Development of a non-amphibious amphibian - an interview with a coquí.

Development without a free-living tadpole is common among Ibero American frogs. The most derived condition is direct development where the tadpole has been eliminated, and the most investigated direct developing frog is . To provide a different point-of-view, an imaginary interview with a coqui is conducted.

The corn snake yolk sac becomes a solid tissue filled with blood vessels and yolk-rich endodermal cells.

The amniote egg was a key innovation in vertebrate evolution because it supports an independent existence in terrestrial environments. The egg is provisioned with yolk, and development depends on the yolk sac for the mobilization of nutrients. We have examined the yolk sac of the corn snake Pantherophis guttatus by the dissection of living eggs.

Amniote yolk sacs: diversity in reptiles and a hypothesis on their origin.

Oviparous amniotes produce a large yolky egg that gives rise to a free-living hatchling. Structural characteristics and functional attributes of the egg are best known for birds, which have a large mass of fluid yolk surrounded by an extraembryonic yolk sac. Yolk nutrients are delivered to the embryo via the vascular yolk sac.

Commitment to nutritional endoderm in Eleutherodactylus coqui involves altered nodal signaling and global transcriptional repression.

The vegetal cells of a Xenopus laevis embryo commit to mesendoderm via the Nodal-signaling pathway. In the direct developing frog Eleutherodactylus coqui, mesendoderm is specified at the marginal zone of the early gastrula, and vegetal core cells transform into nutritional endoderm. Nutritional endoderm, a novel tissue, consists of transient, yolky cells that provide nutrition but remain undifferentiated.

Characterization of the nutritional endoderm in the direct developing frog Eleutherodactylus coqui.

Unlike Xenopus laevis, Eleutherodactylus coqui develops without a tadpole. The yolk-rich vegetal region of the embryo forms a transient nutritive tissue, the nutritional endoderm (NE). The definitive endoderm (DE) in E.

Expression of cyclin D1, cyclin D2, and N-myc in embryos of the direct developing frog Eleutherodactylus coqui, with a focus on limbs.

Species of frogs that develop directly have removed the tadpole from their ontogeny and form adult structures precociously. To see whether cell cycle regulators could be involved in this altered embryogenesis, we examined the expression of ccnd1, ccnd2, and mycn in embryos of the direct developing frog, Eleutherodactylus coqui. Notable differences compared to embryos of Xenopus laevis, a species with a tadpole, included prominent expression of ccnd2 in the midbrain and ccnd1 in the mandibular neural crest.

Metamorphosis in a frog that does not have a tadpole.

The evolutionary removal of the tadpole from the frog life history is a very successful strategy, particularly in the tropics. These direct developers form limbs and a frog-like head early in embryogenesis, and they have reduced or lost tadpole-specific structures, like gills, a long, coiled intestine, and tadpole teeth and jaws. Despite the apparently continuous development to the frog morphology, the direct developer, Eleutherodactylus coqui, undergoes a cryptic metamorphosis requiring thyroid hormone.

Expression of a cardiac myosin gene in non-heart tissues of developing frogs.

Direct developing frogs, like Eleutherodactylus coqui, provide opportunities to investigate limb early development in anuran amphibians that are less available in species with tadpoles. We have found that myosin heavy chain 6 (myh6), a myosin gene usually considered heart-specific in Xenopus and other animals, is expressed in limbs of E. coqui embryos.

Developmental diversity of amphibians.

The current model amphibian, Xenopus laevis, develops rapidly in water to a tadpole which metamorphoses into a frog. Many amphibians deviate from the X. laevis developmental pattern.

Germ plasm in Eleutherodactylus coqui, a direct developing frog with large eggs.

Background: RNAs for embryo patterning and for germ cell specification are localized to the vegetal cortex of the oocyte of Xenopus laevis. In oocytes of the direct developing frog Eleutherodactylus coqui, orthologous RNAs for patterning are not localized, raising the question as to whether RNAs and other components of germ plasm are localized in this species.

Methods: To identify germ plasm, E.

Novel regulation of yolk utilization by thyroid hormone in embryos of the direct developing frog Eleutherodactylus coqui.

Thyroid hormone (TH) is required for metamorphosis of the long, coiled tadpole gut into the short frog gut. Eleutherodactylus coqui, a direct developing frog, lacks a tadpole. Its embryonic gut is a miniature adult form with a mass of yolky cells, called nutritional endoderm, attached to the small intestine.

Molecular Haeckel.

More than a century ago, Ernst Haeckel created embryo drawings to illustrate the morphological similarity of vertebrate early embryos. These drawings have been both widely presented and frequently criticized. At the same time that the idea of morphological similarity was recently attacked, there has been a growing realization of molecular similarities in the development of tissues and organs.

Lbx1 expression and frog limb development.

In order to identify prospective limb muscle cells in a frog, we cloned Lbx1 from the direct developing frog Eleutherodactylus coqui. Like in embryos of the frog Xenopus laevis but unlike in other vertebrates, EcLbx1 is expressed in all trunk somites. Like in embryos of chick, mouse, and zebrafish, cells expressing EcLbx1 are then found in limb buds, consistent with migration of those cells from somites.

Raldh expression in embryos of the direct developing frog Eleutherodactylus coqui and the conserved retinoic acid requirement for forelimb initiation.

