Whether gene flow acts as a creative or constraining force on local adaptation is a fundamental question in evolutionary biology, yet its role in the evolution of sexually selected traits remains poorly understood. Here we combine whole-genome and functional analyses to investigate how geological isolation is associated with the evolution of sexually selected traits. We focused on the stag beetle genus Prosopocoilus of the Izu Islands, Japan. In the mainland population (MLP), males possess sexually selected enlarged mandibles. In contrast, across the Izu Islands, relative mandible size decreases progressively with distance from the mainland, and this trend is most pronounced in the southernmost Hachijo-jima island population (HJP), which exhibits markedly dwarfed mandibles. Gene flow is estimated to have occurred from the MLP to the intermediate Izu island population (IIP), while the HJP remained isolated for approximately 149,000 years and evolved a distinct nuclear genetic composition. A genome-wide scan identified the insulin receptor gene InR2 as one of the HJP-specifically differentiated genes. The HJP's InR2 region shows a signature consistent with positive selection. In contrast, in the IIP, where gene flow occurred, this same region was not differentiated. RNAi-mediated gene knockdown of InR2 in MLP males reduced male mandible size to resemble that of HJP males. Our results suggest geological isolation facilitates the evolution of genes associated with sexually selected traits, through reduced gene flow and subsequent local adaptation to the island's specific nutritional resources.

Despite the escalating biodiversity crisis, many species remain unknown to science and may even disappear unnoticed. This is particularly true for many island populations. We illustrate the problem of detecting overlooked species and its consequences by exploring a rare and geographically restricted migratory songbird. We find that this consists of two—hence even rarer—species: the Japanese endemic Ijima's Leaf Warbler Phylloscopus ijimae from the Izu Islands and the Tokara Leaf Warbler from the Tokara Islands. We describe the latter as a new cryptic species, ie one that is morphologically highly similar to, but genetically distinct from, a known species. The genetic divergence is revealed by analyses of nuclear genome-wide and mitochondrial DNA and supported by differences in vocalizations, while the morphological differences are minimal. We evaluate key conservation genomic indicators, showing that both species show low levels of genetic diversity and signs of a decrease of effective population size. Our genome-wide analysis revealed short runs of homozygosity and a low estimated deleterious load, suggesting limited recent inbreeding and possible purging of harmful alleles—indicators of genetic recovery after past demographic fluctuations. Ijima's Leaf Warbler is already classified as Vulnerable as well as a “Natural Monument” in Japan, and we propose that the Tokara Leaf Warbler should retain this status, with continued focused monitoring. Our study not only highlights the importance of integrating genomics with taxonomy for uncovering cryptic avian diversity but also provides a critical foundation for future conservation efforts.

ABSTRACT Island Columbiformes exhibit diverse evolutionary trajectories related to flight ability. Although a reduction in flight ability is a common feature of island avifauna, some species retain or even increase flight ability after colonizing islands, thereby enabling movement between islands within archipelagos. Understanding the movement ecology of mobile island pigeons is essential for evaluating avian adaptations and ecological functions in island ecosystems. In this study, we conducted satellite tracking of the Black Wood Pigeon ( Columba janthina ) in the Izu Islands, Japan, to clarify its interisland movement patterns and habitat use, which may influence the movement decisions related to breeding. Among the 18 pigeons tracked from 2022 to 2025, 12 individuals moved to different islands. Single interisland flights ranged from approximately 4 km (Hachijo to Hachijokojima) to 170 km (Hachijo to Ohshima). Individuals on Aogashima island tended to disperse to various islands after the breeding season and return the following spring. Analysis of habitat selection across land‐use categories revealed a preference for afforestation areas and Sasa shrubs during the breeding period. Habitat shifts were also observed in association with interisland movements. These findings suggest that Black Wood Pigeons may be partially migratory or nomadic, with flexible use of the entire archipelago, depending on individual decisions regarding migration. This study will facilitate further research to better understand the patterns and functions of bird movements at the archipelagic scale.

