Unrelated Host Plants Research Articles

New species, names, and type designation of rust fungi from New Zealand

ABSTRACT Rust fungi are obligate plant pathogens, usually restricted to a single host species or genus, although some species such as myrtle rust (Austropuccinia psidii) and some cereal and grass rusts (Puccinia spp.) can infect many members of the families Myrtaceae and Poaceae, respectively. The rust life cycle can involve up to five spore stages, both sexual and asexual. The cycle may be completed on a single host (autoecious rust) or require two, unrelated host plants (heteroecious rust) for completion. About 300 rust species are known in New Zealand, with approximately half comprising native rusts (usually pathogenic on native plants) and half being introduced rusts (usually pathogenic on exotic plants). In this paper, 26 new species of rust fungi discovered in New Zealand are described, comprising one species each of Aecidium and Milesina, two of Pucciniastrum, four of Uromyces and 18 of Puccinia. Puccinia ferocis-forsteri is a new heteroecious species producing uredinia and telia on the endemic Carex forsteri and aecia on endemic Urtica ferox. This is only the third native heteroecious rust fungus reported in New Zealand. In addition, two species are transferred to the genus Milesina, three to Uromyces and seven to Puccinia (including a nom. nov.), either following discovery of the sexual (telial) stage of these rusts or considering the most appropriate sexual genus. A new combination is proposed for Coleosporium based on morphology of its urediniospores and molecular data. Epitypes are designated for eight species, lectotypes are selected for five species and a neotype is designated. Apart from one new species on the introduced Fuchsia magellanica all the rusts are considered native to New Zealand. Where appropriate, DNA was extracted and the 28S region (nuclear large subunit ribosomal DNA [nuLSU]) was amplified with the resulting LSU sequences aligned with sequences from GenBank representing most of the major clades.

Host Adaptation and Virulence in Heteroecious Rust Fungi.

Rust fungi (Pucciniales, Basidiomycota) are obligate biotrophic pathogens that cause rust diseases in plants, inflicting severe damage to agricultural crops. Pucciniales possess the most complex life cycles known in fungi. These include an alternation of generations, the development of up to five different sporulating stages, and, for many species, the requirement of infecting two unrelated host plants during different parts of their life cycle, termed heteroecism. These fungi have been extensively studied in the past century through microscopy and inoculation studies, providing precise descriptions of their infection processes, although the molecular mechanisms underlying their unique biology are poorly understood. In this review, we cover recent genomic and life cycle transcriptomic studies in several heteroecious rust species, which provide insights into the genetic tool kits associated with host adaptation and virulence, opening new avenues for unraveling their unique evolution.

Distinct Transcriptomic Reprogramming in the Wheat Stripe Rust Fungus During the Initial Infection of Wheat and Barberry.

Puccinia striiformis f. sp. tritici is the causal agent of wheat stripe rust that causes severe yield losses all over the world. As a macrocyclic heteroecious rust fungus, it is able to infect two unrelated host plants, wheat and barberry. Its urediniospores infect wheat and cause disease epidemic, while its basidiospores parasitize barberry to fulfill the sexual reproduction. This complex life cycle poses interesting questions on the different mechanisms of pathogenesis underlying the infection of the two different hosts. In the present study, transcriptomes of P. striiformis f. sp. tritici during the initial infection of wheat and barberry leaves were qualitatively and quantitatively compared. As a result, 142 wheat-specifically expressed genes (WEGs) were identified, which was far less than the 2,677 barberry-specifically expressed genes (BEGs). A larger proportion of evolutionarily conserved genes were observed in BEGs than that in WEGs, implying a longer history of the interaction between P. striiformis f. sp. tritici and barberry. Additionally, P. striiformis f. sp. tritici differentially expressed genes (DEGs) between wheat at 1 and 2 days postinoculation (dpi) and barberry at 3 and 4 dpi were identified by quantitative analysis. Gene Ontology analysis of these DEGs and expression patterns of P. striiformis f. sp. tritici pathogenic genes, including those encoding candidate secreted effectors, cell wall-degrading enzymes, and nutrient transporters, demonstrated that urediniospores and basidiospores exploited distinct strategies to overcome host defense systems. These results represent the first analysis of the P. striiformis f. sp. tritici transcriptome in barberry and contribute to a better understanding of the evolutionary processes and strategies of different types of rust spores during the infection process on different hosts.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Gut bacterial communities and their contribution to performance of specialist Altica flea beetles.

