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The development and the
evolution of the stomodeum and neurohypophysis in ascidians:
insight into the origin of the olfactory and
adenohypophysial placodes of vertebrates
Conodonts and the
origin of the vertebrate skeleton
Phylogenetic analyses based on existing knowledge of
euconodont soft tissues consistently place conodonts as more
derived than lampreys and, thereby, within the crown group
vertebrates. It has been argued that the phosphatic feeding
apparatus of conodonts represents an independent
evolutionary experiment in skeletonization from that of the
remainder of the vertebrates, but, given that conodonts
resolve as the sister group to other skeletonized
vertebrates, this contention is not parsimonious. There are
very close histological similarities between the crown
tissue of euconodont elements and enamel, and between
euconodont basal bodies and dentine, and it is apparent that
euconodont elements grew in an identical manner to the
enamel-dentine system in other vertebrate taxa. It has
recently been demonstrated that this condition holds for the
biomineralized elements of the most primitive of
euconodonts. Thus, palaeontological evidence indicates that
the earliest condition of the mineralized vertebrate
skeleton is as odontodes of dentine plus enamel or of
dentine alone in the oro/pharyngeal cavities of euconodonts
or of their putative ancestors, the paraconodonts.
Functional considerations demonstrate that the conodont
feeding array was adapted for predation/scavenging, so the
vertebrate skeleton was first employed for offensive, not
defensive, purposes.
Apoptosis and recognition of senescent cells in a
compound ascidian:
Programmed cell death by apoptosis is a
fundamental process in the develop-ment and tissue
homeostasis of Vertebrates. Although widespread among
meta-zoans, few data are available on the role of apoptosis
in invertebrates.
A cladogram for the Deuterostomia based on molecular-biological
and fossil evidence On
molecular-biological evidence from extant animals, the
basic cladogram for deuterostomes reads: (((hemichordates +
echinoderms) Xenoturbella) chordates). Moreover there
is now molecular evidence that, if the likely effects of
long-distance attraction are discounted, the basic cladogram
for chordates should read:
Vertebrate evolution has been punctuated by three episodes of widespread gene or genome duplication, each of which coincides with an apparent burst of character acquisition and increase in phenotypic complexity. The three duplications have been linked with the origin of vertebrates, gnathostomes and teleosts, respectively, three evolutionary events taken by many people to mark the establishment of new bodyplans within vertebrates. Despite what seems to be clear coincidence between the timing of gen(om)e duplications and evolutionary events, this does not necessarily imply causality. Furthermore the pattern of congruence is based on a dataset that encompasses living taxa alone. When palaeontological data are incorporated into the phylogenetic milieu, apparent bursts of character acquisition and diversity are diminished. Thus, while there may be grounds for ignoring extinct taxa in attempts to unravel genomic evolution, analyses of the phenotypic consequences should be based on total evidence, integrating data from both living and fossil taxa. From the perspective of total evidence, the association of phenotypic and genomic evolution is far from clear and it quite possible that the two are not associated, or that the retention of paralogues is an effect arising from – rather than the cause of – phenotypic complexity.
The most ancient members of a clade are of importance because fossils closer in time to the ancestor are likely to be closer to it also morphologically and may reveal anatomical characters later lost. Affinities of the geologically oldest undoubted chordates, the conodonts, are supported by their V-shaped myomeres in laterally compressed body, and possible homology of the oral apparatus elements crown tissue with enamel. Such myomeres were also reported in Metaspriggina from the Burgess Shale. Though Pikaia from the same strata is more widely accepted as an ancient chordate, its similarity to the early Cambrian worm Myoscolex suggests that it was rather a polychate (as originally interpreted). Sinuous myomeres were proposed to occur in Myllokunmingia-Haikouichthys from the early Cambrian Chengjiang fauna, a still inadequately known organism that shares its overall body shape and the presence of gill arches with the much better documented Yunnanozoon-Haikouella from the same strata. These Chengjiang fossils show the original three-dimensional disposition of body organs, with the sediment fill of the oesophagus separating left and right series of gill arches and gonads. The gonads are usually dark-stained and preserved in a positive relief on some rock slabs in result of differential collapse of particular organs under the sediment load. The notochord was also resistant to collapse, but it shows a darkening only on its outline. This suggests the original presence of a collagenous envelope surrounding a hydraulic skeleton built of organic matter-poor, vacuolarized cells. Various specimens of Yunnanozoon show different representation of their anatomy depending on local taphonomic conditions. In the Haikou locality, blood vessels supporting the gill arches and the head are well preserved. Probably the collageneous basement membrane of the endothelial cells promoted the staining. In rare specimens complex head structures are represented, suggestive of large eyes. The most controversial and enigmatic aspect of the anatomy of Yunnanozoon is its serial, laterally flattened dorsal body unit. It seem that these were muscular chambers with a thin envelope of the basement membrane, filled with a fluid or fluid-rich tissue. Being located dorsally of the notochord, these chambers differ from myomeres of adult chordates, but resemble the embryonic Anlagen of the myomeres in amphioxus. There was an attempt to restore the ancestral chordate body plan with dorsally located serial muscular chambers; the gill slits proposed to originate from the intestinal caeca. The ancestral chordate depicted in this way fits thus the anatomy of Yunnanozoon in respect to the dorsal location of probable muscular chambers, but is even more similar to the body plan of the Ediacarian dipleurozoans. They show a dorsal series of paired muscular chambers, serial metameric intestinal caeca, and lateral rows of probable oval gonads. The proposed dipleurozoan-yunnanozoan-conodont evolutionary sequence, if true, places all the more recent deuterostomian taxa, traditionally believed to fill the space between the protostomes and typical vertebrates, on side branches of the main evolutionary lineage.
