Quantitative Tests of General Models for the Evolution of Development

Comparative developmental biologists have proposed models to describe patterns of conserved features in vertebrate ontogeny. The hourglass model suggests evolutionary change is most difficult at an intermediate "phylotypic" stage, the adaptive penetrance model suggests change is easiest at an intermediate stage, and the early conservation model suggests change is easier later in ontogeny. Although versions of some of these models have been discussed since the nineteenth century, quantitative approaches have been proposed only recently. Here we present quantitative phylogenetic approaches to evaluating trends in the evolution of ontogeny. We apply these approaches to the proposed models and demonstrate that an existing approach to assessing these models is biased. We show that the hourglass, adaptive penetrance, and early conservation models are unnecessarily complex explanations of the patterns observed in developmental event data for 14 species of vertebrates. Rather, a simpler model that postulates that evolutionary change is easier between ontogenetically adjacent events is adequate.

Embryonic and larval development in the caecilian Ichthyophis kohtaoensis (Amphibia, gymnophiona): a staging table

Little is known about the developmental biology of caecilians-tropical, elongate, limbless, mostly fossorial amphibians that are members of the Order Gymnophiona. Ichthyophis kohtaoensis (Family Ichthyophiidae; southeast Asia) is an oviparous species in which maternal care of the clutch is provided. The clutch is laid in a burrow on land, and the embryos develop in their egg membranes, curved around a large yolk mass. Larvae are aquatic and exhibit characteristic features that are not present in the terrestrial adults. Because accurate descriptions of ontogenies and the establishment of standardized stages of embryonic and larval development are useful for both experimental and comparative embryology, a staging table for I.kohtaoensis was developed based on external morphological features. Development from the end of neurulation to metamorphosis was divided into 20 stages. Principal diagnostic features include development of the lateral line organs, formation of three pairs of external gills, development of the eyes, changes in yolk structure, changes in the structure of the cloacal aperture and growth of the tail, including the formation and regression of the tail fin. This study provides a comparison with descriptions of embryonic stages of I.glutinosus and Hypogeophis rostratus and with a recent staging table for the aquatic, viviparous caecilian Typhlonectes compressicauda, the only other caecilians for which reasonably complete ontogenetic information exists in the literature. Comparisons with established staging tables for selected frogs and salamanders are also presented.

Oviduct structure and function and reproductive modes in amphibians

The structure and function of the oviducts of members of the three Orders of the Class Amphibia (Anura, frogs and toads; Urodela, salamanders and newts; Gymnophiona, caecilians) are well described for only a few species. Further, the majority of such descriptions relate only to temperate species that breed in water, lay their eggs there, and have free-living larvae, the presumed ancestral condition of oviparity. Many species of amphibians have derived reproductive modes. Such modes include breeding terrestrially and arboreally, making foam nests, parental transport of eggs and/or tadpoles, direct development (copulating on land, laying the eggs in terrestrial sites, fully metamorphosed juveniles hatching, obviating the free-living larval stage). Other derived modes are ovoviviparity (developing embryos retained in the oviducts, born at a diversity stages of development, no maternal nutrition in addition to yolk) and viviparity (oviductal retention of developing young, maternal nutrition after yolk is resorbed, young born as fully metamorphosed juveniles). The amphibian oviduct is regionally differentiated to secrete varying numbers of layers of material around each egg, which function in fertilization, etc.; it is responsive to endocrine output and environmental mediation during the reproductive cycle; and it maintains developing embryos in some members of all three orders, some with oviductal epithelial secretion of nutrients. However, little is known of the structure-function relationships of the oviduct in species with derived reproductive modes. A comparison of oviduct morphology, function, endocrinology, ecology and phylogeny in amphibians with diverse reproductive modes suggests a number of highly productive avenues of investigation.

Development of the tectum in Gymnophiones, with comparison to other amphibians

Tectal development in a number of caecilian (Gymnophiona: Amphibia) species was examined and compared with that in frogs and salamanders. The caecilian optic tectum develops along the same rostrocaudal and lateromedial gradients as those of frogs and salamanders. However, differences exist in the time course of development. Our data suggest that, as in salamanders, simplification of morphological complexity in caecilians is due to a retardation or loss of late developmental stages. Differences in the time course of development (heterochrony) among different caecilian species are correlated with phylogenetic history as well as with variation in life histories. The most pronounced differences in development occur between the directly developing Hypogeophis rostratus and all other species examined. In this species, the increase in the degree of morphological complexity is greatly accelerated. J. Morphol. 236:233-246, 1998. © 1998 Wiley-Liss, Inc.

Motor nuclei of nerves innervating the tongue and hypoglossal musculature in a caecilian (amphibia:gymnophiona), as revealed by HRP transport

The organization of the motor nuclei of the glossopharyngeal, vagal, occipital, first spinal and second spinal nerves of Typhlonectes natans (Amphibia: Gymnophiona: Caeciliaidae: Typhlonectinae) was studied by using horseradish peroxidase reaction staining. Each nucleus has discrete patterns of cytoarchitecture and of topography. Nuclei are elongate and some overlap anteroposteriorly. The brainstem is elongate, with no distinct demarcation of brainstem from spinal cord. The occipital nerve emerges through a separate foramen from that for the vagus and glossopharyngeal nerves in the species studied, is distinct from both, and its nucleus is more similar to spinal nuclei in cytoarchitecture. The occipital nerve fuses with spinal nerves 1 and 2 to contribute to the hypoglossal trunk. A spinal accessory nerve is absent.

Evolutionary diversification of cranial and spinal nerves and their targets in the gymnophione amphibians

The ancestral gymnophione (caecilian) became elongate and limbless early in its history. Further structural reduction, including loss of components, followed by redirection and occasionally cooption of remaining structures, particularly muscles and nerves, has evolved. Function may remain that of the ancestral condition, or may change dramatically. Examples include the evolution of the tentacle, the jaw-closing apparatus, and the hypobranchial musculature, including their innervation. 'Regressive' evolution has provided opportunity for new structural-functional relationships and for increased variation.

Patterns of peripheral innervation of the tongue and hyobranchial apparatus in caecilians (Amphibia: Gymnophiona)

The innervation of the musculature of the tongue and the hyobranchial apparatus of caecilians has long been assumed to be simple and to exhibit little interspecific variation. A study of 14 genera representing all six families of caecilians demonstrates that general patterns of innervation by the trigeminal, facial, glossopharyngeal, and vagus nerves are similar across taxa but that the composition of the "hypoglossal" nerve is highly variable. Probably in all caecilians, spinal nerves 1 and 2 contribute to the hypoglossal. In addition, in certain taxa, an "occipital," the vagus, and/or spinal 3 appear to contribute fibers to the composition of the hypoglossal nerve. These patterns, the lengths of fusion of the contributing elements, and the branching patterns of the hypoglossal are assessed according to the currently accepted hypothesis of phylogenetic relationships of caecilians, and of amphibians. An hypothesis is proposed that limblessness and a simple tongue, with concomitant reduced complexity of innervation of muscles associated with limbs and the tongue, has released a constraint on pattern of innervation. As a consequence, a greater diversity and, in several taxa, greater complexity of neuroanatomical associations of nerve roots to form the hypoglossal are expressed.