Hydrangea macrophylla (Thunb.) Ser., an important genetic resource for the development of hydrangea cultivars, is endemic to a limited number of regions in Japan; however, there is a lack of comprehensive phytogeographical information essential for the conservation and effective utilization of this valuable genetic resource. In this study, we investigated the occurrence of natural hybridization between H. macrophylla and H. serrata within the distribution range of H. macrophylla by analyzing the internal transcribed spacer (ITS) regions of nuclear ribosomal DNA. Analysis of the H. macrophylla genome database revealed approximately 300 tandem repeats of ITS sequences in the genome. Two major ITS sequences, accounting for 55% to 59% of the total ITS sequences in the genome of H. macrophylla Aogashima-1 (the individual used for genome assembly), appeared to be inferable from electropherogram data obtained by direct sequencing of PCR products. From a total of 320 samples of H. macrophylla and H. serrata, 10 ITS types were identified. As a result of phylogenetic analysis, the ITS sequence types of H. macrophylla, H. serrata var. yesoensis, var. serrata, var. angustata, and some var. acuminata samples were grouped into a single cluster. Samples were collected from four of the Izu Islands, 16 populations on the Izu Peninsula, and 12 populations on the Boso Peninsula to estimate the occurrence of interspecific hybridization within the H. macrophylla distribution range. Direct sequencing results for PCR products of ITS sequences and morphological data showed that little interspecific hybridization had occurred in the Izu Islands; however, hybridization had occurred in many populations on the Boso and Izu Peninsulas. Furthermore, our findings suggest that habitat topography influences the frequency of interspecific hybridization among H. macrophylla populations. To conserve H. macrophylla genetic resources in the Boso and Izu Peninsulas, it is important to consider not only population decline, but also the impact of natural interspecific hybridization.

The whole mitochondrial genomes (mitogenomes) excluding the control region were sequenced for 42 samples of the ten species of scincid lizards of the genus Plestiodon occurring in Japan, and the mitochondrial genealogy of the East Asian Plestiodon was reconstructed on the basis of the mitogenomic dataset. A comparison with a species phylogeny based on five nuclear DNA fragments revealed that P. latiscutatus, a species distributed on the Izu Peninsula of the Japanese Main Islands, was a sister to the clade consisting of P. japonicus and P. finitimus, species occurring in the remaining parts of the Main Island in the species phylogeny, whereas the former species was a sister to the P. capito group occurring in continental China in the mitogenomic phylogeny. A tree reconciliation analysis of the species and the mitochondrial phylogenies revealed horizontal transfer of mitochondria, suggesting that the position of P. latiscutatus was caused by past introgression of mtDNA from the ancestral P. capito group to the ancestral P. latiscutatus. This further suggests a past long-distance dispersal of the skink from the Eurasian continent to the Izu Peninsula and Izu Islands beyond the western part of the Japanese Main Islands in the Miocene.

Oceanic island ecosystems are highly vulnerable to human activities. Oceanic islands near the mainland offer insights into natural processes, including overseas dispersal from the mainland. However, most studies examining community changes on oceanic islands focus on human activities, while natural colonisation from the mainland has received limited attention. To clarify the mechanisms of insular community changes due to human activities and natural colonisation, we investigated bird assemblages on 10 islands during 2016 and 2021 and compared them to the assemblages during 1970 and 1973 on the Izu Islands, Japan. We explored how the species traits and mainland distribution changes of species are associated with distribution changes of each species on the 10 islands using phylogenetic generalised least squares (PGLS). Furthermore, bird community structures on each island were examined based on species richness and functional and phylogenetic structures. A null modelling approach was used to examine functional and phylogenetic cluster/overdispersion structures; clustering and overdispersion imply environmental filtering and limiting similarity, respectively. We analysed the effects of geographical and topographical characteristics of the islands, introduced predators and human-induced landscape transformations on changes in community structures using linear models (LMs). Species that expanded their mainland distribution and with large clutches colonised many islands. Raptors disappeared from many islands. Species richness decreased, and functional and phylogenetic structures became clustered on almost all islands, which were only statistically associated with geographical and topographical characteristics. The distribution changes in species on islands were associated with their mainland distribution changes, which indicates that mainland community dynamics influence insular communities via overseas colonisation. Despite the colonisations, the net loss of species richness, clustered phylogenetic and functional structures, and disappearance of top predators on many islands suggest that bird communities across the archipelago degraded. Previous studies have demonstrated that introduced weasels negatively affect birds through competition for food resources and direct predation on the introduced islands. These degraded bird communities due to introduced weasels would have transboundary biodiversity effects on communities across the archipelago via inter-island movements of birds, which suggests the need for comprehensive conservation strategies across the archipelago to effectively conserve insular biodiversity.

