Biology subjects

Hirohashi, N.

Publications and source records attributed to Hirohashi, N..

6 recordsLinked to original sources

bioRxiv2026

Intra-male reduction in sperm size persists during growth in cuttlefish despite the absence of insemination-site dimorphism.

In certain groups of squids and cuttlefish, males display alternative reproductive tactics (ARTs). In squids, size-associated male dimorphism appears in mating posture, spermatophore transfer site (insemination dimorphism), and sperm flagellar length (sperm dimorphism). Sperm dimorphism is closely linked to insemination dimorphism, in which sperm are deposited either at the females external buccal mass or within her mantle cavity. Insemination dimorphism shapes post-copulatory sperm environments, including the mode of storage, the risk of sperm competition, and the site of fertilization. Therefore, sperm dimorphism is regarded as an adaptive consequence of insemination dimorphism. Conversely, in cuttlefish, both large consorts and small female-mimicking (sneaker) males deposit spermatophores in the same region of the female buccal mass, indicating the absence of insemination dimorphism. In Sepia esculenta and Sepia lycidas, sperm located at the distal end of the male reproductive tract possess longer flagella than those near the testis. This pattern is consistent across all male sizes, suggesting that as males grow, they produce sperm with shorter flagella. Additionally, these within-individual differences in sperm size are greater in smaller males and are positively correlated with relative testis mass. These findings indicate that insemination dimorphism is not required for the evolution of dimorphic sperm in cuttlefish. In both squids and cuttlefish, sperm from smaller males must either enter the female receptacle for extended storage or swim faster to compete with the more abundant consort sperm. These unfavorable conditions associated with the sneaker tactic may drive costly sperm evolution.

animal behavior and cognition↗

bioRxiv2025

Synchronous and asynchronous counterillumination by three types of photophores in the firefly squid, Watasenia scintillans

Countershading--animal body coloration that is darker on the dorsum than the ventrum--is a form of crypsis achieved through a gradient in skin pigmentation. Counterillumination is somewhat similar, achieving crypsis in midwater marine organisms by emitting downward-directed bioluminescence. Although mesopelagic squids are assumed to use counterillumination, high-resolution spatiotemporal changes in illumination, particularly for each photophore type, have not been elucidated. Watasenia scintillans has three types of photophores on its ventral surfaces: blue- and green-emitting ventral photophores and ocular photophores. It is demonstrated here that all three types of photophores are synchronously, reversibly, and repeatedly involved in counterillumination in response to changes in dim overhead lighting. Furthermore, the green-emitting photophores maintained luminescence in darkness, and their on/off switching is precisely regulated within the ambient temperature range. The photoreceptors responsible for counterillumination are extraocular photosensitive vesicles (EOPs) rather than eyes. Exposure of EOPs to visible light (blue, green, and yellow) elicited counterillumination in proportion to light intensity. No color shifts (such as from green to blue) were observed in any of the photophores. While synchronous illumination seems to support the crypsis hypothesis, the ability to independently control each photophore type could also support a previous hypothesis of conspecific signaling. Significance StatementBioluminescence is widespread among deep-sea organisms, yet the specific functions of individual light-producing organs distributed across the body surface remain underexplored. In this study, three types of ventral photophores in the firefly squid, Watasenia scintillans, were video-recorded, revealing their synchronous involvement in counterillumination--a mechanism that obscures the body silhouette when viewed from below against blue downwelling surface light. Notably, green-emitting photophores remained illuminated for extended periods in darkness, indicating a potential role in visual communication among conspecifics. The video-based techniques employed to quantify individual photophore activity provide new opportunities to investigate photo-signaling in animals inhabiting the mesopelagic zone.

animal behavior and cognition↗

bioRxiv2021

Inhibiting SARS-CoV-2 infection in vitro by suppressing its receptor, angiotensin-converting enzyme 2, via aryl-hydrocarbon receptor signal

