Hideki Ueno

Extreme variation in the prevalence of inherited male-killing microorganisms between three populations of Harmonia axyridis (Coleoptera: Coccinellidae)

Females from three populations of Harmonia axyridis (Pallas) were assayed for two phenotypical indicators of the presence of male-killing endosymbionts: low egg hatch-rates and strongly female-biased progenic sex ratios. Samples from Sapporo City, Japan, and the Altai Mountains, Mongolia, but not from Novosibirsk, Russia, were found to contain some females displaying both of these traits. Furthermore, there was a profound difference in the prevalence of infection between the Japanese and Mongolian populations. The proportion of females infected from the Japanese sample was approximately 0.49, whereas that from Altai was only 0.02. The trait was inherited by more than 99% of the progeny of infected females. The trait was inherited maternally with the same high efficiency over five generations. Treatment with antibiotics produced a small increase in the production of males from the male-killing lines, suggesting that the male-killer is bacterial in nature. Causes of variation in prevalence are discussed, and the consequences of high levels of infection with an efficiently transmitted male-killing microorganism on host population demography are considered.

Genetic architecture for normal and novel host‐plant use in two local populations of the herbivorous ladybird beetle, Epilachna pustulosa

Trade‐offs in host‐plant use are thought to promote the evolution of host specificity. However, usually either positive or no genetic correlations have been found. Whereas factors enhancing variation in overall viability have been claimed to mask negative genetic correlations, alternative hypotheses emphasize the sequential changes in genetic correlation in the course of host‐range evolution. In this study, the genetic architectures of performances on different hosts were compared in two populations of the herbivorous ladybird beetle, Epilachna pustulosa, using three host plants, one being normal for both, one novel for only one population, and the other novel for both populations. The genetic correlations between larval periods on normal hosts were significantly positive whereas those between normal and novel hosts were not different from zero. There was no evidence for reduced genetic variation on the normal host‐plants. These results suggest that the host‐range is not restricted by the antagonistic genetic associations among exploitation abilities on different plant species, but rather that selection of different host‐plants may improve the coordination between genes responsible for the use of different plants.

Genetic variation in larval period and pupal mass in an aphidophagous ladybird beetle (Harmonia axyridis) reared in different environments

Abstract Genetic trade‐offs for host plant use are hypothesized to facilitate the diversification of insect populations through specialization to their host plants. Previous studies mainly estimated the architecture of genetic variances and covariances in herbivorous species with discrete and limited types of host species. In contrast to herbivores, the relative abundance of resources for predatory species fluctuates in time and space, causing a more unpredictable encounter with prey species. The ecological characteristics of resource use might result in a differential mode of selection for herbivorous and predatory species, which could be reflected in a differential genetic architecture of developmental traits such as the duration of larval stage (henceforth referred to as larval period) and size of pupa (measured as pupal weight). This paper presents results from a study on the genetic architecture of larval period and pupal mass of an aphidophagous ladybird beetle, Harmonia axyridis Pallas, in different resource environments. Beetles reared on Acyrthosiphon pisum (Harris) showed a shorter developmental period and a heavier pupal mass than their siblings on Aphis craccivora Koch or on artificial diet, while the average larval period and pupal mass on A. craccivora and the artificial diet were similar. Further analyses of the genetic architecture suggest that the developmental traits on the two aphid species are genetically correlated, while there are only weak or no genetic correlations between these two traits on the two aphid preys and the artificial diet. Thus, the results suggest that the patterns of genotypic relationships between developmental traits differ from the phenotypic ones. The effects of past selection on the genetic architecture and the possible cause of the genetic correlation are discussed, as well as consequences for mass rearing for biological control.

Genetic basis for established and novel host plant use in a herbivorous ladybird beetle, Epilachna vigintioctomaculata

Genetic trade‐offs in host plant use are thought to promote the evolution of host specificity. Experiments on a range of herbivorous insects, however, have found negative genetic correlation in host plant use in only a limited number of species. To account for the general lack of negative genetic correlation, recent hypotheses advocate that different stages in evolution of host use must be distinguished: initial performance on a novel host in comparison with the established host, and performance on both hosts after the insect population has interacted with both hosts for a long time. The hypotheses suggest that genetic correlation may not necessarily be negative at the initial stage. The present study examines growth performance on both the established and a novel host in a herbivorous ladybird beetle, Epilachna vigintioctomaculata Motschulsky (Coccinellidae, Epilachninae). The results show that traits of growth performance across hosts were positively or neutrally correlated, but there was no evidence of a negative genetic correlation. In addition, significant genetic variance of growth performance on each host was detected, suggesting that E. vigintioctomaculata can potentially respond to selection for increased performance on both plant species. These results and similar results from experiments on other herbivores suggest that host expansion may not be constrained genetically, at least at the initial stage of host range evolution.

Genetic basis for different host use in Epilachna pustulosa , a herbivorous ladybird beetle

The genetic basis for different host plant use was studied in a herbivorous ladybird beetle, Epilachna pustulosa, that exhibits interpopulational variation in host plant utilization. It usually depends on thistle, but one of the local forms occurs on both thistle and blue cohosh, which differ at the infraclass taxonomic level. In this local population, genetic association between the developmental performance on the two plants was neutral, suggesting genetic independence across the host plants. The form of reaction norms indicated some changes in relative rank position. Genetic variation among individuals on each host plant was detected. These results suggest that different genotypes are selected on different host plants and that a substantial proportion of the overall phenotypic plasticity is contributed by genotype-dependent environmental effects.

