A periodic pattern generator for dental diversity

Continuous tooth replacement: what can teleost fish teach us?

Most tooth-bearing non-mammalian vertebrates have the capacity to replace their teeth throughout life. This capacity was lost in mammals, which replace their teeth only once at most. Not surprisingly, continuous tooth replacement has attracted much attention. Classical morphological studies (e.g. to analyse patterns of replacement) are now being complemented by molecular studies that investigate the expression of genes involved in tooth formation. This review focuses on ray-finned fish (actinopterygians), which have teeth often distributed throughout the mouth and pharynx, and more specifically on teleost fish, the largest group of extant vertebrates. First we highlight the diversity in tooth distribution and in tooth replacement patterns. Replacement tooth formation can start from a distinct (usually discontinuous and transient) dental lamina, but also in the absence of a successional lamina, e.g. from the surface epithelium of the oropharynx or from the outer dental epithelium of a predecessor tooth. The relationship of a replacement tooth to its predecessor is closely related to whether replacement is the result of a prepattern or occurs on demand. As replacement teeth do not necessarily have the same molecular signature as first-generation teeth, the question of the actual trigger for tooth replacement is discussed. Much emphasis has been laid in the past on the potential role of epithelial stem cells in initiating tooth replacement. The outcome of such studies has been equivocal, possibly related to the taxa investigated, and the permanent or transient nature of the dental lamina. Alternatively, replacement may result from local proliferation of undifferentiated progenitors, stimulated by hitherto unknown, perhaps mesenchymal, factors. So far, the role of the neurovascular link in continuous tooth replacement has been poorly investigated, despite the presence of a rich vascularisation surrounding actinopterygian (as well as chondrichthyan) teeth and despite a complete arrest of tooth replacement after nerve resection. Lastly, tooth replacement is possibly co-opted as a process to expand the number of teeth in a dentition ontogenetically whilst conserving features of the primary dentition. That neither a dental lamina, nor stem cells appear to be required for tooth replacement places teleosts in an advantageous position as models for tooth regeneration in humans, where the dental lamina regresses and epithelial stem cells are considered lost.

Morphological description of spontaneous pelvic fin loss in a neotropical cichlid fish

Pelvic fins are a characteristic structure of the vertebrate Bauplan. Yet, pelvic fin loss has occurred repeatedly across a wide diversity of other lineages of tetrapods and at least 48 times in teleost fishes. This pelvic finless condition is often associated with other morphological features such as body elongation, loss of additional structures, and bilateral asymmetry. However, despite the remarkable diversity in the several thousand cichlid fish species, none of them are characterized by the complete absence of pelvic fins. Here, we examined the musculoskeletal structure and associated bilateral asymmetry in Midas cichlids (Amphilophus cf. citrinellus) that lost their pelvic fins spontaneously in the laboratory. Due to this apparent mutational loss of the pelvic girdle and fins, the external and internal anatomy are described in a series of "normal" Midas individuals and their pelvic finless sibling tankmates. First, other traits associated with teleost pelvic fin loss, the genetic basis of pelvic fin loss, and the potential for pleiotropic effects of these genes on other traits in teleosts were all reviewed. Using these traits as a guide, we investigated whether other morphological differences were associated with the pelvic girdle/fin loss. The mean values of the masses of muscle of the pectoral fin, fin ray numbers in the unpaired fins, and oral jaw tooth numbers did not differ between the two pelvic fin morphotypes. However, significant differences in meristic values of the paired traits assessed were observed for the same side of the body between morphotypes. Notably, bilateral asymmetry was found exclusively for the posterior lateral line scales. Finally, we found limited evidence of pleiotropic effects, such as lateral line scale numbers and fluctuating asymmetry between the Midas pelvic fin morphotypes. The fast and relatively isolated changes in the Midas cichlids suggest minor but interesting pleiotropic effects could accompany loss of cichlid pelvic fins.

Morphological characteristics of beak-like tooth in spotted knifejaw (Oplegnathus punctatus) and mechanisms of dental development regulation by the Wnt, BMP, FGF and SHH signalling pathways

Oplegnathus punctatus is a fish species with beak-like tooth that feeds on algae, oysters, sea urchins, and other organisms attached to rocks. Currently, there are no research reports on the development and regulatory mechanisms of O. punctatus beak-like tooth. This present study firstly elucidated the nesting structure pattern of the beak-like tooth with dental formula (4, 15-16, 10-1) for O. punctatus. Four critical periods during early beak-like tooth development (28dph, 40dph, 50dph, 60dph) were also identified. In addition, 11 key genes (bmp2, bmpr2, smad1, wnt5a, msx, axin2, fgfr1a, fgfr2, pitx2, ptch1, cyp27a1) closely related to the development of beak-like tooth were discovered, with the highest expression levels in the initial stages of functional teeth and replacement teeth development, and expression in the mesenchymal and epithelial tissues of the teeth. Further research found that the cyp27a1 gene, related to vitamin D metabolism and calcium accumulation, was expressed in the maxilla and base of the tooth in O. punctatus. This study provides support for the biological theory of tooth development and healing and provides a reference for the adaptive evolution of tooth healing in special habitats.

Single Cell RNA Sequencing Reveals Deep Homology of Dental Cell Types Across Vertebrates

Like most mammals, humans replace their teeth once throughout their lives and have limited regenerative capabilities. In contrast, mice continually renew tissues lost due to gnawing through a well characterized population of stem cells on the labial surface of the incisor. Most non-mammalian vertebrates replace teeth throughout life; the cellular and molecular mechanisms of successional tooth replacement are largely unknown. Here we use single nuclei RNA sequencing (snRNA-seq) of replacement teeth and adjacent oral lamina in Lake Malawi cichlids, species with lifelong whole–tooth replacement, to make two main discoveries. First, despite hundreds of millions of years of evolution, we demonstrate conservation of cell type gene expression across vertebrate teeth (fish, mouse, human). Second, we used an approach that combines marker gene expression and developmental potential of dental cells to uncover the transcriptional signature of stem-like cells in regenerating teeth. Our work underscores the importance of a comparative framework in the study of vertebrate oral and regenerative biology.

An epithelial signalling centre in sharks supports homology of tooth morphogenesis in vertebrates

Development of tooth shape is regulated by the enamel knot signalling centre, at least in mammals. Fgf signalling regulates differential proliferation between the enamel knot and adjacent dental epithelia during tooth development, leading to formation of the dental cusp. The presence of an enamel knot in non-mammalian vertebrates is debated given differences in signalling. Here, we show the conservation and restriction of fgf3, fgf10, and shh to the sites of future dental cusps in the shark (Scyliorhinus canicula), whilst also highlighting striking differences between the shark and mouse. We reveal shifts in tooth size, shape, and cusp number following small molecule perturbations of canonical Wnt signalling. Resulting tooth phenotypes mirror observed effects in mammals, where canonical Wnt has been implicated as an upstream regulator of enamel knot signalling. In silico modelling of shark dental morphogenesis demonstrates how subtle changes in activatory and inhibitory signals can alter tooth shape, resembling developmental phenotypes and cusp shapes observed following experimental Wnt perturbation. Our results support the functional conservation of an enamel knot-like signalling centre throughout vertebrates and suggest that varied tooth types from sharks to mammals follow a similar developmental bauplan. Lineage-specific differences in signalling are not sufficient in refuting homology of this signalling centre, which is likely older than teeth themselves.