Pumice drifting poses substantial risks to maritime navigation and coastal communities. While traditional ocean-current-based simulations effectively predict drifting patterns, they are resource-intensive and unsuitable for real-time use following abrupt eruptions. This study proposes a data-driven framework that enables rapid, low-cost pumice drift prediction, leveraging daily-reported Kuroshio Current axis (KCA) patterns and conducting similarity searches on pre-existing simulation datasets. Focusing on eruptions at Bayonnaise Rocks in the Izu Islands, Japan, we demonstrate that the dynamic time warping distance, a measure of similarity between the current KCA pattern and historical KCA patterns, accurately predicts drifting scenarios within the critical first 10 days post-eruption. This method reliably identifies high-risk cases, including those affecting coastal regions, without requiring new simulations. By refining simulation datasets and enhancing prediction accuracy, this framework can become a practical tool for hazard assessments, offering a scalable solution for proactive disaster-risk management in response to unpredictable pumice eruptions.

Abstract A moderate‐magnitude earthquake swarm occurred in the remote Izu Islands region of Japan between October 1 and 8, 2023. The swarm included 151 shallow earthquakes cataloged by the U.S. Geological Survey, which notably included a roughly 2.5‐hr episode of 15 successive magnitude (M) < 5.5 earthquakes. Origin times were coincident with regionally recorded tsunami waves, but tsunamigenesis for moderate‐magnitude earthquakes is uncommon, indicating that volcanic activity generated the ocean displacements. Leveraging a surface‐wave relative relocation approach, we estimate precise epicentroid locations for the remote swarm. Final epicentroids and caldera analogs indicate a three‐stage model to explain swarm activity: (a) caldera pressurization due to magma intrusion, (b) depressurization via dike propagation away from the caldera, and (c) eruption corresponding with caldera reactivation either by collapse or additional intrusion.

Diversity of invertebrates within the burrows of the streaked shearwater (Calonectris leucomelas) on Toshima Island, Izu Islands, Japan

Enneapterygius pallidoserialis Fricke, 1997, previously considered a valid species, is regarded as a junior synonym of E. erythrosoma Shen, 1994. Re-examination of type specimens of both nominal species revealed that the meristic and morphometric characters of the holotype and seven paratypes of E. pallidoserialis examined closely matched those of the holotype, paratype, and non-type specimens of E. erythrosoma. Furthermore, the nuptial coloration of male specimens of E. pallidoserialis, including the holotype, was consistent with that of E. erythrosoma. This synonymization resulted in E. erythrosoma being newly recorded from Miyake-jima island (Izu Islands, Japan), and Putic and Siayan islands (Philippines) on the basis of specimens previously identified as E. pallidoserialis.

Several juveniles of millipedes, considered to be the alien millipede introduced from Taiwan, Chamberlinius hualienensis, were collected on July 9, 2023 from Aoga-shima Island, Izu Islands. The juvenile was incubated, and the molted adult male was identified as C. hualienensis by examining morphological diagnostic characters of the genus Chamberlinius. This is the first record of the species from Aoga-shima Island.

Adaptive introgression plays a vital role in allowing recipient species to adapt and colonize new environments. However, our understanding of such environment-dependent introgressions is primarily limited to specific plant taxa in particular settings. In Japan, two related orchid species, the autonomously self-pollinating Pogonia minor and the outcrossing Pogonia japonica, typically inhabit dry grasslands and wetlands, respectively. Intriguingly, an island ecotype of P. japonica exists in arid, wind-swept, open sites on volcanic mountain slopes on Kozu Island, in the oceanic Izu Islands. To investigate potential introgression and its implications between P. japonica and P. minor on Kozu Island, we applied a comprehensive approach that included examining morphological traits, genome-wide SNP data, and plastid DNA sequences. We also examined the breeding systems of these species on Kozu Island through artificial pollination experiments to determine if introgression from P. minor has endowed the P. japonica ecotype with selfing capabilities. Extensive sampling on Kozu Island revealed that all P. japonica specimens exhibit signs of introgression from P. minor, suggesting the absence of pure P. japonica populations on the island. Furthermore, the chloroplast haplotypes of the insular P. japonica ecotype consistently match those of P. minor, indicating a predominantly asymmetrical initial hybridization with P. minor acting mainly as the maternal parent in the formation of F1 hybrids. Despite the advantages of self-fertilization in isolated environments, the insular P. japonica does not exhibit autogamy. Consequently, the scarcity of moist habitats, rather than selection pressure for selfing, likely contributes to the observed widespread introgression. Our study strongly suggests that the arid-environment-adapted P. minor has introgressed into the insular ecotype of P. japonica, enabling its successful colonization of arid volcanic mountain slopes of the oceanic island.

