Early development of the neural plate, neural crest and facial region of marsupials

Translating the Timing of Developmental Benchmarks in Short-Tailed Opossums (Monodelphisdomestica) to Facilitate Comparisons with Commonly Used Rodent Models

INTRODUCTION

The gray short-tailed opossum, Monodelhis domestica (M. domestica), is a widely used marsupial model species that presents unique advantages for neurodevelopmental studies. Notably their extremely altricial birth allows manipulation of postnatal pups at timepoints equivalent to embryonic stages of placental mammals. A robust literature exists on the development of short-tailed opossums, but many researchers working in the more conventional model species of mice and rats may find it daunting to identify the appropriate age at which to conduct experiments.

METHODS

Here, we present detailed staging diagrams taken from photographic observations of 40 individual pups, in 6 litters, over 25 timepoints across postnatal development. We also present a comparative neurodevelopmental timeline of short-tailed opossums (M. domestica), the house mouse (Mus musculus), and the laboratory rat (Rattus norvegicus) during embryonic as well as postnatal development, using timepoints taken from this study and a review of existing literature, and use this dataset to present statistical models comparing the opossum to the rat and mouse.

RESULTS

One aim of this research was to aid in testing the generalizability of results found in rodents to other mammalian brains, such as the more distantly related metatherians. However, this broad dataset also allows the identification of potential heterochronies in opossum development compared to rats and mice. In contrast to previous work, we found broad similarity between the pace of opossum neural development with that of rats and mice. We also found that development of some systems was accelerated in the opossum, such as the forelimb motor plant, oral motor control, and some aspects of the olfactory system, while the development of the cortex, some aspects of the retina, and other aspects of the olfactory system are delayed compared to the rat and mouse.

DISCUSSION

The pace of opossum development is broadly similar to that of mice and rats, which underscores the usefulness of this species as a compliment to the more commonly used rodents. Many features that differ the most between opossums and rats and mice were either clustered around the day of birth and were features that have functional importance for the pup immediately after or during birth, or were features that have reduced functional importance for the pup until later in postnatal development, given that it is initially attached to the mother.

Evolutionary mechanisms modulating the mammalian skull development

Mammals possess impressive craniofacial variation that mirrors their adaptation to diverse ecological niches, feeding behaviour, physiology and overall lifestyle. The spectrum of craniofacial geometries is established mainly during embryonic development. The formation of the head represents a sequence of events regulated on genomic, molecular, cellular and tissue level, with each step taking place under tight spatio-temporal control. Even minor variations in timing, position or concentration of the molecular drivers and the resulting events can affect the final shape, size and position of the skeletal elements and the geometry of the head. Our knowledge of craniofacial development increased substantially in the last decades, mainly due to research using conventional vertebrate model organisms. However, how developmental differences in head formation arise specifically within mammals remains largely unexplored. This review highlights three evolutionary mechanisms acknowledged to modify ontogenesis: heterochrony, heterotopy and heterometry. We present recent research that links changes in developmental timing, spatial organization or gene expression levels to the acquisition of species-specific skull morphologies. We highlight how these evolutionary modifications occur on the level of the genes, molecules and cellular processes, and alter conserved developmental programmes to generate a broad spectrum of skull shapes characteristic of the class Mammalia. This article is part of the theme issue 'The mammalian skull: development, structure and function'.

Postnatal and Adult Neurogenesis in Mammals, Including Marsupials

In mammals, neurogenesis occurs during both embryonic and postnatal development. In eutherians, most brain structures develop embryonically; conversely, in marsupials, a number of brain structures develop after birth. The exception is the generation of granule cells in the dentate gyrus, olfactory bulb, and cerebellum of eutherian species. The formation of these structures starts during embryogenesis and continues postnatally. In both eutherians and marsupials, neurogenesis continues in the subventricular zone of the lateral ventricle (SVZ) and the dentate gyrus of the hippocampal formation throughout life. The majority of proliferated cells from the SVZ migrate to the olfactory bulb, whereas, in the dentate gyrus, cells reside within this structure after division and differentiation into neurons. A key aim of this review is to evaluate advances in understanding developmental neurogenesis that occurs postnatally in both marsupials and eutherians, with a particular emphasis on the generation of granule cells during the formation of the olfactory bulb, dentate gyrus, and cerebellum. We debate the significance of immature neurons in the piriform cortex of young mammals. We also synthesize the knowledge of adult neurogenesis in the olfactory bulb and the dentate gyrus of marsupials by considering whether adult-born neurons are essential for the functioning of a given area.

Prolonged Myocardial Regenerative Capacity in Neonatal Opossum

BACKGROUND

Early neonates of both large and small mammals are able to regenerate the myocardium through cardiomyocyte proliferation for only a short period after birth. This myocardial regenerative capacity declines in parallel with withdrawal of cardiomyocytes from the cell cycle in the first few postnatal days. No mammalian species examined to date has been found capable of a meaningful regenerative response to myocardial injury later than 1 week after birth.

METHODS

We examined cardiomyocyte proliferation in neonates of the marsupial opossum (Monodelphis domestica) by immunostaining at various times after birth. The regenerative capacity of the postnatal opossum myocardium was assessed after either apex resection or induction of myocardial infarction at postnatal day 14 or 29, whereas that of the postnatal mouse myocardium was assessed after myocardial infarction at postnatal day 7. Bioinformatics data analysis, immunofluorescence staining, and pharmacological and genetic intervention were applied to determine the role of AMPK (5'-AMP-activated protein kinase) signaling in regulation of the mammalian cardiomyocyte cell cycle.