High-Resolution Histology for Craniofacial Studies on Zebrafish and Other Teleost Models

In the era of molecular biology, identification of cells and even tissues mostly relies on the presence of fluorescent tags, or of "marker gene" expression. We list a number of caveats and present a protocol for embedding, sectioning, and staining semithin plastic sections. The method is neither new nor innovative, but is meant to revive skills that tend to get lost.This easy-to-use and inexpensive protocol (1) yields high-resolution images in transmitted and polarized light, (2) can be utilized simultaneously for transmission electron microscopy, and (3) is applicable to any type of material (wild type, morphants, mutants, transgenic, or pharmacologically treated animals as well as all of their controls), provided the sample size is kept under a limit. Thus, we hope to encourage researchers to use microanatomy and histology to complement molecular studies investigating, e.g., gene function.

The remarkable dynamics in the establishment, rearrangement, and loss of dentition during the ontogeny of the sterlet sturgeon

BACKGROUND

Sturgeons belong to an early-branching lineage often used as a proxy of ancestor-like traits of ray-finned fishes. However, many features of this lineage, such as the transitory presence and the eventual loss of dentition, exemplify specializations that, in fact, provide important information on lineage-specific evolutionary dynamics.

RESULTS

Here, we introduce a detailed overview of the dentition during the development of the sterlet sturgeon. The dentition is composed of tooth fields at oral, palatal, and anterior pharyngeal regions. Oral fields are single-rowed, non-renewed and are shed early. Palatal and pharyngeal fields are multi-rowed and renewed from the adjacent superficial epithelium without the presence of the successional dental lamina. The early loss of oral fields and subsequent establishment of palatal and pharyngeal fields leads to a translocation of the functional dentition from the front to the rear of the oropharyngeal cavity until the eventual loss of all teeth.

CONCLUSIONS

Our survey shows the sterlet dentition as a dynamic organ system displaying differential composition at different time points in the lifetime of this fish. These dynamics represent a conspicuous feature of sturgeons, unparalleled among extant vertebrates, and appropriate to scrutinize developmental and evolutionary underpinnings of vertebrate odontogenesis.

The conundrum of pharyngeal teeth origin: the role of germ layers, pouches, and gill slits

There are several competing hypotheses on tooth origins, with discussions eventually settling in favour of an 'outside-in' scenario, in which internal odontodes (teeth) derived from external odontodes (skin denticles) in jawless vertebrates. The evolution of oral teeth from skin denticles can be intuitively understood from their location at the mouth entrance. However, the basal condition for jawed vertebrates is arguably to possess teeth distributed throughout the oropharynx (i.e. oral and pharyngeal teeth). As skin denticle development requires the presence of ectoderm-derived epithelium and of mesenchyme, it remains to be answered how odontode-forming skin epithelium, or its competence, were 'transferred' deep into the endoderm-covered oropharynx. The 'modified outside-in' hypothesis for tooth origins proposed that this transfer was accomplished through displacement of odontogenic epithelium, that is ectoderm, not only through the mouth, but also via any opening (e.g. gill slits) that connects the ectoderm to the epithelial lining of the pharynx (endoderm). This review explores from an evolutionary and from a developmental perspective whether ectoderm plays a role in (pharyngeal) tooth and denticle formation. Historic and recent studies on tooth development show that the odontogenic epithelium (enamel organ) of oral or pharyngeal teeth can be of ectodermal, endodermal, or of mixed ecto-endodermal origin. Comprehensive data are, however, only available for a few taxa. Interestingly, in these taxa, the enamel organ always develops from the basal layer of a stratified epithelium that is at least bilayered. In zebrafish, a miniaturised teleost that only retains pharyngeal teeth, an epithelial surface layer with ectoderm-like characters is required to initiate the formation of an enamel organ from the basal, endodermal epithelium. In urodele amphibians, the bilayered epithelium is endodermal, but the surface layer acquires ectodermal characters, here termed 'epidermalised endoderm'. Furthermore, ectoderm-endoderm contacts at pouch-cleft boundaries (i.e. the prospective gill slits) are important for pharyngeal tooth initiation, even if the influx of ectoderm via these routes is limited. A balance between sonic hedgehog and retinoic acid signalling could operate to assign tooth-initiating competence to the endoderm at the level of any particular pouch. In summary, three characters are identified as being required for pharyngeal tooth formation: (i) pouch-cleft contact, (ii) a stratified epithelium, of which (iii) the apical layer adopts ectodermal features. These characters delimit the area in which teeth can form, yet cannot alone explain the distribution of teeth over the different pharyngeal arches. The review concludes with a hypothetical evolutionary scenario regarding the persisting influence of ectoderm on pharyngeal tooth formation. Studies on basal osteichthyans with less-specialised types of early embryonic development will provide a crucial test for the potential role of ectoderm in pharyngeal tooth formation and for the 'modified outside-in' hypothesis of tooth origins.

Genome-enabled discovery of evolutionary divergence in brains and behavior

Lake Malawi cichlid fishes exhibit extensive divergence in form and function built from a relatively small number of genetic changes. We compared the genomes of rock- and sand-dwelling species and asked which genetic variants differed among the groups. We found that 96% of differentiated variants reside in non-coding sequence but these non-coding diverged variants are evolutionarily conserved. Genome regions near differentiated variants are enriched for craniofacial, neural and behavioral categories. Following leads from genome sequence, we used rock- vs. sand-species and their hybrids to (i) delineate the push-pull roles of BMP signaling and irx1b in the specification of forebrain territories during gastrulation and (ii) reveal striking context-dependent brain gene expression during adult social behavior. Our results demonstrate how divergent genome sequences can predict differences in key evolutionary traits. We highlight the promise of evolutionary reverse genetics-the inference of phenotypic divergence from unbiased genome sequencing and then empirical validation in natural populations.

Tooth Removal in the Leopard Gecko and the de novo Formation of Replacement Teeth

Many reptiles are able to continuously replace their teeth through life, an ability attributed to the existence of epithelial stem cells. Tooth replacement occurs in a spatially and temporally regulated manner, suggesting the involvement of diffusible factors, potentially over long distances. Here, we locally disrupted tooth replacement in the leopard gecko (Eublepharis macularius) and followed the recovery of the dentition. We looked at the effects on local patterning and functionally tested whether putative epithelial stem cells can give rise to multiple cell types in the enamel organs of new teeth. Second generation teeth with enamel and dentine were removed from adult geckos. The dental lamina was either left intact or disrupted in order to interfere with local patterning cues. The dentition began to reform by 1 month and was nearly recovered by 2-3 months as shown in μCT scans and eruption of teeth labeled with fluorescent markers. Microscopic analysis showed that the dental lamina was fully healed by 1 month. The deepest parts of the dental lamina retained odontogenic identity as shown by PITX2 staining. A pulse-chase was carried out to label cells that were stimulated to enter the cell cycle and then would carry BrdU forward into subsequent tooth generations. Initially we labeled 70-78% of PCNA cells with BrdU. After a 1-month chase, the percentage of BrdU + PCNA labeled cells in the dental lamina had dropped to 10%, consistent with the dilution of the label. There was also a population of single, BrdU-labeled cells present up to 2 months post surgery. These BrdU-labeled cells were almost entirely located in the dental lamina and were the likely progenitor/stem cells because they had not entered the cell cycle. In contrast fragmented BrdU was seen in the PCNA-positive, proliferating enamel organs. Homeostasis and recovery of the gecko dentition was therefore mediated by a stable population of epithelial stem cells in the dental lamina.

