Osmoregulation in Elasmobranchs

Osmoregulatory mechanisms were examined in major groups of fishesincluding hagfish, holocephalans, elasmobranchs, the coelacanth,and ray-fin fishes. Four main patterns of body fluid compositionemerged. These represent the three main osmoregulatory processesin the marine environment and the one in fresh water. Some possibleinterrelationships among these four types are discussed. Urearetention in marine elasmobranchs and the coelacanth, althoughsimilar, may have been acquired independently during evolution.The importance of gills and the rectal gland in elasmobranchosmoregulation is discussed. The importance of amino acids inintracellular osmoregulation in elasmobranchs is also reviewed.Recent studies on water and electrolyte regulation in freshwaterstingrays are summarized.     

Patterns in the Evolution of Ungulate Jaw Shape

A survey of biomechanically significant aspects of the jaw apparatusin 13 orders of recent and fossil ungulates indicates that anexpanded angular process evolved independently at least 12 times.Temporal fossa size is reduced inmost ungulatesof modern aspect,but not in many early ungulates. Expansion of the masseter andinternal pterygoid musculature (inferred from the expanded angularprocess) may represent an adaptation for enhanced control ofgrinding action in mastication.     

The Acoustical Biology of Elasmobranchs

A report on the auditory capabilities and their associated functions of elasmobranch fishes along with a brief review of the physics of underwater sound as it relates to hearing by fishes is provided. The inner ears of elasmobranchs possess structures that are no different from other fishes, except for an enlarged macula neglecta, which is unique among fishes. Hearing abilities among sharks (the only elasmobranchs examined) have demonstrated highest sensitivity to low frequency sound (40Hz to approximately 800Hz), which is sensed solely through the particle-motion component of an acoustical field. Free-ranging sharks are attracted to sounds possessing specific characteristics: irregularly pulsed, broad-band (most attractive frequencies: below 80Hz), and transmitted without a sudden increase in intensity. Such sounds are reminiscent of those produced by struggling prey. A sound, even an attractive one, can also result in immediate withdrawal by sharks from a source, if its intensity suddenly increases 20dB [10 times] or more above a previous transmission. Present evidence also shows that the lateral line system does not respond to normal acoustical stimuli. Morphological and physiological evidence suggest that the maculae neglecta possess acoustical relevance and may explain directionality of response despite physical principles that provide still other hypotheses for acoustical directionality. Brain centers controlling acoustical response, particularly among sharks, must be explored in the near future with careful consideration of the habits of subjects based on indications of species-differences regarding the importance of acoustical stimuli to their activities. Numerous facts and ideas about the acoustical biology of elasmobranch fishes make certain that future research will be most rewarding.     

Ovarian Control in Gyclostomes and Elasmobranchs

All vertebrates show some degree of periodicity in their reproductivephysiology, and there is evidence that light and temperatureare the external stimuli most commonly involved in the regulationof reproductive cycles. Furthermore, except in cyclostomes andelasmobranchs it is already evident that these cues are integratedby the hypothalamus which in turn controls the gonadotrophicactivity of the pituitary gland. The existence of such mechanismshas not as yet been demonstrated in these two groups thoughmore work is essential before it can be said that they do notexist. In lampreys the gonadotrophic function of the pituitary appearsto be restricted to controlling rate of growth of the oocytesand steroidogenesis. In hagfishes the situation is less wellunderstood though potentially more interesting, since both corporalutea and corpora atretica are encountered in these animals. The main, if not the only, gonadotrophic region of the elasmobranchpituitary is its ventral lobe. Nothing is known of the mechanismswhich control the cyclical activity of this lobe, though suchactivity must be postulated in at least some elasmobranchs.Vitellogenesis appears to be under pituitary control since removalof the ventral lobe prevents it and causes the granulosa tolose its vitellogenic function and become phagocytic. It alsoseems likely that steroidogenesis by the elasmobranch ovaryis under pituitary control though this has not been demonstrateddirectly. Only further work will reveal whether parts of thepituitary other than the ventral lobe are also implicated inovarian control in elasmobranchs.     

Biochemical Mechanisms for Regulating Protrusion by Nematode Major Sperm Protein

Crawling motion is ubiquitous in eukaryotic cells and contributes to important processes such as immune response and tumor growth. To crawl, a cell must adhere to the substrate, while protruding at the front and retracting at the rear. In most crawling cells protrusion is driven by highly regulated polymerization of the actin cytoskeleton, and much of the biochemical network for this process is known. Nematode sperm utilize a cytoskeleton composed of Major Sperm Protein (MSP), which is considered to form a simpler, yet similar, crawling motility apparatus. Key components involved in the polymerization of MSP have been identified; however, little is known about the chemical kinetics for this system. Here we develop a model for MSP polymerization that takes into account the effects of several of the experimentally identified cytosolic and membrane-bound proteins. To account for some of the data, the model requires force-dependent polymerization, as is predicted by Brownian ratchet mechanisms. Using the tethered polymerization ratchet model with our biochemical kinetic model for MSP polymerization, we find good agreement with experimental data on MSP-driven protrusion. In addition, our model predicts the force-velocity relation that is expected for in vitro protrusion assays.     

