Origin Of Jawed Vertebrates Research Articles

The oldest complete jawed vertebrates from the early Silurian of China.

Molecular studies suggest that the origin of jawed vertebrates was no later than the Late Ordovician period (around 450 million years ago (Ma))1,2. Together with disarticulated micro-remains of putative chondrichthyans from the Ordovician and early Silurian period3-8, these analyses suggest an evolutionary proliferation of jawed vertebrates before, and immediately after, the end-Ordovician mass extinction. However, until now, the earliest complete fossils of jawed fishes for which a detailed reconstruction of their morphology was possible came from late Silurian assemblages (about 425 Ma)9-13. The dearth of articulated, whole-body fossils from before the late Silurian has long rendered the earliest history of jawed vertebrates obscure. Here we report a newly discovered Konservat-Lagerstätte, which is marked by the presence of diverse, well-preserved jawed fishes with complete bodies, from the early Silurian (Telychian age, around 436 Ma) of Chongqing, South China. The dominant species, a 'placoderm' or jawed stem gnathostome, which we name Xiushanosteus mirabilis gen. et sp. nov., combines characters from major placoderm subgroups14-17 and foreshadows the transformation of the skull roof pattern from the placoderm to the osteichthyan condition10. The chondrichthyan Shenacanthus vermiformis gen. et sp. nov. exhibits extensive thoracic armour plates that were previously unknown in this lineage, and include a large median dorsal plate as in placoderms14-16, combined with a conventional chondrichthyan bauplan18,19. Together, these species reveal a previously unseen diversification of jawed vertebrates in the early Silurian, and provide detailed insights into the whole-body morphology of the jawed vertebrates of this period.

Loss in the making: absence of pelvic fins and presence of paedomorphic pelvic girdles in a Late Devonian antiarch placoderm (jawed stem-gnathostome).

Within jawed vertebrates, pelvic appendages have been modified or lost repeatedly, including in the most phylogenetically basal, extinct, antiarch placoderms. One Early Devonian basal antiarch, Parayunnanolepis, possessed pelvic girdles, suggesting the presence of pelvic appendages at the origin of jawed vertebrates; their absence in more derived antiarchs implies a secondary loss. Recently, paired female genital plates were identified in the Late Devonian antiarch, Bothriolepis canadensis, in the position of pelvic girdles in other placoderms. We studied these putative genital plates along an ontogenetic series of B. canadensis; ontogenetic changes in their morphology, histology and elemental composition suggest they represent endoskeletal pelvic girdles composed of perichondral and endochondral bone. We suggest that pelvic fins of derived antiarchs were lost, while pelvic girdles were retained, but reduced, relative to Parayunnanolepis This indicates developmental plasticity and evolutionary lability in pelvic appendages, shortly after these elements evolved at the origin of jawed vertebrates.

Rapid genome reshaping by multiple-gene loss after whole-genome duplication in teleost fish suggested by mathematical modeling

Whole-genome duplication (WGD) is believed to be a significant source of major evolutionary innovation. Redundant genes resulting from WGD are thought to be lost or acquire new functions. However, the rates of gene loss and thus temporal process of genome reshaping after WGD remain unclear. The WGD shared by all teleost fish, one-half of all jawed vertebrates, was more recent than the two ancient WGDs that occurred before the origin of jawed vertebrates, and thus lends itself to analysis of gene loss and genome reshaping. Using a newly developed orthology identification pipeline, we inferred the post-teleost-specific WGD evolutionary histories of 6,892 protein-coding genes from nine phylogenetically representative teleost genomes on a time-calibrated tree. We found that rapid gene loss did occur in the first 60 My, with a loss of more than 70-80% of duplicated genes, and produced similar genomic gene arrangements within teleosts in that relatively short time. Mathematical modeling suggests that rapid gene loss occurred mainly by events involving simultaneous loss of multiple genes. We found that the subsequent 250 My were characterized by slow and steady loss of individual genes. Our pipeline also identified about 1,100 shared single-copy genes that are inferred to have become singletons before the divergence of clupeocephalan teleosts. Therefore, our comparative genome analysis suggests that rapid gene loss just after the WGD reshaped teleost genomes before the major divergence, and provides a useful set of marker genes for future phylogenetic analysis.

