate scientific question or questions that Drs. Nüsslein-Volhard and Wieschaus’ work addresses? [paper + video]
Drosophila and human lineages diverged about 600 million years ago and yet the two groups share many aspects of basic biology. How has the work of Drs. Nüsslein-Volhard and Wieschaus influenced our understanding of inherited human developmental disorders? [students’ independent research]

Answers

The most straightforward evidence that the authors did indeed find most of the genes for embryonic development appears in Fig. 5 (p.280). This graph depicts a drop off in the discovery of new loci above a certain threshold in the number of total mutants isolated by the screen.1984 Paper, p. 281: “Although we cannot claim to have reached saturation for such loci in our screen, our calculations presented above indicate that our collection represents the great majority of such loci especially of those which mutate to a striking and unique embryonic visible phenotype.”
b and cChoice (b) is correct.
1980 Paper, p.795: “The phenotypes of the mutant embryos indicate that the process of segmentation involves three levels of spatial organization: the entire egg as developmental unit, a repeat unit with the length of two segments, and the individual segment.”

Choice (c) is correct. Table 1, p.798 “Loci affecting segmentation in Drosophila”

This table shows 3 classes of mutants: Segment polarity, Pair-rule, and Gap.

Choice (a) lists 3 mutants that the authors identified, but they do not represent different levels of spatial organization.

Choice (d) is incorrect. The authors do not discuss an association between spatial organization and chromosome number.
a, b, c
Choice a. Quote 1, p. 268 “DTS91 is one of the dominant temperature-sensitive mutants …”

Choice b. Quote 2, p. 268 “The F3 progeny was scored for the absence of white-eyed, straight-winged flies(cn bw sp homozygotes) .”

Choice c. Quote 3, p.268 “These matings were set at 29°C for the first three days, after which the parents were discarded with help of a vacuum cleaner.”
Sample answer:
In his talk, Dr. Wieschaus describes how he and Christiane were looking for a “global picture.” They wanted to identify “everything .. all the genes [involved in embryonic development]” and “see what happened.” As the first slide title reads, “Make random mutations. No preconceptions.” Sometimes in science a specific question is asked and the results indicate a specific answer. Other times, as in this experiment, and in much of the work that was done at the beginning of the genomics era, one has no choice but to gather a ton of data and “see what happens.” Drs. Wieschaus and Nüsslein-Volhard’s work provides an example of how careful exploratory work can spark countless question-driven research programs.
b; The larva fits the description of even-skipped, which belongs to the pair-rule class of mutants (1980 Research Paper, p.796 and Fig. 1).
Sample answerVideo (8:11) “The numbers are small because of the peculiar biological features… the way flies develop. It turns out that flies’ embryonic development is very rapid, the mothers puts everything the embryo needs… [everything] that she can possibly put into the egg is put in by the mother, and the transcriptional requirements early during these patterning stages are very minimal. So, the fly has stripped down its transcriptional requirements to the rare genes that can’t be supplied by the mother–that must be supplied by transcription in this cell and not in this cell.”
Other possible explanations may include:

The structure of the embryo is relatively simple compared to the structure of an adult fly; – Many of the genes required for organ function are not needed in the embryo.
Segment pairs could represent an initial division of the epidermal primordium, which, at 40 cells long, cannot be accurately divided into 11 segments.
1980 Research Paper, p. 800: “The discovery of a mutant class affecting corresponding regions in every other segment was not expected …”
c; 1984 Paper, p.269: “Crosses were performed between mutants with a similar phenotype. Eggs were collected from three pairs of flies using the block system and unhatched eggs inspected under oil. In doubtful cases, the parents were recovered from the egg-laying tubes and progeny raised. They were checked for transheterozygotes.”
Choices a and b are true, but did not help with exclusion of duplicates.
Sample answer
Their work explores the nature of the “construction of complex form from similar repeating units.” 1980 Paper, p. 795
Sample answer:
Reference: Goodrich, LV, MP Scott. 1998. Hedgehog and Patched Review in Neural Development and Disease. Neuron 21:1243-1257.

https://www.ncbi.nlm.nih.gov/pubmed/9883719

Developmental geneticists throughout the globe have based decades of research upon studying the genes Drs. Nüsslein-Volhard and Wieschaus discovered and their homologs. Vertebrate homologs of the genes controlling embryonic patterning in Drosophila are involved in dozens of human developmental disorders and cancers. For example, mutations in genes related to hedgehog and patch are now known to be involved in childhood brain tumors, and human Hypoprosencephaly, a disorder leading to severe craniofacial malformation and central nervous system defects.