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Maggs X. A synthetic review: natural history of amniote reproductive modes in light of comparative evolutionary genomics. Biol Rev Camb Philos Soc 2024. [PMID: 39300750 DOI: 10.1111/brv.13145] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/05/2022] [Revised: 09/02/2024] [Accepted: 09/04/2024] [Indexed: 09/22/2024]
Abstract
There is a current lack of consensus on whether the ancestral parity mode was oviparity (egg-laying) or viviparity (live-birth) in amniotes and particularly in squamates (snakes, lizards, and amphisbaenids). How transitions between parity modes occur at the genomic level has primary importance for how science conceptualises the origin of amniotes, and highly variable parity modes in Squamata. Synthesising literature from medicine, poultry science, reproductive biology, and evolutionary biology, I review the genomics and physiology of five broad processes (here termed the 'Main Five') expected to change during transitions between parity modes: eggshell formation, embryonic retention, placentation, calcium transport, and maternal-fetal immune dynamics. Throughout, I offer alternative perspectives and testable hypotheses regarding proximate causes of parity mode evolution in amniotes and squamates. If viviparity did evolve early in the history of lepidosaurs, I offer the nucleation site hypothesis as a proximate explanation. The framework of this hypothesis can be extended to amniotes to infer their ancestral state. I also provide a mechanism and hypothesis on how squamates may transition from viviparity to oviparity and make predictions about the directionality of transitions in three species. After considering evidence for differing perspectives on amniote origins, I offer a framework that unifies (i) the extended embryonic retention model and (ii) the traditional model which describes the amniote egg as an adaptation to the terrestrial environment. Additionally, this review contextualises the origin of amniotes and parity mode evolution within Medawar's paradigm. Medawar posited that pregnancy could be supported by immunosuppression, inertness, evasion, or immunological barriers. I demonstrate that this does not support gestation or gravidity across most amniotes but may be an adequate paradigm to explain how the first amniote tolerated internal fertilization and delayed egg deposition. In this context, the eggshell can be thought of as an immunological barrier. If serving as a barrier underpins the origin of the amniote eggshell, there should be evidence that oviparous gravidity can be met with a lack of immunological responses in utero. Rare examples of two species that differentially express very few genes during gravidity, suggestive of an absent immunological reaction to oviparous gravidity, are two skinks Lampropholis guichenoti and Lerista bougainvillii. These species may serve as good models for the original amniote egg. Overall, this review grounds itself in the historical literature while offering a modern perspective on the origin of amniotes. I encourage the scientific community to utilise this review as a resource in evolutionary and comparative genomics studies, embrace the complexity of the system, and thoughtfully consider the frameworks proposed.
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Affiliation(s)
- X Maggs
- Richard Gilder Graduate School at The American Museum of Natural History, 200 Central Park West, New York, NY, 10024, USA
- Christopher S. Bond Life Science Center at the University of Missouri, 1201 Rollins St, Columbia, MO, 65201, USA
- School of Life and Environmental Sciences at the University of Sydney, Heydon-Laurence Building A08, Sydney, NSW, 2006, Australia
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2
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Gordon S, Roberti A, Kaufmann SHE. Mononuclear Phagocytes, Cellular Immunity, and Nobel Prizes: A Historic Perspective. Cells 2024; 13:1378. [PMID: 39195266 PMCID: PMC11352343 DOI: 10.3390/cells13161378] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/19/2024] [Revised: 08/11/2024] [Accepted: 08/12/2024] [Indexed: 08/29/2024] Open
Abstract
The mononuclear phagocyte system includes monocytes, macrophages, some dendritic cells, and multinuclear giant cells. These cell populations display marked heterogeneity depending on their differentiation from embryonic and bone marrow hematopoietic progenitors, tissue location, and activation. They contribute to tissue homeostasis by interacting with local and systemic immune and non-immune cells through trophic, clearance, and cytocidal functions. During evolution, they contributed to the innate host defense before effector mechanisms of specific adaptive immunity emerged. Mouse macrophages appear at mid-gestation and are distributed throughout the embryo to facilitate organogenesis and clear cells undergoing programmed cell death. Yolk sac, AGM, and fetal liver-derived tissue-resident macrophages persist throughout postnatal and adult life, supplemented by bone marrow-derived blood monocytes, as required after injury and infection. Nobel awards to Elie Metchnikoff and Paul Ehrlich in 1908 drew attention to cellular phagocytic and humoral immunity, respectively. In 2011, prizes were awarded to Jules Hoffmann and Bruce Beutler for contributions to innate immunity and to Ralph Steinman for the discovery of dendritic cells and their role in antigen presentation to T lymphocytes. We trace milestones in the history of mononuclear phagocyte research from the perspective of Nobel awards bearing directly and indirectly on their role in cellular immunity.
