Papers by Daniel Blackburn
Placental specializations of the mountain spiny lizard Sceloporus jarrovi
Journal of Morphology, Sep 9, 2010
The lizard Sceloporus jarrovi (Phrynosomatidae) is one of the most widely studied viviparous rept... more The lizard Sceloporus jarrovi (Phrynosomatidae) is one of the most widely studied viviparous reptiles of North America. Past research has assumed that placentation in this species is relatively simple and functions mainly in gas exchange. Our examination of the late stage placenta via transmission electron microscopy reveals that S. jarrovi has a unique combination of placental characteristics, with unusual specializations for secretion and absorption. In the chorioallantoic placenta, chorionic and uterine tissues are directly apposed through eggshell loss, and their epithelia are greatly attenuated, enhancing gas exchange; this placenta shows evidence of both nutrient transfer and endocrine function. Contrary to past inferences, a yolk sac placenta forms from the avascular omphalopleure and persists through the end of gestation. The uterine epithelium is enlarged and secretory, and the fetal omphalopleure shows branching absorptive channels and other specializations for uptake. Elsewhere, the omphalopleure develops elongated folds that protrude into a coagulum of degenerating shell membrane and other organic material. Uterine tissue in this region shows specializations for absorption. Placental features in S. jarrovi have unexpected functional implications, and challenge assumptions that specializations for nutrient transfer are confined to matrotrophic species. J. Morphol. 271:1153–1175, 2010. © 2010 Wiley‐Liss, Inc.
Reproduction in the lizard <i>Mabuya heathi</i> (Scincidae): a commentary on viviparity in new world <i>Mabuya</i>
Canadian Journal of Zoology, Dec 1, 1983
The tropical Brazilian skink Mabuya heathi is viviparous, producing ova which increase in wet mas... more The tropical Brazilian skink Mabuya heathi is viviparous, producing ova which increase in wet mass by 53 800% and in dry mass by 38 400% during gestation. Ovulation occurs during October–January, rapid growth in embryos takes place during June–October, and parturition occurs in September–November, between 9 and 12 months after ovulation. Female and male reproduction is cyclic and is synchronous between the sexes. Brood size is 2–9 [Formula: see text] and is significantly correlated to female size. Fat bodies of females are largest when embryos are small, and decrease in mass as embryos increase in size. Testes in males are largest during September–February, associated with a decrease in fat body size. Mabuya heathi is the second species of lizard in the caatinga herpetofauna shown to be strongly cyclical in reproduction and to reproduce only once per year.
American zoologist, Apr 1, 1992
Quantitative analyses based upon the superimposition of phylogenetic and reproductive data have r... more Quantitative analyses based upon the superimposition of phylogenetic and reproductive data have revealed that viviparity has originated on at least 132 independent occasions among vertebrates, with 98 of these origins having occurred among reptiles. The viviparous lineages have given rise to at least 24 matrotrophic clades, all but four of which are anamniotes. Traditional scenarios assume progressive, gradualistic evolution from oviparity to lecithotrophic viviparity to matrotrophic viviparity. However, mammalian evidence indicates that matrotrophy can precede the evolution of viviparity. Moreover, data on reptiles seem to be consistent with a punctuated equilibrium model for viviparity and a saltatory model for incipient matrotrophy and placentation. Among the specializations for fetal nutrition, strong convergence is evident at organismal, organological, and cytological levels. Examples include yolk sac placentation, trophotaeniae, and adaptations for embryonic cannibalism. Certain lizards of the genera Mabuya and Chalcides have converged strongly on eutherian mammals with respect to morphology of the chorioallantoic placenta. Placental specializations that have evolved independently in some eutherians and matrotrophic lizards include placentomes, giant binucleate cells, deciduate maternal tissue, and chorionic areolae.
Herpetological Monographs, 2006
For over a century, research has been conducted on squamates in order to reveal how viviparity ha... more For over a century, research has been conducted on squamates in order to reveal how viviparity has evolved in mammals and other vertebrates. The recent proliferation of studies has yielded much information on anatomical, physiological, ecological, and evolutionary aspects, allowing a reassessment of squamates as model organisms for the study of viviparity. Strong support for the ''squamate model'' comes from phylogenetic analyses that have shown that squamates have evolved viviparity with great frequency (. 108 origins), at low taxonomic levels, and in geologically recent times. However, available data also indicate that viviparity has evolved by different chronologies and mechanisms in squamates, fishes, and mammals. Further, generalizations about squamates are difficult to make, given the diverse mechanisms by which they achieve viviparity. Thus, similarities between squamates must be demonstrated empirically, and generalizations should be based on quantitative, phylogenetic analyses of multiple lineages. Explanations for similarities between squamate clades can invoke such concepts as evolutionary constraints, exaptations, and selection pressures, and should distinguish between adaptations, correlated attributes, and features that predate viviparity. However, homocentric assumptions of an orthogenetic transformation towards the eutherian condition should be abandoned, along with untested assumptions that viviparity squamates and mammals is similar. The value of the squamate model ultimately may lie in insights it provides into physiological problems rather than in universality of specific mechanisms that have evolved to meet those problems.
