An examination of the variation in maternal placentae across the genus Poeciliopsis (Poeciliidae)

Placentae show considerable diversity in a number of nonmammalian, viviparous organisms, including amphibians, reptilian sauropsids, teleost fish, and chondrichthyes. However, the evolutionary processes driving the evolution of placenta are still debated. In teleost fishes, the genus Poeciliopsis (Poeciliidae) offers a rare opportunity for studying placental evolution: extensive placentation has evolved three independent times within the last 750,000 years and there is substantial interspecific variation in the degree of embryonic, maternal nutrient provisioning and development of the placenta. In poeciliids, the placenta is composed of a hypertrophied maternal follicular epithelium apposed to a highly vascularized embryonic pericardial sac. To better understand placental evolution, we have undertaken a comprehensive comparative study of the maternal follicle in eight closely related Poeciliopsis species that span the range in postfertilization, embryonic, maternal nutrient provisioning (from lecithotrophs, to moderate matrotrophs, to extensive matrotrophs). Using light and scanning electron microscopy, we found that the species that provide extensive postfertilization maternal nutrient provisioning (extensive matrotrophs) have thicker follicles and more extensive folding of the follicular epithelium compared to the lecithotrophs and moderate matrotrophs. Follicle sections and histology revealed that epithelial folds of the extensive matrotrophs are comprised primarily of cuboidal and columnar cells and are richly supplied with capillaries. Among the extensive matrotrophs, enhancements of follicle traits corresponded with increases in the level of maternal nutrient provisioning. Hypertrophied maternal follicles with richly vascularized folds can serve to increase the surface area and, thus, facilitate the transfer of substances between the mother and developing embryo. Finally, we found egg envelopes in the lecithotrophs and moderate matrotrophs, but not in the extensive matrotrophs. Morphological studies, like this one, can provide a better understanding of the natural variation in the structure and functioning of maternal and offspring traits associated with matrotrophy and, thus, insights into the processes driving placental evolution. J. Morphol. 276:707–720, 2015. © 2015 Wiley Periodicals, Inc.     

Allantoplacental ultrastructure of an Andean population of Mabuya (Squamata, Scincidae)

Mabuya species are highly matrotrophic viviparous lizards with Type IV epitheliochorial allantoplacenta. The allantoplacenta of an Andean population of this genus, currently assigned to Mabuya sp., possesses specializations related to histotrophic nutrition at the embryonic hemisphere (placentome, paraplacentome, and chorionic areolas), while at the abembryonic hemisphere it has a mixed function: histotrophic transfer (absorptive plaques) and hemotrophic nutrition (gas exchange in respiratory segments). These placental specializations were studied using high-resolution light microscopy and transmission electron microscopy, and were compared with those found in other squamate reptiles and eutherian mammals. Cytological features of the placentome suggest that this is an important region for nutritional provision; the paraplacentome also shows characteristics for nutrient transfer, especially lipids. Chorionic areolas allow the absorption of glandular products, as well as uterine and chorionic cellular debris produced by lysis of some cells of both epithelia during areola formation. In the absorptive plaques both uterine and chorionic epithelia are firmly attached and their cellular apices exhibit electron-dense granules that could be related to autocrine and paracrine functions. The short interhemal distance found in the respiratory segments confirms their role in gas exchange. A common feature of all regional specializations in the Mabuya sp. allantoplacenta is the presence of lipids in the interacting chorionic and uterine epithelia, suggesting that lipids are transferred throughout the entire embryonic chamber; placental transfer of lipids may be the principal fetal energy and lipid source in this species. In spite of this feature, each one of the specialized areas of the allantoplacenta has different features suggesting particular functions in the transfer of nutrients (as ions, lipids, proteins, amino acids, sugar, water, and gases), and in the possible synthesis of hormones and proteins. The placental complexity observed in this species of Mabuya is greater than in any other reptile, and resembles that of eutherian mammals: Each one of these specializations of the placental membranes in Mabuya sp. is similar to those found among different eutherian mammals, indicating a very impressive evolutionary convergence at the histological and cytological levels between both clades. However, no eutherian mammal species simultaneously displays all of these specializations in the embryonic chamber as does Mabuya sp.     

