Questions: Model Organisms

Question 1

Why was Drosophila melanogaster the organism in which the genetic logic of body plan patterning was first worked out?

• A. Drosophila is the most complex organism and therefore the most informative
• B. Drosophila combines rapid generation time, powerful forward genetics (mutagenesis screens), externally developing embryos with visible segmentation, and a compact genome — enabling systematic identification of genes controlling body plan patterning
• C. Drosophila was the only available research organism at the time
• D. Drosophila development is simpler than all other animals and therefore easier to study

Answer: (B)

Nusslein-Volhard and Wieschaus performed saturating mutagenesis screens in Drosophila (Nobel Prize, 1995), systematically identifying genes required for embryonic patterning. This was possible because of Drosophila's short generation time (10 days), the ease of screening thousands of mutagenized lines for visible body plan defects (missing segments, homeotic transformations), and the compact genome that made it feasible to identify mutations by genetic mapping. The segmented body plan provided a clear readout — disrupted segment pattern was immediately visible. These practical advantages, combined with decades of prior genetic work, made Drosophila the system where the molecular logic of development was first cracked.

Question 2

C. elegans has an invariant cell lineage — every individual of the species undergoes exactly the same sequence of cell divisions, producing the same 959 somatic cells. This means cell-cell signaling plays no role in C. elegans development.

• T. True
• F. False

Answer: False

The invariant lineage might suggest that development is entirely cell-autonomous (determined by lineage history alone), but cell-cell signaling is essential at many points. Vulval induction requires EGF signaling from the anchor cell, and lateral inhibition via Notch ensures the correct vulval cell pattern. Asymmetric cell divisions in the early embryo depend on cell-cell interactions. The invariant lineage reflects the outcome of both cell-autonomous determinants and stereotyped cell-cell signaling events — the signaling is just as invariant as the lineage itself. If signaling is disrupted (by mutation or laser ablation of the inducing cell), the lineage becomes abnormal.

Question 3

What experimental advantages does the zebrafish offer that neither Drosophila nor mouse can provide?

Model answer: Zebrafish embryos are optically transparent, allowing live imaging of every cell division, migration, and differentiation event in a developing vertebrate in real time. Combined with fluorescent transgenic reporters, this enables watching gene expression dynamics, cell lineage tracing, and morphogenetic movements in vivo. Zebrafish also develop externally (unlike mice) and rapidly (major organs form within 24 hours), are small enough for high-throughput drug screens (embryos fit in 96-well plates), and are genetically tractable (forward genetic screens and CRISPR editing). This combination of vertebrate biology, optical transparency, and high-throughput tractability is unique to zebrafish.

Zebrafish have become the organism of choice for in vivo imaging of vertebrate development and for large-scale chemical screens to identify drugs that affect specific developmental processes. Their transparency solves a fundamental problem: in mice, development occurs inside the mother and inside opaque tissues, making live observation impossible without invasive procedures.