Differentiation of ambisexual gonads and immunohistochemical localization of P450 cholesterol side-chain cleavage enzyme during gonadal sex differentiation in the protandrous anemonefish, Amphiprion clarkii

Anemonefishes: A model system for evolutionary genomics

Anemonefishes are an iconic group of coral reef fish particularly known for their mutualistic relationship with sea anemones. This mutualism is especially intriguing as it likely prompted the rapid diversification of anemonefish. Understanding the genomic architecture underlying this process has indeed become one of the holy grails of evolutionary research in these fishes. Recently, anemonefishes have also been used as a model system to study the molecular basis of highly complex traits such as color patterning, social sex change, larval dispersal and life span. Extensive genomic resources including several high-quality reference genomes, a linkage map, and various genetic tools have indeed enabled the identification of genomic features controlling some of these fascinating attributes, but also provided insights into the molecular mechanisms underlying adaptive responses to changing environments. Here, we review the latest findings and new avenues of research that have led to this group of fish being regarded as a model for evolutionary genomics.

Switches, stability and reversals in the evolutionary history of sexual systems in fish

Sexual systems are highly diverse and have profound consequences for population dynamics and resilience. Yet, little is known about how they evolved. Using phylogenetic Bayesian modelling and a sample of 4614 species, we show that gonochorism is the likely ancestral condition in teleost fish. While all hermaphroditic forms revert quickly to gonochorism, protogyny and simultaneous hermaphroditism are evolutionarily more stable than protandry. In line with theoretical expectations, simultaneous hermaphroditism does not evolve directly from gonochorism but can evolve slowly from sequential hermaphroditism, particularly protandry. We find support for the predictions from life history theory that protogynous, but not protandrous, species live longer than gonochoristic species and invest the least in male gonad mass. The distribution of teleosts' sexual systems on the tree of life does not seem to reflect just adaptive predictions, suggesting that adaptations alone may not fully explain why some sexual forms evolve in some taxa but not others (Williams' paradox). We propose that future studies should incorporate mating systems, spawning behaviours, and the diversity of sex determining mechanisms. Some of the latter might constrain the evolution of hermaphroditism, while the non-duality of the embryological origin of teleost gonads might explain why protogyny predominates over protandry in teleosts.

The possible role of catalase in innate immunity and diminution of cellular oxidative stress: Insights into its molecular characteristics, antioxidant activity, DNA protection, and transcriptional regulation in response to immune stimuli in yellowtail clownfish (Amphiprion clarkii)

Catalase, a key enzyme in the antioxidant defense grid of organisms, scavenges free radicals to curtail their harmful effects on the host, supporting proper immune function. Herein, we report the identification and characterization of a catalase homolog from Amphiprion clarkii (ClCat), followed by its functional characterization. An open reading frame was identified in the cDNA sequence of ClCat at 1581 bp, which encodes a protein of 527 amino acids (aa) with a molecular mass of 60 kDa. In silico analyses of ClCat revealed characteristic features of the catalase family and a lack of a signal peptide. Multiple sequence alignment of ClCat indicated the conservation of functionally important residues among its homologs. According to phylogenetic analysis, ClCat was of vertebrate origin, positioned within the teleost clade. During native conditions, ClCat mRNA was highly expressed in blood, followed by the liver and kidney. Moreover, significant changes in ClCat transcription were observed after stimulation with LPS, poly I:C, and Vibrio harveyi, in a time-dependent manner. Recombinant ClCat (rClCat) was characterized, and its peroxidase activity was determined. Furthermore, the optimum temperature and pH for rClCat were determined to be 30-40 °C and pH 7, respectively. Oxidative stress tolerance and chromatin condensation assays indicated enhanced cell survival and reduced apoptosis, resulting from reactive oxygen species scavenging by rClCat. The DNA-protective function of rClCat was further confirmed via a metal-catalyzed oxidation assay. Taken together, our findings propose that rClCat plays an essential role in maintaining cellular oxidative homeostasis and host immune protection.

Steroids in teleost fishes: A functional point of view

Steroid hormones are involved in the regulation of a variety of processes like embryonic development, sex differentiation, metabolism, immune responses, circadian rhythms, stress response, and reproduction in vertebrates. Teleost fishes and humans show a remarkable conservation in many developmental and physiological aspects, including the endocrine system in general and the steroid hormone related processes in particular. This review provides an overview of the current knowledge about steroid hormone biosynthesis and the steroid hormone receptors in teleost fishes and compares the findings to the human system. The impact of the duplicated genome in teleost fishes on steroid hormone biosynthesis and perception is addressed. Additionally, important processes in fish physiology regulated by steroid hormones, which are most dissimilar to humans, are described. We also give a short overview on the influence of anthropogenic endocrine disrupting compounds on steroid hormone signaling and the resulting adverse physiological effects for teleost fishes. By this approach, we show that the steroidogenesis, hormone receptors, and function of the steroid hormones are reasonably well understood when summarizing the available data of all teleost species analyzed to date. However, on the level of a single species or a certain fish-specific aspect of physiology, further research is needed.

