Evolutionary alteration in anterior patterning: otx2 expression in the direct developing frog Eleutherodactylus coqui

Area of Science:

• Developmental Biology
• Evolutionary Developmental Biology
• Genetics

Background:

• The homeobox gene otx2 is crucial for vertebrate head development.
• In Xenopus, otx2 specifies the cement gland, an anterior larval organ.
• Direct-developing frogs like Eleutherodactylus coqui lack a cement gland, prompting investigation into otx2's role.

Purpose of the Study:

• To investigate the role of otx2 gene expression in the evolutionary loss of the cement gland in Eleutherodactylus coqui.
• To compare the expression patterns and functional capacity of E. coqui otx2 (Ecotx2) with its Xenopus homologue.

Main Methods:

• Cloning of the E. coqui otx2 homologue (Ecotx2).
• Analysis of Ecotx2 spatiotemporal expression patterns during embryonic development.
• Misexpression experiments of Ecotx2 in Xenopus laevis ectoderm to assess functional conservation.

Main Results:

• Ecotx2 protein's homeodomain is highly conserved, identical to mouse/zebrafish Otx2 and differing by one amino acid from Xenopus Otx2.
• Ecotx2 RNA expression is restricted to the anterior neuroectoderm (forebrain/midbrain), unlike Xenopus otx2, which also includes the cement gland primordium.
• Ecotx2 retains the ability to activate cement gland-specific gene expression (XCG, XAG1) when misexpressed in Xenopus ectoderm.

Conclusions:

• The evolutionary loss of the cement gland in E. coqui correlates with a lack of anterior expansion of otx2 expression.
• Functional studies indicate that the Ecotx2 protein can still initiate cement gland formation pathways.
• Modifications in developmental pathways upstream of otx2 expression likely underlie the absence of a cement gland in E. coqui.