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Affiliation(s)
- Siamon Gordon
- Department of Microbiology and Immunology, College of Medicine, Chang Gung University, Taoyuan 333, Taiwan
- Sir William Dunn School of Pathology, University of Oxford, Oxford OX1 3RE, UK;
| | - Annabell Roberti
- Sir William Dunn School of Pathology, University of Oxford, Oxford OX1 3RE, UK;
| | - Stefan H. E. Kaufmann
- Max Planck Institute for Infection Biology, 10117 Berlin, Germany;
- Max Planck Institute for Multidisciplinary Sciences, 37077 Göttingen, Germany
- Hagler Institute for Advanced Study, Texas A&M University, College Station, TX 77843, USA
- Charité—Universitätsmedizin Berlin, Corporate Member of Freie Universität Berlin and Humboldt-Universität zu Berlin, 10117 Berlin, Germany
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3
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Maliuk A, Marghoub A, Williams CJA, Stanley E, Kéver L, Vickaryous M, Herrel A, Evans SE, Moazen M. Comparative analysis of osteoderms across the lizard body. Anat Rec (Hoboken) 2024. [PMID: 38396371 DOI: 10.1002/ar.25418] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/21/2023] [Revised: 12/12/2023] [Accepted: 02/11/2024] [Indexed: 02/25/2024]
Abstract
Osteoderms (ODs) are mineralized tissue embedded within the skin and are particularly common in reptiles. They are generally thought to form a protective layer between the soft tissues of the animal and potential external threats, although other functions have been proposed. The aim of this study was to characterize OD variation across the lizard body. Adults of three lizard species were chosen for this study. After whole body CT scanning of each lizard, single ODs were extracted from 10 different anatomical regions, CT scanned, and characterized using sectioning and nanoindentation. Morphological analysis and material characterization revealed considerable diversity in OD structure across the species investigated. The scincid Tiliqua gigas was the only studied species in which ODs had a similar external morphology across the head and body. Greater osteoderm diversity was found in the gerrhosaurid Broadleysaurus major and the scincid Tribolonotus novaeguineae. Dense capping tissue, like that reported for Heloderma, was found in only one of the three species examined, B. major. Osteoderm structure can be surprisingly complex and variable, both among related taxa, and across the body of individual animals. This raises many questions about OD function but also about the genetic and developmental factors controlling OD shape.