Evolutionary developmental genetics of teeth and odontodes in jawed vertebrates: a perspective from the study of elasmobranchs

Most extant vertebrates display a high variety of tooth and tooth-like organs (odontodes) that vary in shape, position over the body and nature of composing tissues. The development of these structures is known to involve similar genetic cascades and teeth and odontodes are believed to share a common evolutionary history. Gene expression patterns have previously been compared between mammalian and teleost tooth development but we highlight how the comparative framework was not always properly defined to deal with different tooth types or tooth developmental stages. Larger-scale comparative analyses also included cartilaginous fishes: sharks display oral teeth and dermal scales for which the gene expression during development started to be investigated in the small-spotted catshark Scyliorhinus canicula during the past decade. We report several descriptive approaches to analyse the embryonic tooth and caudal scale gene expressions in S. canicula. We compare these expressions wih the ones reported in mouse molars and teleost oral and pharyngeal teeth and highlight contributions and biases that arise from these interspecific comparisons. We finally discuss the evolutionary processes that can explain the observed intra and interspecific similarities and divergences in the genetic cascades involved in tooth and odontode development in jawed vertebrates.

Oral and Palatal Dentition of Axolotl Arises From a Common Tooth-Competent Zone Along the Ecto-Endodermal Boundary

Vertebrate dentitions arise at various places within the oropharyngeal cavity including the jaws, the palate, or the pharynx. These dentitions develop in a highly organized way, where new tooth germs are progressively added adjacent to the initiator center, the first tooth. At the same time, the places where dentitions develop house the contact zones between the outer ectoderm and the inner endoderm, and this colocalization has instigated various suggestions on the roles of germ layers for tooth initiation and development. Here, we study development of the axolotl dentition, which is a complex of five pairs of tooth fields arranged into the typically tetrapod outer and inner dental arcades. By tracking the expression patterns of odontogenic genes, we reason that teeth of both dental arcades originate from common tooth-competent zones, one present on the mouth roof and one on the mouth floor. Progressive compartmentalization of these zones and a simultaneous addition of new tooth germs distinct for each prospective tooth field subsequently control the final shape and composition of the axolotl dentition. Interestingly, by following the fate of the GFP-labeled oral ectoderm, we further show that, in three out of five tooth field pairs, the first tooth develops right at the ecto-endodermal boundary. Our results thus indicate that a single tooth-competent zone gives rise to both dental arcades of a complex tetrapod dentition. Further, we propose that the ecto-endodermal boundary running through this zone should be accounted for as a potential source of instruction factors instigating the onset of the odontogenic program.

The role of core and variable Gene Regulatory Network modules in tooth development and evolution

Among the developmental processes that have been proposed to influence the direction of evolution, the modular organization of developmental gene regulatory networks (GRNs) has shown particular promise. In theory, GRNs have core modules comprised of essential, conserved circuits of genes, and sub-modules of downstream, secondary circuits of genes that are more susceptible to variation. While this idea has received considerable interest as of late, the field of evo-devo lacks the experimental systems needed to rigorously evaluate this hypothesis. Here, we introduce an experimental system, the vertebrate tooth, that has great potential as a model for testing this hypothesis. Tooth development and its associated GRN have been well studied and modeled in both model and non-model organisms. We propose that the existence of modules within the tooth GRN explains both the conservation of developmental mechanisms and the extraordinary diversity of teeth among vertebrates. Based on experimental data, we hypothesize that there is a conserved core module of genes that is absolutely necessary to ensure tooth or cusp initiation and development. In regard to tooth shape variation between species, we suggest that more relaxed sub-modules activated at later steps of tooth development, e.g., during the morphogenesis of the tooth and its cusps, control the different axes of tooth morphological variation.

Species specificity and sexual dimorphism in tooth shape among the three sympatric haplochromine species in Lake Kivu cichlids

Tooth shape is used to differentiate between morphologically similar species of vertebrates, including fish. This study aimed to quantify tooth shape of three sympatric species: Haplochromis kamiranzovu, H. insidiae, and H. astatodon endemic to Lake Kivu, whose existing identification criteria are currently only qualitative. A quantitative tooth shape analysis was performed based on digitized tooth outline data with a subsequent elliptic Fourier analysis to test for differences among the three species. We looked at crown shape and size differences within H. kamiranzovu and H. insidiae at geographical, habitat, and gender levels. No comparison at habitat level was done for H. astatodon because it is found only in littoral zone. The analysis revealed significant tooth shape differences among the three species. Haplochromis astatodon had a significantly longer major cusp height and a longer and larger minor cusp than that of H. insidiae. It had also a longer major cusp height and a longer and larger minor cusp than that of H. kamiranzovu. Tooth shape differences of H. kamiranzovu and H. insidiae species were not significantly different between littoral and pelagic fish (p > .05) while differences were significant between southern and northern Lake Kivu populations (p < .05). Tooth sizes in H. kamiranzovu and H. insidiae were significantly different, both in height and width as well as in their ratios, and this was true at sex and geographic levels (p < .05), but not at habitat level (p > .05). Tooth shape was also significantly different with sharp teeth for males compared with females of southern populations versus northern ones. These shape- and size-related differences between sexes suggest differences in the foraging strategies toward available food resources in the lake habitat. Further research should explain the genetic basis of the observed pattern.

Novel and Bizarre Abnormalities of the Tooth Row in Side-Blotched Lizards (Uta) and Rock Lizards (Petrosaurus)

Multiple tooth rows along the dentary or maxilla are present in many bony and cartilaginous fishes but occur infrequently in amniotes. Although two partially overlapping rows of teeth or a few doubled tooth positions were both previously reported in extant and extinct lizards, multiple rows of teeth across the entire dental or maxillary shelf were not previously documented in any lizard taxon. Here, we report one specimen of the side-blotched lizard Uta stansburiana and two specimens of the rock lizard Petrosaurus mearnsi that have two or more marginal tooth rows that overlap for most of the dental shelf. We also describe several other phrynosomatid lizard specimens with one or two doubled tooth positions per marginal tooth-bearing skeletal element. We examined 520 total specimens of pleurodont iguanian lizards and verified that aberrant tooth row and tooth position morphologies were almost exclusively present in phrynosomatid lizards. We hypothesize that developmental irregularities in the number of odontogenic bands or in the function of zones of inhibition may have resulted in the observed tooth row abnormalities. The expression of multiple tooth rows or doubled tooth positions may be phylogenetically informative morphologies of phrynosomatid lizards. This bears further investigation from developmental, genetic, ecological, and phylogenetic perspectives. Anat Rec, 2019. © 2019 American Association for Anatomy Anat Rec, 303:2014-2025, 2020. © 2019 American Association for Anatomy.

Developmental plasticity of epithelial stem cells in tooth and taste bud renewal

Nearly one-third of adults over the age of 65 have lost all their teeth. We set out to understand tooth renewal in animals that have replacement and regeneration capabilities. Using cichlid fishes and mouse models, we discovered plasticity between tooth and taste bud progenitor cell derivatives, mediated by BMP. Our results suggest that oral organs have surprising regenerative capabilities and can be manipulated to express characteristics of different tissue types.

In Lake Malawi cichlids, each tooth is replaced in one-for-one fashion every ∼20 to 50 d, and taste buds (TBs) are continuously renewed as in mammals. These structures are colocalized in the fish mouth and throat, from the point of initiation through adulthood. Here, we found that replacement teeth (RT) share a continuous band of epithelium with adjacent TBs and that both organs coexpress stem cell factors in subsets of label-retaining cells. We used RNA-seq to characterize transcriptomes of RT germs and TB-bearing oral epithelium. Analysis revealed differential usage of developmental pathways in RT compared to TB oral epithelia, as well as a repertoire of genome paralogues expressed complimentarily in each organ. Notably, BMP ligands were expressed in RT but excluded from TBs. Morphant fishes bathed in a BMP chemical antagonist exhibited RT with abrogated shh expression in the inner dental epithelium (IDE) and ectopic expression of calb2 (a TB marker) in these very cells. In the mouse, teeth are located on the jaw margin while TBs and other oral papillae are located on the tongue. Previous study reported that tongue intermolar eminence (IE) oral papillae of Follistatin (a BMP antagonist) mouse mutants exhibited dysmorphic invagination. We used these mutants to demonstrate altered transcriptomes and ectopic expression of dental markers in tongue IE. Our results suggest that vertebrate oral epithelium retains inherent plasticity to form tooth and taste-like cell types, mediated by BMP specification of progenitor cells. These findings indicate underappreciated epithelial cell populations with promising potential in bioengineering and dental therapeutics.