RESULTS

Opossum neonates were found to manifest cardiomyocyte proliferation for at least 2 weeks after birth at a frequency similar to that apparent in early neonatal mice. Moreover, the opossum heart at postnatal day 14 showed substantial regenerative capacity both after apex resection and after myocardial infarction injury, whereas this capacity had diminished by postnatal day 29. Transcriptomic and immunofluorescence analyses indicated that AMPK signaling is activated in postnatal cardiomyocytes of both opossum and mouse. Pharmacological or genetic inhibition of AMPK signaling was sufficient to extend the postnatal window of cardiomyocyte proliferation in both mouse and opossum neonates as well as of cardiac regeneration in neonatal mice.

CONCLUSIONS

The marsupial opossum maintains cardiomyocyte proliferation and a capacity for myocardial regeneration for at least 2 weeks after birth. As far as we are aware, this is the longest postnatal duration of such a capacity among mammals examined to date. AMPK signaling was implicated as an evolutionarily conserved regulator of mammalian postnatal cardiomyocyte proliferation.

The influence of divergent reproductive strategies in shaping modularity and morphological evolution in mammalian jaws

Marsupial neonates are born at an earlier developmental stage than placental mammals, but the rapid development of their forelimbs and cranial skeleton allows them to climb to the pouch, begin suckling and complete their development ex utero. The mechanical environment in which marsupial neonates develop is vastly different from that of placental neonates, which exhibit a more protracted development of oral muscles and bones. This difference in reproductive strategy has been theorized to constrain morphological evolution in the oral region of marsupials. Here, we use 3D morphometrics to characterize one of these oral bones, the lower jaw (dentary), and assess modularity (pattern of covariation among traits), morphological disparity and rates of morphological evolution in two clades of carnivorous mammals: the marsupial Dasyuromorphia and placental fissiped Carnivora. We find that dasyuromorph dentaries have fewer modules than carnivorans and exhibit tight covariation between the angular and coronoid processes, the primary attachment sites for jaw-closing muscles. This pattern of modularity may result from the uniform action of muscles on the developing mandible during suckling. Carnivorans are free from this constraint and exhibit a pattern of modularity that more strongly reflects genetic and developmental signals of trait covariation. Alongside differences in modularity, carnivorans exhibit greater disparity and faster rates of morphological evolution compared with dasyuromorphs. Taken together, this suggests dasyuromorphs have retained a signal of trait covariation that reflects the outsized influence of muscular force during early development, a feature that may have impacted the ability of marsupial carnivores to explore specialized regions of morphospace.

Postnatal development in a marsupial model, the fat-tailed dunnart (Sminthopsis crassicaudata; Dasyuromorphia: Dasyuridae)

Marsupials exhibit unique biological features that provide fascinating insights into many aspects of mammalian development. These include their distinctive mode of reproduction, altricial stage at birth, and the associated heterochrony that is required for their crawl to the pouch and teat attachment. Marsupials are also an invaluable resource for mammalian comparative biology, forming a distinct lineage from the extant placental and egg-laying monotreme mammals. Despite their unique biology, marsupial resources are lagging behind those available for placentals. The fat-tailed dunnart (Sminthopsis crassicaudata) is a laboratory based marsupial model, with simple and robust husbandry requirements and a short reproductive cycle making it amenable to experimental manipulations. Here we present a detailed staging series for the fat-tailed dunnart, focusing on their accelerated development of the forelimbs and jaws. This study provides the first skeletal developmental series on S. crassicaudata and provides a fundamental resource for future studies exploring mammalian diversification, development and evolution.

Establishment of Long-Term Primary Cortical Neuronal Cultures From Neonatal Opossum Monodelphis domestica

Primary dissociated neuronal cultures have become a standard model for studying central nervous system (CNS) development. Such cultures are predominantly prepared from the hippocampus or cortex of rodents (mice and rats), while other mammals are less used. Here, we describe the establishment and extensive characterization of the primary dissociated neuronal cultures derived from the cortex of the gray South American short-tailed opossums, Monodelphis domestica. Opossums are unique in their ability to fully regenerate their CNS after an injury during their early postnatal development. Thus, we used cortex of postnatal day (P) 3–5 opossum to establish long-surviving and nearly pure neuronal cultures, as well as mixed cultures composed of radial glia cells (RGCs) in which their neurogenic and gliogenic potential was confirmed. Both types of cultures can survive for more than 1 month in vitro. We also prepared neuronal cultures from the P16–18 opossum cortex, which were composed of astrocytes and microglia, in addition to neurons. The long-surviving opossum primary dissociated neuronal cultures represent a novel mammalian in vitro platform particularly useful to study CNS development and regeneration.

Anatomical comparison across heads, fore- and hindlimbs in mammals using network models

Animal body parts evolve with variable degrees of integration that nonetheless yield functional adult phenotypes: but, how? The analysis of modularity with Anatomical Network Analysis (AnNA) is used to quantitatively determine phenotypic modules based on the physical connection among anatomical elements, an approach that is valuable to understand developmental and evolutionary constraints. We created anatomical network models of the head, forelimb, and hindlimb of two taxa considered to represent a 'generalized' eutherian (placental: mouse) and metatherian (marsupial: opossum) anatomical configuration and compared them with our species, which has a derived eutherian configuration. In these models, nodes represent anatomical units and links represent their physical connection. Here, we aimed to identify: (1) the commonalities and differences in modularity between species, (2) whether modules present a potential phylogenetic character, and (3) whether modules preferentially reflect either developmental or functional aspects of anatomy, or a mix of both. We predicted differences between networks of metatherian and eutherian mammals that would best be explained by functional constraints, versus by constraints of development and/or phylogeny. The topology of contacts between bones, muscles, and bones + muscles showed that, among all three species, skeletal networks were more similar than musculoskeletal networks. There was no clear indication that humans and mice are more alike when compared to the opossum overall, even though their musculoskeletal and skeletal networks of fore- and hindlimbs are slightly more similar. Differences were greatest among musculoskeletal networks of heads and next of forelimbs, which showed more variation than hindlimbs, supporting previous anatomical studies indicating that in general the configuration of the hindlimbs changes less across evolutionary history. Most observations regarding the anatomical networks seem to be best explained by function, but an exception is the adult opossum ear ossicles. These ear bones might form an independent module because the incus and malleus are involved in forming a functional primary jaw that enables the neonate to attach to the teat, where this newborn will complete its development. Additionally, the human data show a specialized digit 1 module (thumb/big toe) in both limb types, likely the result of functional and evolutionary pressures, as our ape ancestors had highly movable big toes and thumbs.

