Vertebral Phenotypes Research Articles

Genetic basis for the evolution of vertebrate mineralized tissue.

Mineralized tissue is vital to many characteristic adaptive phenotypes in vertebrates. Three primary tissues, enamel (enameloid), dentin, and bone, are found in the body armor of ancient agnathans and mammalian teeth, suggesting that these two organs are homologous. Mammalian enamel forms on enamel-specific proteins such as amelogenin, whereas dentin and bone form on collagen and many acidic proteins, such as SPP1, coordinately regulate their mineralization. We previously reported that genes for three major enamel matrix proteins, five proteins necessary for dentin and bone formation, and milk caseins and salivary proteins arose from a single ancestor by tandem gene duplications and form the secretory calcium-binding phosphoprotein (SCPP) family. Gene structure and protein characteristics show that SCPP genes arose from the 5' region of ancestral sparcl1 (SPARC-like 1). Phylogenetic analysis on SPARC and SPARCL1 suggests that the SCPP genes arose after the divergence of cartilaginous fish and bony fish, implying that early vertebrate mineralization did not use SCPPs and that SPARC may be critical for initial mineralization. Consistent with this inference, we identified SPP1 in a teleost genome but failed to find any genes orthologous to mammalian enamel proteins. Based on these observations, we suggest a scenario for the evolution of vertebrate tissue mineralization, in which body armor initially formed on dermal collagen, which acted as a reinforcement of dermis. We also suggest that mammalian enamel is distinct from fish enameloid. Their similar nature as a hard structural overlay on exoskeleton and teeth is because of convergent evolution.

Frimousse – a spontaneous ascidian mutant with anterior ectodermal fate transformation

Among vertebrates, the cellular and molecular mechanisms of neural induction appear largely divergent [1], thus providing little indication of the ancestral chordate mode. Ascidians, which belong to a sub-phylum of the chordates (Urochordata) long separated from the lineage leading to the vertebrates, may provide insight to these questions [2,3]. We have investigated neural specification in ascidians using a genetic approach. Ascidians are hermaphrodites, and the ability of ascidians of the genus Ciona to self-fertilize provides a rapid means for identifying recessive zygotic mutations [4,5]. We screened Ciona intestinalis for naturally occurring mutations, which can be detected in a large percentage of the wild population One such mutant is frimousse (frm). In the ascidian larva, the sensory vesicle, the putative homolog of the vertebrate foreand midbrain [6], is the most prominent feature of the anterior CNS, and contains two pigmented sensory organs, the ocellus and the otolith, which are sensitive to light and gravity, respectively (Figure 1A). The frimousse (frm) mutant was initially identified in larvae of a self-fertilized brood due to the absence of the pigmented sensory structures in ~25% of individuals (Figure 1B). The founder was subsequently crossed to wild-type individuals, and the mutant line has been maintained for over six generations without a change in phenotype. Conservationists have good reason to ring alarm bells about the decline in numbers of many species worldwide. Most are linked to the increasing intrusion from human activity, but one decline has proved a major puzzle: all around the world there appears to be a decline in numbers of amphibians and often in habitats with minimal human influence. A new study has looked at the falling numbers of the mountain yellow-legged frog (Rana muscosa) in the Sierra Nevada in California. Reporting in the Proceedings of the National Academy of Sciences (published online) a researcher has for once got some good news. Not only does he determine one major factor in the frog population's decline, he describes how to reverse it. The key factor was thought by Vance Vredenburg, at the University of California, Berkeley, to be the widespread introduction of both the rainbow trout and brook trout into lakes in these mountains where the frogs live. His study experimentally manipulated the presence and absence of these two fish species. From 1996 to 2003, the introduced trout were removed from five lakes in a remote protected area, and 16 nearby lakes were used as controls, eight with introduced trout and eight without. To determine the vulnerable life stages, rainbow trout were placed in cages in three lakes containing amphibians. The study found that removal of introduced trout resulted in rapid recovery of frog populations and in the caging experiment, tadpoles were found to be vulnerable to trout predation. The results suggest that the widespread introduction of these fish has had a major impact on frog populations in this region of California, but that removal of them can lead to a rapid and dramatic reversal in the amphibians' fortunes. Other observations in Costa Rica, Spain and field experiments in Australia implicate introduced trout in the decline of local frog populations, but this study has shown for this frog species what has actually been happening and what can be done. Trout out

Thermal plasticity of skeletal muscle phenotype in ectothermic vertebrates and its significance for locomotory behaviour.