Host plant shifts are a common mode of speciation in herbivorous insects. Although insects can evolve adaptations to successfully incorporate a new host plant, it is becoming increasingly recognized that the gut bacterial community may play a significant role in allowing insects to detoxify novel plant chemical defenses. Here, we examined differences in gut bacterial communities between Altica flea beetle species that feed on phylogenetically unrelated host plants in sympatry. We surveyed the gut bacterial communities of three closely related flea beetles from multiple locations using 16S rRNA amplicon sequencing. The results showed that the beetle species shared a high proportion (80.7%) of operational taxonomic units. Alpha-diversity indicators suggested that gut bacterial diversity did not differ among host species, whereas geography had a significant effect on bacterial diversity. In contrast, analyses of beta-diversity showed significant differences in gut bacterial composition among beetle species when we used species composition and relative abundance metrics, but there was no difference in composition when species presence/absence and phylogenetic distance indices were used. Within host beetle species, gut bacterial composition varied significantly among sites. A metagenomic functionality analysis predicted that the gut microbes had functions involved in xenobiotic biodegradation and metabolism as well as metabolism of terpenoids and polyketides. These predictions, however, did not differ among beetle host species. Antibiotic curing experiments showed that development time was significantly prolonged, and there was a significant decline in body weight of newly emerged adults in beetles lacking gut bacteria, suggesting the beetles may receive a potential benefit from the gut microbe-insect interaction. On the whole, our results suggest that although the gut bacterial community did not show clear host-specific patterns among Altica species, spatiotemporal variability is an important determinant of gut bacterial communities. Furthermore, the similarity of communities among these beetle species suggests that microbial facilitation may not be a determinant of host plant shifts in Altica.

Species diversity, taxonomy, and phylogeny of Gymnosporangium in China

ABSTRACT Gymnosporangium is a group of plant fungal pathogens that cause rust diseases on many economically important fruit trees. Most Gymnosporangium are heteroecious and demicyclic, producing four morphologically diverse spore stages on two taxonomically unrelated host plants, the Cupressaceae and Rosaceae. The complex life cycle and heteroecism make it difficult to investigate the species within Gymnosporangium. To determine the taxonomy, phylogeny, and species diversity of Gymnosporangium in China, a large collection of 672 specimens were analyzed using a combination of morphological observations and phylogenetic analyses. In total, 27 Gymnosporangium species from China are documented here, including 22 known species, one new combination, one new record, and three new species. The study also documents a novel aeciospore surface structure with an irregular surface that is described here as “surfy.”

Further Support of Conspecificity of Oak and Mango Powdery Mildew and First Report of Erysiphe quercicola and Erysiphe alphitoides on Mango in Mainland Europe.

Mango leaves and inflorescences infected by powdery mildew in southern Spain were analyzed using multigene sequencing (ITS + 4 single-copy coding genes) to identify the causal agent. Erysiphe quercicola was detected in 97% out of 140 samples, collected in six different orchards in the Malaga region. Among these, a small proportion also yielded E. alphitoides (8% of all samples) and E. alphitoides was found alone in 3% of samples. A phylogenetic approach was completed by cross inoculations between oak and mango, which led to typical symptoms, supporting the conspecificity of oak and mango powdery mildews. To our knowledge, this is the first report of E. quercicola and E. alphitoides causing powdery mildew on mango trees in mainland Spain, and thus mainland Europe, based on unequivocal phylogenetic and biological evidence. Our study thus confirmed the broad host range of both E. quercicola and E. alphitoides. These results have practical implications in terms of the demonstrated ability for host range expansion in powdery mildews. They also open interesting prospects to the elucidation of molecular mechanisms underlying the ability to infect single versus multiple and unrelated host plants since these two closely related powdery mildew species belong to a small clade with both generalist and specialist powdery mildews.