Ontogenetic origins of cranial convergence between the extinct marsupial thylacine and placental gray wolf

Phenotypic convergence, describing the independent evolution of similar characteristics, offers unique insights into how natural selection influences developmental and molecular processes to generate shared adaptations. The extinct marsupial thylacine and placental gray wolf represent one of the most extraordinary cases of convergent evolution in mammals, sharing striking cranial similarities despite 160 million years of independent evolution. We digitally reconstructed their cranial ontogeny from birth to adulthood to examine how and when convergence arises through patterns of allometry, mosaicism, modularity, and integration. We find the thylacine and wolf crania develop along nearly parallel growth trajectories, despite lineage-specific constraints and heterochrony in timing of ossification. These constraints were found to enforce distinct cranial modularity and integration patterns during development, which were unable to explain their adult convergence. Instead, we identify a developmental origin for their convergent cranial morphologies through patterns of mosaic evolution, occurring within bone groups sharing conserved embryonic tissue origins. Interestingly, these patterns are accompanied by homoplasy in gene regulatory networks associated with neural crest cells, critical for skull patterning. Together, our findings establish empirical links between adaptive phenotypic and genotypic convergence and provides a digital resource for further investigations into the developmental basis of mammalian evolution.

Migratory patterns and evolutionary plasticity of cranial neural crest cells in ray-finned fishes

The cranial neural crest (CNC) arises within the developing central nervous system, but then migrates away from the neural tube in three consecutive streams termed mandibular, hyoid and branchial, respectively, according to the order along the anteroposterior axis. While the process of neural crest emigration generally follows a conserved anterior to posterior sequence across vertebrates, we find that ray-finned fishes (bichir, sterlet, gar, and pike) exhibit several heterochronies in the timing and order of CNC emergence that influences their subsequent migratory patterns. First, emigration of the cranial neural crest in these fishes occurs prematurely compared to other vertebrates, already initiating during early neurulation and well before neural tube closure. Second, delamination of the hyoid stream occurs prior to the more anterior mandibular stream; this is associated with early morphogenesis of key hyoid structures like external gills (bichir), a large opercular flap (gar) or first forming cartilage (pike). In sterlet, the hyoid and branchial CNC cells form a single hyobranchial sheet, which later segregates in concert with second pharyngeal pouch morphogenesis. Taken together, the results show that despite generally conserved migratory patterns, heterochronic alterations in the timing of emigration and pattern of migration of CNC cells accompanies morphological diversity of ray-finned fishes.

The embryo of the silky shrew opossum, Caenolestes fuliginosus (Tomes, 1863): First description of the embryo of Paucituberculata

The development of caenolestid marsupials (order Paucituberculata) is virtually unknown. We provide here the first description of Caenolestes fuliginosus embryos collected in the Colombian Central Andes. Our sample of four embryos comes from a single female caught during a fieldtrip at Río Blanco (Manizales, Caldas), in 2014. The sample was processed for macroscopic description using a Standard Event System and for histological descriptions (sectioning and staining). The grade of development of the lumbar flexure and coelomic closure differed between embryos, two of them being more advanced than the others (similar to McCrady's stages 30 and 29, respectively). The pericardial and peritoneal cavities were present, the hepatic anlage was organized in hepatic cords, the heart was in its final position, and the mesonephros was functional. Compared to other Neotropical marsupials, an early appearance of the frontonasal-maxillary fusion and the cervical growth (thickness) was observed; however, absorption of the pharyngeal arches into the body and lung development was delayed. Besides these differences, embryos were similar to equivalent stages in Didelphis virginiana and Monodelphis domestica. Previous proposals of litter size of four for C. fuliginosus are supported.

Quantitative Analysis of the Timing of Development of the Cerebellum and Precerebellar Nuclei in Monotremes, Metatherians, Rodents, and Humans

We have used a quantitative statistical approach to compare the pace of development in the cerebellum and precerebellar systems relative to body size in monotremes and metatherians with that in eutherians (rodents and humans). Embryos, fetuses, and early postnatal mammals were scored on whether key structural events had been reached in the development of the cerebellum itself (CC-corpus cerebelli; 10 milestones), or the pontine and inferior olivary precerebellar nuclear groups (PC; 4 milestones). We found that many early cerebellar and precerebellar milestones (e.g., formation of Purkinje cell layer and deep cerebellar nuclei) were reached at a smaller absolute body length in both metatherians and eutherians together, compared to monotremes. Some later milestones (e.g., formation of the external granular layer and primary fissuration) were reached at a smaller body length in metatherians than eutherians. When the analysis was performed with proportional body length expressed as a natural log-transformed ratio of length at birth, milestones were reached at a much smaller proportional body length in rodents and humans than in the metatherians or monotremes. The findings are consistent with the slower pace of metabolic activity and embryonic development in monotremes. They also indicate slightly advanced maturation of some early features of the cerebellum in some metatherians (i.e., early cerebellar development in dasyurids relative to body size), but do not support the notion of an accelerated development of the cerebellum to cope with the demands of early birth. Anat Rec, 2019. © 2019 American Association for Anatomy Anat Rec, 303:1998-2013, 2020. © 2019 American Association for Anatomy.

