Patterns of allometric and heterochronic changes in the early ontogeny of species of Physalaemus (Anura: Leptodactylidae).

We studied the relationship between shape, size, and developmental time in the embryonic ontogeny of 15 species of the frog genus Physalaemus. As in other anuran exotrophic embryos, shape changes are correlated with size increase and mainly concern tail elongation, decrease in body height, and increase in fin height. Size ranges and developmental times vary interspecifically. Embryos of the P. signifer Clade and the P. gracilis Group are among the largest, are slightly peramorphic, and develop fast regarding congeneric species. Embryos of P. cicada combine the smallest sizes with fast development and the most peramorphic shapes. The paedomorphic shapes of embryos of P. biligonigerus and P. henselii groups are correlated with fast vs. slow developmental times respectively. Trajectories in the P. cuvieri Group are diverse and in general differ in size and developmental time. The embryos of P. cristinae and from the Argentinean lineage of P. cuvieri stand out with the longest development. Sequences of developmental events are overall conserved in the genus, and main differences concern mouthpart ontogeny. This study constitutes the first attempt to evaluate morphological, allometric, and heterochronic parameters of the early ontogeny of anurans and how these can vary and contribute to diversification in taxonomic groups.

Elevated ammonia cues hatching in red-eyed treefrogs: A mechanism for escape from drying eggs.

Egg dehydration can kill terrestrial frog embryos, and this threat is increasing with climate change and deforestation. In several lineages that independently evolved terrestrial eggs, and retained aquatic tadpoles, embryos accelerate hatching to escape from drying eggs, entering the water earlier and less developed. However, the cues that stimulate drying-induced early hatching are unknown. Ammonia is a toxic, water-soluble metabolic waste that accumulates within eggs as embryos develop and concentrates as eggs dehydrate. Thus, increasing ammonia concentration may be a direct threat to embryos in drying eggs. We hypothesized that it could serve as a cue, stimulating embryos to hatch and escape. The embryos of red-eyed treefrogs, Agalychnis callidryas, hatch early to escape from many threats, including dehydration, and are known to use mechanosensory, hypoxia, and light cues. To test if they also use high ammonia as a cue to hatch, we exposed stage-matched pairs of hatching-competent, well-hydrated sibling embryos to ammonia and control solutions in shallow water baths and recorded their behavior. Control embryos remained unhatched while ammonia-exposed embryos showed a rapid, strong hatching response; 95% hatched, on average in under 15 min. This demonstrates that elevated ammonia can serve as a hatching cue for A. callidryas embryos. This finding is a key step in understanding the mechanisms that enable terrestrial frog embryos to escape from egg drying, opening new possibilities for integrative and comparative studies on this growing threat.

From the Belousov-Zhabotinsky reaction to biochemical clocks, traveling waves and cell cycle regulation.

In the last 20 years, a growing army of systems biologists has employed quantitative experimental methods and theoretical tools of data analysis and mathematical modeling to unravel the molecular details of biological control systems with novel studies of biochemical clocks, cellular decision-making, and signaling networks in time and space. Few people know that one of the roots of this new paradigm in cell biology can be traced to a serendipitous discovery by an obscure Russian biochemist, Boris Belousov, who was studying the oxidation of citric acid. The story is told here from an historical perspective, tracing its meandering path through glycolytic oscillations, cAMP signaling, and frog egg development. The connections among these diverse themes are drawn out by simple mathematical models (nonlinear differential equations) that share common structures and properties.

From egg to embryo in marsupial frogs.

Marsupial frogs (Hemiphractidae) evolved exceptional mechanisms for the conquest of terrestrial life. These adaptations include very large eggs. In some species eggs reach 10mm in diameter, and are considered to be the largest in frogs. Females have reproductive modifications for the incubation of embryos in their bodies. Modifications of embryos include adaptations for development inside the body of the mother, and changes in the developmental pattern. Moreover, in some species, oocytes are multinucleated instead of having a single germinal vesicle as in most vertebrates. This chapter provides an overview of the adaptations of marsupial frogs associated with terrestrial life, with a discussion of gastrulation and multinucleated oogenesis.

Experiences with the marsupial frogs: reminiscences of a developmental biologist.

This article provides a brief account of the career of Eugenia M. del Pino. Casual events and serendipity played important roles in modeling her career as a developmental biologist. In collaboration with colleagues and students, she analyzed the biology and development of the marsupial frog Gastrotheca riobambae (family: Hemiphractidae) in comparison with Xenopus laevis and tropical frogs. The emphasis was placed on oogenesis and the early stages of development. Topics include the mono- and multi-nucleated modes of oogenesis. She described two modes of gastrulation in frogs, gastrulation modes one and two, according to the timing of notochord elongation. She was able to establish a pioneer laboratory for the comparative analysis of frog development in Ibero America at the Pontifical Catholic University of Ecuador, in Quito. Her contributions to society include her influence in the establishment of the National Academy of Sciences of Ecuador, and efforts toward the conservation of the Galápagos Archipelago. She is part of a pioneer group of professors that placed Biology as an academic discipline in Ecuador. The experiences of her career reveal that we all face difficulties in our jobs. However, nothing is impossible when we follow a passion. Her work reveals that the key to success is to turn obstacles into opportunities.

Parallel evolution of direct development in frogs - Skin and thyroid gland development in African Squeaker Frogs (Anura: Arthroleptidae: Arthroleptis).