Seasonal cooling can modify the thermal preferenda of ectothermic vertebrates and elicit a variety of physiological responses ranging from winter dormancy to an acclimation response that partially compensates for the effects of low temperature on activity. Partial compensation of activity levels is particularly common in aquatic species for which seasonal temperature changes provide a stable cue for initiating the response. Thermal plasticity of locomotory performance has evolved independently on numerous occasions, and there is considerable phylogenetic diversity with respect to the mechanisms at the physiological and molecular levels. In teleosts, neuromuscular variables that can be modified include the duration of motor nerve stimulation, muscle activation and relaxation times, maximum force and unloaded shortening velocity (V(max)), although not all are modified in every species. Thermal plasticity in V(max) has been associated with changes in myosin ATPase activity and myosin heavy chain (MyHC) composition and/or with a change in the ratio of myosin light chain isoforms. In common carp (Cyprinus carpio), there are continuous changes in phenotype with acclimation temperature at lower levels of organisation, such as MyHC composition and V(max), but a distinct threshold for an effect in terms of locomotory performance. Thus, there is no simple relationship between whole-animal performance and muscle phenotype. The nature and magnitude of temperature acclimation responses also vary during ontogeny. For example, common carp acquire the ability to modify MyHC composition with changes in acclimation temperature during the juvenile stage. In contrast, the thermal plasticity of swimming performance observed in tadpoles of the frog Limnodynastes peronii is lost in the terrestrial adult stage. Although it is often assumed that the adjustments in locomotory performance associated with temperature acclimation enhance fitness, this has rarely been tested experimentally. Truly integrative studies of temperature acclimation are scarce, and few studies have considered both sensory and motor function in evaluating behavioural responses. Developmental plasticity is a special case of a temperature acclimation response that can lead to temporary or permanent changes in morphology and/or physiological characteristics that affect locomotory performance.

A role for HNF-3 in the regulation of the HNF-1 gene of the Atlantic salmon.

Hepatocyte nuclear factors -1 (HNF-1) and -3 (HNF-3) are hepatocyte-enriched transcription factors that are central to the establishment and maintenance of the liver phenotype in vertebrates. In the present study we demonstrate that, in the Atlantic salmon, asHNF-3 regulates the expression of the gene for asHNF-1. Multiple putative binding sites for asHNF-3 were identified within the 5' flanking region of the HNF-1 gene using a computer-based algorithm, and these were confirmed to be functional by electrophoretic mobility shift assays. In transient transfection assays it was shown that co-expression of asHNF-3 leads to a decrease in the promoter activity of the 5' flanking region of the asHNF-1 gene.

The role of Bapx1 (Nkx3.2) in the development and evolution of the axial skeleton.

The bagpipe-related homeobox-containing genes are members of the NK family, bagpipe (bap) was first identified in Drosophila and there are three different bagpipe-related genes in vertebrates. Only two of these are found in mammals, the Nkx3.1 and the Bapxl (Nkx3.2) gene. The targeted mutation in the mouse Bapxl gene shows a vertebral phenotype in which the ventromedial elements are lacking; these are the centra and the intervertebral discs. In addition, a region of gastric mesenchyme is abnormal. This mesenchyme surrounds the posterior region of the presumptive stomach and duodenum, and in the mutant fails to support normal development of the spleen. In Drosophila, bagpipe has a role in gut mesoderm and the mutant embryos have no midgut musculature. Thus bap related genes in mouse and Drosophila have roles in patterning gut mesoderm; however, neither of the mammalian genes has a discernible role in the gut musculature. In contrast, both mammalian genes have roles in developmental processes that have appeared recently in evolution. The Bapxl gene found in fish, amphibians, birds and mammals appears to have derived vertebrate specific functions sometime after the split between the jawless fish and gnathostomes.