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Affiliation(s)
- Anastasiia Maliuk
- Department of Mechanical Engineering, University College London, London, UK
- Department of Zoology, National Museum of Natural History, NAS of Ukraine, Kyiv, Ukraine
| | - Arsalan Marghoub
- Department of Mechanical Engineering, University College London, London, UK
| | - Catherine J A Williams
- Department of Biomedical Sciences, University of Guelph, Guelph, Ontario, Canada
- Department of Biology, Aarhus University, Aarhus, Denmark
- Department of Animal and Veterinary Sciences, Aarhus University, Tjele, Denmark
| | - Edward Stanley
- Department of Natural History, Florida Museum of Natural History, Gainesville, Florida, USA
| | - Loïc Kéver
- Département Adaptations du Vivant, UMR7179 CNRS/MNHN, Paris, France
| | - Matthew Vickaryous
- Department of Biomedical Sciences, University of Guelph, Guelph, Ontario, Canada
| | - Anthony Herrel
- Département Adaptations du Vivant, UMR7179 CNRS/MNHN, Paris, France
- Department of Biology, Evolutionary Morphology of Vertebrates, Ghent University, Ghent, Belgium
- Department of Biology, University of Antwerp, Wilrijk, Belgium
- Naturhistorisches Museum Bern, Bern, Switzerland
| | - Susan E Evans
- Department of Cell and Developmental Biology, University College London, London, UK
| | - Mehran Moazen
- Department of Mechanical Engineering, University College London, London, UK
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4
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Ray EJ, Maruska KP. Sensory Mechanisms of Parent-Offspring Recognition in Fishes, Amphibians, and Reptiles. Integr Comp Biol 2023; 63:1168-1181. [PMID: 37488679 DOI: 10.1093/icb/icad104] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/29/2023] [Revised: 07/14/2023] [Accepted: 07/21/2023] [Indexed: 07/26/2023] Open
Abstract
Parental care is important for offspring survival and success. Recognition of offspring by parents is critical to ensure parents direct care behaviors at related offspring and minimize energy lost by caring for unrelated young. Offspring recognition of parents prevents possible aggressive interactions between young and unrelated adults and allows offspring to direct begging behaviors toward the correct adult. Despite its importance and widespread nature, much of the current research has focused on a small range of species, particularly mammals and birds. We review the existing literature on the sensory mechanisms of parent-offspring recognition in fishes, amphibians, and reptiles. Within these groups, there is diversity in the presence and strategies for parent-offspring recognition. Future studies should continue to identify these mechanisms, as well as the neural and endocrine underpinnings in non-model organisms to expand our knowledge of this behavior and inform our understanding of the evolution of parent-offspring recognition.
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Affiliation(s)
- Emily J Ray
- Department of Biological Sciences, 202 Life Sciences Building, Louisiana State University, Baton Rouge, LA 70803, USA
| | - Karen P Maruska
- Department of Biological Sciences, 202 Life Sciences Building, Louisiana State University, Baton Rouge, LA 70803, USA
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5
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Chuva de Sousa Lopes SM, Roelen BAJ, Lawson KA, Zwijsen A. The development of the amnion in mice and other amniotes. Philos Trans R Soc Lond B Biol Sci 2022; 377:20210258. [PMID: 36252226 PMCID: PMC9574641 DOI: 10.1098/rstb.2021.0258] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/24/2022] Open
Abstract
The amnion is an extraembryonic tissue that evolutionarily allowed embryos of all amniotes to develop in a transient and local aquatic environment. Despite the importance of this tissue, very little is known about its formation and its molecular characteristics. In this review, we have compared the basic organization of the extraembryonic membranes in amniotes and describe the two types of amniogenesis, folding and cavitation. We then zoom in on the atypical development of the amnion in mice that occurs via the formation of a single posterior amniochorionic fold. Moreover, we consolidate lineage tracing data to better understand the spatial and temporal origin of the progenitors of amniotic ectoderm, and visualize the behaviour of their descendants in the extraembryonic–embryonic junctional region. This analysis provides new insight on amnion development and expansion. Finally, using an online-available dataset of single-cell transcriptomics during the gastrulation period in mice, we provide bioinformatic analysis of the molecular signature of amniotic ectoderm and amniotic mesoderm. The amnion is a tissue with unique biomechanical properties that deserves to be better understood. This article is part of the theme issue ‘Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom’.