The alternative regenerative strategy of bearded dragon unveils the key processes underlying vertebrate tooth renewal

Deep understanding of tooth regeneration is hampered by the lack of lifelong replacing oral dentition in most conventional models. Here, we show that the bearded dragon, one of the rare vertebrate species with both polyphyodont and monophyodont teeth, constitutes a key model for filling this gap, allowing direct comparison of extreme dentition types. Our developmental and high-throughput transcriptomic data of microdissected dental cells unveils the critical importance of successional dental lamina patterning, in addition to maintenance, for vertebrate tooth renewal. This patterning process happens at various levels, including directional growth but also gene expression levels, dynamics, and regionalization, and involves a large number of yet uncharacterized dental genes. Furthermore, the alternative renewal mechanism of bearded dragon dentition, with dual location of slow-cycling cells, demonstrates the importance of cell migration and functional specialization of putative epithelial stem/progenitor niches in tissue regeneration, while expanding the diversity of dental replacement strategies in vertebrates.

All multicellular organisms, from lizards to humans, must be able to repair and regrow damaged tissue. This includes not only healing after an injury, but also replacing parts of the body that suffer wear and tear. For example, many animals shed and replace worn out teeth throughout their life, but the number of times this occurs varies greatly between species.

Much of the understanding about how teeth grow and develop has come from researching mice. However, mice only develop one set of teeth, making them a poor ‘model’ for studying how species such as fish and reptiles can re-grow and replace their teeth. Recent studies of these species has shown that regenerating teeth relies on a specialised structure known as the dental lamina. In mice, the dental lamina forms but then quickly disappears, preventing new sets of teeth from developing. In most animals that regrow their teeth, however, the dental lamina keeps growing beyond the most recently produced tooth to create an area where its replacement will emerge. Now, Salomies et al. have identified other strategies involved in tooth replacement from studying the bearded dragon lizard, a rare example of an animal that continuously regenerates some, but not all, of its teeth.

Analysing the cells in different parts of the re-growing teeth from bearded dragon lizards revealed new features of the dental lamina. Specifically, Salomies et al. found that a previously uncharacterized set of genes within the dental lamina could determine whether or not a tooth will be replaced. Further experiments using microscope imaging revealed that bearded dragon lizards use two distinct groups of stem cells – specialised cells that have the potential to develop into various cell types in the body – to re-grow their teeth. These experiments demonstrate how the bearded dragon lizard uses a previously unknown mechanism to regenerate its teeth, combining elements used by gecko lizards and sharks.

These findings are an important step towards understanding the different strategies animals can use to maintain and regenerate their teeth. The knowledge gained could one day help design better therapies for patients suffering from inherited dental disorders or tooth loss.

Non-syndromic cleft lip with or without palate-susceptible SNPs is associated with hyperdontia

BACKGROUND

Non-syndromic supernumerary teeth (NSST) or hyperdontia may share common genetic determinants with non-syndromic cleft lip with or without palate (NSCL/P). The aim of this study was to test the associations between five genome-wide-associated NSCL/P-susceptible single nucleotide polymorphisms (SNPs) (rs2235371, rs7078160, rs8049367, rs4791774, and rs13041247) and the occurrence of NSST.

MATERIALS AND METHODS

A total of 163 cases and 326 controls were recruited and their genomic DNA was extracted from blood samples. Five NSCL/P-susceptible SNPs (rs2235371, rs7078160, rs8049367, rs4791774, and rs13041247) were genotyped by TaqMan method. Odds ratio (OR) and 95% confidence interval (CI) were used to estimate the associations between the SNPs and the risk of NSST by PLINK software.

RESULTS

Rs4791774 (A > G) and rs13041247 (T > C) were associated with risk of NSST (rs4791774: P_add  = 0.011, OR, 95% CI = 0.62, 0.43-0.90; rs13041247: P_homo  = 0.031, OR, 95% CI = 1.79, 1.05-3.05) and one supernumerary tooth (rs4791774: P_dom  = 0.009, OR, 95% CI = 0.56, 0.36-0.87; rs13041247: P_homo  = 0.034, OR, 95% CI = 1.82, 1.05-3.15). Rs4791774 (A > G) was also showed association with risk of upper arch supernumerary teeth only (P_add  = 0.010, OR, 95% CI = 0.60, 0.41-0.89).

CONCLUSION

Non-syndromic cleft lip with or without palate-susceptible loci rs4791774 (A > G) and rs13041247 (T > C) were associated with the risk of supernumerary teeth.

Trunk dental tissue evolved independently from underlying dermal bony plates but is associated with surface bones in living odontode-bearing catfish

Although oral dental tissue is a vertebrate attribute, trunk dental tissue evolved in several extinct vertebrate lineages but is rare among living species. The question of which processes trigger dental-tissue formation in the trunk remains open, and would shed light on odontogenesis evolution. Extra-oral dental structures (odontodes) in the trunk are associated with underlying dermal bony plates, leading us to ask whether the formation of trunk bony plates is necessary for trunk odontodes to emerge. To address this question, we focus on Loricarioidei: an extant, highly diverse group of catfish whose species all have odontodes. We examined the location and cover of odontodes and trunk dermal bony plates for all six loricarioid families and 17 non-loricarioid catfish families for comparison. We inferred the phylogeny of Loricarioidei using a new 10-gene dataset, eight time-calibration points, and noise-reduction techniques. Based on this phylogeny, we reconstructed the ancestral states of odontode and bony plate cover, and find that trunk odontodes emerged before dermal bony plates in Loricarioidei. Yet we discovered that when bony plates are absent, other surface bones are always associated with odontodes, suggesting a link between osteogenic and odontogenic developmental pathways, and indicating a remarkable trunk odontogenic potential in Loricarioidei.

Tooth germ initiation patterns in a developing dentition: An in vivo study of Xenopus laevis tadpoles

The transparency of soft tissue in Xenopus laevis tadpoles and the anterior-posterior orientation of their developing tooth germs in the upper jaw offer a unique opportunity for the in vivo charting of the first 15-20 days of the developing dentition. Twenty-two X. laevis tadpoles were anesthetized daily and their mouths opened to record the first appearance, position, and development of tooth germs in the upper jaw. The initiation patterns revealed considerable variability between animals, and even between the jaw quadrants in the same animal. This variability appears within a structural boundary and the results are consistent with the presence of an odontogenic band. The final length of dental rows far exceeded the jaw growth for each quadrant during the recording period. This in vivo investigation underlines the limits of cross-sectional studies, and in particular the assumption that tooth germs initiate at the same position in the dental row. The tooth germ initiation patterns in this study did not align with the predictions of standard models for the development of the dentition-Zahnreihen, Clone, and New Progress Zone theories.

Sonic Hedgehog Signaling and Development of the Dentition

Sonic hedgehog (Shh) is an essential signaling peptide required for normal embryonic development. It represents a highly-conserved marker of odontogenesis amongst the toothed vertebrates. Signal transduction is involved in early specification of the tooth-forming epithelium in the oral cavity, and, ultimately, in defining tooth number within the established dentition. Shh also promotes the morphogenetic movement of epithelial cells in the early tooth bud, and influences cell cycle regulation, morphogenesis, and differentiation in the tooth germ. More recently, Shh has been identified as a stem cell regulator in the continuously erupting incisors of mice. Here, we review contemporary data relating to the role of Shh in odontogenesis, focusing on tooth development in mammals and cartilaginous fishes. We also describe the multiple actions of this signaling protein at the cellular level.

