A revised hypothesis on the evolutionary origin of the vertebrate dentition

Despite claims to the contrary, the evolutionary origin of teeth has not been definitely established. The classical ‘outside in’ theory stating that teeth derive from odontodes that invaded the oral cavity in conjunction with the origin of jaws has been challenged by an alternative, ‘inside out’, hypothesis suggesting that teeth evolved from pharyngeal denticles, as endodermal derivatives, prior to the origin of jaws. We propose a third scenario, a revised ‘outside in’ hypothesis ( Huysseune et al., 2009 ). Our hypothesis is consistent with the current data and avoids speculations about convergent tooth evolution. We suggest that teeth may indeed have arisen before the origin of jaws, a pillar of the ‘inside out’ hypothesis, but not from the endodermally lined posterior pharynx. Rather, teeth would have been the result of competent, odontode‐forming ectoderm invading the oropharyngeal cavity through the mouth as well as through the gill slits, interacting with neural‐crest derived mesenchyme. Arguments in support of this hypothesis are: (i) the observation that pharyngeal teeth are present only in species known to possess gill slits, and disappear from the pharyngeal region in early tetrapods concomitant with the closure of gill slits; (ii) the assumption that endoderm alone, together with neural crest, cannot form teeth; (iii) observations on pharyngeal tooth and gill slit formation in extant species; (iv) the observation that the dental lamina (sensu Reif, 1982 ) is not a prerequisite for tooth formation; (v) evidence that patterning does not distinguish pharyngeal from skin denticles, and (vi) the observation on zebrafish mutants affected in the dermal skeleton. This ‘modified outside in’ hypothesis can be tested both on paleontological data (it predicts a correlation of the presence of pharyngeal teeth and of gill slits), and on developmental data in extant species (it predicts the necessity of an ectodermal signal to make [pharyngeal] teeth).     

The evolutionary origins of Syngnathidae: pipefishes and seahorses

Despite their importance as evolutionary and ecological model systems, the phylogenetic relationships among gasterosteiforms remain poorly understood, complicating efforts to understand the evolutionary origins of the exceptional morphological and behavioural diversity of this group. The present review summarizes current knowledge on the origin and evolution of syngnathids, a gasterosteiform family with a highly developed form of male parental care, combining inferences based on morphological and molecular data with paleontological evidence documenting the evolutionary history of the group. Molecular methods have provided new tools for the study of syngnathid relationships and have played an important role in recent conservation efforts. Despite recent insights into syngnathid evolution, however, a survey of the literature reveals a strong taxonomic bias towards studies on the species-rich genera Hippocampus and Syngnathus, with a lack of data for many morphologically unique members of the family. The study of the evolutionary pressures responsible for generating the high diversity of syngnathids would benefit from a wider perspective, providing a comparative framework in which to investigate the evolution of the genetic, morphological and behavioural traits of the group as a whole.     

Patterns of tooth size variability in the dentition of primates

Published data on tooth size in 48 species of non-human primates have been analyzed to determine patterns of variability in the primate dentition. Average coefficients of variation calculated for all species, with males and females combined, are greatest for teeth in the canine region. Incisors tend to be somewhat less variable, and cheek teeth are the least variable. Removing the effect of sexual dimorphism, by pooling coefficients of variation calculated for males and females separately, reduces canine variability but does not alter the basic pattern. Ontogenetic development and position in functional fields have been advanced to explain patterns of variability in the dentition, but neither of these appears to correlate well with patterns documented here. We tentatively suggest another explanation. Variability is inversely proportional to occlusal complexity of the teeth. This suggests that occlusal complexity places an important constraint on relative variability within the dentition. Even when the intensity of natural selection is equal at all tooth positions, teeth with complex occlusal patterns must still be less variable than those with simple occlusion in order to function equally well. Hence variability itself cannot be used to estimate the relative intensity of selection. Low variability of the central cheek teeth ( \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm M}\frac{1}{1} $\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm M}\frac{2}{2} $\end{document}) makes them uniquely important for estimating body size in small samples, and for distinguishing closely related species in the fossil record.     

Involuntary defection and the evolutionary origins of empathy

An occasional involuntary defection (IVD)--being unable, rather than unwilling, to donate help to others--is an intrinsic attribute of reciprocal cooperation (2003. J. Theor. Biol. 225-285). In fact, it is easy to see that--barring special circumstances--individuals that can donate help whenever requested do not need help of the same kind from others. That is, it is by no means clear why such individuals should participate in symmetric reciprocity interactions. In this paper, I analyse the consequences of introducing IVD into direct reciprocity modeling and show that a simple form of empathy--not retaliating after being punished for IVD--is a prerequisite for evolutionarily stable cooperation. Furthermore: we will see that the stability of this, empathic retaliator, strategy increases with the number of opportunities for cooperative exchanges in the life of an average individual.     

