In Drosophila, the anterior-posterior axis is specified by maternally deposited bicoid mRNA localized at the anterior pole. If bicoid mRNA is injected at the posterior pole of a wild-type embryo, what develops?
ANothing changes — the normal anterior structures override the ectopic bicoid
BA second set of head structures develops at the posterior, producing a bicephalic (two-headed) embryo
CThe entire embryo becomes anterior tissue with no posterior structures
DThe bicoid protein is immediately degraded at the posterior pole
Bicoid is a morphogen: a transcription factor whose concentration gradient specifies position along the AP axis. At the anterior, high Bicoid activates head-specific genes; at the posterior, absence of Bicoid (and presence of Nanos) allows abdominal and posterior genes. Injecting bicoid mRNA at the posterior creates a second concentration peak that activates anterior gene expression locally, producing ectopic head structures. This classic experiment by Driever and Nusslein-Volhard demonstrated that Bicoid is sufficient to specify anterior identity and that positional information in the embryo is determined by morphogen concentration.
Question 2 True / False
In mammals, the anterior-posterior axis is specified by maternal mRNA determinants deposited in the egg, just as in Drosophila.
TTrue
FFalse
Answer: False
Mammalian axis formation differs fundamentally from Drosophila. The mammalian egg lacks obvious maternal mRNA asymmetries for axis specification. Instead, the AP axis is established relatively late, through cell-cell interactions within the inner cell mass and signaling from extraembryonic tissues (the anterior visceral endoderm). This regulative mode of development means that mammalian blastomeres remain remarkably flexible — they can be separated and each can form a complete embryo (the basis of identical twinning). The reliance on cell interactions rather than maternal determinants for axis specification is a key distinction between regulative (mammalian) and mosaic (Drosophila) development strategies.
Question 3 Short Answer
What establishes left-right asymmetry in vertebrate embryos?
Think about your answer, then reveal below.
Model answer: Left-right asymmetry is initiated by cilia-driven leftward fluid flow at the embryonic node (in mice) or equivalent structure. Motile cilia rotate clockwise, creating a leftward current across the node that generates asymmetric distribution of signaling molecules (like Nodal). This triggers the Nodal-Pitx2 signaling cascade specifically on the left side: Nodal activates Pitx2, a transcription factor that drives left-side-specific organ morphogenesis (heart looping, gut rotation, spleen placement). Disrupting ciliary function (as in Kartagener syndrome / primary ciliary dyskinesia) randomizes left-right asymmetry, resulting in situs inversus (mirror-reversed organs) in approximately half of affected individuals.
Left-right asymmetry is the last axis to be established and is mechanistically fascinating because it breaks an apparent molecular symmetry. The nodal flow model explains how a mechanical process (ciliary rotation) translates into biochemical asymmetry (lateralized Nodal signaling), which is then interpreted by transcription factors to produce asymmetric organ morphogenesis.