The main Metazoan Radiation: a Pre-Cambrian event It is now certain that the pre-Cambrian
(Ediacaran) Kimberella quadrata is a very primitive
Mollusc. The occurrence of a true member of a living phylum
proves that the radiation of the main phyla did not occur in
the Cambrian, but not later than the middle Ediacaran.
Moreover, as Molluscs are typical trochophorates and
spiralians, either the separation between protostomians and
deuterostomians is not valid or must have occurred even
earlier, possibly even before the beginning of the
Ediacarian, this strongly arguing against maintaining the
reality of the "Cambrian Explosion".
The early
organogenesis stages in metazoans differ drastically between
higher order taxa such as phyla and classes. The segmented
germ band stage in insects, the nauplius stage of
crustaceans, and the neurula/pharyngula stage in vertebrates
are examples of this diversification. In striking contrast
with this divergence, is the similarity of these stages
within these taxa, i.e., within insects, crustaceans, and
vertebrates. The early organogenesis stages, have, thus,
remained very similar among most species within higher taxa
since the evolutionary origin of the latter. These stages
are considerably more similar to each other than to the
earlier stages of cleavage and gastrulation. Cleavage and
gastrulation stages display not only great variability, but
also striking examples of apparent convergence among species
in different phyla, for example in the many cases of
epiblastic cleavage in yolk-rich eggs. This leads to the
paradoxical situation that the overall similarity of
cleavage and gastrulation stages is in general higher among
metazoans than of the early stages of organogenesis, but
within phyla and classes the situation is the reverse. I
will discuss data and evaluate possible causes for
conservation, homoplasy, and diversification in an attempt
to throw light on this paradoxical situation.
Vascular regeneration
in a tunicate recalls vertebrate angiogenesis Among
chordates, the regenerative potentiality is particularly
developed in the tunicate ascidians, which are able to
regenerate tunic and the embedded blood vessels with a sort
of process recalling the angiogenesis occurring during
physiological or pathological conditions in vertebrates.
The vertebrate nervous
system is patterned by genes in the Zic and Gli gene
families, however, the ortholgues of these genes play no
part in patterning the Drosophila
nervous system, instead Odd-paired (Zic), patterns visceral
mesoderm, while Cubitus interruptus (Gli) regulates Hedgehog
signaling. Here we show that Zic has a primitive role in
patterning the neural cell lineages in the Metazoa.
Hatschek's work continues
- fossils confirm that L-R asymmetry was important in
chordate origins Hatschek
and others demonstrated that the ontogeny of the pharynx
in amphioxus is R-L assymmetrical with primary, secondary
and tertiary sets of gill slits. The primary slits are left
slits and are the only ones to appear during larval life;
the secondary slits are right slits and arise at
metamorphosis; and the tertiary slits are left and right
gill slits and arise during the post larval stage, posterior
to the slits already there. All this is remarkable since the
fossil cornutes, such as Cothurnocystis, had left
gill slits only like the primary slits of amphioxus; while
the fossil mitrates had right and left gill slits, the right
slits corrsponding to the secondary slits of amphioxus; and,
the mitrate Lagynocystis had median gill slits
corresponding to the tertiary slits of amphioxus. The
fossils therefore suggest that the L-R asymmetry of the gill
slits in amphioxus is a striking case of Haeckelian
recapitulation. The sudden origin of the secondary gill
slits in ontogeny could well result from a macromutation in
phylogeny and as such would repay further developmental
research. R-L asymmetries may result from an episode in the
ancestry of all deuterostomes when a bilaterally symmetrical
ancestor lay down on the right.