Development of the orangutan permanent dentition: Assessing patterns and variation in tooth development

This study examines dental formation and alveolar emergence in a large cross-sectional sample composed primarily of wild-reared orangutans (N = 89) in order to provide information on the development of the permanent dentition in this hominoid and to address questions of variation in individual tooth formation, between teeth and between individuals. All specimens have been radiographed in lateral aspect and stages of crown and root formation recorded for all teeth. The ranges of crown and root formation of I¹₁, C¹₁, P⁴₄, and M³₃ have been calculated relative to the stage of M₁¹ development within a specific tooth quadrant. Then, for each specimen, BMDP scatterplot and nonparametric statistics have been used to graph changes in stages of these teeth relative to M¹₁ stages and to examine relationships between pairs of upper and lower dental counterparts and between teeth of each jaw. Results indicate 1) high correlations between upper and lower tooth pairs and between many of the permanent teeth within individuals, 2) a relatively large range of variability in individual tooth development (multistage ranges relative to M¹₁), 3) greater variation in root development at emergence than earlier reports, and 4) evidence of variability within the sequence emergence pattern of the orangutan. © 1996 Wiley-Liss, Inc.     

New X-ray diffraction observations on vertebrate muscle: organisation of C-protein (MyBP-C) and troponin and evidence for unknown structures in the vertebrate A-band

Previous low-angle X-ray diffraction studies of various vertebrate skeletal muscles have shown the presence of two rich layer-line patterns, one from the myosin heads and based on a 429 A axial repeat, and one from actin filaments and based on a repeat of about 360-370 A. In addition, meridional intensities have been seen from C-protein (MyBP-C; at about 440 A and its higher orders) and troponin (at about 385 A and its orders). Using preparations of intact, relaxed, bony fish fin muscles and the ID-02 low-angle X-ray camera at the ESRF with a 10 m camera length we have now seen numerous, hitherto unreported, sampled, X-ray layer-lines many of which do not fit onto the previously observed repeats and which require interpretation. The new reflections all fall on the normal ("vertical") hexagonal lattice row-lines in the highly sampled, almost "crystalline", low-angle diffraction X-ray patterns from bony fish muscle, indicating that they all arise from the muscle A-band. However, they do not fall on a single axial repeat. In direct confirmation of our previous analysis, some of these new reflections are explained by the interaction in resting muscle between the N-terminal ends of myosin-bound C-protein molecules with adjacent actin filaments, possibly through the Pro-Ala-rich region. Other newly observed reflections lie on a much longer repeat, but they are most easily interpreted in terms of the arrangement of troponin on the actin filaments. If this is so, then the implication is that the actin filaments and their troponin complexes are systematically arranged in the fish muscle A-band lattice relative to the myosin head positions, and that these newly observed X-ray reflections, when fully analysed, will report on the shape and distribution of troponin molecules in the resting muscle A-band. The less certain contributions of titin and nebulin to these new reflections have also been tested and are described. Many of the new reflections do not appear to come from these known structures. There must be structural features of the A-band that have not yet been described.     

Structure and evolutionary origins of the CMG complex

The CMG (Cdc45–MCM–GINS) complex is the eukaryotic replicative helicase, the enzyme that unwinds double-stranded DNA at replication forks. All three components of the CMG complex are essential for its function, but only in the case of MCM, the molecular motor that harnesses the energy of ATP hydrolysis to catalyse strand separation, is that function clear. Here, we review current knowledge of the three-dimensional structure of the CMG complex and its components and highlight recent advances in our understanding of its evolutionary origins.     

The evolutionary origins of nematodes within the order Strongylida are related to predilection sites within hosts

The evolutionary relationships of the different groups of nematodes within the order Strongylida based on morphological data have been speculative and the subject of conjecture. In this paper, we present a multigene phylogenetic analysis, using sequence data of the 18S and 28S ribosomal RNA genes from representatives of all four suborders and seven superfamilies of the Strongylida, to test existing hypotheses proposed for the relationships of the suborders based on morphological data sets. The results obtained demonstrated that the Strongylida is a monophyletic assemblage, with only the Metastrongylina (but not the other suborders) forming a distinct monophyletic clade. We show that, in contrast to all previous hypotheses, one major lineage comprises taxa which occur exclusively in the pulmonary, circulatory or nervous systems of marsupial and eutherian mammals, whereas a second lineage comprises species occurring in the gastrointestinal tracts or perirenal tissues of vertebrates, or in the lungs of birds. The findings suggest that the predilection site of adult nematodes and host type reflect the evolutionary origin of the different taxonomic groups within the Strongylida.     

