記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Elsevier BV  

Electric eels (Electrophorus) are renowned for their ability to generate electric discharge, which is used for prey capture and defense. Their electric organs (EOs) are located along the lateral-ventral region of the tail and contain electrocytes, which are multinucleated syncytium cells. The developmental origin of the electrocyte is mesodermal lineage cells observed in the ventral part of the myotome. However, it is unclear whether these precursor cells are also maintained in later stages and contribute to electric organ growth in adulthood. In this study, we report regional differences in cell morphology within the main EO (mEO) and identify a candidate cell population for electrocyte progenitors at the juvenile stages of Electrophorus. The cell morphology and distribution from the ventral terminal to the dorsal region of the mEO suggest the segregation of progenitors from the ventral cluster and their gradual transformation into mature multinucleated electrocytes via cell fusion and layering along the dorsal axis. Immunohistochemistry revealed strong expression of sodium-potassium adenosine triphosphatase (Na+/K+-ATPase), a key component in generating electric discharge in the mEO, across most mEO regions, except in the ventral cluster cells. Based on these observations, we propose that electrocyte progenitors develop from ventral cluster cells in the mEO and differentiate into mature multinucleated cells as they migrate dorsally. This is the first report to approach the developmental process of Electrophorus electrocytes from cell morphology and genetic profiles. Our findings represent a breakthrough in understanding the differentiation of electrocytes during the growth stages of Electrophorus.

DOI： 10.1016/j.ydbio.2025.05.003

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担当区分：最終著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Wiley  

Fish exhibit a diverse array of reproductive strategies adapted to various ecological niches. Parental egg-care, including live-bearing, mouth-brooding, and male egg protection by brood pouches, represents an effective strategy for ensuring larval survival and has emerged independently in multiple lineages. Despite the recognised evolutionary bias that favours a strategy transition from non-carer to egg-carer, the genetic mechanisms underlying this bias and the commonalities among parental egg-care species remain elusive. This study explores the relationship between egg-care and the chorion hardening system crucial for protecting eggs in non-care species. By analysing whole genome sequences of 240 species of Acanthopterygii across 25 orders, we discovered that multiple genes associated with chorion hardening have become pseudogenes in various egg-care species, indicating a collapse of the chorion hardening system in these fish. These findings suggest that the evolutionary bias in fish reproductive strategies not only aims to enhance survival efficiency but also imposes a constraint on egg-care species, preventing them from reverting to a reproductive strategy relying on a hardened chorion. In particular, alveolin, previously characterised as a single mutant resulting in significantly fragile chorion in medaka, suggests a strong correlation between egg-care strategy and gene loss. Our results suggest an evolutionary dead-end because gene loss may impose an evolutionary constraint at the behavioural level. The observed association between gene loss and reproductive strategies provides insights into suitable reproductive environments for each species and may facilitate non-invasive estimation of reproductive strategies in species with unknown breeding strategies.

DOI： 10.1111/mec.17816

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担当区分：最終著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）  

Cichlids that have undergone adaptive radiation are genetically close but exhibit extreme ecological and morphological diversity, making them useful for understanding speciation mechanisms. Vomeronasal type 1 receptors (V1R) are highly conserved among teleost fish at the amino acid sequence level and are believed to play a fundamental role in reproduction. We previously reported the surprisingly high sequence diversity of V1Rs among certain cichlid species, suggesting a possible role for V1Rs in their speciation. In this study, we investigated the process of evolutionary diversification of all 6 V1Rs (V1R1-6) by using the genome data of 528 cichlid species, encompassing nearly all lineages. In the case of V1R2, two highly divergent alleles (1.17%: variant sites/coding sequence [CDS] length) without recombination were preserved and shared among cichlids found in all of the East African Great Lakes. In the case of V1R6, numerous highly variable alleles that could be derived from multiple recombination events between two highly divergent alleles (1.39%: variant sites/CDS length) were found among the Lake Victoria cichlids. Additionally, we identified highly divergent alleles of V1R1 within the tribe Tropheini, and of both V1R3 and V1R6 within Trematocarini and Ectodini. However, despite extensive investigations, we could not identify the source lineages for these introgressions, implying that they may have become extinct. This study revealed the potential role of introgression in explaining the remarkable diversity of V1Rs in East African cichlids.

