Development of the mandibular, hyoid arch and gill arch skeleton in the Chinese barb Puntius semifasciolatus: comparisons of ossification sequences among Cypriniformes.

The morphogenesis and sequence of ossification and chondrification of skeletal elements of the jaws, and hyoid arch and gill arches of Puntius semifasciolatus are described. These data provide a baseline for further studies and enable comparisons with other described cypriniforms. Some general patterns of ossification in the hyoid arch and branchial arches in cypriniforms were notable. First, the overall development is from anterior to posterior, with the exception of the fifth ceratobranchial bone, which ossifies first. Second, where ossification of iterated elements is sequential, it tends to proceed from posterior to anterior, even when more posterior chondrifications are the smallest in the series. Ossification of the ceratobranchial, epibranchial and pharyngobranchial bones tends to proceed from ventral to dorsal. The comparisons revealed small sets of skeletal elements whose ossification sequence appears to be relatively conserved across cyprinid cypriniforms. Several potentially key timing changes in the ossification sequence of the jaws, hyoid arch and gill arches were identified, such as the accelerated timing of ossification of the fifth ceratobranchial bone, which may be unique to cypriniforms.

Gdf5 is expressed in the developing skeleton of median fins of late-stage zebrafish, Danio rerio.

The dynamic expression of Gdf5 is described in the developing skeleton of the median fins of late-stage zebrafish, Danio rerio (6-45 days post-fertilization). In situ hybridization revealed expression in the mesenchyme between cartilage condensations of the endoskeletal supports of the dorsal, anal, and caudal fins. As development proceeds, the expression domains expand distally to surround tips of developing cartilages, consistent with a role in cartilage growth and differentiation. Gdf5 is later expressed in the segmenting regions of the dorsal and anal fin radials, which may indicate a role in segmentation. After growth to 7.5 mm, Gdf5 transcripts can no longer be detected in any of the median fins, nor is Gdf5 expression reinitiated in later development of the median fin skeleton.

An experimental study of intraspecific variation, developmental timing, and heterochrony in fishes.

Heterochrony is widely regarded as an important evolutionary mechanism, one that may underlie most, if not all, morphological evolution, yet relatively few studies have examined variation in the sequence of development. Even fewer studies have been designed so that intraspecific variation in the relative sequence of developmental events can be assessed, although this variation must be the basis for evolutionary change. Intraspecific variation in developmental ossification sequences was documented from the zebrafish (Danio rerio) by Cubbage and Mabee (1996) and from the Siamese fighting fish (Betta splendens) by Mabee and Trendler (1996), but a quantitative analysis of the patterns within this variation was not made. Here, we quantify the effect of rearing temperature on the sequence of ossification and characterize the levels and patterns of intraspecific variation in these fishes. For Danio, there were no temperature effects on the sequence of bone development across the cranium, cranial region development, cartilage versus dermal bones, or lateral line bone versus nonassociated bones. Likewise the level of variation in relative sequence (position) of ossification was low, about two ranks, across temperatures. At higher temperatures, we found higher levels of variation in iterated cranial bones and less in bones forming early in the sequence. No temperature effects on variation were found among regions, between lateral line-associated bones and nonassociated bones, between median and paired bones, or across the entire sequence, indicating concordant variability among the three temperatures. Individual bones with the highest levels of variability were not consistent among temperatures. Baseline patterns of intraspecific variation in Danio were compared to those of Betta. For both species, the level of intraspecific variation in sequence position was low and the variability of cranial bones was concordant. Individual bones with the highest levels of variability were not consistent between species. In both species, variation was widespread (distributed evenly across the sequence). We used comparisons (among regions, between dermal and cartilage bones, between lateral line-associated and other bones, between median and paired bones, between iterated and noniterated bones, between feeding-associated bones and others) to see which subsets were most variable and thus potentially useful in predicting high levels of evolutionary change. The only subset of bones that was significantly more variable than others was cartilage bones. If interspecific patterns are parallel to these intraspecific differences, cartilage bones would be expected to show higher levels of heterochrony. Although concordance across the cranial ossification sequence and among regions in Danio, Betta, and two other teleosts, Oryzias and Barbus, suggests an evolutionarily conserved pattern of ossification, identity in sequence position across taxa was not observed for any bone. Thus, variation existed in sequence position across temperatures and species. Intraspecific variation of this sort may influence the morphological outcome and evolutionary trajectories of species.

A dual embryonic origin for vertebrate mechanoreceptors.

Neuromasts, the mechanoreceptors of the lateral line system of fishes and aquatic amphibians, have previously been thought to develop exclusively from embryonic epidermal placodes. Use of fate mapping techniques shows that neuromasts of the head and body of zebrafish, Siamese fighting fish, and Xenopus are also derived from neural crest. Neural crest migrates away from the neural tube in developing vertebrates to form much of the peripheral nervous system, pigment cells, and skeletal elements of the head. The data presented here demonstrate that neuromasts are derived from both neural crest and epidermal placodes.