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Abstract Some aphid species produce a soldier caste with enlarged forelegs and horns (weapons). It has been hypothesised that the evolution of morphological specialization by soldiers in social aphids is accelerated by high predation pressure, but this possibility has not been tested. Here, we investigated the relationship between local predator abundance and soldiers’ weapon size and aggressiveness in a prey–predator system comprising a eusocial aphid, Ceratovacuna japonica, and its predators (larvae of the butterfly Taraka hamada and of the moth Atkinsonia ignipicta) in two populations with different predator abundances. We found that the soldiers in the predator-abundant population had larger weapons and were more aggressive than those in the population with lower predator abundance. Furthermore, the soldiers’ defensive prowess (evaluated as the survival of aphids in the presence of predators) was greater in the predator-abundant population. These results provide the first evidence that a population of eusocial aphids experiencing high predation pressure has soldiers with pronounced defensive traits and defensive prowess.

To understand the reproductive ecology of a plant species with both selfing and outcrossing reproductive systems, it is important to know how selfed individuals contribute to the maintenance of a population. Further, examination of the magnitude of inbreeding depression after pollination is essential for the accurate measurement of reproductive success. In this study, we investigated the characteristics of viable seeds and seedlings of Rhododendron kaempferi Planch. in a region where the bumblebee pollinator Bombus ardens Smith, 1879 are abundant. In a hand-pollination experiment, the fruit set from self-pollination was 32%, significantly lower than that from outcross-pollination (81%). The number of seeds per fruit from self-pollination was one sixth the number from outcross-pollination. However, the number of seeds per fruit was negatively correlated with the mass per seed. In addition, the germination rate was low when the seed mass was light. Although selfed seeds could germinate, 89% of the seedlings died within a year. The inbreeding depression coefficient one year after sowing was very high (0.89). These results indicate that R. kaempferi shows extremely strong selection for outcrossing but maintains a slight selfing potential in a natural population in central Japan. Key words: bumblebee, germination, inbreeding depression, rhododendron, seed. Afin de comprendre l'ecologie de la reproduction d'especes vegetales dont les systemes de reproduction sont a la fois endogames et par croisements eloignes, il est important de savoir comment les individus endogames contribuent au maintien d'une population. De plus, l'examen de l'ampleur de la depression de consanguinite apres la pollinisation est essentiel pour mesurer precisement le succes de la reproduction. Dans cette etude, les auteurs ont examine les caracteristiques des graines et des plantules viables de Rhododendron kaempferi Planch. dans une region ou le bourdon pollinisateur Bombus ardens Smith, 1879 est abondant. Lors d'une experience de pollinisation manuelle, la nouaison issue de l'autopollinisation etait de 32 %, soit nettement moins que celle issue de la pollinisation croisee (81 %). Le nombre de graines par fruit provenant de l'autopollinisation etait un sixieme de celui provenant de la pollinisation croisee. Cependant, le nombre de graines par fruit etait correle negativement avec le poids par graine. Par ailleurs, le taux de germination etait faible lorsque le poids des graines etait leger. Bien que les graines endogames aient pu germer, 89 % des plantules mouraient au cours de la premiere annee. Le coefficient de depression de consanguinite un an apres le semis etait tres eleve (0,89). Ces resultats indiquent que R. kaempferi presente une selection extremement forte pour les croisements eloignes, mais maintient un leger potentiel d'autofecondation dans une population naturelle du centre du Japon. [Traduit par la Redaction] Mots-cles : bourdon, germination, depression de consanguinite, rhododendron, graine., Introduction Inbreeding depression, defined as the reduction in fitness of progeny derived from inbreeding relative to that of progeny derived from outcrossing, has long been an important topic in the [...]

Background and aims – Previous researchers have demonstrated that geographic variation in pollinator community composition can generate diversity in the floral traits of animal-pollinated plants. Our study focused on the bumblebee-pollinated white dead-nettle Lamium album var. barbatum. Geographic variation in corolla length of this species is known to be correlated with regional pollinator size. The aim of this study is to clarify whether size-matching between flower and pollinator affects seed set in L. album.Material and methods – In the present study, we investigated two L. album populations on Mount Norikura, central Japan. We determined the pollinator community composition and corolla length during the flowering period of L. album and recorded seed set after a single visit by different pollinator categories.Key results – We observed that the main pollinators of L. album were bumblebee queens and workers. Bumblebee queens visited flowers more frequently than workers during peak flowering. Furthermore, size-matching between flowers and bumblebee queens, but not workers, strongly promoted seed set. These results suggest that L. album flower size is adapted to bumblebee queens, the main pollinator during peak flowering season in our study sites.

