Zoological Science最新文献

The Acoela is a notable taxon in terms of the early evolution of bilaterians and the photosynthetic symbiosis between animals and microalgae. There are approximately 416 described species of Acoela worldwide, which are classified into 16 families. In total, 21 species have been reported in Japan, of which five have been reported in the Seto Inland Sea. We surveyed acoels in the intertidal zone of beaches along the Seto Inland Sea coast of Hiroshima Prefecture and collected specimens. A comparison of mitochondrial cytochrome oxidase subunit I (COI) sequences and molecular phylogenetic analysis suggested that they could be divided into 12 species. Molecular phylogenetic analysis using 18S rRNA sequences suggested that these species belonged to five families: Convolutidae, Otocelididae, Dakuidae, Actinoposthiidae, and Isodiametridae. There have been no previous reports of Dakuidae or Actinoposthiidae in Japan and no reports of Isodiametridae in the Seto Inland Sea. These results suggested that the diversity of Acoela in Japan and the Seto Inland Sea is far richer than is currently known.

Daphnids survive unsuitable conditions by producing resting eggs. Resting eggs contribute to population persistence and coexistence by enabling temporal dispersal strategies. Two genotypes of asexual Daphnia cf. pulex (JPN1 and JPN2) differ in the frequency with which they produce resting eggs, despite occurring simultaneously in the same lake. To clarify molecular mechanisms underlying differences in diapause frequency, we hypothesized that juvenile hormone (JH) is involved in resting egg production in Daphnia and that variation in resting egg production between genotypes may reflect differences in responsiveness to JH. We exposed these two D. cf. pulex genotypes to JH analogs and methyl farnesoate (MF), which act on the JH receptor. We found that JH analogs and MF exposure increased resting egg production and did not induce male offspring in the JPN2 genotype. However, in the JPN1 genotype, JH exposure induced male offspring, despite slightly increased resting egg production. Our findings suggest that variation in resting egg production between these two genotypes is partly explained by differences in responsiveness to endogenous JH, potentially influencing population coexistence and dynamics. Finally, we determined that the JH-sensitive period which induces resting egg production occurs about two instars before egg holding. This is slightly earlier than the JH-sensitive period for male production.

In this study, hydrozoan medusae collected from a home aquarium in Sanming, China, were tentatively identified as Cladonema cf. californicum Hyman, 1947, based on DNA barcoding. Morphologically, these specimens differed from known Cladonema medusae, including C. californicum, by possessing a manubrium with protuberances and tentacles with one to three (typically two) adhesive branches, along with three to six (usually four) stinging branches emerging laterally from the main tentacle shaft. These morphological distinctions were supported by molecular phylogenetic analyses using the mitochondrial 16S rRNA and cytochrome c oxidase subunit I (COI) gene sequences. Nonetheless, COI sequences revealed that C. cf. californicum clustered with juvenile Cladonema californicum medusae from California, USA, with a minimal genetic distance of 0.008, indicating potential conspecificity. Given the morphological similarities among juvenile Cladonema medusae, the Californian specimens may have been misidentified. Further investigations focusing on the polyp stage and geographic distribution are necessary to fully resolve the taxonomic status of these medusae and clarify their relationship with C. californicum.

Feather coloration changes over time, but the underlying mechanisms remain poorly understood. In this study, using the reddish throat feathers of the barn swallow, Hirundo rustica, a well-known sexually selected trait, we examined whether within-season feather color change reflected the change of the major pigment, pheomelanin. As predicted, the reddish feathers faded over time, as their coloration became significantly less red with increasing intervals from the initial capture date to recapture date. Likewise, the pheomelanin content of these feathers significantly decreased with increasing intervals between captures. Neither feather color change nor pheomelanin content change depended on study year, sex, or initial capture date. However, feather color change, but not pheomelanin content change, depended on body condition at the initial capture. This condition-dependent feather color change, together with the condition-independent pheomelanin loss, indicates that seasonal change of the major pigment is insufficient to explain the observed patterns of feather color change. Future study should pay attention to additional colorants that may impact the signal honesty of animal coloration.

The molecular cascade of sex determination in insects exhibits remarkable diversity in its upstream factors while conserving the downstream doublesex (dsx) gene across various taxa. dsx encodes a transcription factor that undergoes sex-specific splicing, playing a central role in sex determination. This sex-specific splicing of dsx is generally regulated by the Transformer (Tra) protein. Functional Tra is exclusively expressed in females, governing the female-specific splicing of dsx and inducing feminization. Although the tra gene is widely conserved across various insect species, with the genome typically containing a single copy of the tra gene, it is lost or duplicated in specific taxa, resulting in lineage-specific variations in numbers. Given the flexibility of the tra gene number and meanwhile the deep conservation of its crucial function in sex determination across most investigated taxa, the tra gene has attracted attention as the key gene for evolutionary change in sex-determining mechanisms. In contrast to this conventional insight, unexpectedly, we discovered that the number of tra genes has expanded through gene duplication in stag beetles (Lucanidae, Coleoptera). In addition, we discovered that the tra gene in several species of stag beetles has the same splicing variant in both sexes. Our findings suggest that the tra gene in the Lucanidae family deviates from the basic pattern observed in other insects regarding copy number and expression profiles. The discovery of tra expansion and expression pattern divergence in the family Lucanidae may provide new insights into the evolutionary diversity of the sex differentiation cascade.

