Suspension and optical properties of the crystalline lens in the eyes of basal vertebrates

We have investigated the apparatus suspending the crystalline lens in the eyes of basal vertebrates. Data are presented for Holocephali (Chondrichthyes) and the actinopterygians Polypteriformes, Polyodontidae (Acipenseriformes), Lepisosteiformes, Amiiformes, and one teleost species, the banded archerfish (Toxotes jaculatrix). We also studied the optical properties of the lens in Polypteriformes, Lepisosteiformes, and the archerfish. Together with previously published results, our findings show that there are three basic types of lens suspension in vertebrates. These are i) a rotationally symmetric suspension (Petromyzontida, lampreys; Ceratodontiformes, lungfishes; Tetrapoda), ii) a suspension with a dorso‐ventral axis of symmetry and a ventral papilla (all Chondrichthyes and Acipenseriformes), and iii) an asymmetric suspension with a ventral muscle and a varying number of ligaments (all Actinopterygii except for Acipenseriformes). Large eyes with presumably high spatial resolution have evolved in all groups. Multifocal lenses creating well‐focused color images are also present in all groups studied. Stable and exact positioning of the lens, in many cases in combination with accommodative changes in lens position or shape, is achieved by all three types of lens suspension. It is somewhat surprising that lens suspensions are strikingly similar in Chondrichthyes and Acipenseriformes (Actinopterygii), while the suspension apparatus in Polypteriformes, usually being regarded as an actinopterygian group more basal than Acipenseriformes, are considerably more teleostean‐like. This study completes a series of investigations on lens suspensions in nontetrapod vertebrates, covering all major groups except for the rare and highly derived coelacanths. J. Morphol. 275:613–622, 2014. © 2013 Wiley Periodicals, Inc.     

The phylogenetic placement of Chondrichthyes: inferences from analysis of multiple genes and implications for comparative studies

Elasmobranch fishes (sharks and rays) have proven valuable for inferring general and specific properties of molecular evolution through comparative studies with crown group vertebrates because they are the most ancient group of gnathostomes. Recent studies have questioned the conventional phylogenetic placement of sharks in the vertebrate tree, however. In this paper I review the importance of the basal position of Chondrichthyes for comparative biology and compile evidence from multiple, independent genes to evaluate the phylogenetic placement of sharks. The results suggests that alternative phylogenetic hypotheses of the relationships among the Chondrichthyes, Actinopterygii and Sarcopterygii can not be refuted with available data, implying that the assumption of the basal placement of sharks in the vertebrate tree is suspect. Resolving the phylogeny of basal vertebrates is important for testing hypotheses about the evolution of vertebrates, and the current lack of a robust phylogeny limits evolutionary inferences that can be gained from comparative studies that include sharks and rays.     

A fibrous membrane suspends the multifocal lens in the eyes of lampreys and African lungfishes

The sharpness and thus information content of the retinal image in the eye depends on the optical quality of the lens and its accurate positioning in the eye. Multifocal lenses create well‐focused color images and are present in the eyes of all vertebrate groups studied to date (mammals, reptiles including birds, amphibians, and ray‐finned fishes) and occur even in lampreys, i.e., the most basal vertebrates with well‐developed eyes. Results from photoretinoscopy obtained in this study indicate that the Dipnoi (lungfishes), i.e., the closest piscine relatives to tetrapods, also possess multifocal lenses. Suspension of the lens is complex and sophisticated in teleosts (bony fishes) and tetrapods. We studied lens suspension using light and electron microscopy in one species of lamprey (Lampetra fluviatilis) and two species of African lungfish (Protopterus aethiopicus aethiopicus and Protopterus annectens annectens). A fibrous and highly transparent membrane suspends the lens in both of these phylogenetically widely separated vertebrate groups. The membrane attaches to the lens approximately along the lens equator, from where it extends to the ora retinalis. The material forming the membrane is similar in ultrastructure to microfibrils in the zonule fibers of tetrapods. The membrane, possibly in conjunction with the cornea, iris, and vitreous body, seems suitable for keeping the lens in the correct position for well‐focused imaging. Suspension of the lens by a multitude of zonule fibers in tetrapods may have evolved from a suspensory membrane similar to that in extant African lungfishes, a structure that seems to have appeared first in the lamprey‐like ancestors of allextant vertebrates. J. Morphol. 271:980–989, 2010. © 2010 Wiley‐Liss, Inc.     