Historical biogeography of the livebearing fish genus Poeciliopsis (Poeciliidae: Cyprinodontiformes)

To assess the historical biogeography of freshwater topminnows in the genus Poeciliopsis, we examined sequence variation in two mitochondrial genes, cytochrome b (1140 bp) and NADH subunit 2 (1047 bp). This widespread fish genus is distributed from Arizona to western Colombia, and nearly half of its 21 named species have distributions that border on the geologically active Trans-Mexican Volcanic Belt (TMVB), a region that defines the uplifted plateau (Mesa Central) of Mexico. We used the parametric bootstrap method to test the hypothesis that a single vicariant event associated with the TMVB was responsible for divergence of taxa found to the north and south of this boundary. Because the single-event hypothesis was rejected as highly unlikely, we hypothesize that at least two geological events were responsible for divergence of these species. The first (8-16 million years ago) separated ancestral populations that were distributed across the present TMVB region. A second event (2.8-6.4 million years ago) was associated with northward dispersal and subsequent vicariance of two independent southern lineages across the TMVB. The geological history of this tectonically and volcanically active region is discussed and systematic implications for the genus are outlined.     

Morphological specializations for fetal maintenance in viviparous vertebrates: An introduction and historical retrospective

In many viviparous vertebrates, pregnant females sustain their developing embryos and provide them with nutrients by means of placentas and a diversity of other types of specializations. With this article, we introduce a virtual (online) issue of the Journal of Morphology that presents 12 recent papers on fetal maintenance in viviparous vertebrates. We also outline the history of research in this area and document the central role of morphology in helping to explain the function and evolution of specializations for fetal nutrition. This virtual issue of the Journal of Morphology is an outgrowth of a symposium held under auspices of the International Congress of Vertebrate Morphology. The included papers reflect a diversity of taxa, research methods, and biological issues. To celebrate the publication of this virtual issue of the Journal of Morphology, the publisher is making freely available to readers a number of other relevant papers published in the journal over the past 128 years. J. Morphol. 276:1–16, 2015. © 2015 Wiley Periodicals, Inc.     

The allantoplacenta of Mabuya mabouya (Sauria, Scincidae)

Analysis of placentation in the final stages of development in Mabuya mabouya shows that the placenta is formed by the apposition of the chorioallantois to the uterine mucosa implicating the entire embryonic chamber, because the allantoic vesicle invades all the exocoelom. The chorioallantoic placenta presents the features proper of a type IV allantoplacenta. However, in the mesometrial area peripheral to the placentome, we found that the paraplacentome is an additional zone specialized for histotrophic transfer, and is separated from the rest of the embryonic chamber by a chorionic invagination formed of polymorphic cells. The chorionic areolae are components of the embryonic hemisphere; they are in apposition to an endometrium with columnar epithelial cells and several glands that secrete toward the cavity of the areolae. They are observed only in the preparturition stage, probably operating in maternal-fetal transfer of nutrients during the last embryonic growth stage. The mesometrial hemisphere possesses specializations related to histotrophic nutrition (placentome, paraplacentome, and chorionic areolae), while in the abembryonic hemisphere there is an allantoplacenta of mixed function, with capacity for histotrophic nutrition and for gas exchange. The absorptive plaques are small rounded areas constituted by chorionic cells similar to the paraplacentomal chorionic cells, in intimate apposition with a secretory uterine epithelium. Separating the absorptive plaques are respiratory segments histologically similar to the type I allantoplacenta. The additional histotrophic areas found for this species demonstrate the great specialization of this allantoplacenta, and support the highest degree of matrotrophy among reptiles reached in the Neotropical Mabuya.     

Morphogenesis of the ovarian follicular epithelium during initial stages of embryogenesis of the viviparous earwig,Hemimerus talpoides

Representatives of the highly specialized earwig family Hemimeridae are epizoic and viviparous. Their embryos develop inside terminal ovarian follicles (termed also embryonic follicles) and rely solely on nutrients transferred from mother tissues. In this report, we present results of ultrastructural and histochemical studies of the initial stage of Hemimerus talpoides development. Our results show that the follicular cells surrounding fully grown oocyte of Hemimerus do not degenerate after initiation of embryogenesis, but transform and gradually form the wall of the incubation chamber in which the embryo develops. We also show that amniotic cells of the early embryo remain in direct contact with transformed follicular cells. In the region of contact, short outgrowths of the amniotic cells associate with irregular surface specializations of the transformed follicular cells. We suggest that extended “postfertilization” activity of hemimerid follicular cells represents an adaptation to viviparity and matrotrophy in this insect lineage.     

