Molecular evidence on the origin of tetrapods and the relationships of the coelacanth

Coelacanths were believed to have gone extinct more than 80 million years ago - until the sensational rediscovery of one surviving member of this leneage, Latimeria chalumanae, in 1938. Since then, plaeontologists and comparative morphologists have argues whether coelacanths or lungfish (two groups of lobe-finned fish) are the living sistergroup of the third extant lineage, the tetrapods. Recent molecular phylogenetic data on this debate tend to favor the hypothesis that lungfish are the closest relatives of land vertebrates. Somewhat surprisingly, the strongest molecular support for this hypothesis stems from mitochondrial rather than nuclear DNA sequences, despite the expectation that the more-slowly evolving nuclear genes should be more appropriate in addressing a phylogenetic issue involving taxonomic groups that diverged around 400 million years ago. This molecular estimate might serve as a framework to test palepntological and physiological innovations and preadaptations that allowed Devanian lobe-finned fish to colonize land.

Morphology, characterization, and distribution of retinal photoreceptors in the Australian lungfish Neoceratodus forsteri (Krefft, 1870)

The Australian lungfish Neoceratodus forsteri (Dipnoi) is an ancient fish that has a unique phylogenetic relationship among the basal Sarcopterygii. Here we examine the ultrastructure, histochemistry, and distribution of the retinal photoreceptors using a combination of light and electron microscopy in order to determine the characteristics of the photoreceptor layer in this living fossil. Similar proportions of rods (53%) and cones (47%) reveal that N. forsteri optimizes both scotopic and photopic sensitivity according to its visual demands. Scotopic sensitivity is optimized by a tapetum lucidum and extremely large rods (18.62 +/- 2.68 microm ellipsoid diameter). Photopic sensitivity is optimized with a theoretical spatial resolving power of 3.28 +/- 0.66 cycles degree(-1), which is based on the spacing of at least three different cone types: a red cone containing a red oil droplet, a yellow cone containing a yellow ellipsoidal pigment, and a colorless cone containing multiple clear oil droplets. Topographic analysis reveals a heterogeneous distribution of all photoreceptor types, with peak cone densities predominantly found in temporal retina (6,020 rods mm(-2), 4,670 red cones mm(-2), 900 yellow cones mm(-2), and 320 colorless cones mm(-2)), but ontogenetic changes in distribution are revealed. Spatial resolving power and the diameter of all photoreceptor types (except yellow cones) increases linearly with growth. The presence of at least three morphological types of cones provides the potential for color vision, which could play a role in the clearer waters of its freshwater environment.

Evolution of myosin filament arrangements in vertebrate skeletal muscle

A survey of skeletal muscles throughout craniates shows basic kinds of myosin filament arrangement, simple-lattice and superlattice, within the A-band of each sarcomere. Distribution of simple- and superlattice arrangements across a phylogeny of craniates suggests that the superlattice arrangement is primitive and that Amia and teleosts are derived in showing simple-lattice arrangements. Two taxa examined (Scyliorhinus and Acipenser) show both lattice types within the same organism implying that there is not a simple evolutionary transformation of one to the other fiber arrangement. We discuss the possible functional significance of the different lattice types. We believe that the crossbridges may have greater competition for actin binding sites in simple-lattice muscles compared to the superlattice types.