Structures Associated with Oogenesis and Embryonic Development during Intraovarian Gestation in Viviparous Teleosts (Poeciliidae)

Membrane molecule bouncer regulates sperm binding activity in immature oocytes in the viviparous teleost species Poecilia reticulata (guppy)

Generally, in vertebrates, the first step toward fertilization is the ovulation of mature oocytes, followed by their binding to sperm cells outside of the ovary. Exceptionally, the oocytes of poeciliid fish are fertilized by sperm cells within the follicle, and the developmental embryo is subsequently released into the ovarian lumen before delivery. In the present study, we aimed to identify the factor(s) responsible for intrafollicular fertilization in a viviparous teleost species, Poecilia reticulata (guppy). Sperm tracking analysis in this regard indicated that in this species, sperm cells reached immature oocytes including the germinal vesicle, and the insemination assay indicated that the immature oocytes robustly adhered to the sperm cells; similar binding was not observed in Danio rerio (zebrafish) and Oryzias latipes (medaka). We also identified the Ly6/uPAR protein bouncer as the factor responsible for the observed sperm binding activity of the immature oocytes in this species. The recombinant bouncer peptide acted as an inhibitory decoy for the sperm-oocyte binding in guppy. On the other hand, ectopic expression of guppy bouncer in zebrafish oocytes resulted in interspecific sperm-oocyte binding. These results argue that bouncer is responsible for sperm-immature oocyte binding. Our findings highlight the unique reproductive strategies of guppy fish and enhance our understanding of the diverse reproductive mechanisms in vertebrates.

Whole Genome Assembly and Annotation of Blackstripe Livebearer Poeciliopsis prolifica

The blackstripe livebearer Poeciliopsis prolifica is a live-bearing fish belonging to the family Poeciliidae with high level of postfertilization maternal investment (matrotrophy). This viviparous matrotrophic species has evolved a structure similarly to the mammalian placenta. Placentas have independently evolved multiple times in Poeciliidae from nonplacental ancestors, which provide an opportunity to study the placental evolution. However, there is a lack of high-quality reference genomes for the placental species in Poeciliidae. In this study, we present a 674 Mb assembly of P. prolifica in 504 contigs with excellent continuity (contig N50 7.7 Mb) and completeness (97.2% Benchmarking Universal Single-Copy Orthologs [BUSCO] completeness score, including 92.6% single-copy and 4.6% duplicated BUSCO score). A total of 27,227 protein-coding genes were annotated from the merged datasets based on bioinformatic prediction, RNA sequencing and homology evidence. Phylogenomic analyses revealed that P. prolifica diverged from the guppy (Poecilia reticulata) ∼19 Ma. Our research provides the necessary resources and the genomic toolkit for investigating the genetic underpinning of placentation.

Distinguishing Between Embryonic Provisioning Strategies in Teleost Fishes Using a Threshold Value for Parentotrophy

The source of embryonic nutrition for development varies across teleost fishes. A parentotrophy index (ratio of neonate: ovulated egg dry mass) is often used to determine provisioning strategy, but the methodologies used vary across studies. The variation in source and preservation of tissue, staging of embryos, and estimation approach impedes our ability to discern between methodological and biological differences in parentotrophy indices inter- and intra-specifically. The threshold value used to distinguish between lecithotrophy and parentotrophy (0.6-1) differs considerably across studies. The lack of a standardised approach in definition and application of parentotrophy indices has contributed to inconsistent classifications of provisioning strategy. Consistency in both methodology used to obtain a parentotrophy index, and in the classification of provisioning strategy using a threshold value are essential to reliably distinguish between provisioning strategies in teleosts. We discuss alternative methods for determining parentotrophy and suggest consistent standards for obtaining and interpreting parentotrophy indices.

Sperm transport and male pregnancy in seahorses: An unusual model for reproductive science

The Syngnathidae (seahorses and pipefishes) are a group of teleost fishes in which, uniquely, developing embryos are hosted throughout pregnancy by males, using a specialized brood pouch situated on the abdomen or tail. Seahorses have evolved the most advanced form of brood pouch, whereby zygotes and embryos are intimately connected to the host's circulatory system and also bathed in pouch fluid. The pouch is closed to the external environment and has to perform functions such as gaseous exchange, removal of waste and maintenance of appropriate osmotic conditions, much like the mammalian placenta. Fertilization of the oocytes occurs within the brood pouch, but unlike the mammalian situation the sperm transport mechanism from the ejaculatory duct towards the pouch is unclear, and the sperm: egg ratio (about 5:1) is possibly the least of any vertebrate. In this review, there is highlighting of the difficulty of elucidating the sperm transport mechanism, based on studies of Hippocampus kuda. The similarities between seahorse pouch function and the mammalian placenta have led to suggestions that the pouch provides important nutritional support for the developing embryos, supplementing the nutritional functions of the yolk sac provided by the oocytes. In this review, there is a description of the recent evidence in support of this hypothesis, and also emphasis, as in mammals, that embryonic development depends on nutritional support from the placenta-like pouch at important stages of the gestational period ("critical windows").