Herpetological Monographs, 2006
Standardized criteria for the recognition of reproductive modes in squamate reptiles
Herpetologica, 1993
... A third potential artifact of the labo-ratory is that some oviparous squamates reportedly can... more ... A third potential artifact of the labo-ratory is that some oviparous squamates reportedly can delay laying eggs when suitable oviposition sites are unavailable (Blanchard, 1937; Cuellar, 1971; Stamps, 1976; also see Huey, 1977; Wood, 1936; Zweifel, 1962). ...
Classics revisited, history of reptile placentology, part IV: Hanni Hrabowski's 1926 monograph on fetal membranes of lizards
Placenta, Jun 1, 2020
In 1926, the German biologist Johanna (Hanni) Hrabowski published a study of the morphology and d... more In 1926, the German biologist Johanna (Hanni) Hrabowski published a study of the morphology and development of the fetal placenta in lizards that has proven to be of historical importance. Her anatomical descriptions and interpretations identified developmental patterns that differ from other amniotes -- features now recognized as unique attributes of squamate (lizards and snakes) development. Her 1926 monograph presented the first histological comparison of fetal membranes in closely-related oviparous and viviparous reptiles, thereby establishing a comparative framework for understanding placental specializations for viviparity. Hrabowski reported that yolk sac development did not differ between oviparous and viviparous species. The novel, shared components of yolk sac development she identified are now recognized as the foundation for the unique yolk sac placenta of reptiles, the omphaloplacenta. In addition, Hrabowski's extensive ontogenetic sampling and the detail and accuracy of her anatomical descriptions set high standards for subsequent studies of comparative evolutionary embryology.
Evolution of viviparity in squamate reptiles: Reversibility reconsidered
Journal Of Experimental Zoology Part B: Molecular And Developmental Evolution, May 13, 2015
Viviparity in squamate reptiles is widely recognized as having evolved convergently from oviparit... more Viviparity in squamate reptiles is widely recognized as having evolved convergently from oviparity more than 100 times. However, questions persist as to whether reversals from viviparity back to oviparity have ever occurred. Based on a theoretical model, a recent paper (Pyron and Burbrink, 2014) has proposed that viviparity is ancestral for squamates and that viviparity-oviparity reversals have far outnumbered origins of viviparity in reproductive history. Close examination of this analysis reveals features that cast doubt on its plausibility, notably the requirement of repeated, sequential transformations back and forth between these reproductive modes, as well as numerous, uncounted evolutionary transformations that have produced inaccurate estimates of parsimony. Evidence derived from studies of anatomy, physiology, and developmental biology strongly supports the inference that oviparity is ancestral for squamates and has given rise to viviparity on numerous occasions. Biological data provide important insights into the likelihood of evolutionary transformations, and deserve to be incorporated fully into future analyses of the evolution of reproductive modes.
Reptilian viviparity: past research, future directions, and appropriate models
Comparative Biochemistry and Physiology A-molecular & Integrative Physiology, Dec 1, 2000
Squamate reptiles represent an ideal group for studies of viviparity, because they have evolved t... more Squamate reptiles represent an ideal group for studies of viviparity, because they have evolved this reproductive pattern frequently, relatively recently, and at low taxonomic levels. A phylogenetic approach shows particular promise in helping us interpret anatomical, physiological, and ecological diversity. This review summarizes four major categories of active investigation: (1) reproductive anatomy and physiology; (2) placental structure and function; (3) reproductive endocrinology; and (4) reproductive and physiological ecology. Evolutionary reconstructions suggest that on many occasions viviparity has evolved concomitantly with functional placentation, through reduction of the shell membrane and hormonal modifications that prolong gestation. Studies of placentotrophic clades as well as reproductively bimodal species offer great potential for explaining the evolution of viviparity and placentation. However, live-bearing squamates are reproductively diverse, and appear to have solved physiological problems associated with viviparity by a variety of mechanisms. Consequently, studies on one or a few squamate species appear increasingly unlikely to yield all-inclusive explanations. Future studies and analyses should abandon assumptions of universal physiological mechanisms and a single historical sequence, in favor of the documentation of diversity in phylogenetic and quantitative terms.