Coevolution of the Monogenoidea (Platyhelminthes) based on a revised hypothesis of parasite phylogeny

A revised hypothesis for the phylogeny of the Subclass Polyonchoinea (Monogenoidea) was contructed employing phylogenetic systematics. The Acanthocotylidae (formerly of the Order Capsalidea) is transferred to the Order Gyrodactylidea based on this analysis. The new phylogeny is used to determine coevolutionary relationships of the familial taxa of Monogenoidea with their hosts. The coevolutionary analysis suggests that the Monogenoidea apparently underwent sympatric speciation or dispersal while parasitic on ancestral Guathostomata, resulting in two primary clades: the Polyonchoinea and the Oligonchoinea + Polystomatoinea. The two parasite clades apparently cospeciated independently with divergence of the Chondrichthyes and Osteichthyes. In the Polyonchoinea, the clade associated with Chondrichthyes experienced primary extiaction within the Holocephala, but coevolved into the Loimoidae and Monocotylidae in the Galeomorphii and Squalea (Elasmobranchii), respectively. Within the Osteichthyes, polyonchoineans experienced primary extinction with the divergence of Sarcopterygii, Polypteriformes and Acipenseriformes. They demonstrate primary dispersal from the Neopterygii into the Squalea (as Amphibdellatinea), Actinistia (as Neodactylodiscinea) and Urodela (as Lagarocotylidea). Secondary dispersals of polyonchoineans occurred in the Gyrodactylidae to the Polypteriformes, Urodela and Anura; in the Acanthocotylidae to the Myxinoidea and Squalea; in the Capsalidae to the Acipenseriformes and Elasmobranchii; and in the Monocotylidae to the Helocephala. The Oligonchoinea and Polystomatoinea developed upon divergence of the Chondrichthyes and Osteichthyes. Oligonchoineans cospeciated within the Chondrichthyes, with the Chimaericolidea developing within the Helocephala and the ancestor of the Diclybothriidea + Mazocraeidea within the Elasmobranchii. Two cases of primary dispersal occurred within this clade: the Diclybothriidae to the Acipenseriformes and the ancestor of mazocracidean families to the Neopterygii (both Osteichthyes). Secondary dispersal within the Oligonchoinea includes host switching of the common ancestor of Callorhynchocotyle (Hexabothriidae) to the Holocephala. Polystomatoineans coevolved within the Osteichthyes, but experienced primary extinctions in the Actinopterygii, Actinistia, Dipnoi and Amniota. Coevolution of the Sphyranuridae and Polystomatidae occurred with divergence of the Urodela and Anura, respectively. Secondary dispersal of polystomatids to the Urodela, Dipnoi and Amniota is suggested. A preliminary phylogenetic analysis of the Polystomatoinea suggests that primary extinction with secondary dispersal of polystomatids to the Dipnoi may not be necessary to explain extant parasite distributions, since Concinnocotyla (Concinnocotylinae) appears to represent the sister taxon of the remaining Polystomatidae + Sphyranuridae.     