Iijima's Leaf Warbler Phylloscopus ijimae is a summer visitor, endemic to the Izu and Tokara Islands, and its habitat range is especially restricted in Tokara. In this study, the detailed distribution and density of the Tokara population were investigated, together with its ecological, morphological, and genetic characteristics. The distribution of singing males was surveyed on five islands within the species' known distribution range, and the warbler was most abundant on the largest, Nakanoshima, but less abundant on Kuchinoshima, Suwanosejima, and Akusekijima, and undetected on Tairajima. During the period of 2001 – 2023, the abundance on Nakanoshima was judged stable from trim analysis based on the annual route census data (averaging 1.57 individuals /ha). The breeding ecology on Nakanoshima was similar to that reported in the Izu Islands, but the nest sites were more restricted to the branches of the bamboo Pleioblastus linearis, growing thickly under the evergreen forest. It is known to be difficult to identify the sex of this species from external characteristics, but the sex of the captured individuals could be clearly determined by DNA- based methods using universal primers. Mean external measurements of the Tokara males were generally smaller than those of the Izu males, and the differences in tarsus and total head lengths were significant (P < 0.05). The results of the probabilistic principal component analysis also supported the trend that the Tokara males are smaller. Female measurements could not be examined due to insufficient samples. Four haplotypes, two each from two regions, were identified from the mitochondrial cytochrome b region partial sequences (936 bp), and haplotypes from each region were placed in two different clusters of the neighbor- joining tree, respectively. The average genetic distance between regional populations was 0.064 in nucleotide substitution rates, suggesting that the two isolated populations are best placed in two independent conservation units.

The fern independent gametophytes that can maintain populations by vegetative reproduction without conspecific sporophytes have been considered an unusual phenomenon found in some epiphytic or epilithic species of Hymenophyllaceae, Pteridaceae, Lomariopsidaceae, and Polypodiaceae. By chance, the discovery of mysterious strap-like gametophytes on Izu-Oshima Island, Japan, has led to the hypothesis that Hymenasplenium murakami-hatanakae, a fern species belonging to Aspleniaceae, can also form independent gametophytes. Our investigation revealed gametophyte populations of H. murakami-hatanakae on three islands in the Izu Islands. Based on chloroplast DNA analysis of the gametophyte and sporophyte populations, the gametophytes were found to be maintained by vegetative reproduction without a new supply of spores from sporophytes. A comparison of the surrounding vegetation at the collection sites showed that environmental factors such as light and humidity may influence the maintenance of gametophyte populations. These results clearly show that H. murakami-hatanakae is one of the ferns capable of forming independent gametophytes. This is the first report of independent gametophytes from the suborder Aspleniineae (eupolypod II). The discovery of the independent gametophyte within a phylogenetic lineage previously thought not to form independent gametophytes will provide important insights into the morphological and functional evolution of gametophytes in ferns.

Abstract Insect community structures and biological interactions vary with the distance from the mainland to islands. Gall inducers are key organisms in local arthropod communities because their galls harbor diverse arthropods. We investigated the parasitoid community of a gall‐inducing cecidomyiid Schizomyia sasakii on the Izu Peninsula and the Izu Islands, Japan. We examined relationships between parasitism by Inostemma sp. and Torymus hirtipennis, and gall characteristics, and analyzed the directional selection on gall characteristics induced by S. sasakii in each locality. The species richness of parasitoids on the Izu Islands decreased with the distance from the Izu Peninsula to each island, and area of respective islands. Inostemma sp. preferably attacked large galls on Ohshima Island, and T. hirtipennis tended to attack relatively small galls in the Izu Peninsula. Directional selection on gall characteristics of S. sasakii favored the induction of larger galls and thicker tissues by S. sasakii on Ohshima Island. In contrast, no directional selection was detected in the gall characteristics on Hachijojima Island. The number of alternative host species of parasitoid may affect variation in parasitoid richness of S. sasakii on distant and small islands, because the species richness of gall midges is necessary to maintain local parasitoid populations. Our study supported the ovipositor limitation hypothesis for the torymid species, related to gall size. The directional selection of parasitoid attacks on the Izu Islands may act to favor the induction of large and hypertrophic galls by S. sasakii to avoid the parasitoids.