Estrogen favors the differentiation of ovarian tissues in the ambisexual gonads of anemonefish Amphiprion clarkii

All undifferentiated gonads of anemonefish first differentiate into ovaries, and then testicular tissue appear among ovarian tissue, and finally form ambisexual gonads with both ovarian and testicular tissues. The role of estradiol-17beta (E2) in differentiation of ovarian cells is well conserved across phyla; however, its role in development of ambisexual gonads is poorly understood. Here we demonstrate that the E2 produced during the differentiation of ovarian cells does not allow testicular cells to differentiate in the prospective ambisexual gonad. We examined the immunolocalization of the steroidogenic enzyme cytochrome aromatase (P450(arom)), which is involved in E2 production. In the gonads, numbers of the P450(arom) -positive cells increased during ovarian differentiation. However, immunopositive cells with weak signal intensity were seen in the interstitial areas among oocytes and between oocytes and testicular tissue undergoing testicular differentiation. In contrast, P450(arom) -positive cells were not found in any testicular tissues of the ambisexual gonads. We also examined changes in E2 production in vitro in the gonads during testicular differentiation. E2 was high in the ovaries before the appearance all of testicular tissue, and decreased accompanying the differentiation of testicular tissue. These results suggest a balance of estrogen/androgen seems to be important during sex differentiation, and then a shift from estrogen to androgen production may induce testicular differentiation in the ovary. Further, exogenous E2 treatment suppressed naturally occurring differentiation of testicular cells forming exclusively ovarian tissues in the gonad in vivo, suggesting the increase of estrogen blocks the differentiation of testicular tissue and the formation of ambisexual gonad.

Chromera velia is endosymbiotic in larvae of the reef corals Acropora digitifera and A. tenuis

Scleractinian corals occur in symbiosis with a range of organisms including the dinoflagellate alga, Symbiodinium, an association that is mutualistic. However, not all symbionts benefit the host. In particular, many organisms within the microbial mucus layer that covers the coral epithelium can cause disease and death. Other organisms in symbiosis with corals include the recently described Chromera velia, a photosynthetic relative of the apicomplexan parasites that shares a common ancestor with Symbiodinium. To explore the nature of the association between C. velia and corals we first isolated C. velia from the coral Montipora digitata and then exposed aposymbiotic Acropora digitifera and A. tenuis larvae to these cultures. Three C. velia cultures were isolated, and symbiosis was established in coral larvae of both these species exposed to all three clones. Histology verified that C. velia was located in the larval endoderm and ectoderm. These results indicate that C. velia has the potential to be endosymbiotic with coral larvae.

Diversity and plasticity of sex determination and differentiation in fishes

Among vertebrates, fishes show an exceptional range of reproductive strategies regarding the expression of their sexuality. Fish sexualities were categorized into gonochorism, synchronous/sequential hermaphrodite, or unisexual reproduction. In gonochoristic fishes, sex is determined genetically or by environmental factors. After sex determination, the gonads are differentiated into ovary or testis, with the sex remaining fixed for the entire life cycle. In contrast, some sequential hermaphrodite fishes can change their sex from male to female (protandrous), female to male (protogynous), or serially (bi-directional sex change) in their life cycle. In many cases, sex change is cued by social factors such as the disappearance of a male or female from a group. This unique diversity in fishes provides an ideal animal model to investigate sex determination and differentiation in vertebrates. This review first discusses genetic-orientated sex determination mechanisms. Then, we address the gonadal sex differentiation process in a gonochoristic fish, using an example of the Nile tilapia. Finally, we discuss various types of sex change that occur in hermaphrodite fishes.

Immunohistochemical evidence for 11beta-hydroxylase (P45011beta) and androgen production in the gonad during sex differentiation and in adults in the protandrous anemonefish Amphiprion clarkii

To obtain basic information on the endocrine mechanisms underlying sex change in the protandrous anemonefish Amphiprion clarkii, we examined the immunolocalization of the steroidogenic enzyme cytochrome 11beta-hydroxylase (P45011beta), which is involved in 11-ketotestosterone (11-KT) production, and analyzed the ability of gonads to produce steroid hormones throughout the sex differentiation process and at the breeding stage. Immunopositive reactions against P45011beta appeared in sexually undifferentiated gonads at 30 days post hatching (dph). The number of immunopositive cells continued to increase during ovarian differentiation (from 60 to 180 dph) and throughout the formation of ambisexual gonads with both ovarian and testicular tissue until 270 dph. In the male phase, strongly immunopositive cells were observed in the cellular interstices of both testicular and ovarian tissues. P45011beta was localized only in the theca cells enclosing developed oocytes in the female phase. In-vitro 11-KT production in the gonads gradually increased with testicular differentiation (before, during, and after differentiation). Production of 11-KT in the gonads was higher in the male phase than during testicular differentiation or in the female phase. Our results suggest that androgen is involved in testicular differentiation during sex differentiation and spermatogenesis.