Molecular phylogenetics of gnathostomous (jawed) fishes: old bones, new cartilage

Cartilaginous fishes (chondrichthyans) have traditionally been taken as an early offshoot among jawed vertebrates. To examine some crucial chondrichthyan relationships, we have sequenced the mitochondrial genomes of the holocephalan Chimaera monstrosa (ratfish) and the basal galeomorph species Heterodontus francisci (horn shark) and analysed them together with the corresponding data set of several other chondrichthyans, teleosts, the coelacanth, the African lungfish and the bichir. The rooting point of the tree was established using unequivocal outgroups, the sea lamprey , the sea lancelet or echinoderms. The phylogenetic analyses identified monophyletic Chondrichthyes in a terminal position in the piscine tree, lending no support to the traditionally accepted basal position of cartilaginous fishes among extant gnathostomes. The findings suggest that the cartilage characterizing extant chondrichthyans is a retention of an embryonic condition, thus representing a derived rather than a primitive phylogenetic and developmental stage. Similarly, the analyses suggest that the open gill slits of neoselachians (sharks and rays) constitute a derived state compared to the operculum (gill cover) characterizing bony fishes and holocephalans. The analyses did not support the so-called Squalea/Galea hypothesis which posits that batomorphs (sharks, rays) have arisen from recent selachians (sharks). Inconsistent with the common understanding of piscine and gnathostome evolution, the two taxa having lungs, the African lungfish and the bichir, had a basal position in the piscine tree. The findings put into question the phylogenetic validity of the taxonomic nomenclature attributed to various vertebrate, notably piscine, clades.     

Pattern formation in development of chondrichthyan dentitions: a review of an evolutionary model

The mode of tooth development displayed in Chondrichthyans (sharks, rays and holocephalans), one of frequent tooth replacement, was possible once a dental lamina had evolved, and since 1982 this has been known as the odontode regulation theory after Reif. Today, Reif's concepts need to be transformed into those of modern biology, the crosstalk between epithelium and mesenchyme, for the regulation of timing, spacing and shape of vertebrate teeth. Although Reif's proposed ‘primordial tissue’ may be the only site of progenitor cells, to restrict odontogenic potential to time-specific sites (protogerms), as has been suggested in the sequential addition tooth (SAT) model, very little data are available. Here, his model of alternate tooth replacement files has been interpreted as an integrated tooth addition unit of two adjacent files (SAT) unit for alternate replacement of teeth, regulated by putative, precisely timed gene expression for activation and inhibition. We have provided new data on patterns of tooth succession in dentitions of extant sharks and rays to compare with those of Reif. Using a phylogeny combined from molecular and morphological data, it is suggested that the alternate tooth addition and replacement model is derived within Chondrichthyes, and diversified from single file tooth addition of the stem chondrichthyans.     

Virgin birth in a hammerhead shark

Parthenogenesis has been documented in all major jawed vertebrate lineages except mammals and cartilaginous fishes (class Chondrichthyes: sharks, batoids and chimeras). Reports of captive female sharks giving birth despite being held in the extended absence of males have generally been ascribed to prior matings coupled with long-term sperm storage by the females. Here, we provide the first genetic evidence for chondrichthyan parthenogenesis, involving a hammerhead shark (Sphyrna tiburo). This finding also broadens the known occurrence of a specific type of asexual development (automictic parthenogenesis) among vertebrates, extending recently raised concerns about the potential negative effect of this type of facultative parthenogenesis on the genetic diversity of threatened vertebrate species.     

Glial architecture of the ghost shark (Callorhinchus milii, Holocephali, Chondrichthyes) as revealed by different immunohistochemical markers

This article presents the first study on the glial architecture of a representative species of Holocephali, Callorhinchus milii (ghost shark). Holocephali are a small subclass of Chondrichthyes, with only a few extant genera, and those are considered to have a brain organization more similar to squalomorph sharks than to galeomorph sharks, skates, and rays. Three different astroglial markers--glial fibrillary acidic protein, S-100 protein, and glutamine synthetase (GS)--were investigated by immunohistochemical methods, applying both diaminobenzidine (DAB) and fluorescent techniques. They revealed similar glial structures, although most of them were detected by immunohistochemical reaction against GS and visualized by DAB. The predominant elements were radial ependymoglia spanning the area between the ventricular and meningeal surfaces, as in squalomorph sharks. Other similar features were the light appearance of myelinated neural tracts devoid of immunoreactivity, and the glial architecture of the reticular formation of the brain stem, cerebellum, and tectum, the latter with recognizable layers. The immunoreactivity of the vascular walls was similar; however, it is believed that different cell types form the blood-brain barrier in chimeras and in elasmobranchs. Some glial structures, however, resembled those of skates, rays, and galeomorph sharks. In C. milii astrocyte-like elements were observed in the telencephalon, using GS and S-100, although typical astrocyte-rich regions were not found. In some areas, especially the telencephalon, not only endfeet but also cell bodies were observed to be attached to the meningeal surface, with processes extending into the brain substance.     