Spatially restricted dental regeneration drives pufferfish beak development

Vertebrate dentitions are extraordinarily diverse in both morphology and regenerative capacity. The teleost order Tetraodontiformes exhibits an exceptional array of novel dental morphologies, epitomized by constrained beak-like dentitions in several families, i.e., porcupinefishes, three-toothed pufferfishes, ocean sunfishes, and pufferfishes. Modification of tooth replacement within these groups leads to the progressive accumulation of tooth generations, underlying the structure of their beaks. We focus on the dentition of the pufferfish (Tetraodontidae) because of its distinct dental morphology. This complex dentition develops as a result of (i) a reduction in the number of tooth positions from seven to one per quadrant during the transition from first to second tooth generations and (ii) a dramatic shift in tooth morphogenesis following the development of the first-generation teeth, leading to the elongation of dental units along the jaw. Gene expression and 1,1'-Dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) lineage tracing reveal a putative dental epithelial progenitor niche, suggesting a highly conserved mechanism for tooth regeneration despite the development of a unique dentition. MicroCT analysis reveals restricted labial openings in the beak, through which the dental epithelium (lamina) invades the cavity of the highly mineralized beak. Reduction in the number of replacement tooth positions coincides with the development of only four labial openings in the pufferfish beak, restricting connection of the oral epithelium to the dental cavity. Our data suggest the spatial restriction of dental regeneration, coupled with the unique extension of the replacement dental units throughout the jaw, are primary contributors to the evolution and development of this unique beak-like dentition.

Development of cyclic shedding teeth from semi-shedding teeth: the inner dental arcade of the stem osteichthyan Lophosteus

The numerous cushion-shaped tooth-bearing plates attributed to the stem group osteichthyan Lophosteus superbus, which are argued here to represent an early form of the osteichthyan inner dental arcade, display a previously unknown and presumably primitive mode of tooth shedding by basal hard tissue resorption. They carry regularly spaced, recumbent, gently recurved teeth arranged in transverse tooth files that diverge towards the lingual margin of the cushion. Three-dimensional reconstruction from propagation phase-contrast synchrotron microtomography (PPC-SRµCT) reveals remnants of the first-generation teeth embedded in the basal plate, a feature never previously observed in any taxon. These teeth were shed by semi-basal resorption with the periphery of their bases retained as dentine rings. The rings are highly overlapped, which evidences tooth shedding prior to adding the next first-generation tooth at the growing edge of the plate. The first generation of teeth is thus diachronous. Successor teeth at the same sites underwent cyclical replacing and shedding through basal resorption, producing stacks of buried resorption surfaces separated by bone of attachment. The number and spatial arrangement of resorption surfaces elucidates that basal resorption of replacement teeth had taken place at the older tooth sites before the addition of the youngest first-generation teeth at the lingual margin. Thus, the replacement tooth buds cannot have been generated by a single permanent dental lamina at the lingual edge of the tooth cushion, but must have arisen either from successional dental laminae associated with the individual predecessor teeth, or directly from the dental epithelium of these teeth. The virtual histological dissection of these Late Silurian microfossils broadens our understanding of the development of the gnathostome dental systems and the acquisition of the osteichthyan-type of tooth replacement.

A compendium of developmental gene expression in Lake Malawi cichlid fishes

BACKGROUND

Lake Malawi cichlids represent one of a growing number of vertebrate models used to uncover the genetic and developmental basis of trait diversity. Rapid evolutionary radiation has resulted in species that share similar genomes but differ markedly in phenotypes including brains and behavior, nuptial coloration and the craniofacial skeleton. Research has begun to identify the genes, as well as the molecular and developmental pathways that underlie trait divergence.

RESULTS

We assemble a compendium of gene expression for Lake Malawi cichlids, across pharyngula (the phylotypic stage) and larval stages of development, encompassing hundreds of gene transcripts. We chart patterns of expression in Bone morphogenetic protein (BMP), Fibroblast growth factor (FGF), Hedgehog (Hh), Notch and Wingless (Wnt) signaling pathways, as well as genes involved in neurogenesis, calcium and endocrine signaling, stem cell biology, and numerous homeobox (Hox) factors-in three planes using whole-mount in situ hybridization. Because of low sequence divergence across the Malawi cichlid assemblage, the probes we employ are broadly applicable in hundreds of species. We tabulate gene expression across general tissue domains, and highlight examples of unexpected expression patterns.

CONCLUSIONS

On the heels of recently published genomes, this compendium of developmental gene expression in Lake Malawi cichlids provides a valuable resource for those interested in the relationship between evolution and development.

Early development and replacement of the stickleback dentition

Teeth have long served as a model system to study basic questions about vertebrate organogenesis, morphogenesis, and evolution. In nonmammalian vertebrates, teeth typically regenerate throughout adult life. Fish have evolved a tremendous diversity in dental patterning in both their oral and pharyngeal dentitions, offering numerous opportunities to study how morphology develops, regenerates, and evolves in different lineages. Threespine stickleback fish (Gasterosteus aculeatus) have emerged as a new system to study how morphology evolves, and provide a particularly powerful system to study the development and evolution of dental morphology. Here, we describe the oral and pharyngeal dentitions of stickleback fish, providing additional morphological, histological, and molecular evidence for homology of oral and pharyngeal teeth. Focusing on the ventral pharyngeal dentition in a dense developmental time course of lab-reared fish, we describe the temporal and spatial consensus sequence of early tooth formation. Early in development, this sequence is highly stereotypical and consists of seventeen primary teeth forming the early tooth field, followed by the first tooth replacement event. Comparing this detailed morphological and ontogenetic sequence to that described in other fish reveals that major changes to how dental morphology arises and regenerates have evolved across different fish lineages. J. Morphol. 277:1072-1083, 2016. © 2016 Wiley Periodicals, Inc.

Cichlid fishes as a model to understand normal and clinical craniofacial variation

We have made great strides towards understanding the etiology of craniofacial disorders, especially for 'simple' Mendelian traits. However, the facial skeleton is a complex trait, and the full spectrum of genetic, developmental, and environmental factors that contribute to its final geometry remain unresolved. Forward genetic screens are constrained with respect to complex traits due to the types of genes and alleles commonly identified, developmental pleiotropy, and limited information about the impact of environmental interactions. Here, we discuss how studies in an evolutionary model - African cichlid fishes - can complement traditional approaches to understand the genetic and developmental origins of complex shape. Cichlids exhibit an unparalleled range of natural craniofacial morphologies that model normal human variation, and in certain instances mimic human facial dysmorphologies. Moreover, the evolutionary history and genomic architecture of cichlids make them an ideal system to identify the genetic basis of these phenotypes via quantitative trait loci (QTL) mapping and population genomics. Given the molecular conservation of developmental genes and pathways, insights from cichlids are applicable to human facial variation and disease. We review recent work in this system, which has identified lbh as a novel regulator of neural crest cell migration, determined the Wnt and Hedgehog pathways mediate species-specific bone morphologies, and examined how plastic responses to diet modulate adult facial shapes. These studies have not only revealed new roles for existing pathways in craniofacial development, but have identified new genes and mechanisms involved in shaping the craniofacial skeleton. In all, we suggest that combining work in traditional laboratory and evolutionary models offers significant potential to provide a more complete and comprehensive picture of the myriad factors that are involved in the development of complex traits.