TGFβ signalling

Mutation phenotypes in vertebrates suggest that establishment of the embryonic midline and of left–right asymmetry are linked. Three recent papers clarify the pathways, and suggest evolutionary conservation across three species. Bisgrove et al.1 Izraeli S. et al. Regulation of midline development by antagonism of lefty and nodal signaling. Nature. 1999; 399: 691-693 Crossref PubMed Scopus (167) Google Scholar look at the role of TGFβ family members, while Izraeli et al.2 Bisgrove B. et al. Nodal signalling and the roles of the transcription factors SnR and Pitx2 in vertebrate left-right asymmetry. Development. 1999; 126: 3253-3262 PubMed Google Scholar and Patel et al.3 Patel K. et al. Mutation phenotypes in vertebrates suggest that establishment of the embryonic midline and of left-right asymmetry are linked. Curr. Biol. 1999; 9: 609-612 Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar focus on genes upstream and downstream of TGFβ signalling. ErratumAlison SnapeTrends in GeneticsNovember 01, 1999In Brief Full-Text PDF

The homeobox gene Phox2b is essential for the development of autonomic neural crest derivatives.

The sympathetic, parasympathetic and enteric ganglia are the main components of the peripheral autonomic nervous system, and are all derived from the neural crest. The factors needed for these structures to develop include the transcription factor Mash1, the glial-derived neurotrophic factor GNDF and its receptor subunits, and the neuregulin signalling system, each of which is essential for the differentiation and survival of subsets of autonomic neurons. Here we show that all autonomic ganglia fail to form properly and degenerate in mice lacking the homeodomain transcription factor Phox2b, as do the three cranial sensory ganglia that are part of the autonomic reflex circuits. In the anlagen of the enteric nervous system and the sympathetic ganglia, Phox2b is needed for the expression of the GDNF-receptor subunit Ret and for maintaining Mash1 expression. Mutant ganglionic anlagen also fail to switch on the genes that encode two enzymes needed for the biosynthesis of the neurotransmitter noradrenaline, dopamine-beta-hydroxylase and tyrosine hydroxylase, demonstrating that Phox2b regulates the noradrenergic phenotype in vertebrates.

Synteny-defined candidate genes for congenital and idiopathic scoliosis

Idiopathic scoliosis (IS) is a common but poorly understood syndrome. Congenital scoliosis (CS) is less common but comparably unexplored. Previous studies suggest that each has a significant genetic component. However, the occurrence of scoliosis in the presence of other hereditary connective tissue syndromes raises the possibility that IS and CS are in fact a heterogeneous group of disorders with varied pathogenetic mechanisms. Mouse mutations have proven informative in identifying genes that are important in the development of the musculoskeletal system and provided important mechanistic insights regarding their roles in human disease. We sought to identify candidate genes for human IS and CS by reviewing mouse mutations with phenotypes affecting the axial skeleton. We performed a systematic review using the Mouse Genome Database (MGD), the Genome Database (GDB), and the Online Mendelian Inheritance in Man (OMIM) world-wide-web sites with additional searches performed based on the results of this initial search. We identified approximately 400 mouse mutations, reviewed approximately 250 of these for vertebral phenotypes, assessed 45 of these for synteny conservation between mouse and man, and identified 28 mouse mutations for which 29 credible candidates for human scoliosis could be identified based on mouse phenotypic and mapping data. For each of these, we have synthesized information about the mouse mutant phenotype, mapping data, information regarding molecular pathogenesis when a specific causative gene has been identified, and information regarding plausible candidates based on map position when the causative gene has not been identified. Among these were three loci for which the mutant gene had been identified and the human homologue was known. Some of the mouse mutants have phenotypes similar to human syndromes.

Paired-like homeodomain proteins, Phox2a and Phox2b, are responsible for noradrenergic cell-specific transcription of the dopamine beta-hydroxylase gene.