Tooth development and histology patterns in lamniform sharks (Elasmobranchii,Lamniformes) revisited

The dentition of lamniforme sharks exhibits several characters that have been used extensively to resolve the phylogenetic relationships of extant taxa, yet some uncertainties remain. Also, the development of different teeth of a tooth file within the jaws of most extant lamniforms has not been documented to date. High‐resolution micro‐computed tomography is used here to re‐evaluate the importance of two dental characters within the order Lamniformes, which were considered not to be phylogenetically informative, the histotype and the number of teeth per tooth file. Additionally, the development and mineralization patterns of the teeth of the two osteodont lamniforms Lamna nasus and Alopias superciliosus were compared. We discuss the importance of these dental characters for phylogenetic interpretations to assess the quality of these characters in resolving lamniform relationships. The dental characters suggest that (1) Lamniformes are the only modern‐level sharks exhibiting the osteodont histotype, (2) the osteodont histotype in lamniform sharks is a derived state in modern‐level sharks (Elasmobranchii), (3) the osteodont type, conversely is convergently achieved when the clade Chondrichthyes is considered and thus might comprise a functional rather than a phylogenetic signal, and (4) there is an increase in the number of teeth per file throughout lamniform phylogeny. Structural development of the teeth of L. nasus and A. superciliosus is congruent with a previous investigation of the lamniform shark Carcharodon carcharias. J. Morphol. 277:1584–1598, 2016. © 2016 Wiley Periodicals, Inc.     

Testing morphologically based phylogenetic theories within the cartilaginous fishes with molecular data, with special reference to the catshark family (Chondrichthyes; Scyliorhinidae) and the interrelationships within them

A molecular phylogenetic investigation was conducted to examine phylogenetic relationships between various members of the catsharks (Chondrichthyes; Carcharhiniformes; Scyliorhinidae), and is the largest chondrichthyan data set yet analysed, consisting of nearly 130,000 nucleotides. Three mitochondrial DNA genes were used to construct the phylogenies, cytochrome b, NADH-2, and NADH-4, with 41 sequences from 18 taxa being novel. These sequences were either used separately or combined into a single data set, and phylogenies were constructed using various methods, however, only the Bayesian inference tree derived from the cytochrome b data set was resolved sufficiently for phylogenetic inferences to be made. Interestingly, the family Scyliorhinidae was not supported by the results and was found to be paraphyletic. The Scyliorhininae and Pentanchinae were supported, whereas the Pentanchini clade was present, but not well supported. The Halaelurini hypothesis was supported with Holohalaelurus identified as the basal genus of that clade, and Haploblepharus edwardsii identified as the basal taxon for that genus. Elsewhere within the Chondrichthyes, the Carcharhiniformes and the Lamniformes were found to be monophyletic, and the Heterodontiformes was placed within the Squalimorphs. The placement of the skates and rays in these analyses support the Batoidea as being sister to the Elasmobranchii.     