An ancient dental gene set governs development and continuous regeneration of teeth in sharks

The evolution of oral teeth is considered a major contributor to the overall success of jawed vertebrates. This is especially apparent in cartilaginous fishes including sharks and rays, which develop elaborate arrays of highly specialized teeth, organized in rows and retain the capacity for life-long regeneration. Perpetual regeneration of oral teeth has been either lost or highly reduced in many other lineages including important developmental model species, so cartilaginous fishes are uniquely suited for deep comparative analyses of tooth development and regeneration. Additionally, sharks and rays can offer crucial insights into the characters of the dentition in the ancestor of all jawed vertebrates. Despite this, tooth development and regeneration in chondrichthyans is poorly understood and remains virtually uncharacterized from a developmental genetic standpoint. Using the emerging chondrichthyan model, the catshark (Scyliorhinus spp.), we characterized the expression of genes homologous to those known to be expressed during stages of early dental competence, tooth initiation, morphogenesis, and regeneration in bony vertebrates. We have found that expression patterns of several genes from Hh, Wnt/β-catenin, Bmp and Fgf signalling pathways indicate deep conservation over ~450 million years of tooth development and regeneration. We describe how these genes participate in the initial emergence of the shark dentition and how they are redeployed during regeneration of successive tooth generations. We suggest that at the dawn of the vertebrate lineage, teeth (i) were most likely continuously regenerative structures, and (ii) utilised a core set of genes from members of key developmental signalling pathways that were instrumental in creating a dental legacy redeployed throughout vertebrate evolution. These data lay the foundation for further experimental investigations utilizing the unique regenerative capacity of chondrichthyan models to answer evolutionary, developmental, and regenerative biological questions that are impossible to explore in classical models.

Making teeth to order: conserved genes reveal an ancient molecular pattern in paddlefish (Actinopterygii)

Ray-finned fishes (Actinopterygii) are the dominant vertebrate group today (+30 000 species, predominantly teleosts), with great morphological diversity, including their dentitions. How dental morphological variation evolved is best addressed by considering a range of taxa across actinopterygian phylogeny; here we examine the dentition of Polyodon spathula (American paddlefish), assigned to the basal group Acipenseriformes. Although teeth are present and functional in young individuals of Polyodon, they are completely absent in adults. Our current understanding of developmental genes operating in the dentition is primarily restricted to teleosts; we show that shh and bmp4, as highly conserved epithelial and mesenchymal genes for gnathostome tooth development, are similarly expressed at Polyodon tooth loci, thus extending this conserved developmental pattern within the Actinopterygii. These genes map spatio-temporal tooth initiation in Polyodon larvae and provide new data in both oral and pharyngeal tooth sites. Variation in cellular intensity of shh maps timing of tooth morphogenesis, revealing a second odontogenic wave as alternate sites within tooth rows, a dental pattern also present in more derived actinopterygians. Developmental timing for each tooth field in Polyodon follows a gradient, from rostral to caudal and ventral to dorsal, repeated during subsequent loss of teeth. The transitory Polyodon dentition is modified by cessation of tooth addition and loss. As such, Polyodon represents a basal actinopterygian model for the evolution of developmental novelty: initial conservation, followed by tooth loss, accommodating the adult trophic modification to filter-feeding.

Species-specific modifications of mandible shape reveal independent mechanisms for growth and initiation of the coronoid

Background

The variation in mandibular morphology of mammals reflects specialisations for different diets. Omnivorous and carnivorous mammals posses large mandibular coronoid processes, while herbivorous mammals have proportionally smaller or absent coronoids. This is correlated with the relative size of the temporalis muscle that forms an attachment to the coronoid process. The role of this muscle attachment in the development of the variation of the coronoid is unclear.

Results

By comparative developmental biology and mouse knockout studies, we demonstrate here that the initiation and growth of the coronoid are two independent processes, with initiation being intrinsic to the ossifying bone and growth dependent upon the extrinsic effect of muscle attachment. A necessary component of the intrinsic patterning is identified as the paired domain transcription factor Pax9. We also demonstrate that Sox9 plays a role independent of chondrogenesis in the growth of the coronoid process in response to muscle interaction.

Conclusions

The mandibular coronoid process is initiated by intrinsic factors, but later growth is dependent on extrinsic signals from the muscle. These extrinsic influences are hypothesised to be the basis of the variation in coronoid length seen across the mammalian lineage.

Coevolutionary patterning of teeth and taste buds

Teeth and taste buds are iteratively patterned structures that line the oro-pharynx of vertebrates. Biologists do not fully understand how teeth and taste buds develop from undifferentiated epithelium or how variation in organ density is regulated. These organs are typically studied independently because of their separate anatomical location in mammals: teeth on the jaw margin and taste buds on the tongue. However, in many aquatic animals like bony fishes, teeth and taste buds are colocalized one next to the other. Using genetic mapping in cichlid fishes, we identified shared loci controlling a positive correlation between tooth and taste bud densities. Genome intervals contained candidate genes expressed in tooth and taste bud fields. sfrp5 and bmper, notable for roles in Wingless (Wnt) and bone morphogenetic protein (BMP) signaling, were differentially expressed across cichlid species with divergent tooth and taste bud density, and were expressed in the development of both organs in mice. Synexpression analysis and chemical manipulation of Wnt, BMP, and Hedgehog (Hh) pathways suggest that a common cichlid oral lamina is competent to form teeth or taste buds. Wnt signaling couples tooth and taste bud density and BMP and Hh mediate distinct organ identity. Synthesizing data from fish and mouse, we suggest that the Wnt-BMP-Hh regulatory hierarchy that configures teeth and taste buds on mammalian jaws and tongues may be an evolutionary remnant inherited from ancestors wherein these organs were copatterned from common epithelium.

Making maxillary barbels with a proximal-distal gradient of Wnt signals in matrix-bound mesenchymal cells

The evolution of specific appendages is made possible by the ontogenetic deployment of general cell signaling pathways. Many fishes, amphibians and reptiles have unique skin appendages known as barbels, which are poorly understood at the cellular and molecular level. In this study, we examine the cell arrangements, cell division patterns, and gene expression profiles associated with the zebrafish maxillary barbel, or ZMB. The earliest cellular organization of the ZMB is an internal whorl of mesenchymal cells in the dermis of the maxilla; there is no epithelial placode, nor any axially-elongated epithelial cells as expected of an apical ectodermal ridge (AER). As the ZMB develops, cells in S-phase are at first distributed randomly throughout the appendage, gradually transitioning to a proliferative population concentrated at the distal end. By observing ZMB ontogenetic stages in a Wnt-responsive transgenic reporter line, TCFsiam, we identified a strongly fluorescent mesenchymal cell layer within these developing appendages. Using an in vitro explant culture technique on developing barbel tissues, we co-localized the fluorescent label in these cells with the mitotic marker EdU. Surprisingly, the labeled cells showed little proliferation, indicating a slow-cycling subpopulation. Transmission electron microscopy of the ZMB located these cells in a single, circumferential layer within the barbel's matrix core. Morphologically, these cells resemble fibroblasts or osteoblasts; in addition to their matrix-bound location, they are identified by their pancake-shaped nuclei, abundant rough endoplasmic reticulum, and cytoplasmic extensions into the surrounding extracellular matrix. Taken together, these features define a novel mesenchymal cell population in zebrafish, the "TCF(+) core cells." A working model of barbel development is proposed, in which these minimally mitotic mesodermal cells produce collagenous matrix in response to ectodermally-derived Wnt signals deployed in a proximal-distal gradient along the appendage. This documents a novel mechanism of vertebrate appendage outgrowth. Similar genetic signals and cell behaviors may be responsible for the independent and repeated evolution of barbel structures in other fish species.