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Affiliation(s)
- Susana M Chuva de Sousa Lopes
- Department of Anatomy and Embryology, Leiden University Medical Centre, Einthovenweg 20, 2333 ZC Leiden, The Netherlands.,Ghent-Fertility and Stem Cell Team (G-FAST), Department of Reproductive Medicine, Ghent University Hospital, Corneel Heymanslaan 10, 9000 Ghent, Belgium
| | - Bernard A J Roelen
- Anatomy and Physiology, Department of Clinical Sciences, Faculty of Veterinary Medicine, Utrecht University, Yalelaan 1, 3584CL Utrecht, The Netherlands.,Department of Biosciences, Biotechnologies & Biopharmaceutics, University of Bari Aldo Moro, Bari, Italy
| | - Kirstie A Lawson
- MRC Human Genetics Unit, IGC, University of Edinburgh, Crewe Road South, Edinburgh EH4 2XU, UK
| | - An Zwijsen
- Cardiovascular Sciences, Center for Molecular and Vascular Biology, KU Leuven, Herestraat 49 box 911, 3000 Leuven, Belgium
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Stoddard MC. Bird eggs. Curr Biol 2022; 32:R1126-R1132. [DOI: 10.1016/j.cub.2022.07.052] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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Legendre LJ, Choi S, Clarke JA. The diverse terminology of reptile eggshell microstructure and its effect on phylogenetic comparative analyses. J Anat 2022; 241:641-666. [PMID: 35758681 PMCID: PMC9358755 DOI: 10.1111/joa.13723] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/24/2022] [Revised: 05/05/2022] [Accepted: 06/16/2022] [Indexed: 11/29/2022] Open
Abstract
Reptile eggshell ensures water and gas exchange during incubation and plays a key role in reproductive success. The diversity of reptilian incubation and life history strategies has led to many clade-specific structural adaptations of their eggshell, which have been studied in extant taxa (i.e. birds, crocodilians, turtles, and lepidosaurs). Most studies on non-avian eggshells were performed over 30 years ago and categorized reptile eggshells into two main types: "hard" and "soft" - sometimes with a third intermediate category, "semi-rigid." In recent years, however, debate over the evolution of eggshell structure of major reptile clades has revealed how definitions of hard and soft eggshells influence inferred deep-time evolutionary patterns. Here, we review the diversity of extant and fossil eggshell with a focus on major reptile clades, and the criteria that have been used to define hard, soft, and semi-rigid eggshells. We show that all scoring approaches that retain these categories discretize continuous quantitative traits (e.g. eggshell thickness) and do not consider independent variation of other functionally important microstructural traits (e.g. degree of calcification, shell unit inner structure). We demonstrate the effect of three published approaches to discretizing eggshell type into hard, semi-rigid, and soft on ancestral state reconstructions using 200+ species representing all major extant and extinct reptile clades. These approaches result in different ancestral states for all major clades including Archosauria and Dinosauria, despite a difference in scoring for only 1-4% of the sample. Proposed scenarios of reptile eggshell evolution are highly conditioned by sampling, tree calibration, and lack of congruence between definitions of eggshell type. We conclude that the traditional "soft/hard/semi-rigid" classification of reptilian eggshells should be abandoned and provide guidelines for future descriptions focusing on specific functionally relevant characteristics (e.g. inner structures of shell units, pores, and membrane elements), analyses of these traits in a phylogenetic context, and sampling of previously undescribed taxa, including fossil eggs.
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Affiliation(s)
- Lucas J. Legendre
- Department of Geological SciencesUniversity of Texas at AustinAustinTexasUSA
| | - Seung Choi
- Department of Earth SciencesMontana State UniversityBozemanMontanaUSA
- Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of SciencesInstitute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of SciencesBeijingChina
| | - Julia A. Clarke
- Department of Geological SciencesUniversity of Texas at AustinAustinTexasUSA
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8
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Abel P, Pommery Y, Ford DP, Koyabu D, Werneburg I. Skull Sutures and Cranial Mechanics in the Permian Reptile Captorhinus aguti and the Evolution of the Temporal Region in Early Amniotes. Front Ecol Evol 2022. [DOI: 10.3389/fevo.2022.841784] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
While most early limbed vertebrates possessed a fully-roofed dermatocranium in their temporal skull region, temporal fenestrae and excavations evolved independently at least twice in the earliest amniotes, with several different variations in shape and position of the openings. Yet, the specific drivers behind this evolution have been only barely understood. It has been mostly explained by adaptations of the feeding apparatus as a response to new functional demands in the terrestrial realm, including a rearrangement of the jaw musculature as well as changes in strain distribution. Temporal fenestrae have been retained in most extant amniotes but have also been lost again, notably in turtles. However, even turtles do not represent an optimal analog for the condition in the ancestral amniote, highlighting the necessity to examine Paleozoic fossil material. Here, we describe in detail the sutures in the dermatocranium of the Permian reptile Captorhinus aguti (Amniota, Captorhinidae) to illustrate bone integrity in an early non-fenestrated amniote skull. We reconstruct the jaw adductor musculature and discuss its relation to intracranial articulations and bone flexibility within the temporal region. Lastly, we examine whether the reconstructed cranial mechanics in C. aguti could be treated as a model for the ancestor of fenestrated amniotes. We show that C. aguti likely exhibited a reduced loading in the areas at the intersection of jugal, squamosal, and postorbital, as well as at the contact between parietal and postorbital. We argue that these “weak” areas are prone for the development of temporal openings and may be treated as the possible precursors for infratemporal and supratemporal fenestrae in early amniotes. These findings provide a good basis for future studies on other non-fenestrated taxa close to the amniote base, for example diadectomorphs or other non-diapsid reptiles.
