It was a zoological sensation when a living specimen of the coelacanth was first discovered in 1938, as this lineage of lobe-finned fish was thought to have gone extinct 70 million years ago. The modern coelacanth looks remarkably similar to many of its ancient relatives, and its evolutionary proximity to our own fish ancestors provides a glimpse of the fish that first walked on land. Here we report the genome sequence of the African coelacanth, Latimeria chalumnae. Through a phylogenomic analysis, we conclude that the lungfish, and not the coelacanth, is the closest living relative of tetrapods. Coelacanth protein-coding genes are significantly more slowly evolving than those of tetrapods, unlike other genomic features . Analyses of changes in genes and regulatory elements during the vertebrate adaptation to land highlight genes involved in immunity, nitrogen excretion and the development of fins, tail, ear, eye, brain, and olfaction. Functional assays of enhancers involved in the fin-to-limb transition and in the emergence of extra-embryonic tissues demonstrate the importance of the coelacanth genome as a blueprint for understanding tetrapod evolution.

Condensation is the pivotal stage in the development of skeletal and other mesenchymal tissues. It occurs when a previously dispersed population of cells gathers together to differentiate into a single cell/tissue type such as cartilage, bone, muscle, tendon, kidney, and lung and is the earliest stage during organ formation when tissue‐specific genes are upregulated. We present a synopsis of our current understanding of how condensations are initiated and grown, how their boundaries and sizes are set, how condensation ceases, and how overt differentiation begins. Extracellular matrix molecules, cell surface receptors and cell adhesion molecules, such as fibronectin, tenascin, syndecan, and N‐CAM, initiate condensation formation and set condensation boundaries. Hox genes (Hoxd‐11‐13) and other transcription factors (CFKH‐1, MFH‐1, osf‐2), modulate the proliferation of cells within condensations. Cell adhesion is ensured indirectly through Hox genes (Hoxa‐2, Hoxd‐13), and directly via cell adhesion molecules (N‐CAM and N‐cadherin). Subsequent growth of condensations is regulated by BMPs, which activate Pax‐2, Hoxa‐2 and Hoxd‐11 among other genes. Growth of a condensation ceases when Noggin inhibits BMP signalling, setting the stage for transition to the next stage of skeletal development, namely overt cell differentiation. BioEssays 22:138–147, 2000. ©2000 John Wiley & Sons, Inc.

Elements of the vertebrate skeleton are initiated as cell condensations, collectively termed the 'membranous skeleton' whether cartilages or bones by Grüneberg (1963). Condensations, which were identified as the basic cellular units in a recent model of morphological change in development and evolution (Atchley and Hall 1991) are reviewed in this paper. Condensations are initiated either by increased mitotic activity or by aggregation of cells towards a centre. Prechondrogenic (limb bud) and preosteogenic (scleral ossicle) condensations are discussed and contrasted. Both types of skeletogenic condensations arise following epithelial-mesenchymal interactions; condensations are identified as the first cellular product of such tissue interactions. Molecular characteristics of condensations are discussed, including peanut agglutinin lectin, which is used to visualize prechondrogenic condensations, and hyaluronan, hyaladherins, heparan sulphate proteoglycan, chondroitin sulphate proteoglycan, versican, tenascin, syndecan, N-CAM, alkaline phosphatase, retinoic acid and homeo-box-containing genes. The importance for the initiation of chondrogenesis or osteogenesis of upper and lower limits to condensation size and the numbers of cells in a condensation are discussed, as illustrated by in vitro studies and by mutant embryos, including Talpid3 in the chick and Brachypod, Congenital hydrocephalus and Phocomelia in the mouse. Evidence that genes specific to the skeletal type are selectively activated at condensation is discussed, as is a recent model involving TGF-beta and fibronectin in condensation formation. Condensations emerge as a pivotal stage in initiation of the vertebrate skeleton in embryonic development and in the modification of skeletal morphology during evolution.