Most flowering plants are visited by various pollinator insects. To understand floral specialization for pollinators, the relative importance of different flower visitors to the focal plant species should be revealed. In the present study, we observed the insects that visited the orchid Platanthera hologlottis throughout the day and night using interval timer photography to reveal the relative importance of diurnal and nocturnal flower visitors. We observed visitation by both diurnal (e.g. the butterfly Ochlodes ochraceus) and nocturnal (e.g. the moth Thysanoplusia intermixta) insects and examined their relative contribution to fruit production and pollinarium removal experimentally. Results showed that the fitness was higher in flowers visited by nocturnal insects than in those visited by diurnal insects. These results suggest that the floral traits of P. hologlottis may be specialized for nocturnal flower visitors rather than diurnal flower visitors.

Examination of the efficacy of pollinators that are not morphologically matched to a flower’s shape can deepen our understanding of the main pollinators of a plant species. In central Japan, Rhododendron japonicum is visited much more frequently by bumblebees than by butterflies although its flower shape is more suited for pollination by large butterflies. Here, we observed flower-visiting behaviour of Bombus diversus queens and the alpine black swallowtail butterfly, Papilio maackii, and compared their efficacy as pollinators. Papilio maackii always touched the stigma during a flower visit, whereas B. diversus queens did so during just 54% of their flower visits. As bumblebees visit neighboring flowers of a R. japonicum tree in sequence not like Papilio maackii, we hypothesized that they deliver self pollen to R. japonicum so that the fruit set would be low. However, the fruit set in a bumblebee-dominated area was 72.5%, significantly higher than that by hand-pollination with self pollen (31.4%). This suggests that R. japonicum can obtain sufficient fruits by B. diversus queens although its flower shape morphologically matches to pollination by swallowtail butterflies.

Abstract Cimicifuga simplex has three genetically and ecologically distinct pollination morphs with different flowering phenology, flower sex expressions, and selfing rates. A previous study showed that strong protandry in hermaphroditic flowers of C. simplex causes there to be seasonal minority sexes; for example, unisexual female ramets are advantageous in the first half of the flowering season and bloom then (minority sex hypothesis). That study, however, did not distinguish among the three pollination morphs of C. simplex. We investigated seasonal sex ratio changes and pollinator environments of the three morphs to verify and expand the minority sex hypothesis. We investigated flowering phenology, pollinator quantity and quality, seasonal population sex ratio dynamics, and stamen/ovule ratios in hermaphroditic flowers. We also examined the seasonal female reproductive success of hermaphroditic flowers, and the effect of male flower excision on fruit set by morph II hermaphroditic flowers. Morph I (mainly hermaphroditic and female ramets) had high pollinator quality and quantity. Fruit set of hermaphroditic ramets was high throughout the flowering season. Morph II (hermaphroditic and andromonoecious ramets) had low pollinator quality, and few pollinators visited near the beginning and end of the season. Removal of male flowers led to a pollen limitation. Morph III (hermaphroditic ramets and a high selfing rate) had very low pollinator abundance throughout the flowering season, and male and female phases largely overlapped. The scarcity of male flowers in morph I is likely a result of the high pollinator quantity and quality. The high fruit set suggests that no pollen limitation existed. The low pollinator quality and quantity of the morph II population caused it to suffer from pollen limitation and may explain the absence of unisexual female ramets in this morph. The high selfing rate of morph III may be due to the extremely low pollinator abundance. Theoretical studies have indicated that the optimal investment allocation to male and female functions depends on whether a pollen limitation exists. In C. simplex, the observed relationships between pollinator environment and reproductive systems are consistent with these theoretical models.

Relationships between flower visitor composition and floral traits help us to understand floral evolution. Swallowtail butterflies have been assumed to be the main pollinators of Rhododendron kaempferi and R. japonicum based on their floral shapes, and R. kaempferi was reported to be pollinated by butterflies in southern Japan. In the mountains of central and northern Japan, however, bumblebees are abundant and candidate pollinators of Rhododendron. We found that visitation frequencies of bumblebees were higher than those of swallowtail butterflies to both of the Rhododendron species at three study sites in the mountains of central Japan. R. japonicum pollen adhered to the bodies of both floral visitors. Further, the nectar sugar concentration of R. kaempferi was in the preference range of bumblebees, whereas that of R. japonicum was suited to both bumblebees and butterflies. Taken together, these findings support the hypothesis that bumblebees are potentially also important pollinators of R. kaempferi and R. japonicum in mountains of central Japan, where they are more abundant than swallowtail butterflies.