The present study describes two new species of the monocotyledon-feeding lace bugs of the genusAgrammaStephens, 1829 (Hemiptera, Heteroptera, Tingidae, Tinginae, Tingini) from small islands of Japan. The first is A. (A.) izuensesp. nov., which was recorded as A. (A.) japonicum (Drake, 1948) from Hachijo Island, the Izu Islands, in a previous study, and is considered an independent species here based on morphological characteristics and molecular data. The second is A. (A.) keramensesp. nov., which has a remarkable spineless head and was discovered from Aka and Geruma islands, Kerama Group, the Ryukyu Islands. Consequently, the following four species ofAgrammawere recognized in Japan: A. (A.) abruptifrons Golub, 1990, A. (A.) izuensesp. nov., A. (A.) japonicum, and A. (A.) keramensesp. nov.Only dozens of submacropterous morphs were confirmed in these two species in the present study, suggesting that both new species are flightless. In addition, an illustrated key for the identification of the four species from Japan and the host plant relationships of the two new species are provided.

Abstract On 8 October 2023, mysterious tsunamis with a maximum wave height of 60 cm were observed in Izu Islands and southwestern Japan, although only seismic events with body‐wave magnitudes mb 4–5 have been documented to the west of Sofugan volcano. To investigate the source process, we analyze tsunami waveforms recorded by an array network of ocean bottom pressure gauges. Stacked waveforms of pressure gauge records suggest recurrent arrivals of multiple wave trains. Deconvolution of the stacked waveforms by tsunami waveforms from an earlier event revealed over 10 source events that intermittently generated tsunamis for ∼1.5 hr. The temporal history of this sequence corresponds to the origin times of T‐phases estimated by an ocean bottom seismometer and of the seismic swarm, implying a common origin. Larger events later in the sequence occurred at intervals comparable to the tsunami wave period, causing amplification of later phases of the tsunami waves.

Faunal studies of oceanic islands provide unique opportunities to study ecology and evolution. Four Armadillidae taxa were recorded from the Izu Islands, a group of oceanic islands off the coast of Japan. The distributions of these taxa and a phylogenetic tree are based on molecular data for 249 specimens from 88 sites. The Spherillo russoi (Arcangeli, 1927) species complex is distributed widely throughout these islands, and dispersal may have occurred via oceanic currents. On the nearby Izuoshima and Toshima Islands, Spherillo dorsalis (Iwamoto, 1943) may have recently migrated from the Izu Peninsula either via currents or been transported by human activity. In the middle islands, Shikinejima, Kozushima, Miyakejima, and Mikurajima, Venezillo hasegawai (Nunomura, 1991) was found, which may be derived from Spherillo obscurus (Budde-Lund, 1885) distributed in the Kanto region on the mainland. On the most southern and distant Hachijojima Island, Spherillo punctatus Nunomura, 2007 was found, and it does not differ genetically from specimens from Izenajima and Tokunoshima Islands in the Nansei Island Group, despite an approximate 1,000-km distance between them; this species may have been transferred by human activity. Our study showed that the Armadillidae assemblages in the Izu Islands were formed by several migration routes.

伊豆諸島式根島には3カ所の裸地が存在し，その周縁ではクロマツの偏形樹やオオシマハイネズ，イソギクなどの海浜植物が生育する独特な植生景観が見られる．本研究では，式根島における裸地の分布およびその成立環境について明らかにすることを目的として調査を行った．調査の結果から式根島の3カ所の裸地は島内でも比較的標高が高い島西部の海食崖の上方の緩斜面に分布しており，裸地周縁ではクロマツをはじめとした矮性低木や先駆植物が確認された．このことから，式根島の裸地では地形的制約のために強風が吹くことで植物の定着および生育が妨げられていることが示された．また，裸地周縁の露頭ではシルトや円礫を含む層が見られ，裸地の地表では鞍部でシルトや砂が堆積していることが確認された．このことから，降雨時に一時的な水流や池が発生することで，表層の土砂や植物の種子が流しだされ，植物の定着が困難となることが推定された．

On October 8, 2023, at 21:40 UTC (6:40 on October 9 local time), a tsunami warning was issued for the Izu Islands and southwest Japan. This tsunami was initially considered to be associated with the Mw 4.7 earthquake at 20:25 UTC (5:25 JST). However, we know events of this magnitude are far too small to generate observed tsunamis from coseismic deformation alone. In this study, we analyzed the ocean-bottom pressure records of DONET and S-net, real-time cabled observation networks on the Pacific coast of Japan. We find that the dominant period of this tsunami was relatively short, 250 sec, and that the largest tsunami occurred at 21:13 (6:13 JST) near Sofu-gan volcano. In addition, T waves, or the ocean-acoustic waves, were clearly observed by DONET – we posit these correspond to a vigorous swarm-like seismic event at the same region of the tsunami source. We formally invert for the tsunami source and find that several tsunami sources with an interval of about 4 min are necessary to reproduce the observed records. These most likely correspond to volcanic eruptions.