Metabolic regulation in meagre, Argyrosomus regius (Asso, 1801): Study of gene-diet interactions on lipid metabolism

Metabolic regulation in meagre, Argyrosomus regius (Asso, 1801): Study of gene-diet interactions on lipid metabolism

Functional Desaturase Fads1 (Δ5) and Fads2 (Δ6) Orthologues Evolved before the Origin of Jawed Vertebrates

Long-chain polyunsaturated fatty acids (LC-PUFAs) such as arachidonic (ARA), eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids are essential components of biomembranes, particularly in neural tissues. Endogenous synthesis of ARA, EPA and DHA occurs from precursor dietary essential fatty acids such as linoleic and α-linolenic acid through elongation and Δ5 and Δ6 desaturations. With respect to desaturation activities some noteworthy differences have been noted in vertebrate classes. In mammals, the Δ5 activity is allocated to the Fads1 gene, while Fads2 is a Δ6 desaturase. In contrast, teleosts show distinct combinations of desaturase activities (e.g. bifunctional or separate Δ5 and Δ6 desaturases) apparently allocated to Fads2-type genes. To determine the timing of Fads1-Δ5 and Fads2-Δ6 evolution in vertebrates we used a combination of comparative and functional genomics with the analysis of key phylogenetic species. Our data show that Fads1 and Fads2 genes with Δ5 and Δ6 activities respectively, evolved before gnathostome radiation, since the catshark Scyliorhinus canicula has functional orthologues of both gene families. Consequently, the loss of Fads1 in teleosts is a secondary episode, while the existence of Δ5 activities in the same group most likely occurred through independent mutations into Fads2 type genes. Unexpectedly, we also establish that events of Fads1 gene expansion have taken place in birds and reptiles. Finally, a fourth Fads gene (Fads4) was found with an exclusive occurrence in mammalian genomes. Our findings enlighten the history of a crucially important gene family in vertebrate fatty acid metabolism and physiology and provide an explanation of how observed lineage-specific gene duplications, losses and diversifications might be linked to habitat-specific food web structures in different environments and over geological timescales.

Birth of the jawed vertebrates

The discovery of embryos in certain fossil fishes not only shows that internal fertilization and live birth evolved early in vertebrate history, but also raises questions about the origin of jawed vertebrates.

Evolution of vertebrate opioid receptors

The opioid peptides and receptors have prominent roles in pain transmission and reward mechanisms in mammals. The evolution of the opioid receptors has so far been little studied, with only a few reports on species other than tetrapods. We have investigated species representing a broader range of vertebrates and found that the four opioid receptor types (delta, kappa, mu, and NOP) are present in most of the species. The gene relationships were deduced by using both phylogenetic analyses and chromosomal location relative to 20 neighboring gene families in databases of assembled genomes. The combined results show that the vertebrate opioid receptor gene family arose by quadruplication of a large chromosomal block containing at least 14 other gene families. The quadruplication seems to coincide with, and, therefore, probably resulted from, the two proposed genome duplications in early vertebrate evolution. We conclude that the quartet of opioid receptors was already present at the origin of jawed vertebrates approximately 450 million years ago. A few additional opioid receptor gene duplications have occurred in bony fishes. Interestingly, the ancestral receptor gene duplications coincide with the origin of the four opioid peptide precursor genes. Thus, the complete vertebrate opioid system was already established in the first jawed vertebrates.

Early vertebrate chromosome duplications and the evolution of the neuropeptide Y receptor gene regions

BackgroundOne of the many gene families that expanded in early vertebrate evolution is the neuropeptide (NPY) receptor family of G-protein coupled receptors. Earlier work by our lab suggested that several of the NPY receptor genes found in extant vertebrates resulted from two genome duplications before the origin of jawed vertebrates (gnathostomes) and one additional genome duplication in the actinopterygian lineage, based on their location on chromosomes sharing several gene families. In this study we have investigated, in five vertebrate genomes, 45 gene families with members close to the NPY receptor genes in the compact genomes of the teleost fishes Tetraodon nigroviridis and Takifugu rubripes. These correspond to Homo sapiens chromosomes 4, 5, 8 and 10.ResultsChromosome regions with conserved synteny were identified and confirmed by phylogenetic analyses in H. sapiens, M. musculus, D. rerio, T. rubripes and T. nigroviridis. 26 gene families, including the NPY receptor genes, (plus 3 described recently by other labs) showed a tree topology consistent with duplications in early vertebrate evolution and in the actinopterygian lineage, thereby supporting expansion through block duplications. Eight gene families had complications that precluded analysis (such as short sequence length or variable number of repeated domains) and another eight families did not support block duplications (because the paralogs in these families seem to have originated in another time window than the proposed genome duplication events). RT-PCR carried out with several tissues in T. rubripes revealed that all five NPY receptors were expressed in the brain and subtypes Y2, Y4 and Y8 were also expressed in peripheral organs.ConclusionWe conclude that the phylogenetic analyses and chromosomal locations of these gene families support duplications of large blocks of genes or even entire chromosomes. Thus, these results are consistent with two early vertebrate tetraploidizations forming a paralogon comprising human chromosomes 4, 5, 8 and 10 and one teleost tetraploidization. The combination of positional and phylogenetic data further strengthens the identification of orthologs and paralogs in the NPY receptor family.