Evolution of vertebrate viviparity and specializations for fetal nutrition: A quantitative and qualitative analysis

Phylogenetic analyses indicate that viviparity (live‐bearing reproduction) has originated independently in more than 150 vertebrate lineages, including a minimum of 115 clades of extant squamate reptiles. Other evolutionary origins of viviparity include 13 origins among bony fishes, nine among chondrichthyans, eight in amphibians, one in Paleozoic placoderms, six among extinct reptiles, and one in mammals. The origins of viviparity range geologically from the mid‐Paleozoic through the Mesozoic to the Pleistocene. Substantial matrotrophy (maternal provision of nutrients to embryos during pregnancy) has arisen at least 33 times in these viviparous clades, with most (26) of these origins having occurred among fishes and amphibians. Convergent evolution in patterns of matrotrophy is widespread, as reflected by multiple independent origins of placentotrophy, histotrophy, oophagy, and embryophagy. Specializations for nutrient transfer to embryos are discontinuously distributed, reflecting the roles of phylogenetic inertia, exaptation (preadaptation), and constraint. Ancestral features that function in gas exchange and nutrition repeatedly and convergently have been co‐opted for nutrient transfer, often through minor modification of their components and changes in the timing of their expression (heterochrony). Studies on functional and evolutionary morphology continue to play a central role in our attempts to understand viviparity and mechanisms of fetal nutrition. J. Morphol. 276:961–990, 2015. © 2014 Wiley Periodicals, Inc.     

The evolution of placentas and superfetation in the fish genus Poecilia (Cyprinodontiformes: Poeciliidae: subgenera Micropoecilia and Acanthophacelus)

Complex adaptations are often found in nature, although our ability to discern how and why such traits evolved is limited because their origin occurred in the distant past and the details of their evolution have been lost through extinction (e.g. all placental mammals inherited their placentas from a single common ancestor that lived over 100 Mya). In poeciliid fishes, placentas have evolved independently multiple times and portions of the path to the evolution of complexity can be found in living species. In the present study, we describe the life histories of six species within the genus Poecilia that includes the subgenera Micropoecilia and Acanthophacelus (the guppy; Poecilia reticulata). We demonstrate that extensive placentotrophy and superfetation, the ability to simultaneously carry more than one developing brood, have evolved within this clade. These fish represent the third clade in which we have discovered the independent origin of a placenta that also includes close relatives that lack a placenta. We discuss possible adaptive advantages of the joint evolution of extensive placentation and superfetation in these fishes. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99 , 784–796.     

Variation in heat-shock proteins among species of desert fishes (Poeciliidae, Poeciliopsis)

Analysis of the heat-shock proteins (hsps) of six closely related species of Poeciliopsis demonstrated the existence of biochemical diversity in the hsp100, hsp70, hsp60, and hsp30 protein families among species. Each species expressed five to seven hsp70-related isoforms. Constitutive 70-kD isoforms were identical among species, but four different patterns of heat-inducible isoforms were seen in these six species. Members of the hsp70 family of molecular chaperones are included among the most highly conserved proteins known, and the possibility of variation in hsp70 among closely related species has rarely been addressed. The hsp30 family is known to be less conserved than the hsp70 family, and, as expected, the Poeciliopsis hsp30 patterns showed more variation. Most of the hsp30 isoforms characteristic of a particular species were unique to that species. Hsp100 and hsp60 were identical in five of the species, but alternate isoforms were found in P. monacha. The small size and limited geographical distribution of the P. monacha population have probably contributed to the uniqueness of the monacha pattern. Two of the species were shown to acquire thermotolerance, the ability to withstand normally lethal temperatures when subjected to a gradual temperature increase. Rapid-heating protocols commonly used to establish critical thermal maxima of organisms do not include this inducible component of thermoresistance and therefore do not adequately assess an organism's capacity to withstand thermal stress.      

Molecular Evidence for Multiple Origins of Hybridogenetic Fish Clones (Poeciliidae: Poeciliopsis)

Hybrid matings between the sexual species Poeciliopsis monacha and Poeciliopsis lucida produced a series of diploid all-female lineages of P. monacha-lucida that inhabit the Río Fuerte of northwestern Mexico. Restriction site analyses of mitochondrial DNA (mtDNA) clearly revealed that P. monacha was the maternal ancestor of these hybrids. The high level of mtDNA diversity in P. monacha was mirrored by similarly high levels in P. monacha-lucida; thus hybridizations giving rise to unisexual lineages have occurred many times. However, mtDNA variability among P. monacha-lucida lineages revealed a geographical component. Apparently the opportunity for the establishment of unisexual lineages varies among tributaries of the Río Fuerte. We hypothesize that a dynamic complex of sexual and clonal fishes appear to participate in a feedback process that maintains genetic diversity in both the sexual and asexual components.     

