Congrats to the Stock lab for a cool paper in PNAS!ÌýÌý
The following article from the has just appeared in the EarlyÌýEdition of :
Aigler SR, Jandzik D, Hatta K, Uesugi K & Stock DW. 2014. Selection andÌýconstraint underlie irreversibility of tooth loss in cypriniform fishes.ÌýProc. Natl. Acad. Sci. USA; published ahead of print May 12 2014.
Significance
The mechanisms underlying Dollo's Law, the assertion that the evolutionaryÌýloss of complex structures is irreversible, remain poorly characterized. InÌýprinciple, such mechanisms could involve the improbability either ofÌýgenerating the mutations required for trait reappearance or of selecting forÌýtheir fixation. Whereas most attention has focused on the former mechanism,Ìýwe used experimental reversal of dentition reduction in cypriniform fishesÌýto provide evidence for the operation of both within a single system.
Abstract
The apparent irreversibility of the loss of complex traits in evolutionÌý(Dollo's Law) has been explained either by constraints on generating theÌýlost traits or the complexity of selection required for their return. Distinguishing between these explanations is challenging,Ìýhowever, andÌýlittle is known about the specific nature of potential constraints. WeÌýinvestigated the mechanisms underlying the irreversibility of trait lossÌýusing reduction of dentition in cypriniform fishes, a lineage that includesÌýthe zebrafish (Danio rerio) as aÌýmodel. Teeth were lost from the mouth andÌýupper pharynx in this group at least 50 million y ago and retained only inÌýthe lower pharynx. We identified regional loss of expression of theÌýEctodysplasin (Eda) signaling ligand as a likely cause of dentitionÌýreduction. InÌýaddition, we found that overexpression of this gene in theÌýzebrafish is sufficient to restore teeth to the upper pharynx but not to theÌýmouth. Because both regions are competent to respond to Eda signaling withÌýtranscriptional output, the likely constraint on theÌýreappearance of oralÌýteeth is the alteration of multiple genetic pathways required for toothÌýdevelopment. The upper pharyngeal teeth are fully formed, but do not exhibitÌýthe ancestral relationship to other pharyngeal structures, suggesting thatÌýthey would not beÌýfavored by selection. Our results illustrate an underlyingÌýcommonality between constraint and selection as explanations for theÌýirreversibility of trait loss; multiple genetic changes would be required toÌýrestore teeth themselves to the oral region and optimallyÌýfunctioning onesÌýto the upper pharynx.
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