How fish eggs are preadapted for the evolution of matrotrophy

Metabolic loads and the costs of metazoan reproduction

Reproduction includes two energy investments-the energy in the offspring and the energy expended to make them. The former is well understood, whereas the latter is unquantified but often assumed to be small. Without understanding both investments, the true energy costs of reproduction are unknown. We present a framework for estimating the total energy costs of reproduction by combining data on the energy content of offspring (direct costs) and the metabolic load of bearing them (indirect costs). We find that direct costs typically represent the smaller fraction of the energy expended on reproduction. Mammals pay the highest reproductive costs (excluding lactation), ~90% of which are indirect. Ectotherms expend less on reproduction overall, and live-bearing ectotherms pay higher indirect costs compared with egg-layers. We show that the energy demands of reproduction exceed standard assumptions.

Ovarian changes and development of the branchial placenta occurring in Jenynsia lineata (Cyprinodontiformes, Anablepidae)

In viviparous teleosts, intraovarian gestation occurs intrafollicularly, as in poeciliids, or intraluminally, as in goodeids and anablepids. Furthermore, there are two different forms of embryonic nutrition: lecithotrophy and matrotrophy; depending on the species, these can be exclusive or coexist during gestation. In matrotrophic species, nutrients are transmitted from the mother to the embryo and are especially important in species with intraluminal gestation. Jenynsia lineata is a South American viviparous teleost with intraluminal gestation, characterized by eggs with scarce yolk, which is resorbed when embryos are 6 mm long, thus developing a branchial placenta. Using histological, histochemical, and immunohistochemical techniques, the present study describes the characteristics and changes of the ovarian mucosa in J. lineata during gestational and nongestational phases, and analyzes the embryonic pharyngeal epithelium in the branchial placenta. The ovaries of 30 adult female specimens were processed using histological techniques and stained with hematoxylin-eosin, Masson's trichrome, and Alcian Blue pH 2.5/periodic acid Schiff reagent. To detect cell proliferation, we used antiproliferating cell nuclear antigen antibody. In nonpregnant females, eosinophilic granular cells (EGCs) and lymphocytes were identified in the lamina propria of the tunica mucosa, and melanomacrophage centers (MMCs) and fibroblasts were identified adjacent to tissue debris in the ovarian folds'. In the cellular debris, an embryo in resorption was observed. In pregnant females, the ovarian mucosa has thin vascularization branches entering the opercular chamber of the embryos, in close contact with the forming gill processes, thereby establishing a branchial placenta. Active cell replacement was observed in these ovarian branches. The identification of fibroblasts, lymphocytes, EGCs, and MMCs adjacent to tissue debris could indicate that these cell types are involved in the embryonic resorption process. Considering the new data obtained in this study on the branchial placenta of J. lineata, we conclude that cell proliferation could be involved in the development of maternal-embryonic interaction.

Primordial Germ Cell Development in the Poeciliid, Gambusia holbrooki, Reveals Shared Features Between Lecithotrophs and Matrotrophs

Metazoans exhibit two modes of primordial germ cell (PGC) specification that are interspersed across taxa. However, the evolutionary link between the two modes and the reproductive strategies of lecithotrophy and matrotrophy is poorly understood. As a first step to understand this, the spatio-temporal expression of teleostean germ plasm markers was investigated in Gambusia holbrooki, a poecilid with shared lecitho- and matrotrophy. A group of germ plasm components was detected in the ovum suggesting maternal inheritance mode of PGC specification. However, the strictly zygotic activation of dnd-β and nanos1 occurred relatively early, reminiscent of models with induction mode (e.g., mice). The PGC clustering, migration and colonisation patterns of G. holbrooki resembled those of zebrafish, medaka and mice at blastula, gastrula and somitogenesis, respectively—recapitulating features of advancing evolutionary nodes with progressive developmental stages. Moreover, the expression domains of PGC markers in G. holbrooki were either specific to teleost (vasa expression in developing PGCs), murine models (dnd spliced variants) or shared between the two taxa (germline and somatic expression of piwi and nanos1). Collectively, the results suggest that the reproductive developmental adaptations may reflect a transition from lecithotrophy to matrotrophy.

The behavioural and physiological ecology of embryos: responding to the challenges of life inside an egg

Adaptations of post-hatching animals have attracted far more study than have embryonic responses to environmental challenges, but recent research suggests that we have underestimated the complexity and flexibility of embryos. We advocate a dynamic view of embryos as organisms capable of responding - on both ecological and evolutionary timescales - to their developmental environments. By viewing embryos in this way, rather than assuming an inability of pre-hatching stages to adapt and respond, we can broaden the ontogenetic breadth of evolutionary and ecological research. Both biotic and abiotic factors affect embryogenesis, and embryos exhibit a broad range of behavioural and physiological responses that enable them to deal with changes in their developmental environments in the course of interactions with their parents, with other embryos, with predators, and with the physical environment. Such plasticity may profoundly affect offspring phenotypes and fitness, and in turn influence the temporal and spatial dynamics of populations and communities. Future research in this field could benefit from an integrated framework that combines multiple approaches (field investigations, manipulative experiments, ecological modelling) to clarify the mechanisms and consequences of embryonic adaptations and plasticity.

Phylogeny and evolutionary history of the amniote egg

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.

