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"VERTEBRATES"
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The African coelacanth genome provides insights into tetrapod evolution
The discovery of a living coelacanth specimen in 1938 was remarkable, as this lineage of lobe-finned fish was thought to have become extinct 70 million years ago. The modern coelacanth looks remarkably similar to many of its ancient relatives, and its evolutionary proximity to our own fish ancestors provides a glimpse of the fish that first walked on land. Here we report the genome sequence of the African coelacanth, Latimeria chalumnae. Through a phylogenomic analysis, we conclude that the lungfish, and not the coelacanth, is the closest living relative of tetrapods. Coelacanth protein-coding genes are significantly more slowly evolving than those of tetrapods, unlike other genomic features. Analyses of changes in genes and regulatory elements during the vertebrate adaptation to land highlight genes involved in immunity, nitrogen excretion and the development of fins, tail, ear, eye, brain and olfaction. Functional assays of enhancers involved in the fin-to-limb transition and in the emergence of extra-embryonic tissues show the importance of the coelacanth genome as a blueprint for understanding tetrapod evolution.
Journal Article
Animal school : what class are you?
Characteristics of the five vertebrate classes--mammal, bird, fish, reptile, and amphibian--are described along with illustrations reminiscent of woodcuts.
Book
Massive increase in visual range preceded the origin of terrestrial vertebrates
The evolution of terrestrial vertebrates, starting around 385 million years ago, is an iconic moment in evolution that brings to mind images of fish transforming into four-legged animals. Here, we show that this radical change in body shape was preceded by an equally dramatic change in sensory abilities akin to transitioning from seeing over short distances in a dense fog to seeing over long distances on a clear day. Measurements of eye sockets and simulations of their evolution show that eyes nearly tripled in size just before vertebrates began living on land. Computational simulations of these animal’s visual ecology show that for viewing objects through water, the increase in eye size provided a negligible increase in performance. However, when viewing objects through air, the increase in eye size provided a large increase in performance. The jump in eye size was, therefore, unlikely to have arisen for seeing through water and instead points to an unexpected hybrid of seeing through air while still primarily inhabiting water. Our results and several anatomical innovations arising at the same time suggest lifestyle similarity to crocodiles. The consequent combination of the increase in eye size and vision through air would have conferred a 1 million-fold increase in the amount of space within which objects could be seen. The “buena vista” hypothesis that our data suggest is that seeing opportunities from afar played a role in the subsequent evolution of fully terrestrial limbs as well as the emergence of elaborated action sequences through planning circuits in the nervous system.
Journal Article
A cold-blooded view of adaptive immunity
The adaptive immune system arose 500 million years ago in ectothermic (cold-blooded) vertebrates. Classically, the adaptive immune system has been defined by the presence of lymphocytes expressing recombination-activating gene (RAG)-dependent antigen receptors and the MHC. These features are found in all jawed vertebrates, including cartilaginous and bony fish, amphibians and reptiles and are most likely also found in the oldest class of jawed vertebrates, the extinct placoderms. However, with the discovery of an adaptive immune system in jawless fish based on an entirely different set of antigen receptors — the variable lymphocyte receptors — the divergence of T and B cells, and perhaps innate-like lymphocytes, goes back to the origin of all vertebrates. This Review explores how recent developments in comparative immunology have furthered our understanding of the origins and function of the adaptive immune system.
Journal Article
Inner ear development in cyclostomes and evolution of the vertebrate semicircular canals
Jawed vertebrates have inner ears with three semicircular canals, the presence of which has been used as a key to understanding evolutionary relationships. Ostracoderms, the jawless stem gnathostomes, had only two canals and lacked the lateral canal
1
–
3
. Lampreys, which are modern cyclostomes, are generally thought to possess two semicircular canals whereas the hagfishes—which are also cyclostomes—have only a single canal, which used to be regarded as a more primitive trait
1
,
4
. However, recent molecular and developmental analyses have strongly supported the monophyly of cyclostomes
5
–
7
, which has left the evolutionary trajectory of the vertebrate inner ear unclear
8
. Here we show the differentiation of the otic vesicle of the lamprey
Lethenteron camtschaticum
and inshore hagfish
Eptatretus burgeri
. This is the first time, to our knowledge, that the development of the hagfish inner ear is reported. We found that canal development in the lamprey starts with two depressions—which is reminiscent of the early developmental pattern of the inner ear in modern gnathostomes. These cyclostome otic vesicles show a pattern of expression of regulatory genes, including OTX genes, that is comparable to that of gnathosomes. Although two depressions appear in the lamprey vesicle, they subsequently fuse to form a single canal that is similar to that of hagfishes. Complete separation of the depressions results in anterior and posterior canals in gnathostomes. The single depression of the vesicle in hagfishes thus appears to be a secondarily derived trait. Furthermore, the lateral canal in crown gnathostomes was acquired secondarily—not by de novo acquisition of an OTX expression domain, but by the evolution of a developmental program downstream of the OTX genes.
The differentiation of the inner ear in the lamprey
Lethenteron camtschaticum
and hagfish
Eptatretus burgeri
sheds light on the evolution of the semicircular canals of jawed vertebrates.
Journal Article
Signalling dynamics in vertebrate segmentation
Key Points
Vertebrate segmentation depends on an oscillator (the segmentation clock) controlling periodic signalling activities of the Notch, WNT and fibroblast growth factor (FGF) pathways, which act on precursors of the somites in the presomitic mesoderm.
Spacing of the response to the periodic signal of the clock is controlled by a system of travelling posterior gradients of FGF and WNT signalling. This system leads to the successive determination of embryonic segments along the anteroposterior axis.
Although the pacemaker of the oscillator has not been fully characterized, delayed negative-feedback loops have been shown to be involved in the control of oscillations in mouse and zebrafish embryos.
Notch signalling is involved in the synchronization of individual cellular oscillators, resulting in coordinated waves travelling along the presomitic mesoderm.
Segmental determination occurs in the presomitic mesoderm when segmentation genes such as mesoderm posterior 2 (
MESP2
) are activated in a striped domain in response to the clock signal. This striped domain specifies the future boundaries of the somite.
Somite formation relies on a molecular oscillator, the segmentation clock, which leads to oscillatory gene expression in the presomitic mesoderm; this is converted into the periodic generation of segments in response to signalling gradients referred to as the wavefront. Recent studies provide insights into the molecular mechanisms behind this intricate developmental system.
Segmentation of the paraxial mesoderm is a major event of vertebrate development that establishes the metameric patterning of the body axis. This process involves the periodic formation of sequential units, termed somites, from the presomitic mesoderm. Somite formation relies on a molecular oscillator, the segmentation clock, which controls the rhythmic activation of several signalling pathways and leads to the oscillatory expression of a subset of genes in the presomitic mesoderm. The response to the periodic signal of the clock, leading to the establishment of the segmental pre-pattern, is gated by a system of travelling signalling gradients, often referred to as the wavefront. Recent studies have advanced our understanding of the molecular mechanisms involved in the generation of oscillations and how they interact and are coordinated to activate the segmental gene expression programme.
Journal Article