DOI： 10.1002/ece3.71467

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担当区分：最終著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）  

Cichlid fishes inhabiting the East African Great Lakes, Victoria, Malawi, and Tanganyika, are textbook examples of parallel evolution, as they have acquired similar traits independently in each of the three lakes during the process of adaptive radiation. In particular, 'hypertrophied lip' has been highlighted as a prominent example of parallel evolution. However, the underlying molecular mechanisms remain poorly understood. In this study, we conducted an integrated comparative analysis between the hypertrophied and normal lips of cichlids across three lakes based on histology, proteomics, and transcriptomics. Histological and proteomic analyses revealed that the hypertrophied lips were characterized by enlargement of the proteoglycan-rich layer, in which versican and periostin proteins were abundant. Transcriptome analysis revealed that the expression of extracellular matrix-related genes, including collagens, glycoproteins, and proteoglycans, was higher in hypertrophied lips, regardless of their phylogenetic relationships. In addition, the genes in Wnt signaling pathway, which is involved in promoting proteoglycan expression, was highly expressed in both the juvenile and adult stages of hypertrophied lips. Our comprehensive analyses showed that hypertrophied lips of the three different phylogenetic origins can be explained by similar proteomic and transcriptomic profiles, which may provide important clues into the molecular mechanisms underlying phenotypic parallelisms in East African cichlids.

DOI： 10.7554/eLife.99160

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担当区分：責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）  

Ancient vomeronasal receptor type-1 (ancV1R), a putative vomeronasal receptor, is highly conserved across a wide range of vertebrates and is expressed in the majority of vomeronasal sensory neurons, co-expressing with canonical vomeronasal receptors, V1Rs and V2Rs. The pseudogenization of ancV1R is closely associated with vomeronasal organ (VNO) degeneration, indicating its critical role in pheromone sensing. However, the specific role of ancV1R remains unknown. In this study, to elucidate the function of ancV1R, we conducted phenotypic analyses of ancV1R-deficient female mice. Behavioral analyses showed that ancV1R-deficient females exhibited rejective responses toward male sexual behavior and displayed no preference for male urine. Physiological analyses demonstrate that the loss-of-function mutation of ancV1R reduced VNO response to various pheromone cues, including male urine, the sexual enhancing pheromone exocrine gland-secreting peptide 1 (ESP1), and β-estradiol 3-sulfate. Pre-exposure to ESP1 did not overcome the rejection behavior caused by ancV1R deficiency. Analysis of neural activity in the vomeronasal system revealed increased responses in the medial amygdala and posteromedial cortical amygdala of mutant females upon contact with males but not in response to male urine alone. Additionally, upon male contacts, ancV1R-deficient females exhibited increased neural activity in the lateral septum, a stress-associated brain region, along with elevated stress hormone levels. Such effects were not observed in females exposed solely to male urine. These findings suggest that, in females, ancV1R facilitates VNO responses to pheromone stimuli and plays a crucial role in perceiving males as mating partners. The absence of ancV1R results in failure of male perception, leading to abnormal sexual behaviors and stress responses upon male contact.

