Izumi Yao

Ant attendance changes the sugar composition of the honeydew of the drepanosiphid aphid Tuberculatus quercicola

Mutualistic interactions between aphids and ants are mediated by the honeydew produced by aphids. Previous work showed that when attended by the ant Formica yessensis, nymphs of the aphid Tuberculatus quercicola developed into significantly smaller adults with lower fecundity than when not ant-attended. This study tested the hypothesis that this cost of ant attendance arises through changes in the quality and quantity of honeydew. Ant-attended and ant-excluded aphid colonies were prepared in the field, and the sugar concentration and sugar composition of the honeydew of ant-attended colonies were compared with those of ant-excluded colonies. The frequency and amount of honeydew excretion were also quantified in the two types of colonies. The aphids excreted smaller droplets of honeydew more frequently in ant-attended colonies than in ant-excluded colonies. There was no significant difference in total sugar concentration between the honeydew of ant-attended aphids and ant-excluded aphids. However, ant-attended aphids produced honeydew containing a significantly lower proportion of glucose and higher proportions of sucrose and trehalose than did ant-excluded aphids. These results suggest that the enhanced rate of honeydew-excretion behavior under ant attendance led to changes in the aphids physiological status. We suggest that the increase in the proportions of sucrose and trehalose in honeydew leads to a shortage of carbohydrates available for energy metabolism, resulting in lower performance of the aphids under ant attendance.

Flexibility in the composition and concentration of amino acids in honeydew of the drepanosiphid aphid Tuberculatus quercicola

Abstract 1. Mutualistic interactions between aphids and ants are mediated by honeydew that aphids produce. Previous work showed that when attended by the ant Formica yessensis Forel (Hymenoptera: Formicidae), nymphs of the aphid Tuberculatus quercicola (Matsumura) (Homoptera: Aphididae) developed into significantly smaller adults with lower fecundity than did nymphs that were not ant attended.

Aphid galls accumulate high concentrations of amino acids: a support for the nutrition hypothesis for gall formation

The nutrition hypothesis for the adaptive significance of insect gall formation postulates that galls accumulate higher concentrations of nutritive compounds than uninfested plant tissue, resulting in a high performance of the gall former. This hypothesis has been supported by some taxa of gall insects, but not by taxa such as cynipid wasps. Aphid galls are expected to require higher levels of nitrogen than other insects’ galls with a single inhabitant, because aphid galls are required to sustain a number of aphids reproducing parthenogenetically over two generations. The present study tested this hypothesis by evaluating aphid performance and amino acid concentration in phloem sap, using the aphid Rhopalosiphum insertum (Walker) (Homoptera: Aphididae), which establishes colonies on leaves of Sorbus commixta Hedlund or in galls of the aphid Sorbaphis chaetosiphon Shaposhnikov (Homoptera: Aphididae). We prepared the gall and non‐gall treatments on trees of S. commixta, in which R. insertum fundatrices were reared and allowed to reproduce. In S. chaetosiphon galls, R. insertum colonies propagated more rapidly, and the second generation grew larger and more fecund than on ungalled leaves. The amount of amino acids exuding from cut galled leaves was fivefold that in ungalled leaves; however, there was no significant difference in the amino acid composition between galled and ungalled leaves. In the intact leaves, total amino acid concentration in the phloem sap declined rapidly from late April to late May; however, the galls retained this high amino acid concentration in developing leaves for 1 month. These results indicate that the improved performance in R. insertum is ascribed to the increased concentration of amino acids in galled leaves. We suggest that S. chaetosiphon galls function to promote the breakdown of leaf protein, leading to an increased performance of gall‐inhabiting aphids.

Seasonal changes in the genetic structure of an aphid-ant mutualism as revealed using microsatellite analysis of the aphid Tuberculatus quercicola and the ant Formica yessensis.