Embryos of the direct developing frog, Eleutherodactylus coqui, provide opportunities to examine frog early limb development that are not available in species with tadpoles. We cloned two retinaldehyde dehydrogenase genes, EcRaldh1 and EcRaldh2, to see which enzyme likely supplies retinoic acid for limb development. EcRaldh1 is expressed in the dorsal retina, otic vesicle, pronephros, and pronephric duct, but not in the limb.

Nutritional endoderm: a way to breach the holoblastic-meroblastic barrier in tetrapods.

The deceptively simple evolutionary transition from complete, holoblastic cleavage of the zygote to incomplete, meroblastic cleavage occurred only once in tetrapods, with the evolution of the amniote egg. By examining the development of a frog with large eggs, we identified a new tissue called the nutritional endoderm, which provides a possible intermediate step to breach the holoblastic-meroblastic barrier. Nutritional endoderm is divided into cells, but the cells disappear and do not contribute to tissues of the frog after the yolk is depleted.

Muscle development in a biphasic animal: the frog.

Knowledge of muscle development in a vertebrate reflects strengths of the particular model system. For example, the origin of mesoderm is very well characterized in Xenopus laevis, where development of somites is less well understood. The major problem in muscle development, presented by frogs, is the complete replacement of larval muscles by adult muscles at thyroid hormone-dependent metamorphosis.

Comparative analysis of Xenopus VegT, the meso-endodermal determinant, identifies an unusual conserved sequence.

The transcription factor, VegT, is the meso-endodermal determinant in Xenopus laevis. We examined VegT orthologs from several anuran amphibians and the urodele amphibian, the Mexican axolotl. In addition to the conserved T-box, the DNA-binding domain, the orthologs share a conserved 57 amino acid domain at the C-terminal.

Nutritional endoderm in a direct developing frog: a potential parallel to the evolution of the amniote egg.

The egg of the direct-developing frog, Eleutherodactylus coqui, has 20 x the volume as that of the model amphibian, Xenopus laevis. Increased egg size led to the origin of nutritional endoderm, a novel cell type that provides nutrition but does not differentiate into digestive tract tissues. As the E.

RNA of AmVegT, the axolotl orthologue of the Xenopus meso-endodermal determinant, is not localized in the oocyte.

The transcription factor, VegT, is the meso-endodermal determinant in Xenopus laevis, and the localization of VegT RNA to the vegetal cortex of the oocyte is an important starting point for embryonic patterning. We have cloned the VegT orthologue from the urodele amphibian, Ambystoma mexicanum, the Mexican axolotl. Axolotl VegT (AmVegT) is expressed zygotically in the presumptive mesoderm and Rohon-Beard neurons as in X.

Development in frogs with large eggs and the origin of amniotes.

The origin of the amniote egg is one of the most significant events in the evolution of terrestrial vertebrates. This innovation was probably driven by increased egg size, and to find potential parallels, we can examine the derived development of extant amphibians with large eggs. The embryo of the Puerto Rican tree frog, Eleutherodactylus coqui, exhibits an alteration of its fate map and a secondary coverage of its yolky cells, reflecting the large 3.

Status of RNAs, localized in Xenopus laevis oocytes, in the frogs Rana pipiens and Eleutherodactylus coqui.

Early development in the frog model, Xenopus laevis, is governed by RNAs, localized to the vegetal cortex of the oocyte. These RNAs include Xdazl RNA, which is involved in primordial germ cell formation, and VegT RNA, which specifies the mesoderm and endoderm. In order to determine whether orthologues of these RNAs are localized and have similar functions in other frogs, we cloned RpDazl and RpVegT from Rana pipiens, a frog that is phylogenetically distant from X.

Antero-posterior tissue polarity links mesoderm convergent extension to axial patterning.

Remodelling its shape, or morphogenesis, is a fundamental property of living tissue. It underlies much of embryonic development and numerous pathologies. Convergent extension (CE) of the axial mesoderm of vertebrates is an intensively studied model for morphogenetic processes that rely on cell rearrangement.

Localization of RNAs in oocytes of Eleutherodactylus coqui, a direct developing frog, differs from Xenopus laevis.

Eleutherodactylus coqui develops directly on land to a frog. The large 3.5-mm oocyte of E.

Parallel microtubules and other conserved elements of dorsal axial specification in the direct developing frog, Eleutherodactylus coqui.

Specification of the dorsal axis in commonly studied frogs, such as Xenopus laevis and Rana pipiens, depends on a microtubule-mediated movement of cytoplasm in the fertilized egg. The Puerto Rican tree frog, Eleutherodactylus coqui, has an egg that is twenty times the volume of that of X. laevis, raising the question as to whether the mechanism of dorsal axial specification is conserved in these large eggs.

Relationship between ectopic germinal vesicle breakdown in Xenopus oocytes and dorsal development of the embryo.

The early development of several species involves the segregation of cytoplasmic components into different regions of the egg. In Xenopus zygotes, a 30° rotation displaces the central animal cytoplasm to the future dorsal side of the embryo. To elucidate the role of the central animal cytoplasm in dorsal determination, we induced germinal vesicle breakdown (GVBD) closer to the equator by cold/centrifugation treatment of oocytes.