Characterization of Eyes absent and sine oculis
genes in amphioxus Cranial placodes are supposed to be unique vertebrate characteristics from which the paired sense organs of the eyes, ears and nose, in addition to the distal parts of some of the cranial sensory ganglia are formed. Vertebrate placodes arise in the cranial ectoderm. Some placodes, like the otic, nasal, and lens placodes, form as visible thickenings that subsequently invaginate. Others, like the trigeminal and epibranchial placodes, are not distinguishable morphologically. One of the key molecular characteristics of vertebrate placodes is the co-expression of Eyes absent/Eya and sine oculis/Six1/Six4 genes. Vertebrate Eya and Six genes encode proteins that function together in a transcriptional complex; Six protein provides DNA-binding function while. In order to gain insight into the evolutionary origin of placodes, we have cloned and characterized Eya, Six1 and Six4 genes in the invertebrate chordate amphioxus (Branchiostoma floridae). We have found areas of co-expression of Eya/Six1 and Eya/Six4 during amphioxus embryonic development that might correspond to vertebrate placodes. In addition, we have found that amphioxus Eya and Six1/4 proteins require their direct protein-protein interaction for proper function. Our results suggest, based on regionalized expression of specific transcription factors and their properties, that the origin of at least some cranial placodes might predate the origin of vertebrates.
To understand how the jaw evolution took place, the lamprey, a jawless vertebrate, was used as a model. Hox gene cognates were isolated and the lamprey’s mandibular arch (MA) was found to be in the Hox code-default state (devoid of Hox transcripts), as is true in gnathostomes. Spatial colinearity was observed for Hox2 and Hox3 cognates in the pharyngeal ectomesenchyme. Thus, the gnathostome jaw and agnathan oral apparatus seem to have evolved independently on the shared embryonic patterns. Some jaw-patterning-related genes were also isolated from the lamprey, and their expression patterns were compared with those in gnathostomes. In gnathostomes, the expression Dlx1 defines the MA domain in the rostral ectomesenchyme and rostral to the MA, there is a Dlx1-negative premandibular region that is not incorporated into the oral patterning. In contrast, the lamprey uses the entire rostral ectomesenchyme for oral patterning. From these data, it was concluded that heterotopic shift of the tissue interaction, based on the same molecular cascade, was a prerequisite for the patterning of the gnathostome jaw. It will also be discussed that, in the transition from agnathan to gnathostome states, the emergence of diplorhiny should have preceded the heterotopy of oral patterning, as an embryological prerequisite.
Stylophorans (cornutes, mitrates) are a class of Palaeozoic (Middle Cambrian - Upper Carboniferous) calcite-plated, marine deuterostomes. They consist of two well-defined regions : a delicate appendage and an asymmetrical, flattened, polyplated body (test). Since more than 40 years, their precise phylogenetic placement within deuterostomes remains highly controversial, as there is no general agreement on the interpretation of several key anatomical features. The recent discovery of putative echinoderm-like fossils in the Early Cambrian of China (vetulicystids) was presented as a strong evidence supporting a relatively basal placement of stylophorans within deuterostomes, either as basal-most members of the echinoderm stem-group or as early chordates (calcichordates). These two closely-related scenarios are based on (1) the absence of the typical five-fold radial symmetry in stylophorans, (2) the interpretation as a tail of the stylophoran appendage, and (3) the interpretation as gill slits of respiratory structures present in some cornutes. However, first, it is risky to deduce the basal phylogenetic position of stylophorans based on the absence of one single character: five-fold symmetry is frequently, repeatedly, and independently lost in various groups of "undisputable" echinoderms in Palaeozoic times (e.g. crinoids, rhombiferans). In each case, the loss of pentaradial symmetry was correlated with the adoption of an epibenthic, free (unattached) mode of life, comparable to that of stylophorans. Consequently, the hypothesis that five-fold symmetry was secondarily lost in these fossils can not be ruled out. Second, examination of the stylophoran appendage shows that its distal portions consisted of two sets of delicate, movable cover plates (left and right) articulated to one series of massive, uniserial ossicles. The presence of articulatory facets on cover plates indicates that they could open in life. Moreover, the upper edges of cover plates are frequently rounded. These two observations are not compatible with the interpretation of the stylophoran appendage as a closed, tightly sutured organ (stem or tail). On the other hand, the presence of two sets of movable cover plates, and the numerous structures present on the internal surface of ossicles are very comparable to the situation in the feeding arms of many "normal" echinoderms (e.g. eocrinoids, crinoids, ophiuroids). Third, respiratory structures present in some cornutes were interpreted as gill slits, because they are preserved as holes in the body wall. However, many similar structures (holes through the body wall) occur in other "undisputable" Palaeozoic echinoderms (e.g. eocrinoids, diploporites), in which they are interpreted as respiratory structures, not as gill slits. "Cothurnopores" and "lamellipores" of cornutes are fundamentally sutural pores (epispires) comparable to those of many eocrinoids, and there is no scientific argument to interpret them differently. In conclusion, the three main arguments suggesting a basal placement of stylophorans within deuterostomes (asymmetry, tail, gill slits) are falsified by close examination of the fossils. Stylophorans more probably correspond to derived echinoderms, which have secondarily lost they five-fold symmetry.