Do pregnant lizards resorb or abort inviable eggs and embryos? Morphological evidence from an Australian skink,Pseudemoia pagenstecheri
Journal of Morphology, Mar 7, 2003
Viviparous placentotrophy in reptiles and the parent-offspring conflict
Journal Of Experimental Zoology Part B: Molecular And Developmental Evolution, Jun 2, 2015
ABSTRACTIn placentotrophic viviparous reptiles, pregnant females deliver nutrients to their devel... more ABSTRACTIn placentotrophic viviparous reptiles, pregnant females deliver nutrients to their developing fetuses by diverse morphological specializations that reflect independent evolutionary origins. A survey of these specializations reveals a major emphasis on histotrophy (uterine secretion and fetal absorption) rather than hemotrophy (transfer between maternal and fetal blood streams). Of available hypotheses for the prevalence of histotrophic transfer, the most promising derives insights from the theoretical parent–offspring conflict over nutrient investment. I suggest that histotrophy gives pregnant females greater control over nutrient synthesis, storage, and delivery than hemotrophic transfer, reflecting maternal preeminence in any potential parent–offspring competition over nutrient investment. One lizard species shows invasive ovo‐implantation and direct contact between fetal tissues and maternal blood vessels, potentially conferring control over nutrient transfer to the embryo. Future research on squamates will benefit from application of parent–offspring conflict theory to the transition from incipient to substantial matrotrophy, as well as by testing theory‐derived predictions on both facultatively and highly placentotrophic forms. J. Exp. Zool. (Mol. Dev. Evol.) 324B: 532–548, 2015. © 2015 Wiley Periodicals, Inc.
In Memory of Rajkumar (Raju) Shivappa Radder With the permission of his family, we dedicated the ... more In Memory of Rajkumar (Raju) Shivappa Radder With the permission of his family, we dedicated the symposium to the memory of Raju Radder (pictured above), an extraordinary young herpetologist who was registered to participate in the symposium but who suffered a fatal heart attack on 31 May 2008, just 2½ months short of the Congress. Raju had focused his attentions on many aspects of reproduction in reptiles and his contributions to the literature were significant and expanding rapidly (Shanbhag et al. 2008). It was his innovation and dedication that led us to ensure that Raju was part of our symposium and his death touched us all deeply. It is for these reasons that we dedicated the symposium, and these written contributions, to Raju's memory.
Viviparity and Placentation in Lizards
Reproductive Biology and Phylogeny of Lizards and Tuatara, 2014
ABSTRACT (Brief summary): In this review chapter, we have two interrelated goals. One is to summa... more ABSTRACT (Brief summary): In this review chapter, we have two interrelated goals. One is to summarize aspects of viviparity in lizards from descriptive, functional, and evolutionary standpoints. The other goal, the main focus of this chapter, is to explore lizard placentation in terms of development, structure, function, and evolution.
Viviparity in Reptiles and Amphibians
Encyclopedia of Reproduction, 2018
Abstract Although most reptiles and amphibians lay eggs, viviparity (live-bearing reproduction) h... more Abstract Although most reptiles and amphibians lay eggs, viviparity (live-bearing reproduction) has originated multiple times in each group. In viviparous squamates (lizards and snakes), the pregnant female sustains her embryos via placentas that accomplish gas exchange. Viviparous amphibians and reptiles have evolved a variety of specializations has evolved for maternal-fetal transfer of nutrients. Questions over how viviparity is accomplished and why it has evolved are subjects of extensive, ongoing research.