Developmental Plasticity of Patterned and Regenerating Oral Organs

In many aquatic vertebrates, including bony and cartilaginous fishes, teeth and taste buds colocalize on jaw elements. In these animals, taste buds are renewed continuously throughout life, whereas teeth undergo cycled whole-organ replacement by various means. Recently, studies of cichlid fishes have yielded new insights into the development and regeneration of these dental and sensory oral organs. Tooth and taste bud densities covary positively across species with different feeding strategies, controlled by common regions of the genome and integrated molecular signals. Developing teeth and taste buds share a bipotent epithelium during early patterning stages, from which dental and taste fields are specified. Moreover, these organs share a common epithelial ribbon that supports label-retaining cells during later stages of regeneration. During both patterning and regeneration stages, dental organs can be converted to taste bud fate by manipulation of BMP signaling. These observations highlight a surprising long-term plasticity between dental and sensory organ types. Here, we review these findings and discuss the implications of developmental plasticity that spans the continuum of craniofacial organ patterning and regeneration.

Distinct developmental genetic mechanisms underlie convergently evolved tooth gain in sticklebacks

Teeth are a classic model system of organogenesis, as repeated and reciprocal epithelial and mesenchymal interactions pattern placode formation and outgrowth. Less is known about the developmental and genetic bases of tooth formation and replacement in polyphyodonts, which are vertebrates with continual tooth replacement. Here, we leverage natural variation in the threespine stickleback fish Gasterosteus aculeatus to investigate the genetic basis of tooth development and replacement. We find that two derived freshwater stickleback populations have both convergently evolved more ventral pharyngeal teeth through heritable genetic changes. In both populations, evolved tooth gain manifests late in development. Using pulse-chase vital dye labeling to mark newly forming teeth in adult fish, we find that both high-toothed freshwater populations have accelerated tooth replacement rates relative to low-toothed ancestral marine fish. Despite the similar evolved phenotype of more teeth and an accelerated adult replacement rate, the timing of tooth number divergence and the spatial patterns of newly formed adult teeth are different in the two populations, suggesting distinct developmental mechanisms. Using genome-wide linkage mapping in marine-freshwater F2 genetic crosses, we find that the genetic basis of evolved tooth gain in the two freshwater populations is largely distinct. Together, our results support a model whereby increased tooth number and an accelerated tooth replacement rate have evolved convergently in two independently derived freshwater stickleback populations using largely distinct developmental and genetic mechanisms.

Hedgehog signaling regulates dental papilla formation and tooth size during zebrafish odontogenesis

BACKGROUND

Intercellular communication by the hedgehog cell signaling pathway is necessary for tooth development throughout the vertebrates, but it remains unclear which specific developmental signals control cell behavior at different stages of odontogenesis. To address this issue, we have manipulated hedgehog activity during zebrafish tooth development and visualized the results using confocal microscopy.

RESULTS

We first established that reporter lines for dlx2b, fli1, NF-κB, and prdm1a are markers for specific subsets of tooth germ tissues. We then blocked hedgehog signaling with cyclopamine and observed a reduction or elimination of the cranial neural crest derived dental papilla, which normally contains the cells that later give rise to dentin-producing odontoblasts. Upon further investigation, we observed that the dental papilla begins to form and then regresses in the absence of hedgehog signaling, through a mechanism unrelated to cell proliferation or apoptosis. We also found evidence of an isometric reduction in tooth size that correlates with the time of earliest hedgehog inhibition.

CONCLUSIONS

We hypothesize that these results reveal a previously uncharacterized function of hedgehog signaling during tooth morphogenesis, regulating the number of cells in the dental papilla and thereby controlling tooth size.

A comparative examination of odontogenic gene expression in both toothed and toothless amniotes

A well-known tenet of murine tooth development is that BMP4 and FGF8 antagonistically initiate odontogenesis, but whether this tenet is conserved across amniotes is largely unexplored. Moreover, changes in BMP4-signaling have previously been implicated in evolutionary tooth loss in Aves. Here we demonstrate that Bmp4, Msx1, and Msx2 expression is limited proximally in the red-eared slider turtle (Trachemys scripta) mandible at stages equivalent to those at which odontogenesis is initiated in mice, a similar finding to previously reported results in chicks. To address whether the limited domains in the turtle and the chicken indicate an evolutionary molecular parallelism, or whether the domains simply constitute an ancestral phenotype, we assessed gene expression in a toothed reptile (the American alligator, Alligator mississippiensis) and a toothed non-placental mammal (the gray short-tailed opossum, Monodelphis domestica). We demonstrate that the Bmp4 domain is limited proximally in M. domestica and that the Fgf8 domain is limited distally in A. mississippiensis just preceding odontogenesis. Additionally, we show that Msx1 and Msx2 expression patterns in these species differ from those found in mice. Our data suggest that a limited Bmp4 domain does not necessarily correlate with edentulism, and reveal that the initiation of odontogenesis in non-murine amniotes is more complex than previously imagined. Our data also suggest a partially conserved odontogenic program in T. scripta, as indicated by conserved Pitx2, Pax9, and Barx1 expression patterns and by the presence of a Shh-expressing palatal epithelium, which we hypothesize may represent potential dental rudiments based on the Testudinata fossil record.

Tooth replacement without a dental lamina: the search for epithelial stem cells in Polypterus senegalus

Most actinopterygians replace their teeth continuously throughout life. To address the question of where and how replacement teeth form in actinopterygians, it is advisable to investigate well-chosen representatives within the lineage. The African bichir, Polypterus senegalus, belongs to the earliest diverged group of the actinopterygian lineage with currently living representatives. Its well characterized dentition, together with its phylogenetic position, make this species an attractive model to answer following questions: (1) when and where does the replacement tooth form and how is it connected with the dental organ of the predecessor, and (2) is there any evidence for the presence of epithelial stem cells, hypothesized to play a role in replacement? Serial sections show that one tooth family can contain up to three members, which are all interconnected by dental epithelium. Replacement teeth develop without the presence of a successional dental lamina. We propose that this is the plesiomorphic condition for tooth replacement in actinopterygians. BrdU pulse-chase experiments reveal cells in the outer and middle dental epithelium, proliferating at the time of initiation of a new replacement tooth. It is tempting to assume that these cell layers provide a stem cell niche. The observed absence of label-retaining cells after long chase times (up to 8 weeks) is held against the light of divergent views on cell cycling properties of stem cells. At present, our data do not support, neither reject, the hypothesis on involvement of epithelial stem cells within the process of continuous tooth replacement.

Parallel developmental genetic features underlie stickleback gill raker evolution

Background

Convergent evolution, the repeated evolution of similar phenotypes in independent lineages, provides natural replicates to study mechanisms of evolution. Cases of convergent evolution might have the same underlying developmental and genetic bases, implying that some evolutionary trajectories might be predictable. In a classic example of convergent evolution, most freshwater populations of threespine stickleback fish have independently evolved a reduction of gill raker number to adapt to novel diets. Gill rakers are a segmentally reiterated set of dermal bones important for fish feeding. A previous large quantitative trait locus (QTL) mapping study using a marine × freshwater F2 cross identified QTL on chromosomes 4 and 20 with large effects on evolved gill raker reduction.

Results

By examining skeletal morphology in adult and developing sticklebacks, we find heritable marine/freshwater differences in gill raker number and spacing that are specified early in development. Using the expression of the Ectodysplasin receptor (Edar) gene as a marker of raker primordia, we find that the differences are present before the budding of gill rakers occurs, suggesting an early change to a lateral inhibition process controlling raker primordia spacing. Through linkage mapping in F2 fish from crosses with three independently derived freshwater populations, we find in all three crosses QTL overlapping both previously identified QTL on chromosomes 4 and 20 that control raker number. These two QTL affect the early spacing of gill raker buds.

Conclusions

Collectively, these data demonstrate that parallel developmental genetic features underlie the convergent evolution of gill raker reduction in freshwater sticklebacks, suggesting that even highly polygenic adaptive traits can have a predictable developmental genetic basis.

