Patterns of Distal-less Gene Expression and Inductive Interactions in the Head of the Direct Developing FrogEleutherodactylus coqui

Abstract

The direct developing frog Eleutherodactylus coqui exhibits radical changes in its embryogenesis. A frog-like head forms directly with no appearance of a cement gland or several jaw cartilages characteristic of tadpoles, and limbs appear early in development. The numerous differences in the embryogenesis of E. coqui provide an opportunity to examine developmental causes for the evolutionary shift from biphasic to direct development. We have cloned DNA fragments corresponding to four E. coqui genes related to the Drosophila distal-less gene Dll. While the expression patterns of the distal-less genes are generally conserved, there are some spatiotemporal differences when embryos of E. coqui are compared to those of Xenopus laevis. The changes in gene expression are correlated with the embryonic changes in head structures including craniofacial cartilages and in particular, the cement gland. We have then examined inductive interactions involved in cement gland formation by interspecific transplants and recombinants. E. coqui embryos can generate signaling that culminates in cement gland formation, but E. coqui ectoderm appears to be incapable of a cement gland response. These results show here that inductive interactions in the anterior region of the E. coqui embryo have been modified during the evolution of direct development, and that changes in the competence of the E. coqui ectoderm may be responsible for the loss of certain tadpole-specific structures, such as cement gland.

Frogs without polliwogs: Evolution of anuran direct development

“…(41,42) Since the head skeleton is derived largely from the cranial neural crest, the migration pattern of this embryonic cell type was studied to determine whether the alterations in cranial morphology could be due to modifications in the positioning of neural crest streams. (33,43) However, the spatial patterning of the cranial neural crest streams does not appear to have been altered during the evolution of direct development, which is surprising given the dramatic alteration in cranial skeletal ontogeny.…”

“…Heterospecific transplants of either presumptive cement gland or lateral line tissue between E. coqui and biphasic amphibians indicated that, while the E. coqui embryo still appears capable of sending out inductive signals for both cement gland and lateral line formation, its ectoderm has lost the ability to respond to these signals by differentiating into these larval structures. (33,34) Identification of the molecules implicated in the generation of tadpole-specific structures in Xenopus has allowed comparative studies to be performed in E. coqui. The distal-less (dll) genes are homeodomain proteins expressed in cranial neural crest cells and in the developing cement gland of Xenopus embryos.…”

“…Whole-mount analysis of dll RNA and protein distribution revealed that, while expression in the cranial neural crest is conserved in E. coqui, the anterior expression domain representing the cement gland is absent from the direct developer. (33) The discovery that overexpression of the homeobox gene otx2 in fertilised Xenopus eggs could induce ectopic cement glands (35) made this molecule an intriguing gene to examine in E. coqui. In Xenopus, otx2 is expressed in the cement gland primordium, and also in the anterior neural plate.…”

“…Lesions in inductive signalling pathways could accrue, as evidenced by the loss of expression domains of distalless and NeuroD, which correlate with the loss of cement gland and lateral line placodes, respectively. (33,34) Over time, progressively more larval structures could be lost in this manner. Why then, would any larval traits be retained?…”

“…From Fang and Elinson. (33) evolution of shorter larval periods, with a concomitant reduction in size at metamorphosis. Continuing reduction in the larval period, accompanied by accumulation of developmental modifications, may eventually result in direct development.…”