Gene expression across mammalian organ development

The evolution of gene expression in mammalian organ development remains largely uncharacterized. Here we report the transcriptomes of seven organs (cerebrum, cerebellum, heart, kidney, liver, ovary and testis) across developmental time points from early organogenesis to adulthood for human, macaque, mouse, rat, rabbit, opossum and chicken. Comparisons of gene expression patterns identified developmental stage correspondences across species, and differences in the timing of key events during the development of the gonads. We found that the breadth of gene expression and the extent of purifying selection gradually decrease during development, whereas the amount of positive selection and expression of new genes increase. We identified differences in the temporal trajectories of expression of individual genes across species, with brain tissues showing the smallest percentage of trajectory changes, and the liver and testis showing the largest. Our work provides a resource of developmental transcriptomes of seven organs across seven species, and comparative analyses that characterize the development and evolution of mammalian organs.

Influence of Temperature on Motor Behaviors in Newborn Opossums (Monodelphis domestica): An In Vitro Study

External thermosensation is crucial to regulate animal behavior and homeostasis, but the development of the mammalian thermosensory system is not well known. We investigated whether temperature could play a role in the control of movements in a mammalian model born very immature, the opossum (Monodelphis domestica). Like other marsupials, at birth the opossum performs alternate and rhythmic movements with its forelimbs (FLs) to reach a teat where it attaches in order to continue its development. It was shown that FL movements can be induced by mechanical stimulation of the snout in in vitro preparations of newborns consisting of the neuraxis with skin and FLs intact. In the present study, we used puff ejections of cold, neutral (bath temperature) and hot liquid directed toward the snout to induce FL responses in such preparations. Either the responses were visually observed under a microscope or triceps muscle activity was recorded. Cold liquid systematically induced FL movements and triceps contractions, but neutral and hot temperatures were less potent to do so. Sections of the trigeminal nerves and removal of the facial skin diminished responses to cold and nearly abolished those to hot and neutral stimulations. Transient receptor potential melastatin 8 (TRPM8) being the major cold receptor cation channel in adult mammals, we employed immunohistochemistry and reverse transcription-polymerase chain reaction (RT-PCR) to test for its expression, but found that it is not expressed before 13 postnatal days. Overall our results indicate that cold thermosensation exerts a strong influence on motor behaviors in newborn opossums.

The tammar wallaby: a non-traditional animal model to study growth axis maturation

Maturation of the growth hormone (GH)/insulin-like growth factor 1 (IGF1) axis is a critical developmental event that becomes functional over the peripartum period in precocial eutherian mammals such as sheep. In mice and marsupials that give birth to altricial young, the GH/IGF1 axis matures well after birth, suggesting that functional maturation is associated with developmental stage, not parturition. Recent foster-forward studies in one marsupial, the tammar wallaby (Macropus eugenii), have corroborated this hypothesis. 'Fostering' tammar young not only markedly accelerates their development and growth rates, but also affects the timing of maturation of the growth axis compared with normal growing young, providing a novel non-traditional animal model for nutritional manipulation. This review discusses how nutrition affects the maturation of the growth axis in marsupials compared with traditional eutherian animal models.

Quantitative Analysis of the Maturation of the Main and Accessory Olfactory Systems in Monotremes and Metatherians in Comparison to Rodents and Humans

We have used an unbiased statistical approach to compare the pace of development in the main and accessory olfactory systems in monotremes and metatherians with that in rodents and humans. We hypothesized that if metatherians and monotremes, which are born at small body size, use olfaction to locate the pouch and/or teat/milk field, then olfactory structures should reach structural maturity in metatherians and monotremes at a smaller size than eutherians like humans and rodents. The achievement of key structural milestones in the development of the main and accessory olfactory systems (11 and 7 milestones, respectively) was scored for 354 specimens and compared against a measure of general somatic growth (body length). We used a statistical approach adapted from Kaplan-Meier analysis to determine median body length at which structural milestones were achieved, and to test the differences for statistical significance (Braslow statistic). The laboratory mouse achieved most main olfactory milestones at a smaller body size than all the metatherians and the monotremes, although the dasyurids (which are born at only 5.0 mm) and mouse achieved accessory olfactory milestones at similar body length. All other metatherians and monotremes reached olfactory milestones at body lengths similar to or larger than the laboratory rat. We therefore reject the hypothesis that metatherians and monotremes as a group exhibit advanced development of the olfactory pathways relative to body size. The findings suggest that, if olfaction is used by metatherians and monotremes at birth, it is achieved with only a rudimentary system without structurally mature central components. Anat Rec, 301:1258-1275, 2018. © 2018 Wiley Periodicals, Inc.

Orbit orientation in didelphid marsupials (Didelphimorphia: Didelphidae)

Usually considered a morphologically conservative group, didelphid marsupials present considerable variation in ecology and body size, some of which were shown to relate to morphological structures. Thus, changes on orbit morphology are likely and could be related to that variation. We calculated orbit orientation in 873 specimens of 16 Didelphidae genera yielding estimates of orbits convergence (their position relative to midsagittal line) and verticality (their position relative to frontal plane). We then compared similarities in these variables across taxa to ecological, morphological and phylogenetic data to evaluate the influencing factors on orbit orientation in didelphids. We found an inverse relation between convergence and verticality. Didelphids orbits have low verticality but are highly convergent, yet orbit orientation differs significantly between taxa, and that variation is related to morphological aspects of the cranium. Rostral variables are the only morphological features correlated with orbit orientation: increasing snout length yields more convergent orbits, whereas increase on snout breadth imply in more vertical orbits. Size and encephalization quotients are uncorrelated with orbit orientation. Among ecological data, diet showed significant correlation whereas locomotion is the factor that less affects the position of orbits. Phylogeny is uncorrelated to any orbital parameters measured. Ecological factors seemingly play a more important role on orbit orientation than previously expected, and differentiation on orbit orientation seems to be more functional than inherited. Thus, despite the apparent homogeneity on didelphid morphology, there is subtle morphological variability that may be directly related to feeding behavior.