Evolution and developmental diversity of tooth regeneration

This review considers the diversity observed during both the development and evolution of tooth replacement throughout the vertebrates in a phylogenetic framework from basal extant chondrichthyan fish and more derived teleost fish to mammals. We illustrate the conservation of the tooth regeneration process among vertebrate clades, where tooth regeneration refers to multiple tooth successors formed de novo for each tooth position in the jaws from a common set of retained dental progenitor cells. We discuss the conserved genetic mechanisms that might be modified to promote morphological diversity in replacement dentitions. We review current research and recent progress in this field during the last decade that have promoted our understanding of tooth diversity in an evolutionary developmental context, and show how tooth replacement and dental regeneration have impacted the evolution of the tooth-jaw module in vertebrates.

Dental patterning in the earliest sharks: Implications for tooth evolution

Doliodus problematicus is the oldest known fossil shark-like fish with an almost intact dentition (Emsian, Lower Devonian, c. 397Ma). We provide a detailed description of the teeth and dentition in D. problematicus, based on tomographic analysis of NBMG 10127 (New Brunswick Museum, Canada). Comparisons with modern shark dentitions suggest that Doliodus was a ram-feeding predator with a dentition adapted to seizing and disabling prey. Doliodus provides several clues about the early evolution of the "shark-like" dentition in chondrichthyans and also raises new questions about the evolution of oral teeth in jawed vertebrates. As in modern sharks, teeth in Doliodus were replaced in a linguo-labial sequence within tooth families at fixed positions along the jaws (12-14 tooth families per jaw quadrant in NBMG 10127). Doliodus teeth were replaced much more slowly than in modern sharks. Nevertheless, its tooth formation was apparently as highly organized as in modern elasmobranchs, in which future tooth positions are indicated by synchronized expression of shh at fixed loci within the dental epithelium. Comparable dental arrays are absent in osteichthyans, placoderms, and many "acanthodians"; a "shark-like" dentition, therefore, may be a synapomorphy of chondrichthyans and gnathostomes such as Ptomacanthus. The upper anterior teeth in Doliodus were not attached to the palatoquadrates, but were instead supported by the ethmoid region of the prechordal basicranium, as in some other Paleozoic taxa (e.g., Triodus, Ptomacanthus). This suggests that the chondrichthyan dental lamina was originally associated with prechordal basicranial cartilage as well as jaw cartilage, and that the modern elasmobranch condition (in which the oral dentition is confined to the jaws) is phylogenetically advanced. Thus, oral tooth development in modern elasmobranchs does not provide a complete developmental model for chondrichthyans or gnathostomes.

Immune-related functions of the Hivep gene family in East African cichlid fishes

Immune-related genes are often characterized by adaptive protein evolution. Selection on immune genes can be particularly strong when hosts encounter novel parasites, for instance, after the colonization of a new habitat or upon the exploitation of vacant ecological niches in an adaptive radiation. We examined a set of new candidate immune genes in East African cichlid fishes. More specifically, we studied the signatures of selection in five paralogs of the human immunodeficiency virus type I enhancer-binding protein (Hivep) gene family, tested their involvement in the immune defense, and related our results to explosive speciation and adaptive radiation events in cichlids. We found signatures of long-term positive selection in four Hivep paralogs and lineage-specific positive selection in Hivep3b in two radiating cichlid lineages. Exposure of the cichlid Astatotilapia burtoni to a vaccination with Vibrio anguillarum bacteria resulted in a positive correlation between immune response parameters and expression levels of three Hivep loci. This work provides the first evidence for a role of Hivep paralogs in teleost immune defense and links the signatures of positive selection to host-pathogen interactions within an adaptive radiation.

Shaping development through mechanical strain: the transcriptional basis of diet-induced phenotypic plasticity in a cichlid fish

Adaptive phenotypic plasticity, the ability of an organism to change its phenotype to match local environments, is increasingly recognized for its contribution to evolution. However, few empirical studies have explored the molecular basis of plastic traits. The East African cichlid fish Astatoreochromis alluaudi displays adaptive phenotypic plasticity in its pharyngeal jaw apparatus, a structure that is widely seen as an evolutionary key innovation that has contributed to the remarkable diversity of cichlid fishes. It has previously been shown that in response to different diets, the pharyngeal jaws change their size, shape and dentition: hard diets induce an adaptive robust molariform tooth phenotype with short jaws and strong internal bone structures, while soft diets induce a gracile papilliform tooth phenotype with elongated jaws and slender internal bone structures. To gain insight into the molecular underpinnings of these adaptations and enable future investigations of the role that phenotypic plasticity plays during the formation of adaptive radiations, the transcriptomes of the two divergent jaw phenotypes were examined. Our study identified a total of 187 genes whose expression differs in response to hard and soft diets, including immediate early genes, extracellular matrix genes and inflammatory factors. Transcriptome results are interpreted in light of expression of candidate genes-markers for tooth size and shape, bone cells and mechanically sensitive pathways. This study opens up new avenues of research at new levels of biological organization into the roles of phenotypic plasticity during speciation and radiation of cichlid fishes.

Sequence analyses of the distal-less homeobox gene family in East African cichlid fishes reveal signatures of positive selection

BACKGROUND

Gen(om)e duplication events are hypothesized as key mechanisms underlying the origin of phenotypic diversity and evolutionary innovation. The diverse and species-rich lineage of teleost fishes is a renowned example of this scenario, because of the fish-specific genome duplication. Gene families, generated by this and other gene duplication events, have been previously found to play a role in the evolution and development of innovations in cichlid fishes - a prime model system to study the genetic basis of rapid speciation, adaptation and evolutionary innovation. The distal-less homeobox genes are particularly interesting candidate genes for evolutionary novelties, such as the pharyngeal jaw apparatus and the anal fin egg-spots. Here we study the dlx repertoire in 23 East African cichlid fishes to determine the rate of evolution and the signatures of selection pressure.

RESULTS

Four intact dlx clusters were retrieved from cichlid draft genomes. Phylogenetic analyses of these eight dlx loci in ten teleost species, followed by an in-depth analysis of 23 East African cichlid species, show that there is disparity in the rates of evolution of the dlx paralogs. Dlx3a and dlx4b are the fastest evolving dlx genes, while dlx1a and dlx6a evolved more slowly. Subsequent analyses of the nonsynonymous-synonymous substitution rate ratios indicate that dlx3b, dlx4a and dlx5a evolved under purifying selection, while signs of positive selection were found for dlx1a, dlx2a, dlx3a and dlx4b.

CONCLUSIONS

Our results indicate that the dlx repertoire of teleost fishes and cichlid fishes in particular, is shaped by differential selection pressures and rates of evolution after gene duplication. Although the divergence of the dlx paralogs are putative signs of new or altered functions, comparisons with available expression patterns indicate that the three dlx loci under strong purifying selection, dlx3b, dlx4a and dlx5a, are transcribed at high levels in the cichlids' pharyngeal jaw and anal fin. The dlx paralogs emerge as excellent candidate genes for the development of evolutionary innovations in cichlids, although further functional analyses are necessary to elucidate their respective contribution.

Structures and Function of Remora Adhesion

Remoras (echeneid fish) reversibly attach and detach to marine hosts, almost instantaneously, to “hitchhike” and feed. The adhesion mechanisms that they use are remarkably insensitive to substrate topology and quite different from the latching and suction cup-based systems associated with other species at similar length scales. Remora adhesion is also anisotropic; drag forces induced by the swimming host increase adhesive strength, while rapid detachment occurs when the remora reverses this shear load. In this work, an investigation of the adhesive system’s functional morphology and tissue properties was carried out initially through dissection and x-ray microtomographic analyses. Resulting finite element models of these components have provided new insights into the adaptive, hierarchical nature of the mechanisms and a path toward a wide range of engineering applications.