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The developmental stages of Oreochromis niloticus are similar to those described in other mouthbreeding tilapias except that, as in zebrafish, no cavity was found in the blastula. Variation in the rate of development of the embryo and larva of O. niloticus was found within a clutch of eggs as well as between clutches. Hatching glands are described for the first time in tilapias. They are widely distributed within the ectoderm covering the head, body, tail, and surface of the yolk sac near its attachment to the embryo. Timing of larval development is similar to that in other mouthbrooding tilapias, but is slower than that found in substrate-spawning tilapias. A pneumatic duct connects the swimbladder to the digestive tract and swimbladder inflation and initiation of feeding occurs at about the same time. The digestive tract of the larva 8 and 9 days after fertilization is similar to that found in the adult, except that there are no digestive glands. An endocrine pancreatic islet was first seen 76 h after fertilization. A prominent thymus gland is present at 100 h. Hematopoietic tissue develops in the vicinity of the pronephros during early larval development. A spleen develops later, 7 days after fertilization.

A series of studies by Edgeworth demonstrated that cranial muscles of gnathostome fishes are embryologically of somitic origin, originating from the mandibular, hyoid, branchial, epibranchial, and hypobranchial muscle plates. Recent experimental studies using quail-chick chimeras support Edgeworth's view on the developmental origin of cranial muscles. One of his findings, the existence of the premyogenic condensation constrictor dorsalis in teleost fishes, has also been confirmed by molecular developmental studies. Therefore, developmental mechanisms for patterning of cranial muscles, as described and implicated by Edgeworth, may serve as structural entities or regulatory phenomena responsible for developmental and evolutionary changes. With Edgeworth's and other studies as background, muscles in the ventral gill arch region of batoid fishes are analyzed and compared with those of other gnathostome fishes. The spiracularis is regarded as homologous at least within batoid fishes, but its status within elasmobranchs remains unclear; developmental modifications of the spiracularis proper are evident in some batoid fishes and in several shark groups. The peculiar ventral extension of the spiracularis in electric rays and some stingrays may represent convergence, probably facilitating ventilation and/or feeding in both groups. The evolutionary origin of the "internus" and "externus" remains uncertain, despite the fact that a variety of forms of the constrictor superficiales ventrales in batoid fishes indicates an actual medio-ventral extension of the "externus." The intermandibularis is probably present only in electric rays. The "X" muscle occurs only in electric rays and is considered to be Edgeworth's intermandibularis profundus. Its association with the adductor mandibular complex in narkinidid and narcinidid electric rays may relate to its functional role in lower jaw movement. Contrary to common belief, in most batoid fishes as well as some sharks, muscles that originate from the branchial muscle plate and extend medially in the ventral gill arches do exist: the medial extension of the interbranchiales in most batoid fishes and some sharks and the "Y" muscle in the pelagic stingrays Myliobatos and Rhinoptera. The latter is another example of the medial extension of the "internus." Whether the interbranchiales and "Y" muscle are homologous within elasmobranchs and whether homologous with the obliques ventrales and/or transversi ventrales of osteichthyan fishes await further research. Four hypobranchial muscles are recognized in batoid fishes: the coracomandibularis, coracohyoideus, coracoarcualis, and coracohyomandibularis. The coracohyoideus is discrete from the coracoarcualis; its complete structural separation from the latter occurs in several groups of batoid fishes.(ABSTRACT TRUNCATED AT 400 WORDS)

The living coelacanth is a lobe-finned fish that represents an early evolutionary departure from the lineage that led to land vertebrates, and is of extreme interest scientifically. It has changed very little in appearance from fossilized coelacanths of the Cretaceous (150 to 65 million years ago), and is often referred to as a "living fossil." An important general question is whether long-term stasis in morphological evolution is associated with stasis in genome evolution. To this end we have used targeted genome sequencing for acquiring 1,612,752 bp of high quality finished sequence encompassing the four HOX clusters of the Indonesian coelacanth Latimeria menadoensis. Detailed analyses were carried out on genomic structure, gene and repeat contents, conserved noncoding regions, and relative rates of sequence evolution in both coding and noncoding tracts. Our results demonstrate conclusively that the coelacanth HOX clusters are evolving comparatively slowly and that this taxon should serve as a viable outgroup for interpretation of the genomes of tetrapod species.genomics | Latimeria menadoensis | BAC clone

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