Difference in the time of mating activity between host‐associated populations of the rice stem borer, Chilo suppressalis (Walker)

Mating activities of the rice stem borer, Chilo suppressalis, were compared between rice‐ and wateroat‐feeding populations, and two potential temporal factors that may act as reproductive barriers were examined. Seasonal data of the number of moths attracted to pheromone traps showed that the peak of emergence at the rice field was approximately 10 days earlier than that at the wateroat vegetation in the first flight season, although there was a broad overlap of emergence at the two locations. Both field observations and a laboratory experiment showed that moths from the rice field started mating earlier than those from the wateroat vegetation. However, whereas the difference was distinctive in the laboratory experiment, mating activity at the wateroat vegetation shifted significantly to an earlier time phase than that observed in the laboratory. Body size data showed that the male moths attracted to the pheromone traps at the wateroat vegetation were significantly larger than those at the rice field, suggesting that the traps at the two locations mainly attracted moths originating from different host plants. However, pheromone‐trapped males at the rice field were significantly larger than those reared from overwintering samples. These results support the idea that males from the wateroat vegetation migrate to the rice field. The differences in seasonal and temporal mating activity and their effects on development of reproductive isolation between host‐associated populations are discussed.

Population differentiation in host-plant use in a herbivorous ladybird beetle, Epilachna vigintioctomaculata

Herbivorous insects that are polyphagous as a species often exhibit monophagy or oligophagy at the population level (Fox & Morrow, 1981). The difference among populations in host-plant use may arise either intrinsically from genetic changes and/or extrinsically from changes in ecological factors such as availability of host-plant species (Rausher, 1982; Tabashnik, 1983; Hare & Kennedy; 1986; Scriber, 1986). Even when genetic changes are not involved initially, differential host use may lead to evolutionary divergence between populations. If local populations of a herbivore species use different host plants, the populations may diverge as each is subjected to natural selection for improved ability to use its own host species. Thus, variation in host-plant use between populations can reveal patterns of local adaptation and suggests the responses to past selective forces. Epilachna vigintioctomaculata(Coleoptera, Coccinellidae, Epilachninae) is a likely candidate in which to detect differential adaptation. Epilachna vigintioctomaculata is one of the closely related herbivorous ladybird beetles ( E. vigintioctomaculata complex) that has diversified greatly in external morphology and host-plant use in and around the Japanese Archipelago. Epilachna vigintioctomaculata mainly feeds onSolanum tuberosumL. (Solanaceae). Although some additional plants in the family Solanaceae are subsidiarily used by adult beetles, larvae develop almost entirely onS. tuberosum (Katakura, 1981). Some populations on Hokkaido (the main northern island of Japan) are found on Schizopepon bryoniaefoliusMaxim. (Cucurbitaceae) but this host is not attacked on Honshu (the main central island). Such variation suggests geographical variation in host-plant use (Katakura, 1981), as the populations on Hokkaido have depended and persisted on S. bryoniaefoliussince their discovery in 1951 (Katakura, 1981). Thus, the Hokkaido populations on S. bryoniaefoliusand Honshu populations may be subject to differential selection on their respective host-plants. Here we conduct reciprocal transplant experiments to test for growth performance differences between E. vigintioctomaculatacollected fromS. bryoniaefolius on Hokkaido andS. tuberosumon Honshu and determined whether the larvae from different populations exhibit differential adaptation to their own host-plant species.

Distribution and Differentiation of Henosepilachna diekei (Coleoptera: Coccinellidae) on Two Host-Plant Species Across Java, Indonesia

ABSTRACT Divergent natural selection on different host plants may be a crucial factor in promoting the remarkable diversity of phytophagous insects, and might occur in any geographical context. Because the intensity and consequences of divergent selection on different hosts can vary depending on the degree of gene flow between conspecific insect populations, elucidating the geographical context and degree of host specificity in the incipient phase of differential host use is indispensable to understanding the diversification process in phytophagous insects. Henosepilachna diekei Jadwiszczak & Węgrzynowicz (Coleoptera: Coccinellidae) is a tropical ladybird beetle occurring mainly on two host species from different plant families, Asteraceae and Lamiaceae. We investigated the geographical distribution of H. diekei across Java, Indonesia, in relation to the availability of the two hosts, and examined the host specificity of beetles in the laboratory. We also investigated genetic relationships among local populations of beetles using mitochondrial NADH dehydrogenase subunit 2 gene sequences. Geographic variation in host use by H. diekei was largely determined by skewed geographical distributions of the hosts, although there was a synergistic effect with extremely divergent host specificity by the beetles. The molecular analyses suggested that genetic differentiation among the beetle populations has occurred and has been maintained by the effects of both geographical distance and divergent host specificity. The geographical distribution of H. diekei populations differing in host specificity suggests that geographical distance, local host-plant availability, and divergent host specificity contribute synergistically to promote the genetic differentiation and subsequent diversification of phytophagous insects on different hosts.