The evolutionary history of haptophytes and cryptophytes: phylogenomic evidence for separate origins

An important missing piece in the puzzle of how plastids spread across the eukaryotic tree of life is a robust evolutionary framework for the host lineages. Four assemblages are known to harbour plastids derived from red algae and, according to the controversial chromalveolate hypothesis, these all share a common ancestry. Phylogenomic analyses have consistently shown that stramenopiles and alveolates are closely related, but haptophytes and cryptophytes remain contentious; they have been proposed to branch together with several heterotrophic groups in the newly erected Hacrobia. Here, we tested this question by producing a large expressed sequence tag dataset for the katablepharid Roombia truncata, one of the last hacrobian lineages for which genome-level data are unavailable, and combined this dataset with the recently completed genome of the cryptophyte Guillardia theta to build an alignment composed of 258 genes. Our analyses strongly support haptophytes as sister to the SAR group, possibly together with telonemids and centrohelids. We also confirmed the common origin of katablepharids and cryptophytes, but these lineages were not related to other hacrobians; instead, they branch with plants. Our study resolves the evolutionary position of haptophytes, an ecologically critical component of the oceans, and proposes a new hypothesis for the origin of cryptophytes.     

Primate origins: evolutionary change in digital ray patterning and segmentation

This study presents evidence that the first primates share with extant lemurs, tarsiers, and anthropoids hand proportions unlike those of their close relatives, the tree shrews (Scandentia), colugos (Dermoptera), and plesiadapiforms. Specifically, early primates as well as modern strepsirhines and haplorhines have relatively short metacarpals and long proximal phalanges giving them a grasping, prehensile hand. Limb development was studied in the primate Microcebus murinus and a comparative sample of rodents, artiodactyls, and marsupials to investigate the role of embryonic patterning in the morphogenesis and evolution of primate hand proportions. Comparative analysis shows that the derived finger proportions of primates are generated during the early phases of digital ray patterning and segmentation, when the interzone cells marking the presumptive metacarpo- and interphalangeal joints first appear. Interspecific variation in relative digit and metapodial proportions therefore has high developmental penetrance; that is, adult differences are observed at early ontogenetic stages. The paleontological, comparative, and developmental data are therefore consistent with the hypothesis that the early Cenozoic origin of primates involved an evolutionary change in digital ray pattern formation ultimately yielding a grasping, prehensile hand.     

Chromosomal polymorphisms in subterranean mole rats: origins and evolutionary significance

Two distinct chromosomal polymorphisms, among others, characterize the short (p) and long (qh) arms of chromosome number 1 of the Spalax ehrenbergi complex in Israel. We have studied the geographic distributions of these two polymorphisms in 60 animals belonging to four chromosomal species (2 n = 52, 54, 58 and 60). These comprised 15 populations: 12 from continuously distributed populations, two from semi-isolates, and one isolate in the northern Negev Desert.
Our results indicate that: (i) the two polymorphisms are widespread not only within populations and species but most strikingly between different bone marrow cells within an individual; this may reflect a diversity generating mechanism; (ii) the two polymorphisms of p and qh, primarily of p, are correlated with climatic factors of water availability and temperature, as well as geographical distances; these correlations are significantly above those expected by chance, (iii) The semi-isolates and the isolate display high levels of polymorphism in both p and qh, but particularly in qh, despite the small size population effects.
We suggest that at least the polymorphism in p is involved in the adaptive radiation of mole rats into diverse climatic regimes. Furthermore, we speculate that the forces generating chromosomal polymorphisms relevant to speciation (i.e. Robertsonian mutations), coupled with the evolutionary forces operating in small peripherally isolated populations, may be appropriate for the origin of new chromosomal species through peripatric speciation.     

Family studies of tooth size factors in the permanent dentition.

Correlations between dimensions of the permanent teeth in Australian Aboriginals were studied by factor analysis to disclose the main sources of shared variability. Findings indicated that in both males and females most of the common variability in the tooth dimensions could be accounted for by factors representing mesiodistal size of anterior teeth, buccolingual size of anterior teeth, generalized size of the premolars and generalized size of the molars. Factor scores derived from the analysis were used to calculate intraclass correlations among brothers and among sisters. These correlations tended to be higher for the factors contributing most to the common variability indicating that the factors might represent fields under direct genetic control. There was no trend for intraclass correlations among siblings derived from multivariate scores to be consistently higher than those based on observed tooth dimensions. The main advantage to the user of factor analysis is the ability to interpret associations between interrelated variables more objectively than is possible by conventional correlation methods.