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Shao, Yi-Ta, Ho, Hsaun-Ching, Yang, Chien-Hui, Chen, I-Shiung (2022): Special Volume of Marine and Freshwater Biodiversity and Ecological Conservation. Zootaxa 5189 (1): 1-2, DOI: https://doi.org/10.11646/zootaxa.5189.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5189.1.1

Male red nuptial coloration is a primary mating signal for three-spined sticklebacks (Gasterosteus aculeatus), and the retinae of both sexes are especially sensitive to red during the breeding season. Red sensitivity is an important aspect of female mate choice in this species, but only when they are ready to spawn and not over the entire breeding period. Here, we aimed to determine if the red sensitivity of female sticklebacks change over their repeat spawning cycle. To this end, we assessed retinal opsin mRNA levels and behavioral red sensitivity in females over this cycle. Both methods indicated that females were more sensitive to red during spawning than in the inter-spawning intervals. Relative expression levels of red color opsin genes (lws) and optical motor sensitivity were high during spawning, decreased after the spawning period, and then increased again 72–96 h later when they were ready to spawn again. Thus, female sticklebacks altered their color sensitivity according to need, but the underlying mechanism remains unclear.

The shallow-water hydrothermal vent system of Kueishan Island has been described as one of the world's most acidic and sulphide-rich marine habitats. The only recorded metazoan species living in the direct vicinity of the vents is Xenograpsus testudinatus, a brachyuran crab endemic to marine sulphur-rich vent systems. Despite the toxicity of hydrogen sulphide, X. testudinatus occupies an ecological niche in a sulphide-rich habitat, with the underlying detoxification mechanism remaining unknown. Using laboratory and field-based experiments, we characterized the gills of X. testudinatus that are the major site of sulphide detoxification. Here sulphide is oxidized to thiosulphate or bound to hypotaurine to generate the less toxic thiotaurine. Biochemical and molecular analyses demonstrated that the accumulation of thiosulphate and hypotaurine is mediated by the sodium-independent sulphate anion transporter (SLC26A11) and taurine transporter (Taut), which are expressed in gill epithelia. Histological and metagenomic analyses of gill tissues demonstrated a distinct bacterial signature dominated by Epsilonproteobacteria. Our results suggest that thiotaurine synthesized in gills is used by sulphide-oxidizing endo-symbiotic bacteria, creating an effective sulphide-buffering system. This work identified physiological mechanisms involving host-microbe interactions that support life of a metazoan in one of the most extreme environments on our planet.

The shallow-water hydrothermal vent system of Kueishan Island has been described as one of the world's most acidic and sulphide-rich marine habitats. The only recorded metazoan species living in the direct vicinity of the vents is Xenograpsus testudinatus, a brachyuran crab endemic to marine sulphur-rich vent systems. Despite the toxicity of hydrogen sulphide, X. testudinatus occupies an ecological niche in a sulphide-rich habitat, with the underlying detoxification mechanism remaining unknown. Using laboratory and field-based experiments, we characterized the gills of X. testudinatus that are the major site of sulphide detoxification. Here sulphide is oxidized to thiosulphate or bound to hypotaurine to generate the less toxic thiotaurine. Biochemical and molecular analyses demonstrated that the accumulation of thiosulphate and hypotaurine is mediated by the sodium-independent sulphate anion transporter (SLC26A11) and taurine transporter (Taut), which are expressed in gill epithelia. Histological and metagenomic analyses of gill tissues demonstrated a distinct bacterial signature dominated by Epsilonproteobacteria. Our results suggest that thiotaurine synthesized in gills is used by sulphide-oxidizing endo-symbiotic bacteria, creating an effective sulphide-buffering system. This work identified physiological mechanisms involving host-microbe interactions that support life of a metazoan in one of the most extreme environments on our planet.