Earlier theories about the evolution of the jaws and cranium in gnathostome vertebrates were largely morphotypic and invoked many preconceived notions about primitive and derived characters in early vertebrates. Until recently, the earliest known fossil gnathostomes did not contradict or refute the traditional dogma of gnathostome phylogeny. However, new fossil discoveries of "basal" gnathostomes are beginning to revolutionize our views on morphology and evolution in the earliest crown group gnathostomes (osteichthyans and chondrichthyans), while developmental studies also cast new light on fundamental questions surrounding vertebrate and gnathostome origins. Some of these discoveries and ideas are reviewed and their implications for future directions of research in lower vertebrate phylogeny are discussed.
The
phylogenetic position of the Cambrian yunnanozoans, or
Yunnazoon and Haikouella, is controversial, with
strong disagreement over whether these animals were
vertebrate-like chordates, or basal deuterostomes but not
chordates, or even cephalochordates related to amphioxus
(Chen JY et al. 1999, Nature 403:519; Mallatt J and Chen JY
2003, Journal of Morphology 258:1; Shu D. et al. 2003
Science 299:1380). Fossils of the soft-bodied yunnanozoans
have been found in numerous locations in the Maotianshan
Shale of southern China, but the exceptional preservational
detail of the original Haikouella lanceolatum
specimens from Ercai village, Haikou, has not been fully
appreciated. In yunnanozoans, the trunk region of the body
has generated most of the challenges to a chordate
interpretation because its vertical segments (proposed
myomeres) project so far dorsal to its rod-like structure
(proposed notochord) - which ould be highly unusual for a
chordate. Yet the body segments of numerous Ercai specimens
show stacks of horizontal lines indicative of muscle
lamellae in myomeres, indicating yunnazoans indeed had
chordate myomeres. Turning to the pharynx region, the Ercai
specimens were the first yunnanozoan material to show
evidence of true gills ("gill rays") on the branchial bars,
and true gills only occur in vertebrates, not in other
deuterostomes. The head region of Ercai Haikouella,
impossible to interpret in any detail in other yunnanozoans,
shows a well-preserved oral hood with large upper lips and a
lower lip (resembling these structures in modern larval
lampreys); a large brain that thins posteriorly into a
spinal nerve cord dorsal to the notochord; and in four
specimens, small lateral eyes or eye lenses (which appear as
either a dark circle around a light spot or a light circle
around a dark spot, depending on how the rock was split into
part or counterpart). All these characters - including
others that were combined in a parsimony-based analysis of
phylogenetic relationships - indicate that yunnanozoans were
the sister group of vertebrates and were not basal
deuterostomes or cephalochordates.
Hair cell in ascidians
and vertebrates: homology or convergent evolution? Recently,
we described a mechanoreceptor organ, the coronal organ, in
the oral siphon of ascidians of the group pleurogonids. It
is constituted of ciliated cells (hair cells) innervated by
the cerebral ganglion, and forming a sensorial row,
recalling the vertebrate lateral line. These sensory cells
vary in morphology, and in some species they strongly
resemble vertebrate hair cells, since they bear cilia
situated at one side of a crescent-shaped bundle of graded
in length stereovilli. Because only primary sensory cells
were known in ascidians and hair cells were considered
exclusive to vertebrates for their morphology and origin
from neurogenic placodes, the evidence of these
mechanoreceptors in the protochordate ascidians opened a
debate regarding the evolution of chordate sensory cells.