Since understanding molecular mechanisms of SARS-CoV-2 infection is extremely important for developing effective therapies against COVID-19, we focused on the internalization mechanism of SARS-CoV-2 via ACE2. Although cigarette smoke is generally believed to be harmful to the pathogenesis of COVID-19, cigarette smoke extract (CSE) treatments were surprisingly found to suppress the expression of ACE2 in HepG2 cells. We thus tried to clarify the mechanism of CSE effects on expression of ACE2 in mammalian cells. Because RNA-seq analysis suggested that suppressive effects on ACE2 might be inversely correlated with induction of the genes regulated by aryl hydrocarbon receptor (AHR), the AHR agonists 6-formylindolo(3,2-b)carbazole (FICZ) and omeprazole (OMP) were tested to assess whether those treatments affected ACE2 expression. Both FICZ and OMP clearly suppressed ACE2 expression in a dose-dependent manner along with inducing CYP1A1. Knock-down experiments indicated a reduction of ACE2 by FICZ treatment in an AHR-dependent manner. Finally, treatments of AHR agonists inhibited SARS-CoV-2 infection into Vero E6 cells as determined with immunoblotting analyses detecting SARS-CoV-2 specific nucleocapsid protein. We here demonstrate that treatment with AHR agonists, including CSE, FICZ, and OMP, decreases expression of ACE2 via AHR activation, resulting in suppression of SARS-CoV-2 infection in mammalian cells.

molecular biology↗

bioRxiv2020

Context-dependent behavioral plasticity compromises disruptive selection on sperm traits in squid

Male animals are not given equal mating opportunities under competitive circumstances. Small males often exhibit alternative mating behaviours and produce spermatozoa of higher quality to compensate for their lower chances of winning physical contests against larger competitors [1]. Because the reproductive benefits of these phenotypes depend on social status/agonistic ranks that can change during growth or aging [2], sperm traits should be developed/switched into fitness optima according to their prospects. However, reproductive success largely relies upon social contexts arising instantaneously from intra- and inter-sexual interactions, which deter males from developing extreme traits and instead favour behavioural plasticity. Nevertheless, the extent to which such plasticity influences developmentally regulated alternative sperm traits remains unexplored. Squids of the family Loliginidae are excellent models to investigate this, because they show sophisticated alternative reproductive tactics (ARTs) by which small males, known as "sneakers", produce longer spermatozoa and perform extra-pair copulation to attach their sperm packages near the female seminal receptacle (SR). In contrast, large "consort" males have shorter spermatozoa and copulate via pair-bonding to insert their sperm packages near the internal female oviduct [3]. In addition, plasticity in male mating behaviour is common in some species while it is either rare or absent in others. Thus, squid ARTs display a broad spectrum of adaptive traits with a complex repertoire in behaviour, morphology and physiology [3].

evolutionary biology↗

bioRxiv2019

Polyandry is extremely rare in the firefly squid, Watasenia scintillans

Although polygamy has versatile benefits for both sexes, many species favor monogamy for reasons with the clarity or unclarity. In cephalopods, all species are regarded to be polygamous, which could be attributed to their common life-history traits. Contrary to this prediction, we show empirical evidence for monogamy in the firefly squid, Watasenia scintillans. The peak spawning season comes after male disappearance owning to long-reserved spermatangia deposited by male at exact locations (bilateral pouches under neck collar) on female with a symmetric distribution. Such a non-random placement of spermatangia prompted us to hypothesize that females engage in lifetime monoandry. Hence we assigned genotypes of female-stored spermatangia and offspring. We found that in 94.7 % females, the spermatangia were delivered from a single male and all embryos in the same egg string sired by sperm from stored spermatangia. Throughout the season, relative testes mass was much smaller in W. scintillans than all other cephalopods previously examined. The mean number of male-stored spermatophores was approximately 30, the equivalent to 2.5 mates. Our demographic and morphometrical data agree with the prediction that monogyny is favored when potential mates are scarce such as absence of female remating. Together, these results suggest the likelihood of mutual monogamy.

ecology↗

bioRxiv2018

SGK regulates meiotic resumption in ovarian starfish oocytes via intracellular pH-increase and cyclin B-Cdk1 activation

Tight regulation of intracellular pH (pHi) is essential for biological processes. Fully-grown oocytes, having a large nucleus called the germinal vesicle, arrest at meiotic prophase-I. Upon hormonal stimulus, oocytes resume meiosis to acquire fertilizability. At this time, pHi increases through Na+/H+ exchanger activity. However, regulation and function of this change remains obscure. Here we show that in starfish oocytes, serum- and glucocorticoid-regulated kinase (SGK) is activated by the PI3K/TORC2/PDK1 signaling after hormonal stimulus, and is required for the pHi increase and cyclin B-Cdk1 activation. Furthermore, when we clamped pHi at 6.7, corresponding to the pHi of unstimulated ovarian oocytes, hormonal stimulus normally induced cyclin B-Cdk1 activation; thereafter, oocytes initiated germinal vesicle breakdown (GVBD), but failed to complete it. Thus, SGK-dependent pHi increase is likely prerequisite for completion of GVBD in ovarian oocytes. We propose a model that SGK drives meiotic resumption through concomitant regulation of pHi and the cell-cycle machinery.

molecular biology↗