Placoderm branchial and hypobranchial muscles and origins in jawed vertebrates

ABSTRACT Muscle morphology in the Placodermi can be inferred by comparison to extant gnathostomes (Chon-drichthyes (Actinopterygii + Sarcopterygii)). Examination of the chondrichthyan hypobranchial musculature suggested a comparable musculature for placoderms, including the coracomandibularis and coracohyoideus, acting separately to depress the mandible and hyoid arch. In sharks, this was associated with the absence of a mandibulohyoid ligament functioning in mandible depression, a characteristic feature of actinopterygians and sarcopterygians. Additionally, the coracobranchiales muscles depress the chondrichthyan ventral gill arches, the clavobranchiales being the corresponding muscles in actinopterygians and sarcopterygians. These muscles and ligaments are not preserved in placoderms, but their attachments can be compared to those of extant gnathostomes. On the placoderm lower jaw, attachment surfaces for the mandibulohyoid ligament are only present in derived taxa and position relative to the jaw joint suggests the ligament functioned poorly (or not at all). This compares to the chondrichthyan condition, but attachment surfaces on the placoderm trunkshield indicate that while some primitive placoderms possessed a coracobranchialis musculature, other derived taxa possessed a clavobranchialis. The Osteostraci are the most closely related agnathans to the jawed vertebrates, and lack muscles involved in active inspiration, the evolution of which was linked to increased activity near the origin of jawed vertebrates. Since overall morphology suggests osteostracans were active animals, absence of these muscles may instead relate to the absence of segmented gill arches and the anterior position of the osteostracan pectoral fin. The latter may affect development of the hypobranchial musculature, questioning homology of these muscles in lampreys and jawed vertebrates.

Ventilation and the origin of jawed vertebrates: a new mouth

Abstract This study investigates the origin of jaws by re-assessing homologies between the oropharyngeal regions of Agnatha and Chondrichthyes. In accordance with classical theory, jaws are interpreted as the most anterior arches of the ventilatory branchial basket. It is proposed that jaws first enlarged for a ventilatory function, i.e. closing the jaws prevented reflux of water through the mouth during forceful expiration. Next, they enlarged further to grasp prey in feeding. As they enlarged, the jaws tilted forward, squeezing the ancestral oral cavity in front of them (‘old mouth’) into a slit between the jaws and lips. Simultaneously, the anterior part of the pharynx behind the jaws was pulled forward and became a ‘new mouth’ (the buccal part of the buccopharyngeal cavity of gnathostomes). During the transition to gnathostomes, the premandibular cheeks and lips of the old mouth remained in place, and are represented in ammocoete lampreys, chimaeroids, and sharks. The stages in the evolution of gnathostomes, driven by selection for increasing activity, are modelled as: ancestral vertebrate (with unjointed branchial arches) to early pre-gnathostome (jointed internal arches and stronger ventilation) to late pre-gnathostome (with mouth-closing, ventilatory ‘jaws’) to early gnathostome (feeding jaws).

Agnathans recent and fossil, and the origin of jawed vertebrates

During the last 15 years we have gained considerably more knowledge about the anatomy, physiology and molecular sequences of the modern agnathans. This knowledge has been analysed with modern systematic techniques which provide clear, unambiguous statements of relationships. At present there is a conflict between the results obtained using morphological/physiological data and that using molecular data. During the next few years it is likely that more molecular sequences will become available for analysis. Whether this will fuel the conflict or resolve the issue remains to be seen. The great increase in our knowledge of the diversity of fossil agnathans is continuing to provide much new anatomical information and this allows more firmly based phylogenies to be constructed. From these we may be able to delimit more precisely the course of evolutionary changes of functional systems in the early history of vertebrates. Many of our decisions concerning primitiveness or degeneracy of the modern agnathans can be gained through study of the ontogenetic development and the variation between the ontogenies from species to species. Lampreys are relatively well known in this respect. However, we lack comparable detailed studies of the development of hagfishes. A major research field is here waiting to be reaped.

Agnathans and the origin of jawed vertebrates

The origins of jawed vertebrates (gnathostomes) lie somewhere within the ranks of long-extinct jawless fishes, represented today as the lampreys and hagfishes. Recent discoveries of hitherto unknown kinds of jawless fishes (agnathans), together with re-examination of known agnathans and advances in systematic methods, have revitalized debates about the relationships of ancient fishes and given fresh insights into early vertebrate history.