DOI： 10.1016/j.cub.2024.10.077

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

The olfactory organ of tetrapods, with few exceptions, comprises the main and accessory organs: olfactory epithelium (OE) and vomeronasal organ (VNO). Unlike tetrapods, teleost fish lack a VNO. However, lungfish, a type of sarcopterygian fish closely related to tetrapods, possesses a lamellar OE similar to the OE of teleosts and a recess epithelium (RecE) resembling the amphibian VNO. The RecE has been hypothesized as a primordial VNO. Olfactory receptors in tetrapods are distinctively expressed in the OE and VNO. For instance, type 2 vomeronasal receptors (V2Rs) in Xenopus are categorized into those exclusively expressed in the OE and those solely expressed in the VNO. It remains unclear whether V2Rs are differentially expressed between the lamellar OE and RecE in lungfish. This study investigated V2R expression in the lamellar OE and RecE of the African lungfish, Protopterus annectens. P. annectens V2Rs were categorized into three groups: those exclusively expressed in the lamellar OE, those exclusively expressed in the RecE, and those expressed in both the lamellar OE and RecE. V2Rs exclusively expressed in the RecE and those expressed in both the lamellar OE and RecE formed a distinct clade in the phylogenetic tree, whereas others were solely expressed in the lamellar OE. These findings suggest that lungfish V2R expression represents an intermediate stage toward complete segregation between V2Rs expressed in the OE and those expressed in the VNO.

DOI： 10.1007/s00441-024-03918-2

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担当区分：最終著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Oxford University Press (OUP)  

Abstract

Within 15,000 years, the explosive adaptive radiation of haplochromine cichlids in Lake Victoria, East Africa, generated 500 endemic species. In the 1980s, the upsurge of Nile perch, a carnivorous fish artificially introduced to the lake, drove the extinction of more than 200 endemic cichlids. The Nile perch predation particularly harmed piscivorous cichlids, including paedophages, cichlids eat eggs and fries, which is an example of the unique trophic adaptation seen in African cichlids. Here, aiming to investigate past demographic events possibly triggered by the invasion of Nile perch and the subsequent impacts on the genetic structure of cichlids, we conducted large-scale comparative genomics. We discovered evidence of recent bottleneck events in four species, including two paedophages, which began during the 1970s–1980s, and population size rebounded during the 1990s-2000s. The timing of the bottleneck corresponded to the historical records of endemic haplochromines’ disappearance and later resurgence, which is likely associated with the introduction of Nile perch by commercial demand to Lake Victoria in the 1950s. Interestingly, among the four species that likely experienced bottleneck, Haplochromis sp. ‘matumbi hunter,’ a paedophagous cichlid, showed the most severe bottleneck signatures. The components of shared ancestry inferred by ADMIXTURE suggested a high genetic differentiation between matumbi hunter and other species. In contrast, our phylogenetic analyses highly supported the monophyly of the five paedophages, consistent with the results of previous studies. We conclude that high genetic differentiation of matumbi hunter occurred due to the loss of shared genetic components among haplochromines in Lake Victoria caused by the recent severe bottleneck.

DOI： 10.1093/molbev/msae093

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掲載種別：研究論文（学術雑誌）   出版者・発行元：MDPI AG  

Baleen whales (Mysticeti) possess the necessary anatomical structures and genetic elements for olfaction. Nevertheless, the olfactory receptor gene (OR) repertoire has undergone substantial degeneration in the cetacean lineage following the divergence of the Artiodactyla and Cetacea. The functionality of highly degenerated mysticete ORs within their olfactory epithelium remains unknown. In this study, we extracted total RNA from the nasal mucosae of common minke whales (Balaenoptera acutorostrata) to investigate ORs’ localized expression. All three sections of the mucosae examined in the nasal chamber displayed comparable histological structure. However, the posterior portion of the frontoturbinal region exhibited notably high OR expression. Neither the olfactory bulb nor the external skin exhibited the expression of these genes. Although this species possesses four intact non-class-2 ORs, all the ORs expressed in the nasal mucosae belong to class-2, implying the loss of aversion to specific odorants. These anatomical and genomic analyses suggest that ORs are still responsible for olfaction within the nasal region of baleen whales, enabling them to detect desirable scents such as prey and potential mating partners.

DOI： 10.3390/ijms25073855

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