Reproductive Mode and Conflict Shape the Evolution of Male Attributes and Rate of Speciation in the Fish Family Poeciliidae

Sexual conflict is caused by differences between the sexes in how fitness is maximized. These differences are shaped by the discrepancy in the investment in gametes, how mates are chosen and how embryos and young are provided for. Fish in the family Poeciliidae vary from completely provisioning eggs before they are fertilized to providing virtually all resources after fertilization via the functional equivalent of a mammalian placenta. This shift in when females provision their young relative to when an egg is fertilized is predicted to cause a fundamental change in when and how sexual conflict is manifested. If eggs are provisioned before fertilization, there should be strong selection for females to choose with whom they mate. Maternal provisioning after fertilization should promote a shift to post-copulatory mate choice. The evolution of maternal provisioning may in turn have cascading effects on the evolution of diverse features of the biology of these fish because of this shift in when mates are chosen. Here we summarize what these consequences are and show that the evolution of maternal provisioning is indeed associated with and appears to govern the evolution of male traits associated with sexual selection. The evolution of placentas and associated conflict does not cause accelerated speciation, contrary to predictions. Accelerated speciation rate is instead correlated with the evolution of male traits associated with sexual selection, which implies a more prominent role of pre-copulatory reproductive isolation in causing speciation in this family.

Molecular Signatures of Placentation and Secretion Uncovered in Poeciliopsis Maternal Follicles

Placentation evolved many times independently in vertebrates. Although the core functions of all placentas are similar, we know less about how this similarity extends to the molecular level. Here, we study Poeciliopsis, a unique genus of live-bearing fish that have independently evolved complex placental structures at least three times. The maternal follicle is a key component of these structures. It envelops yolk-rich eggs and is morphologically simple in lecithotrophic species but has elaborate villous structures in matrotrophic species. Through sequencing, the follicle transcriptome of a matrotrophic, Poeciliopsis retropinna, and lecithotrophic, P. turrubarensis, species we found genes known to be critical for placenta function expressed in both species despite their difference in complexity. Additionally, when we compare the transcriptome of different river populations of P. retropinna, known to vary in maternal provisioning, we find differential expression of secretory genes expressed specifically in the top layer of villi cells in the maternal follicle. This provides some of the first evidence that the placental structures of Poeciliopsis function using a secretory mechanism rather than direct contact with maternal circulation. Finally, when we look at the expression of placenta proteins at the maternal–fetal interface of a larger sampling of Poeciliopsis species, we find expression of key maternal and fetal placenta proteins in their cognate tissue types of all species, but follicle expression of prolactin is restricted to only matrotrophic species. Taken together, we suggest that all Poeciliopsis follicles are poised for placenta function but require expression of key genes to form secretory villi.

Domperidone treatment attenuates stress-induced suppression of reproduction in viviparous mosquitofish Gambusia affinis

The aim of this study was to determine the effect of stress on reproduction and the possible involvement of dopaminergic systems in the reproductive stress response in the mosquitofish Gambusia affinis. Exposure of fish to aquaculture stressors (four 10 min episodes of stress, each corresponding to a different stressor such as handling, chasing, frequent netting and low water levels), for a period of 30 days caused reduction in the mean numbers of stage I-IV follicles associated with lower number of pregnant females and embryos in most of the developmental stages compared with experimental controls. Besides, increase in the intensity of labelling and the per cent area of tyrosine hydroxylase (TH; a rate-limiting enzyme in the biosynthetic pathway of catecholamines)- immunoreactive (ir) neurons was observed in the preoptic area (POA) and the nucleus preopticus (NPO) regions of the brain concomitant with reduction in the labelling of gonadotropin releasing hormone-immunoreactive (GnRH-ir) fibres in the proximal pars distalis (PPD) of the pituitary gland in stressed fish compared with experimental controls. Treatment of domperidone (DOM) caused an increase in the number of stage II and V follicles and promoted pregnancy rate concomitant with an increase in the number of embryos at various developmental stages compared with those of experimental controls. Similar treatment to stressed fish caused an increase in the number of stages I-V follicles compared with those in stress alone group. The GnRH fibres showed increased immunolabelling in stress + DOM treated fish compared with stress alone fish. On the other hand, TH-immunoreactivity in the POA and the NPO regions was reduced in stress + DOM treated fish compared with stress-alone group. These results suggest that stress inhibits follicular development and subsequent hatching success through the suppression of GnRH and that the inhibition appears to be mediated through dopamine, for the first time in a viviparous fish.

How fish eggs are preadapted for the evolution of matrotrophy

Teleost fishes evolved livebearing via egg retention 14 times. Matrotrophy has evolved within 12 of those lineages. By contrast, squamate reptiles evolved livebearing over 115 times, but only two to four of those lineages are known to have evolved matrotrophy. Is the discrepancy between these organisms in the probability of this transition caused by differences in their eggs? We show that the eggs of oviparous species in the superorder Atherinomorpha can acquire small organic molecules from their surrounding environment against a concentration gradient via mechanisms of active transport. Uptake rates were inhibited by competing radiolabelled amino acids against unlabelled versions of themselves. Transport was non-specific as uptake rates were similar for l-leucine and its biologically uncommon enantiomer d-leucine. Eggs are also capable of transporting larger microspheres across the membrane, but transport is inhibited at temperatures below 4°C, suggesting active transport occurs via pinocytosis. Conflict theory predicts that the ability of the egg to acquire maternal resources will facilitate the embryo-parent arms race that leads to the evolution of matrotrophy following the transition to livebearing. The shelled eggs of amniotes lack such access to maternal resources when retained in the evolution of viviparity.