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9
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Hung CM, Tsao SH, Chiang PL, Wu SP, Tuanmu MN, Juang JY. Functional connections between bird eggshell stiffness and nest characteristics through risk of egg collision in nests. Ecol Lett 2022; 25:1421-1431. [PMID: 35291055 PMCID: PMC9315002 DOI: 10.1111/ele.14001] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/14/2021] [Revised: 02/13/2022] [Accepted: 03/01/2022] [Indexed: 11/29/2022]
Abstract
Eggs and nests are two critical traits for the ecological success of birds. Their functional interactions, however, remain unclear. Here, we examined the functional connections between egg stiffness and nest attachment, site and structure for 1350 avian species. We revealed high eggshell stiffness for eggs in nests with a pensile attachment, located on non-tree vegetation or having a domed shape, suggesting that birds produce stiffer eggs in response to higher egg-collision risk in unstable or enclosed nests. Interdependence models suggested that the evolution of eggshell stiffness was more likely to be driven by than drive that of nest characters. Our results implied a trade-off between investment in competing for established nesting niches and producing stiff eggs to explore novel niches with high collision risk, possibly mediated by predation or thermoregulation. This study highlights an overlooked connection between nests and eggshells that may have broadened the ecological niches of birds.
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Affiliation(s)
- Chih-Ming Hung
- Biodiversity Research Center, Academia Sinica, Taipei, Taiwan
| | - Shu-Han Tsao
- Department of Mechanical Engineering, National Taiwan University, Taipei, Taiwan
| | - Pei-Lin Chiang
- Department of Mechanical Engineering, National Taiwan University, Taipei, Taiwan
| | - Shang-Ping Wu
- Department of Mechanical Engineering, National Taiwan University, Taipei, Taiwan
| | - Mao-Ning Tuanmu
- Biodiversity Research Center, Academia Sinica, Taipei, Taiwan.,Thematic Center for Systematics and Biodiversity Informatics, Biodiversity Research Center, Academia Sinica, Taipei, Taiwan
| | - Jia-Yang Juang
- Department of Mechanical Engineering, National Taiwan University, Taipei, Taiwan.,Program in Nanoengineering and Nanoscience, Graduate School of Advanced Technology, National Taiwan University, Taipei, Taiwan
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10
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Uribe MC, Cerda-Jardón PI, Blackburn DG. Morphological basis for maternal nutrient provision to embryos in the viviparous fish Ataeniobius toweri (Teleostei: Goodeidae). J Morphol 2021; 282:1575-1586. [PMID: 34355417 DOI: 10.1002/jmor.21407] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/03/2021] [Revised: 07/31/2021] [Accepted: 08/03/2021] [Indexed: 11/10/2022]
Abstract
In viviparous Mexican fishes of the family Goodeidae, embryos develop in the maternal ovarian lumen. They typically absorb maternal nutrients during gestation by means of "trophotaeniae," that is, specialized, elongated extensions of the hindgut that are exposed to the fluids, which occupy the ovarian lumen. The sole exception is Ataeniobius toweri, whose embryos lack trophotaeniae but are nevertheless matrotrophic. Thus, how its embryos obtain maternal nutrients is unclear. We studied a series of non-pregnant and pregnant ovaries of A. toweri using histology to identify the mechanism of maternal-embryo nutrient transfer. By early-gestation, embryos have depleted their yolk supplies. Yolks are released into the ovarian lumen and are ingested by the developing embryos, as shown by yolk material in their digestive tracts. The embryonic gut is lined by an epithelium consisting of columnar cells with apical microvilli, providing a means for nutrient absorption. Contrary to statements in the literature, embryos develop minuscule trophotaenial rudiments that extend slightly into the ovarian lumen. These structures are formed of an absorptive epithelium that overlies a vascular stroma, similar to the trophotaeniae of other goodeids. Through late gestation, vitellogenic follicles form and oocytes are discharged into the ovarian lumen, contributing to embryonic nutrition. Thus, histological evidence suggests that embryos chiefly obtain nutrients from ingestion of yolk and maternal secretions released into the ovarian lumen. This function possibly is supplemented by uptake via the small hindgut protrusions and other absorptive surfaces (e.g., the skin and the gill epithelium). Our observations are consistent with two evolutionary interpretations of the hindgut protrusions: (a) that they are rudimentary, evolutionary precursors of trophotaeniae formed by exteriorized hindgut; and (b) that they are vestigial remnants of trophotaeniae that were lost during a switch to a form of matrotrophy involving nutrient ingestion.
