Rest Of Vertebrates Research Articles

Extraordinary variability in gene activation and repression programs during gonadal sex differentiation across vertebrates.

Genes involved in gonadal sex differentiation have been traditionally thought to be fairly conserved across vertebrates, but this has been lately questioned. Here, we performed the first comparative analysis of gonadal transcriptomes across vertebrates, from fish to mammals. Our results unambiguously show an extraordinary overall variability in gene activation and repression programs without a phylogenetic pattern. During sex differentiation, genes such as dmrt1, sox9, amh, cyp19a and foxl2 were consistently either male- or female-enriched across species while many genes with the greatest expression change within each sex were not. We also found that downregulation in the opposite sex, which had only been quantified in the mouse model, was also prominent in the rest of vertebrates. Finally, we report 16 novel conserved markers (e.g., fshr and dazl) and 11 signaling pathways. We propose viewing vertebrate gonadal sex differentiation as a hierarchical network, with conserved hub genes such as sox9 and amh alongside less connected and less conserved nodes. This proposed framework implies that evolutionary pressures may impact genes based on their level of connectivity.

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

Viviparity in teleosts involves, invariably, the ovary in a gestational role. This type of viviparity is due to the combination of unique aspects, different from those found in the rest of vertebrates. These aspects are: The ovary has a saccular structure; the germinal epithelium lines the ovarian lumen; the absence of oviducts; and the intraovarian insemination, fertilization, and gestation. The communication of the germinal zone of the ovary to the exterior is via the caudal zone of the ovary—the gonoduct. The germinal epithelium is composed of oogonia and oocytes scattered individually or in cell nests among somatic epithelial cells. In the ovarian stroma the follicles are included which are formed by the oocyte, which is surrounded by follicular cells and the vascularized theca. The oogenesis comprises three stages: chromatin-nucleolus, previtellogenesis, and vitellogenesis. There is no ovulation, as the oocyte is retained in the follicle. During the insemination, the spermatozoa enter into the ovarian lumen and the intrafollicular fertilization occurs, followed by intrafollicular gestation. The intraovarian gestation of poeciliids involves morphological characteristics associated with the intrafollicular embryogenesis and types of nutrition, such as lecithotrophy and matrotrophy. In lecithotrophy, the nutrients come from the yolk reserves stored during oogenesis, whereas in matrotrophy the nutrients are provided by supplies from maternal tissues to the embryo during gestation. The maternal–embryonic metabolic interchanges converge through the development of the association of maternal and embryonic blood vessels, establishing a follicular placenta.

Change of lecithotrophic to matrotrophic nutrition during gestation in the viviparous teleost Xenotoca eiseni (Goodeidae).

Teleosts possess unique features of the female reproductive system compared with the rest of vertebrates, features that define the characteristics of their viviparity. Viviparity involves new maternal-embryonic relationships detailing the most diverse structures during gestation that include embryonic nutrition. In order to analyze the morphological features of the complex nutrition in viviparous teleosts during intraovarian gestation, this study utilizes the goodeid Xenotoca eiseni as a model. Ovarian gestation in X. eiseni, as in all goodeids, is intraluminal; the early embryo moves from the follicle to the ovarian lumen where gestation continues. The scarce yolk in the oocytes implies that the initial lecithotrophy is replaced by matrotrophy, with nutrients provided via maternal tissues. The nutrients are absorbed by the embryo mainly by trophotaenia, extensions of the embryonic intestine into the ovarian lumen. This histological study analyses the structures involved in these two types of nutrition and when they occur during gestation in X. eiseni. The morphology displayed in this study demonstrated the extended simultaneity of lecithotrophy and matrotrophy during gestation with the progressive reduction of lecithotrophy and increase of matrotrophy. Similarly, it describes the development of complex embryonic structures for metabolic exchange with the maternal tissues associated with matrotrophy; specifically the branchial placenta and mainly the trophotaenia.

Comparative morphology and development of extra-ocular muscles in the lamprey and gnathostomes reveal the ancestral state and developmental patterns of the vertebrate head.

The ancestral configuration of the vertebrate head has long been an intriguing topic in comparative morphology and evolutionary biology. One peculiar component of the vertebrate head is the presence of extra-ocular muscles (EOMs), the developmental mechanism and evolution of which remain to be determined. The head mesoderm of elasmobranchs undergoes local epithelialization into three head cavities, precursors of the EOMs. In contrast, in avians, these muscles appear to develop mainly from the mesenchymal head mesoderm. Importantly, in the basal vertebrate lamprey, the head mesoderm does not show overt head cavities or signs of segmental boundaries, and the development of the EOMs is not well described. Furthermore, the disposition of the lamprey EOMs differs from those the rest of vertebrates, in which the morphological pattern of EOMs is strongly conserved. To better understand the evolution and developmental origins of the vertebrate EOMs, we explored the development of the head mesoderm and EOMs of the lamprey in detail. We found that the disposition of lamprey EOM primordia differed from that in gnathostomes, even during the earliest period of development. We also found that three components of the paraxial head mesoderm could be distinguished genetically (premandibular mesoderm: Gsc+/TbxA–; mandibular mesoderm: Gsc–/TbxA–; hyoid mesoderm: Gsc–/TbxA+), indicating that the genetic mechanisms of EOMs are conserved in all vertebrates. We conclude that the tripartite developmental origin of the EOMs is likely to have been possessed by the latest common ancestor of the vertebrates. This ancestor’s EOM developmental pattern was also suggested to have resembled more that of the lamprey, and the gnathostome EOMs’ disposition is likely to have been established by a secondary modification that took place in the common ancestor of crown gnathostomes.Electronic supplementary materialThe online version of this article (doi:10.1186/s40851-016-0046-3) contains supplementary material, which is available to authorized users.

What we can learn from hagfish embryology

The cyclostomes, or hagfishes and lampreys, represent the most basal lineage of vertebrates, providing a key to understand craniofacial evolution. While the monophyly of cyclostomes has been supported by recent molecular analyses, phenotypic traits of the hagfish, especially the lack of some vertebrate‐defining features, as well as endodermal origin of the adenohypophysis, have been interpreted as exhibiting more ancestral state than the rest of vertebrates. It is also true that adult craniofacial anatomy of this animal cannot be compared easily with that of the lamprey. To clarify this, the craniofacial developmental pattern of staged embryos of a Japanese inshore hagfish, Eptatretus burgeri, will be described to show that their adenohypophysis arises ectodermally as in other vertebrates, consistent with the molecular phylogenetic data. The above finding also allowed us to identify a pan‐cyclostome embryonic pattern, not shared by crown gnathostomes, and comparative analyses implied that many of the hagfish‐specific traits can be explained by changes secondarily introduced into the hagfish lineage. Since the craniofacial morphology of some basal gnathostomes retains some cyclostome‐like features, we conclude that the pan‐cyclostome pattern would likely reflect the ancestral program for the craniofacial development for the entire living vertebrates.