Cranial sensory placodes have long been
considered a key character of vertebrates. Despite their
importance for understanding vertebrate origins, the
evolutionary origin of placodes has remained obscure. We
used a panel of molecular markers from the Six, Eya, Pax,
Dach, and COE gene families to examine the tunicate Ciona
intestinalis for evidence of structures homologous to
vertebrate placodes. Our results identify two domains of
Ciona ectoderm that are marked by the genetic cascade
that regulates vertebrate placode formation. The first is
just anterior to the brain, and we suggest this territory is
equivalent to the olfactory/adenohypophyseal placodes of
vertebrates. The second is a bilateral domain adjacent to
the posterior brain and includes cells fated to form the
atrial siphon of an adult Ciona. We show this bares
most similarity to placodes fated to form the vertebrate
acoustico-lateralis system. We also examine a panel of
pro-neurogenic and neurogenic markers from the FoxI, ngn,
phox2 and pouIV gene families. Surprisingly, their
expression was not limited to the embryonic stages and in
some cases was induced in the developping siphons at
metamorphic stages.
Getting a grip on the
bare bones of skeletal origins
Incorporation of fossil and extant data gives the most
complete perspective on the origin of the skeleton at the
chordate vertebrate transition, but skeletal characters may
be the only information available for some taxa. Both, a
framework of topographic position of the skeletal tissue and
rigorously tested phylogenies of relationships based on
whole body information are necessary to propose the order of
origin of skeletal characters and the polarity of change.
The status of ‘primary mineralised skeleton’ is not
established and can be any of the axial, neurocranial,
visceral-splanchnic, or dermal. Priority based on origin
from embryonic cell type, mesodermal versus neural crest, is
part of that debate. The recent demonstration of
neural-crest-like cells in ascidian urochordates, although
known not to have skeletogenic potential, may influence
views on the earliest skeleton.
An important mechanism for the evolution of phenotypic complexity, diversity, and innovation and the origin of novel gene functions is the duplication of genes and entire genomes. Recent phylogenomic studies suggest that, during the evolution of vertebrates, the entire genome was duplicated in two rounds (2R) of duplication. Later, ~350 mya, in the stem lineage of ray-finned (actinopterygian) fishes, but not in that of the land vertebrates, a third genome duplication occurred – the fish-specific genome duplication (FSGD or 3R), leading, at least initially, to up to eight copies of the ancestral deuterostome genome. Therefore, the sarcopterygian (lobe-finned fishes and tetrapods) genome possessed originally only half as many genes compared to the derived fishes, just like the most basal and species-poor lineages of extant fishes that diverged from the fish stem lineage before the 3R duplication. Most duplicated genes were secondarily lost, yet some evolved new functions. The genomic complexity of the teleosts might be causally related to their evolutionary success and astounding biological diversity.
Our goal is to describe the order of
acquisition of innovations in selected phases of amphibian
evolution. Both skull and cranial muscle development are in
focus. I will present selected parts of this ongoing project
including; cranial muscle differentiation and morphogenesis
in the Australian lungfish and in the Mexican axolotl,
cranial muscle and skeleton development in Xenopus laevis
and its relative Hymenochirus boettgeri, as well as skull
development and morphology in a caecilian, Ichthyophis
kohtaoensis. To determine the onset of differentiation we
use antibodies against desmin and optical sectioning using
confocal laser scanning microscopy on whole-mount
immunostained embryos. Antibodies against acetylated tubulin
are used to clarify muscle innervation patterns, and
collagen II staining gives an overview of developing
cartilage. This technique makes it possible to document head
development in three dimensions while keeping the specimens
intact. To obtain an appreciation of complicated
three-dimensional structures in the head, we use
reconstructions based on serial sections (two different
methods will be shown). The project provides a morphological
foundation for further studies of head skeleton as well as
cranial muscle cell fate and early differentiation in a
comparative approach. The focus is on understanding the
developmental origins of morphological innovations.