Morphology and evolution of the oral shield in marsupial neonates including the newborn monito del monte (Dromiciops gliroides, Marsupialia Microbiotheria) pouch young

Newborn marsupials can be arranged into three grades of developmental complexity based on their external form, as well as based on their organ systems and their cytology. The dasyurids are considered the least developed marsupials at birth, while didelphids and peramelids are intermediate, and macropods are the most developed. Currently there is still little information on caenolestid and microbiotherid development at birth. Developmental stages can be graded as G1, G2 and G3, with G1 being the least developed at birth, and G3 the most developed. Marsupials are also characterized by having an extremely developed craniofacial region at birth compared with placentals. However, the facial region is also observed to vary in development between different marsupial groups at birth. The oral shield is a morphological structure observed in the oral region of the head during late embryological development, which will diminish shortly after birth. Morphological variation of the oral shield is observed and can be arranged by developmental complexity from greatly developed, reduced to vestigial. In its most developed state, the lips are fused, forming together with the rhinarium, a flattened ring around the buccal opening. In this study, we examine the external oral shield morphology in different species of newborn marsupials (dasyurids, peramelids, macropods and didelphids), including the newborn monito del monte young (Dromiciops gliroides - the sole survivor of the order Microbiotheria). The adaptive value of the oral shield structure is reviewed, and we discuss if this structure may be influenced by developmental stage of newborn, pouch cover, species relatedness, or other reproductive features. We observe that the oral shield structure is present in most species of Marsupialia and appears to be exclusively present in this infraclass. It has never been described in Monotremata or Eutherians. It is present in unrelated taxa (e.g. didelphids, dasyurids and microbiotherids). We observe that a well-developed oral shield may be related to ultra altricial development at birth, large litter size (more than two), and is present in most species that lack a pouch in reproductive adult females or have a less prominent or less developed pouch with some exceptions. We try to explore the evolution of the oral shield structure using existing databases and our own observations to reconstruct likely ancestral character states that can then be used to estimate the evolutionary origin of this structure and if it was present in early mammals. We find that a simple to develop oral shield structure (type 2-3) may have been present in marsupial ancestors as well as in early therians, even though this structure is not present in the extant monotremes. This in turn may suggest that early marsupials may have had a very simple pouch or lacked a pouch as seen in some living marsupials, such as some dasyurids, didelphids and caenolestids. The study's results also suggest that different morphological stages of the oral shield and hindlimb development may be influenced by species size and reproductive strategy, and possibly by yet unknown species-specific adaptations.

FGF2, FGF3 and FGF4 expression pattern during molars odontogenesis in Didelphis albiventris

Odontogenesis is guided by a complex signaling cascade in which several molecules, including FGF2-4, ensure all dental groups development and specificity. Most of the data on odontogenesis derives from rodents, which does not have all dental groups. Didelphis albiventris is an opossum with the closest dentition to humans, and the main odontogenesis stages occur when the newborns are in the pouch. In this study, D. albiventris postnatals were used to characterize the main stages of their molars development; and also to establish FGF2, FGF3 and FGF4 expression pattern. D. albiventris postnatals were processed for histological and indirect immunoperoxidase analysis of the tooth germs. Our results revealed similar dental structures between D. albiventris and mice. However, FGF2, FGF3 and FGF4 expression patterns were observed in a larger number of dental structures, suggesting broader functions for these molecules in this opossum species. The knowledge of the signaling that determinates odontogenesis in an animal model with complete dentition may contribute to the development of therapies for the replacement of lost teeth in humans. This study may also contribute to the implementation of D. albiventris as model for Developmental Biology studies.

The taming of the neural crest: a developmental perspective on the origins of morphological covariation in domesticated mammals

Studies on domestication are blooming, but the developmental bases for the generation of domestication traits and breed diversity remain largely unexplored. Some phenotypic patterns of human neurocristopathies are suggestive of those reported for domesticated mammals and disrupting neural crest developmental programmes have been argued to be the source of traits deemed the 'domestication syndrome'. These character changes span multiple organ systems and morphological structures. But an in-depth examination within the phylogenetic framework of mammals including domesticated forms reveals that the distribution of such traits is not universal, with canids being the only group showing a large set of predicted features. Modularity of traits tied to phylogeny characterizes domesticated mammals: through selective breeding, individual behavioural and morphological traits can be reordered, truncated, augmented or deleted. Similarly, mammalian evolution on islands has resulted in suites of phenotypic changes like those of some domesticated forms. Many domesticated mammals can serve as valuable models for conducting comparative studies on the evolutionary developmental biology of the neural crest, given that series of their embryos are readily available and that their phylogenetic histories and genomes are well characterized.

Expression of TrkC receptors in the developing brain of the Monodelphis opossum and its effect on the development of cortical cells

In this study, we investigated the distribution, localization and several various functions of TrkC receptors during development of the Monodelphisopossum brain. Western blotting analysis showed that two different forms of the TrkC receptor, the full-length receptor and one of its truncated forms, are abundantly expressed in the opossum brain. The expression of TrkC receptors was barely detected in the brain of newborn opossums. At postnatal day (P) 3, the expression of full-length TrkC remained at low levels, while moderate expression of the TrkC truncated form was detected. The expression levels of both forms of this protein gradually increased throughout development, peaking at P35. We found that in different neocortical areas located both at the rostral and caudal regions of the cortex, up to 98% of BrdU-labeled cells forming cortical layers (II-VI) had prominently expressed TrkC. To assess which developmental processes of cortical cells are regulated by TrkC receptors, three different shRNAs were constructed. The shRNAs were individually tested in transfected cortical progenitor cells grown on culture plates for 2 days. The effects of the shRNA-TrkC constructs were similar: blockade of TrkC receptors decreased the number of Ki67-positive and apoptotic cells, and it did not change the number of TUJ-positive neurons in vitro. Thus, the lack of TrkC receptors in cultured progenitor cells provided insight on the potential role of these receptors in the regulation of proliferation and cell survival but not in the differentiation of cortical cells.