Abstract The present study examined whether the mutualistic relationship between the aphid Tuberculatus quercicola (Matsumura) (Homoptera: Aphididae) and the attending ant Formica yessensis Forel (Hymenoptera: Formicidae) has had any mutual effects on the microgeographical genetic population structure of both partner species. The aphids and the attending ants were collected in June, August, and October 2004 from six trees of the Daimyo oak Quercus dentata Thunberg (Fagales: Fagaceae) and were genotyped using microsatellite loci. Significant genetic differentiation was detected among T. quercicola populations on the respective trees across seasons (an average of pairwise FST = 0.183). Similarly, significant genetic differentiation was found among populations of F. yessensis that attended aphid colonies on the respective host trees, though the averages of pairwise FST were lower (an average of pairwise FST = 0.070). An analysis of molecular variance and two-way ANOVA detected a significantly large genetic difference between spring and summer samples in F. yessensis but not in T. quercicola, indicating that changes in genetic composition occurred in the F. yessensis colony. In spite of a drastic seasonal change in the genetic difference in F. yessensis, principle coordinate analysis showed that the relative position among the six populations was maintained from spring to summer, suggesting that the tree where honeydew was available for a long time was occupied by F. yessensis over the same period and that the honeydew sources were inherited at the level of the ant colony. It is hypothesized that the suitability of host trees for the aphid T. quercicola may have an affect on the genetic structure of the attending ant F. yessensis. Within a colony of aphids, clonal diversity decreased significantly as the season progressed. The reduction in clonal diversity may be due to an increase in identical genotypes by parthenogenesis or selective pressure from host plant deterioration.

Costs and constraints in aphid-ant mutualism

While many studies have demonstrated that ants provide beneficial services to aphids, Bristow (Ant-plant interactions, Oxford University Press, Oxford, 104–119, 1991) first questioned why so few aphid species are ant-attended. Phylogenetic trees have demonstrated multiple gains and loss of ant-attendance in the course of aphid-ant interactions, implying that mutualisms easily form and dissolve. Several studies have reported the factors that influence the formation and maintenance of aphid-ant interactions. Examples include the physiological costs of ant attendance, competition for mutualistic ants, ant predation on aphids, the influence of host plants, and parasitoid wasps. Recent physiological techniques have also revealed the chemical component of aphid-ant mutualisms. The honeydew of ant-attended aphids contains melezitose (a trisaccharide), which has an important role in aphid-ant interactions. Studies of cuticular hydrocarbons on aphids and ants have clarified the underlying mechanisms of ant predation on aphids. Attending ants also reduce aphid dispersal ability, causing the formation of fragmented aphid populations with low genetic diversity in each population. The reduced aphid dispersal could be partly explained by higher wing loading and reduction of flight apparatus due to ant attendance. Whether ant attendance is associated with the range of host plants of aphids or genetic variation in microorganism in aphids remain to be explored.

Contrasting patterns of genetic structure and dispersal ability in ant-attended and non-attended Tuberculatus aphids.

Aphid species within the genus Tuberculatus exhibit a variety of interactions with ants, ranging from close associations to non-attendance. An ant-attended species, Tuberculatus quercicola, and two non-attended species, Tuberculatus japonicus and Tuberculatus paiki, are sympatric and hosted by the tree species Quercus dentata (Fagaceae). An undescribed ant-attended species of Tuberculatus (sp. A) and several non-attended Tuberculatus species are found on Quercus crispula trees. The population genetic structure was examined for the species sympatric on 11 Q. dentata trees and on 11 Q. crispula trees using five microsatellite loci. To determine the extent to which ant-attended or non-attended species migrate between subpopulations, flight intercept traps were placed in the study sites. Ant-attended species exhibited lower allelic richness and showed increased genetic differentiation between subpopulations compared with those of non-attended species. The number of non-attended species caught in traps increased with seasonal abundance; however, few ant-attended species were trapped, despite their abundance. These results suggest that populations of ant-attended aphids are composed of fragmented local subpopulations that are connected by low dispersal rates, leading to considerable population differentiation.