BACKGROUND: Cases of parallel evolution offer the possibility to identify adaptive traits and to uncover developmental constraints on the evolutionary trajectories of these traits. The independent evolution of direct development from the ancestral biphasic life history in frogs is such a case of parallel evolution. In frogs, aquatic larvae (tadpoles) differ profoundly from their adult forms and exhibit a stunning diversity regarding their habitats, morphology and feeding behaviors. The transition from the tadpole to the adult is a climactic, thyroid hormone (TH)-dependent process of profound and fast morphological rearrangement called metamorphosis. One of the organ systems that experiences the most comprehensive metamorphic rearrangements is the skin. Direct-developing frogs lack a free-swimming tadpole and hatch from terrestrial eggs as fully formed froglets. In the few species examined, development is characterized by the condensed and transient formation of some tadpole-specific features and the early formation of adult-specific features during a "cryptic" metamorphosis. RESULTS: We show that skin in direct-developing African squeaker frogs (Arthroleptis) is also repatterned from a tadpole-like to an adult-like histology during a cryptic metamorphosis. This repatterning correlates with histological thyroid gland maturation. A comparison with data from the Puerto Rican coqui (Eleutherodactylus coqui) reveals that the evolution of direct development in these frogs is associated with a comparable heterochronic shift of thyroid gland maturation. CONCLUSION: This suggests that the development of many adult features is still dependent on, and possibly constrained by, the ancestral dependency on thyroid hormone signaling.

Tadpoles of hybridising fire-bellied toads (B. bombina and B. variegata) differ in their susceptibility to predation.

The role of adaptive divergence in the formation of new species has been the subject of much recent debate. The most direct evidence comes from traits that can be shown to have diverged under natural selection and that now contribute to reproductive isolation. Here, we investigate differential adaptation of two fire-bellied toads (Anura, Bombinatoridae) to two types of aquatic habitat. Bombina bombina and B. variegata are two anciently diverged taxa that now reproduce in predator-rich ponds and ephemeral aquatic sites, respectively. Nevertheless, they hybridise extensively wherever their distribution ranges adjoin. We show in laboratory experiments that, as expected, B. variegata tadpoles are at relatively greater risk of predation from dragonfly larvae, even when they display a predator-induced phenotype. These tadpoles spent relatively more time swimming and so prompted more attacks from the visually hunting predators. We argue in the discussion that genomic regions linked to high activity in B. variegata should be barred from introgression into the B. bombina gene pool and thus contribute to gene flow barriers that keep the two taxa from merging into one.

Introduction of Developmental Biology at Utkal University, (Odisha, India).

The paper deals with the background and the establishment of a Developmental Biology Laboratory in Utkal University in Odisha state. It describes the process from a humble beginning with limited facilities into a leading research centre, initially for amphibians and later for the endangered olive ridley (Lepidochelys olivacea) turtle. Starting from the biology, reproduction and development in many anurans, the laboratory took up research on regeneration, especially on super-regeneration in tadpoles under the influence of morphogens such as vitamin A (retinoids). Treatment with vitamin A after amputation of the tail inhibited tail regeneration but unexpectedly induced homeotic transformation of tails into limbs in many anurans, starting with the marbled balloon frog Uperodon systoma. This was the first observation of homeotic transformation in any vertebrate. The laboratory continues research on histological and molecular aspects of this phenomenon. In addition, taking advantage of the largest rookery of olive ridley sea turtles in Gahirmatha, in the same state the laboratory has contributed significantly to the biology, breeding patterns, development and especially the temperature-dependent sex determination phenomenon (TSD). This research was extended to biochemical and ultrastructural aspects during development for the first time for any sea turtle. The laboratory has contributed significantly to the conservation of olive ridleys as well as the saltwater crocodile (Crocodylus porosus). Recognition and awards for the laboratory have been received from both national and international bodies.

Studies on tail regeneration and homeotic transformation in anuran tadpoles.

Anuran tadpoles are excellent models for regeneration studies. The tail, an organ essential for swimming for the aquatic tadpole, regenerates completely following injury or amputation. However, treatment with the morphogen, vitamin A or retinoic acid inhibits normal tail regeneration and induces homeotic transformation of tail to limbs. This phenomenon was discovered for the first time in the Indian marbled balloon frog Uperodon systoma in the Developmental Biology laboratory of Utkal University (Odisha, India) in the year 1992. In this paper, we present the results of morphological, histological, biochemical and molecular (immonohistochemistry) investigations of vitamin A induced homeotic transformation in different anuran species. In addition, we discuss the putative role of fibroblast growth factor 1 during spinal cord regeneration in the tadpoles of the Indian tree frog, Polypedates maculatus, an ideal model for regeneration studies in an Indian context.

Ontogeny of the anuran urostyle and the developmental context of evolutionary novelty.

Developmental novelties often underlie the evolutionary origins of key metazoan features. The anuran urostyle, which evolved nearly 200 MYA, is one such structure. It forms as the tail regresses during metamorphosis, when locomotion changes from an axial-driven mode in larvae to a limb-driven one in adult frogs. The urostyle comprises of a coccyx and a hypochord. The coccyx forms by fusion of caudal vertebrae and has evolved repeatedly across vertebrates. However, the contribution of an ossifying hypochord to the coccyx in anurans is unique among vertebrates and remains a developmental enigma. Here, we focus on the developmental changes that lead to the anuran urostyle, with an emphasis on understanding the ossifying hypochord. We find that the coccyx and hypochord have two different developmental histories: First, the development of the coccyx initiates before metamorphic climax whereas the ossifying hypochord undergoes rapid ossification and hypertrophy; second, thyroid hormone directly affects hypochord formation and appears to have a secondary effect on the coccygeal portion of the urostyle. The embryonic hypochord is known to play a significant role in the positioning of the dorsal aorta (DA), but the reason for hypochordal ossification remains obscure. Our results suggest that the ossifying hypochord plays a role in remodeling the DA in the newly forming adult body by partially occluding the DA in the tail. We propose that the ossifying hypochord-induced loss of the tail during metamorphosis has enabled the evolution of the unique anuran bauplan.