Myrmecophilus crickets (Myrmecophilidae, Orthoptera) are typical ant guests. In Japan, about 10 species are recognized on the basis of morphological and molecular phylogenetic frameworks. We focused on two of these species, M. kinomurai and M. kubotai, and compared their host and habitat use. Previous work based on a limited sampling effort suggested that these two species share some ant species as hosts, but that their habitat (open versus shaded) preferences differ. Here, on the basis of exhaustive sampling across Japan, we confirmed that M. kinomurai and M. kubotai do not differ in their host ant preferences: both prefer formicine ants as hosts. As for habitat preferences, M. kubotai occurred significantly more often in open habitats than in shaded ones (P < 0.05). In contrast, M. kinomurai showed no habitat preference in areas where M. kubotai did not occur. However, M. kinomurai showed an obvious preference for shaded environments in areas of sympatry with its potential competitor M. kubotai. This pattern suggests that interspecific competition between M. kinomurai and M. Kubotai is a factor causing habitat differentiation in areas of sympatry.

Seed dispersal by ants (myrmecochory) is an important life phase in flowering herbs of temperate deciduous forests, and long dispersal is basically preferable in terms of plant fitness. Although it is known that larger ants transport seeds farther, previous studies conflicted whether larger seeds are preferred by large ants. We examine how seed size and ant body size influence removal rate of seeds and their dispersal distance by ants. The obligate myrmecochorous, large-seed bearing Viola hondoensis and ant-ballistic diplochorous, small-seed bearing Viola spp. and Corydalis pallida were comparatively investigated. 1) Ant preference: attraction rate and transportation rate of large V. hondoensis seeds by ants were consistently high regardless of the disperser ant size whilst those rates of small diplochorous seeds were conspicuously lower for large Aphaenogaster ants than for small Pheidole ants, suggesting small seeds are not adapted to be carried by large ants. 2) Dispersal distance: large V. hondoensis seeds were transported for a long distance because they were carried by both large and small ants while the small diplochorous seeds were transported for a short distance because they were carried only by small ants. These results suggest that large seeds are advantageous to attract large ants and to be farther dispersed as far as ant-mediated dispersal is concerned. On the other hand, the diplochorous small seeds do not attract large ants, and so are transported short by ants although they disperse their seeds by ballistic dispersal.

A previously reported mitochondrial DNA (mtDNA) phylogeny of Crematogaster (subgenus Decacrema) ants inhabiting Macaranga myrmecophytes indicated that the partners diversified synchronously and their specific association has been maintained for 20 million years. However, the mtDNA clades did not exactly match morphological species, probably owing to introgressive hybridization among younger species. In this study, we determined the congruence between nuclear simple sequence repeat (SSR, also called microsatellite) genotyping and mtDNA phylogeny to confirm the suitability of the mtDNA phylogeny for inferring the evolutionary history of Decacrema ants. Analyses of ant samples from Lambir Hills National park, northeastern Borneo, showed overall congruence between the SSR and mtDNA groupings, indicating that mtDNA markers are useful for delimiting species, at least at the local level. We also found overall high host-plant specificity of the SSR genotypes of Decacrema ants, consistent with the specificity based on the mtDNA phylogeny. Further, we detected cryptic genetic assemblages exhibiting high specificity toward particular plant species within a single mtDNA clade. This finding, which may be evidence for rapid ecological and genetic differentiation following a host shift, is a new insight into the previously suggested long-term codiversification of Decacrema ants and Macaranga plants.

Predators generally have traits that enable them to efficiently capture their prey and thus improve their survival. Natural selection should also favor traits of predators that improve the survival rate of their eggs, which are immobile and incapable of active resistance. We hypothesized that eggs of Atkinsonia ignipicta, a specialist predator of the eusocial aphid Ceratovacuna japonica, exhibit a defensive trait against aphid soldiers. We found that the hatchability of A. ignipicta eggs did not differ significantly between the experimental treatments with and without soldiers, which suggests that the eggs have a defensive trait that protects them from soldier aphids. Moreover, although the soldiers occasionally exhibited attack behavior when they encountered an egg, they did not continue the attack. We have observed a similar interruption of attack behavior by soldiers that attacked their aphid siblings by mistake, suggesting that the eggs may chemically mimic the soldiers' siblings. This study thus provides evidence for adaptation in a specialist predator of a eusocial aphid.