Size variation, growth strategies, and the evolution of modularity in the mammalian skull

Allometry is a major determinant of within-population patterns of association among traits and, therefore, a major component of morphological integration studies. Even so, the influence of size variation over evolutionary change has been largely unappreciated. Here, we explore the interplay between allometric size variation, modularity, and life-history strategies in the skull from representatives of 35 mammalian families. We start by removing size variation from within-species data and analyzing its influence on integration magnitudes, modularity patterns, and responses to selection. We also carry out a simulation in which we artificially alter the influence of size variation in within-taxa matrices. Finally, we explore the relationship between size variation and different growth strategies. We demonstrate that a large portion of the evolution of modularity in the mammalian skull is associated to the evolution of growth strategies. Lineages with highly altricial neonates have adult variation patterns dominated by size variation, leading to high correlations among traits regardless of any underlying modular process and impacting directly their potential to respond to selection. Greater influence of size variation is associated to larger intermodule correlations, less individualized modules, and less flexible responses to natural selection.

The marsupial pouch: implications for reproductive success and mammalian evolution

Extant mammals are divided into sub- and infraclasses that are distinguished by their mode of reproduction. The monotremes lay eggs, the marsupials give birth to altricial young that typically develop in a pouch, and the eutherians have prolonged in utero development, resulting in well developed young at birth. The three groups exhibit what appears to be a nice progression of evolution towards the well developed newborn young of eutherian mammals. However, marsupials do not represent a step in the progression of producing well developed young, but maintain a reproductive strategy that has evolved to prosper in their specific niche. The production of undeveloped young with increased development in the pouch (or counterpart) provides specific advantages to those species living in diverse environments. The evolution of this reproductive strategy provides a clever solution to the uncertain and often adverse conditions encountered by many species, and the survival of the developing young in a pouch containing potentially harmful microorganisms is truly remarkable. In this review, we explore the unique features of the pouch, highlight the research questions that remain unanswered regarding this unique marsupial attribute and discuss the advantages of the marsupial reproductive strategy and the potential role of the pouch in mammalian diversification.

Mapping the prion protein distribution in marsupials: insights from comparing opossum with mouse CNS

The cellular form of the prion protein (PrP^C) is a sialoglycoprotein widely expressed in the central nervous system (CNS) of mammalian species during neurodevelopment and in adulthood. The location of the protein in the CNS may play a role in the susceptibility of a species to fatal prion diseases, which are also known as the transmissible spongiform encephalopathies (TSEs). To date, little is known about PrP^C distribution in marsupial mammals, for which no naturally occurring prion diseases have been reported. To extend our understanding of varying PrP^C expression profiles in different mammals we carried out a detailed expression analysis of PrP^C distribution along the neurodevelopment of the metatherian South American short-tailed opossum (Monodelphis domestica). We detected lower levels of PrP^C in white matter fiber bundles of opossum CNS compared to mouse CNS. This result is consistent with a possible role for PrP^C in the distinct neurodevelopment and neurocircuitry found in marsupials compared to other mammalian species.

Disparate Igf1 expression and growth in the fore- and hind limbs of a marsupial mammal (Monodelphis domestica)

Proper regulation of growth is essential to all stages of life, from development of the egg into an embryo to the maintenance of normal cell cycle progression in adults. However, despite growth's importance to basic biology and health, little is known about how mammalian growth is regulated. In this study, we investigated the molecular basis of the highly disparate growth of opossum fore- and hind limbs in utero. We first used a novel, opossum-specific microarray to identify several growth-related genes that are differentially expressed in opossum fore- and hind limbs of comparable developmental stages. These genes included Igf1. Given Igf1's role in the growth of other systems, we further investigated the role of Igf1 in opossum limb growth. Supporting the microarray results, RT-PCR indicated that Igf1 levels are approximately two times higher in opossum fore- than hind limbs. Consistent with this, while Igf1 transcripts were readily detectable in opossum forelimbs using whole-mount in situ hybridization, they were not detectable in opossum hind limbs. Furthermore, opossum limbs treated with exogenous Igf1 protein experienced significantly greater cellular proliferation and growth than control limbs in vitro. Taken together, results suggest that the differential expression of Igf1 in developing opossum limbs contributes to their divergent rate of growth, and the unique limb phenotype of opossum newborns. This study establishes the opossum limb as a new mammalian model system for study of organ growth.

Breaking evolutionary and pleiotropic constraints in mammals: On sloths, manatees and homeotic mutations

Background

Mammals as a rule have seven cervical vertebrae, except for sloths and manatees. Bateson proposed that the change in the number of cervical vertebrae in sloths is due to homeotic transformations. A recent hypothesis proposes that the number of cervical vertebrae in sloths is unchanged and that instead the derived pattern is due to abnormal primaxial/abaxial patterning.

Results

We test the detailed predictions derived from both hypotheses for the skeletal patterns in sloths and manatees for both hypotheses. We find strong support for Bateson's homeosis hypothesis. The observed vertebral and rib patterns cannot be explained by changes in primaxial/abaxial patterning. Vertebral patterns in sloths and manatees are similar to those in mice and humans with abnormal numbers of cervical vertebrae: incomplete and asymmetric homeotic transformations are common and associated with skeletal abnormalities. In sloths the homeotic vertebral shift involves a large part of the vertebral column. As such, similarity is greatest with mice mutant for genes upstream of Hox.

Conclusions

We found no skeletal abnormalities in specimens of sister taxa with a normal number of cervical vertebrae. However, we always found such abnormalities in conspecifics with an abnormal number, as in many of the investigated dugongs. These findings strongly support the hypothesis that the evolutionary constraints on changes of the number of cervical vertebrae in mammals is due to deleterious pleitropic effects. We hypothesize that in sloths and manatees low metabolic and activity rates severely reduce the usual stabilizing selection, allowing the breaking of the pleiotropic constraints. This probably also applies to dugongs, although to a lesser extent.