Gen(om)e duplication
and vertebrate origins: the problems of stem vertebrates and
the meaning of complexity Vertebrate
evolution was punctuated by three episodes of widespread
gene or genome duplication, the first of which, between
amphioxus and hagfish, has been implicated in the origin of
vertebrates. Unfortunately, of the three episodes, this is
the most problematic. The gen(om)e duplication appears to be
associated with an evolutionary jump in phenotypic
complexity but, like the other duplications (see Donoghue
and Purnell abstract), we suspect that the apparent
congruence between duplication and evolution is a
consequence of ignoring fossils. Only the pattern of
character evolution through the vertebrate stem can reveal
the true picture, but this is confused by preservational
biases and difficulties inherent in assigning fossils to the
stems of clades with deep divergence times. Exceptionally
preserved fossils, such as Yunnanozoon, Haikouella, and
Haikouichthys from the Chengjiang lagerstätte, provide clear
illustrations of this. Although exquisitely detailed, these
fossils are the products of capricious fossilization -
carbonized residues and mineral replacements of partially,
and variably, decomposed carcasses. Anatomy cannot simply be
read directly from the fossil, and serious complications
arise because interpretation is necessarily predicated on
comparisons with extant organisms, which inevitably colour
subsequent anatomical determinations and hypotheses of
affinity.
The placodes of vertebrates
give rise to different cell types and contribute to many
cranial sensory organs and ganglia. Recent studies indicate
that all placodes originate from a panplacodal primordium,
defined by the expression of Six1, Six4 and Eya
genes, which may promote generic placodal properties such as
neuronal specification and cell shape changes. Our
experiments in Xenopus suggest that this primordium
is induced in rostral nonneural ectoderm by a combination of
neural and mesodermal signals. The common developmental
origin of placodes suggests that all placodes may have
evolved – possibly in several steps - from a common
precursor. Traditionally, placodes were considered to be
evolutionary innovations of vertebrates, but recent studies
in ascidians and amphioxus suggested that some placodes
evolved earlier in the chordate lineage. Many cellular and
molecular components of placodes (e.g., sensory and
neurosecretory cell types, regionalized expression of
transcription factors) indeed predate the origin of
vertebrates. However, there is presently little evidence
that these components are integrated into placodes in
protochordates. I propose instead that all placodes evolved
from an adenohypophyseal-olfactory protoplacode, which may
have originated in the vertebrate ancestor from the anlage
of a rostral neurosecretory organ retained in present day
amphioxus as Hatschek’s pit.
Phylogeny of
early deuterostomes and origin of vertebrates Knowledge from study of the Early
Cambrian Chengjiang Largestatte from 1984 to 1994 was
restricted to protostomes. More intensive investigation and
careful examination in the following ten years have yielded
many discoveries of deuterostomes, which include not only
the earliest-known representatives of the major branches in
all three extant phyla, such as vetulocystids, yunnanozoans,
Cheungkongella, Cathaymyrus, Myllokunmingia
and Haikouichtys belonging to the most primitive
vertebrates, but also an extinct phylum Vetulicolia. This
allows us to propose a tentative deuterostome phylogeny with
vetulicolians at the bottom (Shu 2003; Shu et al.
2004). As the most recent common ancestor of all extant
deuterostomes would have been a segmented animal with
pharyngeal slits (Gee 2001), vetulicolians with both
primitive segmentation and gill slits should represent such
kind of ancestor. If R-L asymmetries have resulted from an
episode in the ancestry of all deuterostomes, the
bilaterally symmetrical vetulicolians would right be or very
close to that ancestor. Since endostyle may be a primitive
feature in deuterostomes, vetulicolians, having no
notochord, should be less likely related to chordates.
Yunnanozoon and Haikouella have no eyes and brain
(Shu and Conway Morris 2003) and even a notochord (Valentine
2004). Instead they share similar boy-plan with
vetulicolians and bear both dorsal and ventral neural cords,
which is characteristic of living hemichordates.
K. Halanych’s comments (2004) are:
Paleontological work of Shu and collaborators (1999, 2001a,
2001b, 2003) is revising our understanding of early
evolution of chordates.
Fox gene duplication
in vertebrate evolution The genome
of higher vertebrates has undergone many duplication events.
Often these duplication events appear to be associated with
phenotypic changes.
Development of a cartilagenous endoskeleton was a pivotal innovation during vertebrate evolution. The underlying molecular genetic mechanisms, however, remain poorly understood. We have taken a comparative approach to this problem by comparing the mechanisms of skeletal development in lamprey and shark embryos. Analysis of a suite of developmental control genes during skeletogenesis reveals that the molecular mechanisms of skeletal development are highly conserved across vertebrates. Comparison of lamprey and gnathostome gene expression patterns also showed evidence for subfunctionalization following gene duplication events, in which the ancestral expression pattern was partitioned among the two duplicated genes. The extent of conservation is surprising, given the reported structural differences between aganthan and gnathostome cartilage, and suggests a more ancient origin of the cartilage developmental program than previously reported.
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