Ant attendance reduces flight muscle and wing size in the aphid Tuberculatus quercicola

In otherwise mutualistic relationships between aphids and ants, attendance by ants often has negative impacts on aphids. For example, in a previous study using traps in the field, the aphid Tuberculatus quercicola, which exhibits mutualistic interactions with ants, showed extremely low dispersal rates, despite having long wings. This study investigates whether components of the flight apparatus (mesonotum length, flight muscle and wings) differ between aphids attended by ants and not attended by ants. Randomized block analysis of variance, using body length as a covariate, showed that ant attendance has a negative influence on aphid flight apparatus. This result indicates that aphids produce honeydew at the expense of resource investment in flight apparatus. Since the dispersal of T. quercicola is limited under ant attendance, the reduction in flight apparatus could precede a decrease in body size. This study also showed that flight apparatus was more developed in aphids under ant-exclusion conditions. This may imply that T. quercicola fly when ants are not available. The maintenance of flight apparatus in T. quercicola might therefore be partly explained by gene flow on the rare occasions that this aphid species disperses.

Phylogenetic Comparative Methods Reveal Higher Wing Loading in Ant-Attended Tuberculatus Aphids (Hemiptera: Aphididae)

Abstract Aphids of the genus Tuberculatus Mordvilko feed on Fagaceae trees and exhibit various interactions with ants, ranging from non-attendance to intermediate or close associations. Despite possession of fully developed wings, two ant-attended species, Tuberculatus quercicola (Matsumura) and Tuberculatus sp. A, exhibited extremely low dispersal. I examined the correlation between wing loading (ratio of body volume to wing area) and ant associations in 20 species of Tuberculatus. Based on a 1317 bp sequence in two mitochondrial regions, cytochrome oxidase I (COI) and NADH dehydrogenase subunit 1 (ND1), phylogenetic trees were reconstructed by neighbor-joining (NJ), most parsimony (MP), maximum likelihood (ML), and Bayesian analyses. AU phylogenetic trees showed that mutualistic interactions with ants have evolved in Tuberculatus at least five times. Comparative analyses based on the NJ, MP, and ML trees showed that increase in wing loading is correlated with ant associations, suggesting that ant-attended aphids have allocated more resources to their bodies than to their wings, resulting in lowered dispersal.

Comparing wing loading, flight muscle and lipid content in ant-attended and non-attended Tuberculatus aphid species

Although all Tuberculatus aphids possess wings, some species associated with ants exhibit extremely low levels of dispersal compared with those not associated with ants. Furthermore, phylogenetic interspecific comparisons find significantly higher wing loading (i.e. higher ratio of body volume to wing area) in ant‐attended species. This observation indicates that ant‐attended species may allocate more of their body resources to reproductive traits (i.e. embryos) rather than flight apparatus (i.e. wings, flight muscle and lipid). The present study focuses on two sympatric aphid species and aims to investigate the hypothesized trade‐off in resource investment between fecundity and the flight apparatus; specifically, the ant‐attended Tuberculatus quercicola (Matsumura) and non‐attended Tuberculatus paiki Hille Ris Lambers. Species differences are compared in: (i) morphology, (ii) embryo production, (iii) triacylglycerol levels and (iv) wing loading and flight muscle. The results show that T. quercicola has a larger body volume, higher fecundity and higher wing loading compared with T. paiki, which has a smaller, slender‐shaped body, lower fecundity and lower wing loading. No significant difference is found between the species with respect to the percentage of triacylglycerol content in dry body weight. The flight muscle development is significantly lower in T. quercicola than in T. paiki. These results indicate that the additive effect of higher wing loading and the lower amount of flight muscle development in T. quercicola may increase the physical difficulty of flight, and hence be responsible for its lower dispersal ability. The trade‐off between fecundity and dispersal documented in wing‐dimorphic insects may therefore be applicable to T. quercicola, which has fully developed wings.

Isolation of eight microsatellite markers from Moina macrocopa for assessing cryptic genetic structure in the wild

We isolated eight polymorphic microsatellite loci from the zooplankton Moina macrocopa (Straus), which is sensitive to pollutants such as insecticides and heavy metals. The isolated loci were polymorphic, with three to seven alleles among 23 individuals. Expected heterozygosities ranged from 0.167 to 0.787. These loci can be used to examine cryptic genetic structure and to infer the connectivity among metapopulations.