Intraovarian Gestation in Viviparous Teleosts: Unique Type of Gestation among Vertebrates

The intraovarian gestation, occurring in teleosts, makes this type of reproduction a such complex and unique condition among vertebrates. This type of gestation of teleosts is expressed in special morphological and physiological characteristic where occurs the viviparity and it is an essential component in the analysis of the evolutionary process of viviparity in vertebrates. In viviparous teleosts, during embryogenesis, there are not development of Müllerian ducts, which form the oviducts in the rest of vertebrates, as a result, exclusively in teleosts, there are not oviducts and the caudal region of the ovary, the gonoduct, connects the ovary to the exterior. The lack of oviducts defines that the embryos develop into the ovary, as intraovarian gestation. The ovary forms the oocytes which may develop different type of oogenesis, according with the storage of diverse amount of yolk, variation observed corresponding to the species. The viviparous gestation is characterized by the possible intimate contact between maternal and embryonic tissues, process that permits their metabolic interchanges. So, the nutrients obtained by the embryos could be deposited in the oocyte before fertilization, contained in the yolk (lecithotrophy), and may be completed during gestation by additional provisioning from maternal tissues to the embryo (matrotrophy). Then, essential requirements for viviparity in poeciliids and goodeids are characterized by: a) the diversification of oogenesis, with the deposition of different amount of yolk in the oocyte; b) the insemination, by the transfer of sperm to the female gonoduct and their transportation from the gonoduct to the germinal region of the ovary where the follicles develop; c) the intrafollicular fertilization; d) the intraovarian gestation with the development of embryos in intrafollicular gestation (as in poeciliids), or intraluminal gestation (as in goodeids); and, e) the origin of embryonic nutrition may be by lecithotrophy and matrotrophy. The focus of this revision compares the general and specific structural characteristics of the viviparity occurring into the intraovarian gestation in teleosts, defining this reproductive strategy, illustrated in this review with histological material in a poeciliid, of the species Poecilia latipinna (Lesueur, 1821) (Poeciliidae), and in a goodeid, of the species Xenotoca eiseni (Rutter, 1896) (Goodeidae).

Morphological basis for maternal nutrient provision to embryos in the viviparous fish Ataeniobius toweri (Teleostei: Goodeidae)

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.

Morphology of yolk and pericardial sacs in lecithotrophic and matrotrophic nutrition in poeciliid fishes

We used histological techniques to describe the morphology of the yolk and pericardial sacs in developing embryos of the lecithotrophic species Girardinus creolus, Gambusia puncticulata, Limia vittata, and Quintana atrizona, in comparison with the extreme matrotrophic Heterandria formosa. In lecithotrophic species, the yolk sac was enlarged and lasted until the final stages of development, while in H. formosa it was completely absorbed soon after fertilization. Lecithotrophic poeciliids showed a pericardial sac with a single layer of blood vessels covering the dorsal surface of the cephalic region only, while H. formosa showed a more complex largely vascularized pericardial sac covering the entire dorsal surface, except the caudal region. In advanced gestation of G. creolus, a vascular plexus of the yolk sac reaches the pharyngeal region, behind the gills, suggesting that the pharynx may play a role in embryonic nutrition in lecithotrophic species. These morphological evidences suggest that matrotrophy derives from lecithotrophy.

Structure of the gonoduct of the viviparous teleost Cnesterodon decemmaculatus (Jenyns, 1842) (Poeciliidae)

During embryogenesis, teleost females do not develop Müllerian ducts, which form the oviducts in all other vertebrates. Thus, when they reach sexual maturity they do not have oviducts. In viviparous teleosts, the lack of oviducts means that the development of the embryos occurs as an intraovarian gestation, unique among vertebrates. The ovary is an unpaired hollow organ whose cavity is continuous with the caudal portion, the gonoduct, characterized by the absence of germinal cells, which opens to the exterior at the gonopore. The gonoduct attains essential function as a barrier between the germinal region of the ovary and the exterior during all reproductive stages. This study describes the functional morphology of the gonoduct in the viviparous teleost Cnesterodon decemmaculatus during non-gestation (previtellogenesis and vitellogenesis) and gestation. The ovaries were processed using histological techniques and stained with hematoxylin-eosin, and periodic acid Schiff. The gonoduct has two regions: cephalic and caudal, and is formed by three histological layers, which are, from inside to the periphery: (a) tunica mucosa; (b) tunica muscularis; and (c) tunica serosa. In the cephalic region there are mucosal folds extending into the lumen and forming a structure similar to a cervix. The histology of the gonoduct indicates essential functions, that is, (a) the control of the luminal diameter in the limit to the germinal region of the ovary by the presence of a cervix; (b) during insemination the gonoduct receives the spermatozoa, may store and transport them to the germinal region; (c) the presence of melano-macrophage centers indicates support of immunological processes, especially during gestation when these centers increase in size; (d) production of exocrine secretions; and (e) it is the birth canal, internally lined by an ciliated epithelium and surrounded by smooth musclesboth tissues supposedly supporting the birth process.