Reproduction in Reptiles
Encyclopedia of Reproduction, 2018
Reptiles share several reproductive features associated with terrestrial reproduction, including ... more Reptiles share several reproductive features associated with terrestrial reproduction, including internal fertilization, an amniotic shelled egg, and complex social behaviors. However, reptiles are highly diverse in terms of reproductive parameters (e.g., sex determination mechanisms, clutch size, and reproductive cycles), as well as physiological mechanisms and life history strategies. In addition, elaborate specializations characterize particular groups, such as sperm storage, viviparity, parental care, and parthenogenesis
Evolution of viviparity in squamate reptiles: Reversibility reconsidered
Journal of experimental zoology. Part B, Molecular and developmental evolution, 2015
Viviparity in squamate reptiles is widely recognized as having evolved convergently from oviparit... more Viviparity in squamate reptiles is widely recognized as having evolved convergently from oviparity more than 100 times. However, questions persist as to whether reversals from viviparity back to oviparity have ever occurred. Based on a theoretical model, a recent paper (Pyron and Burbrink, 2014) has proposed that viviparity is ancestral for squamates and that viviparity-oviparity reversals have far outnumbered origins of viviparity in reproductive history. Close examination of this analysis reveals features that cast doubt on its plausibility, notably the requirement of repeated, sequential transformations back and forth between these reproductive modes, as well as numerous, uncounted evolutionary transformations that have produced inaccurate estimates of parsimony. Evidence derived from studies of anatomy, physiology, and developmental biology strongly supports the inference that oviparity is ancestral for squamates and has given rise to viviparity on numerous occasions. Biological...
Chorioallantoic placentation in squamate reptiles: Structure, function, development, and evolution
Journal of Experimental Zoology, 1993
Allantoplacentae in lizards and snakes form during the evolution of viviparity through apposition... more Allantoplacentae in lizards and snakes form during the evolution of viviparity through apposition of the chorioallantois and a vestige of the shell membrane to the uterine lining. Generalized squamate allantoplacentae are epitheliochorial, diffuse, adeciduate, and highly vascular, accomplish maternal‐fetal gas exchange, and possibly transfer small quantities of organic and inorganic nutrients. Placental gas exchange presumably is enhanced by the thinning of tissues lying between fetal and maternal capillaries, a progressive increase in placental vascularity, and in some species, by differences in oxygen affinity of fetal and maternal blood. A few saurian genera (e.g., Chalcides, Mabuya, and Pseudemoia) include species with specialized placentae that transfer large quantities of nutrients. Specializations of the allantoplacentae of these lizards include interdigitating, hypertrophied uterine and chorioallantoic tissues, and enlarged absorptive chorionic epithelia. South American Mabu...
Viviparous placentotrophy in reptiles and the parent–offspring conflict
Journal of Experimental Zoology Part B: Molecular and Developmental Evolution, 2015
ABSTRACTIn placentotrophic viviparous reptiles, pregnant females deliver nutrients to their devel... more ABSTRACTIn placentotrophic viviparous reptiles, pregnant females deliver nutrients to their developing fetuses by diverse morphological specializations that reflect independent evolutionary origins. A survey of these specializations reveals a major emphasis on histotrophy (uterine secretion and fetal absorption) rather than hemotrophy (transfer between maternal and fetal blood streams). Of available hypotheses for the prevalence of histotrophic transfer, the most promising derives insights from the theoretical parent–offspring conflict over nutrient investment. I suggest that histotrophy gives pregnant females greater control over nutrient synthesis, storage, and delivery than hemotrophic transfer, reflecting maternal preeminence in any potential parent–offspring competition over nutrient investment. One lizard species shows invasive ovo‐implantation and direct contact between fetal tissues and maternal blood vessels, potentially conferring control over nutrient transfer to the embr...
Evolution of placental specializations in viviparous African and South American lizards
Journal of experimental zoology. Part A, Comparative experimental biology, 2003
Phylogenetic information offers an important resource in analyses of reproductive diversity, incl... more Phylogenetic information offers an important resource in analyses of reproductive diversity, including interpretations of fetal membrane evolution. In this paper, we draw upon ongoing studies of South American and African lizards to consider the value of combining phylogenetic and reproductive evidence in the construction of evolutionary interpretations. South American lizards of the genus Mabuya exhibit several reproductive specializations that are convergent on those of eutherian mammals, including viviparity, long gestation periods, ovulation of tiny eggs, and placental supply of the nutrients for development. Studies of placental morphology and development indicate that New World Mabuya share several other derived features, including chorionic areolae and a "Type IV" epitheliochorial placenta with a villous, mesometrial placentome. Some characteristics of these lizards are shared by two African skinks, M. ivensii and Eumecia anchietae, including minuscule eggs, placent...
Reproduction in Viviparous South American Lizards of the Genus Mabuya
Reproductive Biology of South American Vertebrates, 1992
... Reproduction in Viviparous South American Lizards of the Genus Mabuya Daniel G. Blackburn and... more ... Reproduction in Viviparous South American Lizards of the Genus Mabuya Daniel G. Blackburn and Laurie J. Viti Introduction Reproduction in most vertebrates requires that the female construct an egg containing all of the nutrients needed to sustain development and that ... Hist. ...
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Papers by Daniel Blackburn