Essential Role of Nr2f Nuclear Receptors in Patterning the Vertebrate Upper Jaw

1. Download high-res image (167KB)
2. Download full-size image

     

Relationships of Fossil and Living Elasmobranchs

Preliminary cladistic analysis corroborates the hypothesis thatthe Elasmobranchii and the Holocephali are sister groups, andthat the Chondrichthyes are more closely related to the Teleostomi(Acanthodi plus Osteichthyes) than either is to the Placodermi.In regard to the Paleozoic and Mesozoic elasmobranchs, a theoryof relationships has been proposed for six better known genera(Xenacanthus, Denaea, Cladoselache, Hybodus, Ctenacanthus andPaleospinax) by evaluation of certain characters in the skull,postcranial axial skeleton, fin supports, and fins.     

Common Mechanisms of Y Chromosome Evolution

Y chromosome evolution is characterized by the expansion of genetic inertness along the Y chromosome and changes in the chromosome structure, especially the tendency of becoming heterochromatic. It is generally assumed that the sex chromosome pair has developed from a pair of homologues. In an evolutionary process the proto-Y-chromosome, with a very short differential segment, develops in its final stage into a completely heterochromatic and to a great extends genetically eroded Y chromosome. The constraints evolving the Y chromosome have been the objects of speculation since the discovery of sex chromosomes. Several models have been suggested. We use the exceptional situation of the in Drosophila mirandato analyze the molecular process in progress involved in Y chromosome evolution. We suggest that the first steps in the switch from a euchromatic proto-Y-chromosome into a completely heterochromatic Y chromosome are driven by the accumulation of transposable elements, especially retrotransposons inserted along the evolving nonrecombining part of the Y chromosome. In this evolutionary process trapping and accumulation of retrotransposons on the proto-Y-chromosome should lead to conformational changes that are responsible for successive silencing of euchromatic genes, both intact or already mutated ones and eventually transform functionally euchromatic domains into genetically inert heterochromatin. Accumulation of further mutations, deletions, and duplications followed by the evolution and expansion of tandem repetitive sequence motifs of high copy number (satellite sequences) together with a few vital genes for male fertility will then represent the final state of the degenerated Y chromosome. This revised version was published online in July 2006 with corrections to the Cover Date.     

Elasmobranchs from the upper Paleocene of Togo

Bulk sampling and surface collecting of two glauconitic horizons located in Southern Togo yielded a diverse elasmobranch fauna described here. This fauna includes 30 species dominated by carcharhiniforms (eleven species), myliobatiforms (nine species) and lamniforms (five species); it also comprises three orectolobiforms, whereas the squatiniforms and rhinopristiforms are represented by one species each. Although the poor preservation of the specimens hampered numerous species-level identifications, the vast majority of taxa identified were formerly reported from the Paleocene–Ypresian interval, four of which being exclusively known from the upper Paleocene. This, along with the six associated benthic foraminifera species, indicates a late Thanetian age for the sampled horizons and provide age constraints on a geographically widespread benchmark horizon in Western Africa. The composition of the elasmobranch assemblage shows strong resemblances with upper Paleocene faunas from Morocco and differs markedly from known assemblages from geographically closer localities in Niger and Nigeria, which suggests strong palaeoenvironmental control on the distribution of Thanetian elasmobranch diversity.     

Ion and Osmoregulation in Prenatal Elasmobranchs: Evolutionary Implications

The elasmobranchs represent a fascinating series of experimentsin the evolution of maternal support for developing embryos.In oviparous species, eggs are enclosed in a tough, fibrouscapsule. The capsule is very permeable and the embryonic tissuesare bathed in a solution ionically similar to sea water withinhours of oviposition. In the primitively viviparous speciesSqualus acanthias, early embryos in egg capsules are retainedin utero and are bathed in a solution similar to maternal plasma.Several months into the 22 month gestation period the embryosare capable of independent iono- and osmoregulation in a uterinesolution that resembles sea water. Embryos of more advancedviviparous species develop in a solution that is similar tomaternal plasma. Iono- and osmoregulation by these embryos wouldappear to beminimal. It is clear that in the oviparous elasmobranchs,the ability of the egg/embryo to maintain salts and urea atappropriate levels is present at the earliest stage of development.The ability of prenatal elasmobranch embryos to iono- and osmoregulatewould allow the evolution ofa diverse array of reproductivestrategies in the elasmobranchs.     

Apoptosis in Unicellular Organisms: Mechanisms and Evolution

Data about the programmed death (apoptosis) in unicellular organisms, from bacteria to ciliates, are discussed. Firstly apoptosis appeared in lower eukaryotes, but its mechanisms in these organisms are different from the classical apoptosis. During evolution, the apoptotic process has been improving gradually, with reactive oxygen species and Ca2+ playing an essential role in triggering apoptosis. All eukaryotic organisms have apoptosis inhibitors, which might be introduced by viruses. In the course of evolution, caspases and apoptosis-inducing factor appeared before other apoptotic proteins, with so-called death receptors being the last among them. The functional analogs of eukaryotic apoptotic proteins take parts in the programmed death of bacteria.