Functional morphology of the gonoduct of the viviparous teleost Poeciliopsis gracilis (Heckel, 1848) (Poeciliidae)

Female teleosts do not develop Müllerian ducts; consequently, the ovary of teleosts contains two zones: germinal and gonoduct. The gonoduct lacks germinal cells, but has relevant functions in the reproductive process. We describe the functional morphology of the gonoduct in the viviparous teleost Poeciliopsis gracilis during nongestation and gestation stages. This study tests the hypothesis that the gonoduct functions as a barrier between the germinal zone and the exterior. By providing information about morphology and function of the gonoduct we show that this part of the ovary has an essential role in the reproduction of teleosts. The ovaries were processed by histological technique and stained with hematoxylin‐eosin (H‐E), Masson's trichrome, toluidine blue and periodic acid‐Schiff (PAS). The gonoduct is divided into three regions: cephalic, middle, and caudal. In the cephalic and middle regions there are mucosal folds that extend into the gonoductal lumen, forming structures similar to a cervix. The caudal region has two portions: the anterior contains a dorsal invagination and exocrine glands among columnar cells; the posterior has a ventral flexion and stratified epithelium with apical secretory cells. The morphology of this epithelium indicates two functions: (a) secretory by the apical columnar cells, and (b) protection through the stratification. Another peculiarity of the caudal region is that both ducts, reproductive and digestive, converge in a common cavity at their caudal ends, forming a cloacal region. The histology of the gonoduct indicates relevant functions including: (1) the control of the luminal diameter by the muscle and the presence of mucosal folds, like a cervix; (2) the relationship with the spermatozoa during insemination and storing them in mucosal folds; (3) the support of immunological processes; (4) secretory activities; (5) forming the duct during birth; and (6) possibly, acts as a barrier against parasite infestations.     

Evolution of sexual mimicry in sperm-dependent all-female forms of Poeciliopsis (Pisces: Poeciliidae)

The twofold advantage of all-female reproduction is limited in many asexual lineages because females are sperm-dependent. Males of a related sexual-host species typically prefer conspecific females as mates. According to the “sexual mimicry” hypothesis, an all-female lineage that closely resembles females of the sexual-host species should have enhanced mating success. Examination of mating success in all-female fish of the genus Poeciliopsis supported this hypothesis. The excellent sexual mimicry of some all-female strains could have evolved through mutations within clonal lineages subsequent to their origins as interspecific hybrids. Alternatively, this mimicry may have been “frozen” from variation in the sexual gene pool when new unisexual lineages first arose. To test the latter hypothesis, we examined laboratory synthesized strains of the hybridogenetic fish P. monacha-lucida. The frozen variation hypothesis was supported by the present results.     

Life-history plasticity under resource stress in a clonal fish (Poeciliidae: Poeciliopsis)

Within-individual plasticity for reproductive investment was examined in a clonal fish (Poeciliidae: Poeciliopsis) grown under six levels of resource stress. Growth, age at first reproduction, egg production, egg size, egg energetic content, and survivorship were measured from fish grown in three dietary and two density treatments. Growth and fecundity decreased with both increased density and food stress. Age at first reproduction increased with increased density, but was unaffected by the diet treatments. Reproductive effort (clutch size per female weight), offspring investment (egg size and egg energetic content), and survivorship were invariant across all treatment combinations. We compare these results with predictions based on theoretical treatments of optimal reproductive investment.     

Scanning electron microscopy of the placental interface in the viviparous lizard Sceloporus jarrovi (Squamata: Phrynosomatidae)

Placental membranes mediate maternal‐fetal exchange in all viviparous reptilian sauropsids. We used scanning electron microscopy to examine the placental interface in the mountain spiny lizard, Sceloporus jarrovi (Phrynosomatidae). From the late limb bud stage until birth, the conceptus is surrounded by placental membranes formed from the chorioallantois and yolk sac omphalopleure. The chorioallantois lies directly apposed to the uterine lining with no intervening shell membrane. Both fetal and maternal sides of the chorioallantoic placenta are lined by continuous layers of flattened epithelial cells that overlie dense capillary networks. The chorioallantoic placenta shows specializations that enhance respiratory exchange, as well as ultrastructural evidence of maternal secretion and fetal absorption. The yolk sac placenta contains enlarged fetal and maternal epithelia with specializations for histotrophic nutrient transfer. This placenta lacks intrinsic vascularity, although the vascular allantois lies against its inner face, contributing to an omphallantoic placenta. In a specialized region at the abembryonic pole, uterine and fetal tissues are separated by a compact mass of shed shell membrane, yolk droplets, and cellular debris. The omphalopleure in this region develops elongate folds that may contribute to sequestration and absorption of this material. Fetal membrane morphogenesis and composition in S. jarrovi are consistent with those of typical squamates. However, this species exhibits unusual placental specializations characteristic of highly placentotrophic lizards. J. Morphol., 2011. © 2011 Wiley‐Liss, Inc.