Dollo’s law:  A biological generalization positing that complex characters, once lost in evolution, are extremely unlikely to reappear and thus the loss of complex characters is virtually always irreversible.

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Noncanonical pathways can be divided into two types: 

1. The “planar cell polarity” pathway

2. The “Wnt/calcium” pathway

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Central player in the canonical Wnt pathway is β-catenin

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Homologs

β-catenin in vertebrates = Armadillo in Drosophila

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“canonical” Wnt signaling uses a second TM protein, LRP that associates with Frizzled in a Wnt signal– dependent manner.

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Homologs

LRP in vertebrates = Arrow in Drosophila

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In the absence of Wnts,

“free” β-catenin are targeted for degradation by a cytosolic destruction complex that is held together by the tumor suppressor protein Axin.