Agnathans and the origin of jawed vertebrates

Forey, Peter L.; Janvier, Philippe

doi:10.1038/361129a0

Cited by 375 publications

(214 citation statements)

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“…Conodonts (naked agnathan in the Late Cambrian; 510 million years ago) seem to have developed the earliest mineralized exoskeleton as an oral feeding apparatus. The conodont elements are almost entirely composed of enamel (lamellar crown), and dentine (basal body) underlays the crown in at least some species (56)(57)(58), whereas dentine is more common in dermal exoskeleton of ostracoderm (59). Thus, the earliest vertebrate history of the mineralized exoskeletal formation is controversial.…”

Section: Discussionmentioning

confidence: 99%

Mineralized tissue and vertebrate evolution: The secretory calcium-binding phosphoprotein gene cluster

Kawasaki

Weiss

2003

Proc. Natl. Acad. Sci. U.S.A.

283

249

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Gene duplication creates evolutionary novelties by using older tools in new ways. We have identified evidence that the genes for enamel matrix proteins (EMPs), milk caseins, and salivary proteins comprise a family descended from a common ancestor by tandem gene duplication. These genes remain linked, except for one EMP gene, amelogenin. These genes show common structural features and are expressed in ontogenetically similar tissues. Many of these genes encode secretory Ca-binding phosphoproteins, which regulate the Ca-phosphate concentration of the extracellular environment. By exploiting this fundamental property, these genes have subsequently diversified to serve specialized adaptive functions. Casein makes milk supersaturated with Ca-phosphate, which was critical to the successive mammalian divergence. The innovation of enamel led to mineralized feeding apparatus, which enabled active predation of early vertebrates. The EMP genes comprise a subfamily not identified previously. A set of genes for dentine and bone extracellular matrix proteins constitutes an additional cluster distal to the EMP gene cluster, with similar structural features to EMP genes. The duplication and diversification of the primordial genes for enamel͞dentine͞bone extracellular matrix may have been important in core vertebrate feeding adaptations, the mineralized skeleton, the evolution of saliva, and, eventually, lactation. The order of duplication events may help delineate early events in mineralized skeletal formation, which is a major characteristic of vertebrates.

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Section: Discussionmentioning

confidence: 99%

Mineralized tissue and vertebrate evolution: The secretory calcium-binding phosphoprotein gene cluster

Kawasaki

Weiss

2003

Proc. Natl. Acad. Sci. U.S.A.

283

249

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“…Owing to their ancient origins, lampreys are considered modern representatives of the original jawless fishes (Forey and Janvier 1993). Accordingly, it has been proposed that the burrow-dwelling and suspension-feeding life style of ammocoetes may resemble that of some of the early jawless vertebrates (Griffith 1991).…”

Section: Discussionmentioning

confidence: 99%

Nitrogenous waste excretion by the larvae of a phylogenetically ancient vertebrate: the sea lamprey (Petromyzon marinus)

Wilkie¹,

Wang²,

Walsh³

et al. 1999

Can. J. Zool.

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Larval sea lampreys (Petromyzon marinus) (ammocoetes) excreted significant quantities of urea, which composed 15-20% of the total nitrogenous waste excreted. Compared with teleosts of similar size, ammonia and urea excretion rates (J Amm and J Urea , respectively) in ammocoetes were relatively low, reflecting the low metabolic rate of these burrow-dwelling suspension feeders. Analyses of liver enzymes indicated that ammocoetes had all the enzymes necessary to produce urea via uricolysis, but not those of the ornithine-urea cycle (OUC). Further, exposure to 2 mmol·L -1 total ammonia for 5 d was accompanied by a 3-fold elevation of J Urea , but did not lead to greater OUC activity. Internal ammonia levels increased markedly, however, exceeding 2000 µmol·L -1 in plasma and 5000 µmol·L -1 in muscle after the 5-d exposure period. This high resistance to internal ammonia accumulation was related to the very high glutamine synthetase activities measured in ammocoete brains. The excretion and production of urea by ammocoetes demonstrates for the first time that agnathans are capable of producing physiologically relevant amounts of urea. Given the ancient origins and conserved evolution of lampreys, these observations also suggest that at least some of the early jawless vertebrates were able to produce and excrete urea.

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“…This phylogenetic pattern implied that cyclostomes could throw light on the early steps of the assembly of the vertebrate body plan and that hagfishes could document the most generalized condition for a number of vertebrate characters (6). However, it soon raised heated debates, because an increasingly large number of molecular sequence data provided increasingly strong support for the old theory of cyclostomes monophyly (7,8); that is, hagfishes and lampreys were actually sister groups that had diverged in the early Paleozoic, up to 500 million years (Myr) ago. Morphologists who defended cylostome paraphyly argued that molecular sequence-based trees were inconclusive because of the uncertainty as to the outgroups of the vertebrates (i.e., their closest relatives and either tunicates of amphioxus) (Fig.…”

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confidence: 99%