Comparative study of the crystallin supramolecular structure in the carp, frog, and rat lenses by small-angle roentgen ray scattering

The supramolecular structure of crystallins in intact ocular lenses of carp, frog and rat as well as in the interior (nuclear) and outer (cortical) parts of these lenses was studied by the small-angle X-ray scattering method. The results show that the supramolecular structure of crystallins substantially varies both in lenses of different vertebrate species and in various parts of the same lens. In carp lens and in the cortical part of rat lens, crystallins have an ordered supramolecular structure, as indicated by a small-angle X-ray diffraction maximum in the region of Bragg distances 15-20 nm, whereas in frog lens and in the nuclear part of rat lens, the supramolecular structure of these proteins is disordered. The power-law X-ray scattering by rat lens nucleus may be evidence of fractal structures in the lens. A comparison of these results with literary data indicates that there is no obvious correlation between the type of supramolecular structure of crystallins and their polypeptide composition in lenses of different vertebrate species. The results suggest that the supramolecular ordering (short-range order) of crystallins is not a necessary condition for lens transparency.     

The ultrastructure of cerebral blood capillaries in the raffish,Chimaera monstrosa

Summary Sharks and skates (Chondrichthyes: Elasmobranchii) have a glial blood-brain barrier, while all other vertebrates examined so far have an endothelial barrier. For comparative reasons it is desirable to examine the blood-brain barrier in species from the other subclass of cartilaginous fish, the holocephalans. The ultrastructure of cerebral capillaries in the chimaera (Chondrichthyes: Holocephali) is described in the present study. The endothelial cells are remarkably thick. Fenestrae and transendothelial channels were not observed. The endothelial cells are joined by elaborate tight junctions. The perivascular glial processes are separated by wide spaces (15–60nm) without obliterating junctional complexes. These findings indicate that the chimaera has an endothelial blood-brain barrier.     

Comparison of acid mucin goblet cell distribution and Hox13 expression patterns in the developing vertebrate digestive tract

The digestive tract of vertebrates is a complex organ system required for the digestion of food and the absorption of nutrients. The colon evolved as a water absorption organ essential for vertebrates to survive on land. In contrast to land vertebrates, the Chondrichthyes (sharks, skates and rays) are nearly iso-osmotic with their ocean environment and do not reabsorb water from food waste. To understand the origin of the vertebrate colon, we examined the distribution of sulfated and sialyated mucus-producing cells in the little skate, Raja erinacea, as an indication of water absorption function in the chondrichthian digestive tract. The percentage of acid mucin producing goblet cells was analyzed in the spiral valve and hindgut of little skate and the small intestine and colon of mouse embryos. Levels of acid mucins in the hindgut of the little skate was comparable to that of the small intestines of terrestrial vertebrates, whereas the distal region of the spiral valve contained high levels of acid mucin producing cells similar to the colon of mouse and chick. The low numbers of acid mucins in the little skate hindgut confirms that a functional colon for water absorption is absent in the Chondrichthyes. Interestingly, the presence of high levels of acid mucins in the posterior spiral valve provides evidence for a possible primordial water-absorbing organ in the elasmobranchs. Hoxd13 patterns acid mucins in the colons of terrestrial vertebrates. Expression of Hoxd13 and Hoxa13 in R. erinacea suggests conserved roles for Hox genes in patterning the early hindgut.     

A biochemical characterization of the photophore lenses of the midshipman fish,Porichthys notatus Girard

The present study is a biochemical characterization of the photophore lenses of the midshipman fish, Porichthys notatus, a species that bears 800 photophores distributed over the body surface. The biochemical properties of the photophore lenses were compared with those of the eye lens with which they share a similar developmental origin and analogous function. To achieve a high refractive index, the vertebrate eye lens has a relatively high concentration of structural proteins (20–50%, depending on species) and a simple protein composition, that is, relatively few proteins are synthesized in comparison to other tissues. Similarly, the photophore lenses of P. notatus had a relatively high protein concentration (average = 29%, n = 5) and approximately 60% of the total soluble protein was represented by two subunit species of 33 kD and 35 kD on denaturing polyacrylamide gels. The structural proteins of the eye lens are of two principle types: 1) and polypeptides which belong to vertebrate lens-specific crystallin families, and, 2) enzymes recruited into the lens which take on the function of structural proteins. Here, we report that the two major photophore lens subunits of 33 kD and 35 kD are biochemically similar to each other, but are clearly distinct from any of the previously characterized crystallins. Therefore, we propose that photophore lenses appear to recruit a novel protein.     