Recently, a murine paired-like homeobox gene, Phox2a, has been identified whose product is critical for the development of several major noradrenergic neuron populations, including the locus coeruleus. In noradrenergic neurons, dopamine beta-hydroxylase (DBH) is a hallmark protein and catalyzes the conversion of dopamine to noradrenaline. Our previous studies have shown that a composite promoter (domain IV), residing at -185 to -150 bp upstream of the transcription start site, is critical for DBH transcription and is comprised of multiple cis-acting elements, including a cyclic AMP response element, a YY1 binding site, and two core motifs of the homeodomain (HD)-binding site. Here, we show that the HD-binding site residing within domain IV is a noradrenergic-specific cis-acting element. In contrast, the cyclic AMP response element is active in all cell lines tested. We provide evidence that Phox2a is expressed only in DBH-positive cell lines and interacts with the HD-binding site. Forced expression of Phox2a robustly activates DBH promoter activity in DBH-negative cell lines (>10-fold), but increased it only marginally (<50%) in DBH-positive cell lines. Furthermore, another protein factor with an identical HD, Phox2b, also activates DBH transcription with an efficiency comparable to that of Phox2a. In contrast, neither Phox2a nor Phox2b was able to transactivate tyrosine hydroxylase transcription, indicating that these transcription factors differentially activate catecholamine-synthesizing gene transcription. Together with the Phox2a knockout experiment, the studies described here make Phox2a and Phox2b the first strong candidate transcription factors for determining a neurotransmitter phenotype in vertebrates.

REPRODUCTIVE ENERGY ALLOCATION AND LONG-TERM ENERGY STORES IN A VIVIPAROUS LIZARD (EULAMPRUS TYMPANUM)

When animals reproduce, does energy for the brood come from stored reserves (from food eaten months or years before), or from food eaten during the current reproductive season? Variation in the duration of energy storage prior to reproduction is an important, but little-studied, axis of life history variation. We experimentally manipulated the availability of resources to females of a long-lived viviparous lizard (Eulamprus tympanum, Scincidae) in the first year of a two-year study to assess the relative importance of current food intake vs. stored reserves as sources of energy for breeding. Females given greater access to high temperatures (and hence, to resources) in the laboratory amassed larger caudal energy stores than did females with lesser opportunities. Energy intake during gestation slightly influenced offspring size, but the magnitude of energy stores prior to ovulation had a more dramatic effect: females with larger caudal energy stores produced much larger litters in the following year. Growth rates of females were unaffected by basking treatments or by reproduction in either year, but females that reproduced in the second year showed a large concomitant decrease in caudal energy stores compared to females that did not reproduce, indicating a large energetic cost of reproduction. Hence, reproductive output within this species can be influenced by resource availability over 12 months prior to reproduction, and simple comparisons of reproductive output with current resource availability may fail to detect the ways in which phenotypically plastic life history traits are influenced by environmental features. In addition to an appropriate temporal framework, models of resource allocation should also incorporate the possible roles of physiological and morphological constraints (e.g., maternal inability, in some species, to modify postovulation reproductive expenditure) and trade-offs with other life history currencies (e.g., decrements in maternal survival due to high relative clutch masses). The time lag between resource acquisition and expenditure may have significant consequences for a population’s demographic response to shifts in resource availability or predation pressure (via tail autotomy). Finally, this study illustrates the way in which thermal aspects of the environment can influence life history phenotypes in ectothermic vertebrates with long-term energy stores.

THE FUNCTIONAL BASIS OF NATURAL SELECTION FOR VERTEBRAL TRAITS OF LARVAE IN THE STICKLEBACK GASTEROSTEUS ACULEATUS.

Previous studies have demonstrated selective predation for vertebral traits of larvae in the stickleback Gasterosteus aculeatus. I tested the hypothesis that this selection results from a direct functional advantage to particular vertebral phenotypes by direct measurement of the burst swimming performance of larvae. Within a narrow window of lengths, burst speed did depend on vertebral phenotype. As in the previous predation experiments, performance was related more directly to the ratio of abdominal to caudal vertebrae (VR) than to the total number of vertebrae (VN), and the optimal VR decreased as larval length increased. Changes with length in the vertebral phenotype frequencies of wild larvae provided evidence of selection for VR and for VN in the wild. Larvae with particular VR increased in frequency in the wild at just those lengths when their relative performance was superior in the laboratory. The observed pattern of length-dependent selection for vertebral number provides an explanation for the widespread trends in vertebral number that occur among populations of related fishes.

Selective Predation for Vertebral Phenotype in Gasterosteus aculeatus: Reversal in the Direction of Selection at Different Larval Sizes

Selective Predation for Vertebral Phenotype in Gasterosteus aculeatus: Reversal in the Direction of Selection at Different Larval Sizes