Skull modularity in neotropical marsupials and monkeys: size variation and evolutionary constraint and flexibility

An organism is built through a series of contingent factors, yet it is determined by historical, physical, and developmental constraints. A constraint should not be understood as an absolute obstacle to evolution, as it may also generate new possibilities for evolutionary change. Modularity is, in this context, an important way of organizing biological information and has been recognized as a central concept in evolutionary biology bridging on developmental, genetics, morphological, biochemical, and physiological studies. In this article, we explore how modularity affects the evolution of a complex system in two mammalian lineages by analyzing correlation, variance/covariance, and residual matrices (without size variation). We use the multivariate response to selection equation to simulate the behavior of Eutheria and Metharia skulls in terms of their evolutionary flexibility and constraints. We relate these results to classical approaches based on morphological integration tests based on functional/developmental hypotheses. Eutherians (Neotropical primates) showed smaller magnitudes of integration compared with Metatheria (didelphids) and also skull modules more clearly delimited. Didelphids showed higher magnitudes of integration and their modularity is strongly influenced by within-groups size variation to a degree that evolutionary responses are basically aligned with size variation. Primates still have a good portion of the total variation based on size; however, their enhanced modularization allows a broader spectrum of responses, more similar to the selection gradients applied (enhanced flexibility). Without size variation, both groups become much more similar in terms of modularity patterns and magnitudes and, consequently, in their evolutionary flexibility.

Biting through constraints: cranial morphology, disparity and convergence across living and fossil carnivorous mammals

Carnivory has evolved independently several times in eutherian (including placental) and metatherian (including marsupial) mammals. We used geometric morphometrics to assess convergences associated with the evolution of carnivory across a broad suite of mammals, including the eutherian clades Carnivora and Creodonta and the metatherian clades Thylacoleonidae, Dasyuromorphia, Didelphidae and Borhyaenoidea. We further quantified cranial disparity across eutherians and metatherians to test the hypothesis that the marsupial mode of reproduction has constrained their morphological evolution. This study, to our knowledge the first to extensively sample pre-Pleistocene taxa, analysed 30 three-dimensional landmarks, focused mainly on the facial region, which were digitized on 130 specimens, including 36 fossil taxa. Data were analysed with principal components (PC) analysis, and three measures of disparity were compared between eutherians and metatherians. PC1 showed a shift from short to long faces and seemed to represent diet and ecology. PC2 was dominated by the unique features of sabre-toothed forms: dramatic expansion of the maxilla at the expense of the frontal bones. PC3, in combination with PC1, distinguished metatherians and eutherians. Metatherians, despite common comparisons with felids, were more similar to caniforms, which was unexpected for taxa such as the sabre-toothed marsupial Thylacosmilus. Contrary to previous studies, metatherian carnivores consistently exhibited disparity which exceeded that of the much more speciose eutherian carnivore radiations, refuting the hypothesis that developmental constraints have limited the morphological evolution of the marsupial cranium.

Evolution of lactation: nutrition v. protection with special reference to five mammalian species

The evolutionary origin of the mammary gland has been difficult to establish because little knowledge can be gained on the origin of soft tissue organs from fossil evidence. One approach to resolve the origin of lactation has compared the anatomy of existing primitive mammals to skin glands, whilst another has examined the metabolic and molecular synergy between mammary gland development and the innate immune system. We have reviewed the physiology of lactation in five mammalian species with special reference to these theories. In all species, milk fulfils dual functions of providing protection and nutrition to the young and, furthermore, within species the quality and quantity of milk are highly conserved despite maternal malnutrition or illness. There are vast differences in birth weight, milk production, feeding frequency, macronutrient concentration, growth rate and length of lactation between rabbits, quokkas (Setonix brachyurus), pigs, cattle and humans. The components that protect the neonate against infection do so without causing inflammation. Many protective components are not unique to the mammary gland and are shared with the innate immune system. In contrast, many of the macronutrients in milk are unique to the mammary gland, have evolved from components of the innate immune system, and have either retained or developed multiple functions including the provision of nourishment and protection of the hatchling/neonate. Thus, there is a strong argument to suggest that the mammary gland evolved from the inflammatory response; however, the extensive protection that has developed in milk to actively avoid triggering inflammation seems to be a contradiction.

Phylogeny, diet, and cranial integration in australodelphian marsupials

Studies of morphological integration provide valuable information on the correlated evolution of traits and its relationship to long-term patterns of morphological evolution. Thus far, studies of morphological integration in mammals have focused on placentals and have demonstrated that similarity in integration is broadly correlated with phylogenetic distance and dietary similarity. Detailed studies have also demonstrated a significant correlation between developmental relationships among structures and adult morphological integration. However, these studies have not yet been applied to marsupial taxa, which differ greatly from placentals in reproductive strategy and cranial development and could provide the diversity necessary to assess the relationships among phylogeny, ecology, development, and cranial integration. This study presents analyses of morphological integration in 20 species of australodelphian marsupials, and shows that phylogeny is significantly correlated with similarity of morphological integration in most clades. Size-related correlations have a significant affect on results, particularly in Peramelia, which shows a striking decrease in similarity of integration among species when size is removed. Diet is not significantly correlated with similarity of integration in any marsupial clade. These results show that marsupials differ markedly from placental mammals in the relationships of cranial integration, phylogeny, and diet, which may be related to the accelerated development of the masticatory apparatus in marsupials.