Lens optical properties in the eyes of large marine predatory teleosts

The optical properties of the crystalline lenses were studied in a variety of large predatory teleosts (bony fishes) that forage in the open ocean, some of them at considerable depths. We found the first fish lenses that are free of measurable longitudinal spherical aberration, i.e., are perfectly monofocal, in contrast to the multifocal lenses that are typical for smaller fishes living close to the surface. In fact, none of the lenses investigated in this study were clearly multifocal. Most, but not all, of the lenses had long normalized focal lengths (focal length/lens radius) of up to 3.3 lens radii. A monofocal lens of long focal length, combined with spectrally suitably placed cone pigments, may be the optimal solution for vision of high spatial and spectral resolutions in a habitat where the available spectrum of light is limited.     

Localization of LewisX, sialyl-LewisX and alpha-galactosyl epitopes on glycosphingolipids in lens tissues

Mammalian lens contains several neutral and acidic glycosphingolipids, the core structures of which are ganglio-, neolacto-, globo-, and isoglobo-series sugar chains. Old World monkey lens shows glycosphingolipid compositions similar to those of human cataractous lens, in particular the presence of Lewisxand sialyl-Lewisxepitopes and the absence of alpha-galactosyl epitope. Dog and pig lenses contain globotriaosylceramide and the sialyl-Lewisxcontaining neolactotetraosylceramide, respectively, which were found in primate lens, together with the alpha-galactosyl epitope containing neolactotetraosylceramide. Thin-layer chromatography immunostaining revealed the enrichment of some neolacto-series glycosphingolipids in the cortical and nuclear fibers, but not in lens epithelia, of dog, pig, and Japanese monkey lenses. Immunohistochemical studies confirmed the expression of Lewisx, sialyl-Lewisx, and alpha-galactosyl epitopes in the inner cortical and nuclear fibers, in association with the differentiation and maturation of lens epithelial cells to lens fibers. Glycobiological approaches thus suggested that some neolacto-series glycosphingolipids are involved in lens fiber development, in which the physiological roles of the alpha-galactosyl epitope are evolutionarily replaced by the Lewisxand sialyl-Lewisxepitopes in Old World monkeys and humans.      

Inter- and intraspecific variation in the distribution and number of pit organs (free neuromasts) of sharks and rays

The distribution of pit organs (free neuromasts) has previously been documented for several species of pelagic sharks, but is relatively poorly known for rays and bottom-dwelling (demersal) sharks. In the present study, the complete distribution of pit organs was mapped in the demersal sharks Heterodontus portusjacksoni, Orectolobus maculatus, Hemiscyllium ocellatum, Chiloscyllium punctatum, and Asymbolus analis, and the rays Rhinobatos typus, Aptychotrema rostrata, Trygonorrhina sp. A, Raja sp. A, and Myliobatis australis. All of these species had pit organs scattered over the dorsolateral surface. The sharks also had "mandibular" pit organs (and "umbilical" pit organs in C. punctatum and A. analis) on the ventral surface, while pit organs were sparse or absent on the ventral surface of rays. All of the species examined here, except for M. australis, also had a "spiracular" group of pit organs adjacent to the eye and/or spiracle. Spiracular pit organs were also recorded for the sawshark Pristiophorus sp. A and the skate Pavoraja nitida, although the remainder of pit organs were not mapped in these species. The distribution and number of pit organs varied both within and among species. Pit organ distribution was asymmetrical in each individual examined, but no particular trend towards left or right "handedness" was observed in any species. Although rays have been thought to have fewer pit organs than sharks in general, this was not the case in the present study. All of the species examined here had few pit organs compared to the pelagic sharks previously documented, but it is not clear whether this is due to ecological or phylogenetic causes.     

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.