Symbiosis with chemosynthetic bacteria is a biological phenomenon that has enabled animals to adapt to deep-sea environments. The deep-sea vent mussel Bathymodiolus septemdierum harbors sulfur-oxidizing symbionts in its gills, which serve as its main source of nutrients. However, the establishment and maintenance of this symbiosis process remain poorly understood, partly because culturing deep-sea mussels and their symbionts is difficult and experimental studies are rare. In the present study, we aimed to establish a rearing method for B. septemdierum and investigate the effects of sodium sulfide (Na2S) addition and gas concentration control in rearing tanks. Three tanks were prepared: a normal tank (tank N), one with Na2S (tank S), and one with Na2S and control dissolved oxygen (tank SO). Mussels were maintained for more than 1000 days, and host survival, gill histology, and the abundance of symbionts were investigated. In tanks N and S, symbionts were depleted within a short period, whereas in tank SO, gill morphology and symbiont abundance were maintained at levels comparable to those of freshly collected individuals, even after 1000 days. However, the survival rate of the host did not correspond to symbiont maintenance. Phylogenetic analysis revealed that the symbiont RuBisCO exists in Form II, which is generally adapted to low-O2 and high-CO2 environments. These results suggest that the long-term maintenance of B. septemdierum symbionts is possible by adding Na2S and controlling the dissolved gas concentration. Further improvements in rearing methods could contribute to a deeper understanding of host-symbiont interactions in this unique deep-sea organism.

A new species of melitid amphipod, Melita pandina sp. nov., is described from the uppermost subtidal and intertidal waters of Osaka and Wakayama Prefectures, Japan. Melita pandina belongs to Group B2, one of the four species groups defined within the genus, but differs from its congeners by the presence of setae on mandibular palp article 1. Molecular phylogenetic analyses indicate that M. pandina sp. nov. and Melita panda Tomikawa, Yamato and Ariyama, 2024-both characterized by distinctive black-and-white disruptive coloration-are not closely related. These results suggest that this color pattern evolved independently at least twice within the Melita genus, indicating a case of convergent evolution.

The Oriental stork Ciconia boyciana is a large endangered migratory bird. Since the extinction of the wild Japanese population, conservation programs including captive-breeding and reintroduction have been conducted. The current Japanese population consisting of captive, released, and wild-born individuals originates from 25 founders provided from Russia and China, and one wild individual migrated from the Eurasian continent. Understanding their genetic diversity is critical for effective population management. The major histocompatibility complex (MHC) plays a central role in the vertebrate immune system. Especially, polymorphisms of MHC-IA and -IIA/IIB genes, which encode antigen-presenting molecules, are remarkable and could represent the genetic diversity of population. We report the diversity of the MHC region in the Japanese population of C. boyciana. Sixty-seven individuals, including 10 founders, were used for the analysis. Genotyping of two MHC-IIB genes (DAB and DBB loci) revealed 15 and nine alleles of exon 2s, respectively. The number of MHC class II haplotypes determined by the combination of two alleles of exon 2s in DAB and DBB was 23. Genotyping of the MHC-IA gene (UAA locus) detected four alleles of exon 3. Analysis of the linkage between MHC class II haplotypes and UAA alleles suggested 28 MHC haplotypes and their frequencies were 0.7-12.7%, indicating that the genetic diversity in the Japanese population of C. boyciana was relatively high compared with that in other endangered avian species such as the Japanese crested ibis Nipponia nippon. These results provide valuable information for the genetic management of C. boyciana conservation programs and future studies on avian MHC evolution.

Dianemobius nigrofasciatus (Orthoptera: Trigonidiidae) shows a circadian rhythm in locomotor activity. In the present study, we recorded the locomotor activity of male adults under naturally fluctuating temperature and light conditions over a long period in both summer and autumn. The results showed that the onset time of locomotor activity was later on warmer days in summer, and the cessation time was earlier on cooler days in autumn. The delay in the onset time was also observed under laboratory conditions when a high temperature was added in the late photophase. When the high temperature period was removed, the onset of activity immediately returned to the original time of day. Thus, the high temperature did not alter the circadian phase but directly suppressed locomotor activity through a masking effect.

Anuran tadpoles demonstrate morphological alterations in response to predation risk, thereby developing predator-specific defensive morphologies that enhance survivorship, compared to tadpoles without such defenses. In aquarium experiments, we assessed whether Rana japonica tadpoles demonstrated morphological changes when exposed to the salamander larvae Hynobius setouchi. They are gape-limited predators that coexist with these tadpoles in natural environments. In the presence of H. setouchi larvae, some R. japonica tadpoles were swallowed tail-first, and smaller individuals demonstrated damaged tail fins because of predator attacks, whereas larger tadpoles had intact tail fins. Tadpoles developed larger body sizes and deeper tail fins only when they were in close proximity to H. setouchi larvae, and smaller tadpoles demonstrated relatively larger tail-to-body ratios. These results imply that larger body sizes and deeper tail fins serve as inducible defense mechanisms, potentially improving swimming ability to evade predators and reducing the likelihood of tail-first ingestion.