Cranial modularity and sequence heterochrony in mammals

Heterochrony, the temporal shifting of developmental events relative to each other, requires a degree of autonomy among those processes or structures. Modularity, the division of larger structures or processes into autonomous sets of internally integrated units, is often discussed in relation to the concept of heterochrony. However, the relationship between the developmental modules derived from studies of heterochrony and evolutionary modules, which should be of adaptive importance and relate to the genotype-phenotype map, has not been explicitly studied. I analyzed a series of sectioned and whole cleared-and-stained embryological and neonatal specimens, supplemented with published ontogenetic data, to test the hypothesis that bones within the same phenotypic modules, as determined by morphometric analysis, are developmentally integrated and will display coordinated heterochronic shifts across taxa. Modularity was analyzed in cranial bone ossification sequences of 12 therian mammals. A dataset of 12-18 developmental events was used to assess if modularity in developmental sequences corresponds to six phenotypic modules, derived from a recent morphometric analysis of cranial modularity in mammals. Kendall's tau was used to measure rank correlations, with randomization tests for significance. If modularity in developmental sequences corresponds to observed phenotypic modules, bones within a single phenotypic module should show integration of developmental timing, maintaining the same timing of ossification relative to each other, despite differences in overall ossification sequences across taxa. Analyses did not find any significant conservation of developmental timing within the six phenotypic modules, meaning that bones that are highly integrated in adult morphology are not significantly integrated in developmental timing.

The molecular origins of species-specific facial pattern

The prevailing approach within the field of craniofacial development is focused on finding a balance between tissues (e.g., facial epithelia, neuroectoderm, and neural crest) and molecules (e.g., bone morphogenetic proteins, fibroblast growth factors, Wnts) that play a role in sculpting the face. We are rapidly learning that neither these tissues nor molecular signals are able to act in isolation; in fact, molecular cues are constantly reciprocating signals between the epithelia and the neural crest in order to pattern and mold facial structures. More recently, it has been proposed that this crosstalk is often mediated and organized by discrete organizing centers within the tissues that are able to act as a self-contained unit of developmental potential (e.g., the rhombomere and perhaps the ectomere). Whatever the molecules are and however they are interpreted by these tissues, it appears that there is a remarkably conserved mechanism for setting up the initial organization of the facial prominences between species. Regardless of species, all vertebrates appear to have the same basic bauplan. However, sometime during mid-gestation, the vertebrate face begins to exhibit species-specific variations, in large part due to differences in the rates of growth and differentiation of cells comprising the facial prominences. How do these differences arise? Are they due to late changes in molecular signaling within the facial prominences themselves? Or are these late changes a reflection of earlier, more subtle alterations in boundaries and fields that are established at the earliest stages of head formation? We do not have clear answers to these questions yet, but in this chapter we present new studies that shed light on this age-old question. This chapter aims to present the known signals, both on a molecular and cellular level, responsible for craniofacial development while bringing to light the events that may serve to create difference in facial morphology seen from species to species.

Craniofacial development in marsupial mammals: Developmental origins of evolutionary change

Biologists have long studied the evolutionary consequences of the differences in reproductive and life history strategies of marsupial and eutherian mammals. Over the past few decades, the impact of these strategies on the development of the marsupial embryo and neonate has received attention. In this review, the differences in development in the craniofacial region in marsupial and eutherian mammals will be discussed. The review will highlight differences at the organogenic and cellular levels, and discuss hypotheses for shifts in the expression of important regulatory genes. The major difference in the organogenic period is a whole-scale shift in the relative timing of central nervous system structures, in particular those of the forebrain, which are delayed in marsupials, relative to the structures of the oral-facial apparatus. Correlated with the delay in development of nervous system structures, the ossification of the bones of the neurocranium are delayed, while those of the face are accelerated. This study will also review work showing that the neural crest, which provides much of the cellular material to the facial skeleton and may also carry important patterning information, is notably accelerated in its development in marsupials. Potential consequences of these observations for hypotheses on constraint, evolutionary integration, and the existence of developmental modules is discussed. Finally, the implications of these results for hypotheses on the genetic modulation of craniofacial patterning are presented.

Cranial neural crest cell migration in cockatielNymphicus hollandicus (Aves: Psittaciformes)

Parrots have developed unique jaw muscles in their evolutionary history. The M. pseudomasseter, which completely covers the lateral side of the jugal bar, is regarded as a jaw muscle unique to parrots. In a previous study, I presented a hypothesis on the relevance of modifications in the regulation of cranial neural crest cell (NCC) development to the generation of this novel jaw muscle based on histological analyses (Tokita [2004] J Morphol 259:69-81). In the present study, I investigated distribution and migration patterns of cranial neural crest cells (NCCs) through parrot embryogenesis with immunohistochemical techniques to further understand the role of cranial NCCs in the evolution of the M. pseudomasseter, and to provide new information on the relative plasticity in cranial NCC migration at early stages of avian development. The basic nature of cranial NCC development was mostly conserved between chick and parrot. In both, cranial NCCs migrated from the dorsal tip of the neural tube in a ventral direction. Three major populations were identified in their cranial NCCs. Migration pathways of these cells were almost identical between chick and parrot. The principal difference was seen in the relative timing of cranial NCC migration. In the parrot, cranial NCC migration into the first pharyngeal arch was more advanced than in the chick at early stages of development. Such a temporal shift in cranial NCC migration might influence architectural patterning of parrot jaw muscles that generates new muscle like M. pseudomasseter.

Role of morphogens in neural crest cell determination

The neural crest is a transient, migratory cell population found in all vertebrate embryos that generate a diverse range of cell and tissue derivatives including, but not limited, to the neurons and glia of the peripheral nervous system, smooth muscle, connective tissue, melanocytes, craniofacial cartilage, and bone. Over the past few years, many studies have provided tremendous insights into understanding the mechanisms regulating the induction and migration of neural crest cell development. This review highlights the surprising and perhaps unexpected roles for morphogens in these distinct processes. A comparison of studies performed in several different vertebrates emphasizes the requirement for coordination between multiple signaling pathways in the induction and migration of neural crest cells in the developing embryo.