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Affiliation(s)
- Mari Carmen Uribe
- Laboratorio de Biología de la Reproducción, Departamento de Biología Comparada, Facultad de Ciencias, Universidad Nacional Autónoma de México, Mexico City, Mexico
| | - Paola-Ivonne Cerda-Jardón
- Laboratorio de Biología de la Reproducción, Departamento de Biología Comparada, Facultad de Ciencias, Universidad Nacional Autónoma de México, Mexico City, Mexico
| | - Daniel G Blackburn
- Department of Biology and Electron Microscopy Center, Trinity College, Hartford, Connecticut, USA
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Starck JM, Stewart JR, Blackburn DG. Phylogeny and evolutionary history of the amniote egg. J Morphol 2021; 282:1080-1122. [PMID: 33991358 DOI: 10.1002/jmor.21380] [Citation(s) in RCA: 15] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/30/2020] [Revised: 05/07/2021] [Accepted: 05/12/2021] [Indexed: 01/02/2023]
Abstract
We review morphological features of the amniote egg and embryos in a comparative phylogenetic framework, including all major clades of extant vertebrates. We discuss 40 characters that are relevant for an analysis of the evolutionary history of the vertebrate egg. Special attention is given to the morphology of the cellular yolk sac, the eggshell, and extraembryonic membranes. Many features that are typically assigned to amniotes, such as a large yolk sac, delayed egg deposition, and terrestrial reproduction have evolved independently and convergently in numerous clades of vertebrates. We use phylogenetic character mapping and ancestral character state reconstruction as tools to recognize sequence, order, and patterns of morphological evolution and deduce a hypothesis of the evolutionary history of the amniote egg. Besides amnion and chorioallantois, amniotes ancestrally possess copulatory organs (secondarily reduced in most birds), internal fertilization, and delayed deposition of eggs that contain an embryo in the primitive streak or early somite stage. Except for the amnion, chorioallantois, and amniote type of eggshell, these features evolved convergently in almost all major clades of aquatic vertebrates possibly in response to selective factors such as egg predation, hostile environmental conditions for egg development, or to adjust hatching of young to favorable season. A functionally important feature of the amnion membrane is its myogenic contractility that moves the (early) embryo and prevents adhering of the growing embryo to extraembryonic materials. This function of the amnion membrane and the liquid-filled amnion cavity may have evolved under the requirements of delayed deposition of eggs that contain developing embryos. The chorioallantois is a temporary embryonic exchange organ that supports embryonic development. A possible evolutionary scenario is that the amniote egg presents an exaptation that paved the evolutionary pathway for reproduction on land. As shown by numerous examples from anamniotes, reproduction on land has occurred multiple times among vertebrates-the amniote egg presenting one "solution" that enabled the conquest of land for reproduction.
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Affiliation(s)
- J Matthias Starck
- Department of Biology, Ludwig-Maximilians-University Munich, Planegg-Martinsried, Germany
| | - James R Stewart
- Department of Biology, Ludwig-Maximilians-University Munich, Planegg-Martinsried, Germany.,Department of Biological Sciences, East Tennessee State University, Johnson City, Tennessee, USA
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