Androgens are important for stimulating male characters, but excessively high levels may suppress the immune system, and in many animals, circulating levels are homeostatically controlled by feedback mechanisms. However, it was shown that there is no compensation of plasma androgen levels in hemi-castrated threespined stickleback Gasterosteus aculeatus. In this study, we investigated to what extent androgens levels are homeostatically regulated in the breeding male three-spined stickleback, and whether aromatization of androgens plays a role in this. To that end, breeding male sticklebacks were either completely castrated, hemicastrated, or sham-operated, and then implanted with different doses of 11-ketoandrostenedione (11KA) and testosterone (T) or with the aromatase inhibitor (AI), fadrozole. Hemi-castration alone diminished androgen levels, while complete castration almost completely removed them. Low doses of 11KA and T increased plasma androgen levels in castrated but not in sham-operated fish. Both low and high doses of 11KA increased plasma 11-ketotestosterone (11KT) in hemi-castrated fish, whereas only the high dose of 11KA did so in sham-operated fish. If aromatization plays a role in homeostatic mechanisms, androgen levels would be expected to rise in sham-operated fish treated with the AI. However, this was not the case. The reduction in plasma androgen levels in fully mature hemi-castrated fish suggests that the remaining testis was unable to further increase its steroidogenesis. However, both 11KA and T treatments increased plasma levels much less in sham-operated fish than in castrated ones, indicating that homeostatic mechanisms are present and act to prevent excessively high plasma androgen levels.

The seasonal reproduction of the three-spined stickleback is stimulated by long day photoperiod. As in other vertebrates, the reproductive system of stickleback is regulated by the brain-pituitary-gonadal (BPG) axis which is largely controlled by feedback effects. Both negative and positive feedback effects on the BPG axis have been found in fish. So far, the roles feedback effects on the BPG axis play in the photoperiodic regulation of seasonal reproduction are still unclear. This thesis focused on the photoperiodic regulation and gonadal feedback effects on the gene expressions of gonadotropin (GtH) and gonadotropin-releasing hormones (GnRH) in the brain and pituitary, and how gonadal feedback regulated the steroid homeostasis in stickleback.Both GnRH2 and GnRH3 mRNA was found in the hypothalamus. Higher expression levels of both GnRH2 and 3 in breeding than in post-breeding males suggested that they are both involved in seasonal reproduction. There was no evidence for a role of GnRH3, which may be the dominating form, in the photoperiodic control of reproduction. However, the polarity of the feedback effect on gnrh3 gene expression may turn from positive to be negative when the males went into post-breeding state. Tapeworm, Schistocephalus solidus, infection inhibited the reproduction of sticklebacks. However, the infection caused higher expression levels of both GnRHs and GtHs genes, which may be due to feedback effect on the BPG axis.Under short day, both lh-β and fsh-β were suppressed by low androgen levels. This negative feedback may inhibit maturation completely, unless a rise of androgens triggers positive feedback under long day. The change in feedback polarity may result in all or nothing maturation. Furthermore, the androgen inhibitory effect on lh-β and fsh-β under short day could be abolished by aromatase inhibitor, which means the estrogen may cause negative feedback in males under short day.There was no compensation effect on plasma androgen level in fully mature hemi-castrated fish. However, both testosterone and 11-ketoandrostenedione treatments increased plasma levels much less in sham-operated fish than in castrated ones, indicating that homeostatic mechanisms are nevertheless present. At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 1: Submitted. Paper 3: Submitted. Paper 4: Submitted.

Androgens are important for stimulating male characters, but excessively high levels may suppress the immune system, and in many animals, circulating levels are homeostatically controlled by feedback mechanisms. However, it was shown that there is no compensation of plasma androgen levels in hemi-castrated threespined stickleback Gasterosteus aculeatus. In this study, we investigated to what extent androgens levels are homeostatically regulated in the breeding male three-spined stickleback, and whether aromatization of androgens plays a role in this. To that end, breeding male sticklebacks were either completely castrated, hemicastrated, or sham-operated, and then implanted with different doses of 11-ketoandrostenedione (11KA) and testosterone (T) or with the aromatase inhibitor (AI), fadrozole. Hemi-castration alone diminished androgen levels, while complete castration almost completely removed them. Low doses of 11KA and T increased plasma androgen levels in castrated but not in sham-operated fish. Both low and high doses of 11KA increased plasma 11-ketotestosterone (11KT) in hemi-castrated fish, whereas only the high dose of 11KA did so in sham-operated fish. If aromatization plays a role in homeostatic mechanisms, androgen levels would be expected to rise in sham-operated fish treated with the AI. However, this was not the case. The reduction in plasma androgen levels in fully mature hemi-castrated fish suggests that the remaining testis was unable to further increase its steroidogenesis. However, both 11KA and T treatments increased plasma levels much less in sham-operated fish than in castrated ones, indicating that homeostatic mechanisms are present and act to prevent excessively high plasma androgen levels.