The importance of pollen chemistry in evolutionary host shifts of bees

Although bee-plant associations are generally maintained through speciation processes, host shifts have occurred during evolution. Understanding shifts between both phylogenetically and morphologically unrelated plants (i.e., host-saltation) is especially important since they could have been key processes in the origin and radiation of bees. Probably far from being a random process, such host-saltation might be driven by hidden constraints associated with plant traits. We selected two clades of oligolectic bees (i.e., Colletes succinctus group and Melitta leporina group) foraging on co-flowering but unrelated host-plants to test this hypothesis. We analyzed floral scent, floral color and chemical composition of pollen from host and non-host plants of these two clades. We did not find evidence for host-plant evolution in the Melitta leporina group driven by one of the assayed floral traits. On the contrary, hosts of the C. succinctus group display similar primary nutritive content of pollen (i.e., amino acids and sterols) but not similar floral scent or color, suggesting that shared pollen chemistry probably mediates saltation in this clade. Our study revealed that constraints shaping floral associations are diverse and clearly depend on species life-history traits, but evidence suggests that pollen chemistry may act as a major floral filter and guide evolutionary host-shifts.

Host-plants of leaf-miners in Australian subtropical rainforest

Leaf-miners are endophytic insect herbivores that are considered to be relatively host-specific compared with other types of insect herbivores, often depending on one or a few congeneric hosts. Because of their degree of host-specificity, they may be particularly vulnerable to environmental change. Despite this, little is known about the host-plants and life histories of the Australian leaf-mining fauna. Here we present new information on the host-plant use of leaf-miners occurring in Australian subtropical rainforest. We repeatedly hand-collected leaf-miners at 14 sampling sites in the ‘Tweed Caldera’ subtropical rainforest region of south-eastern Queensland and north-eastern New South Wales, Australia. Leaf-miners and their host-plants were identified to species (or morphospecies in the case of some leaf-miners). Within the region, a total of 106 plant species was recorded as leaf-miner hosts, on which a total of 12 679 individual leaf-miners was counted, belonging to 50 different species. We measured the local host-plant range of each leaf-miner species for which we had reliable incidence records across sampling sites (24 species). Local host-specificity was relatively high with 66.7 % of species recorded from a single or two congeneric host-plants. 16.7 % of species were restricted to a single plant family and 16.7 % were recorded on a few to several plants of the same plant order or across a range of unrelated host-plants.

Editorial: Genomics Research on Non-model Plant Pathogens: Delivering Novel Insights into Rust Fungus Biology.

The diversity among rust fungi is simply astounding: over 7000 species that evolved to colonize niches all over the plant kingdom. This diversification likely involved major host jumps, especially considering that the life cycles of heteroecious rusts, such as the cereal rusts, involve sexual and asexual stages that take place on completely unrelated host plants (Savile, 1976; McTaggart et al., 2015), but also co-evolution in diverse natural settings, establishing equilibrium (e.g., Thrall et al., 2012). These interactions resulted in complex life styles including for many rusts, the production of up to five different spore types, each requiring very different developmental programs and hence gene expression.

Phylogeny, evolution and classification of gall wasps: the plot thickens.

Gall wasps (Cynipidae) represent the most spectacular radiation of gall-inducing insects. In addition to true gall formers, gall wasps also include phytophagous inquilines, which live inside the galls induced by gall wasps or other insects. Here we present the first comprehensive molecular and total-evidence analyses of higher-level gall wasp relationships. We studied more than 100 taxa representing a rich selection of outgroups and the majority of described cynipid genera outside the diverse oak gall wasps (Cynipini), which were more sparsely sampled. About 5 kb of nucleotide data from one mitochondrial (COI) and four nuclear (28S, LWRh, EF1alpha F1, and EF1alpha F2) markers were analyzed separately and in combination with morphological and life-history data. According to previous morphology-based studies, gall wasps evolved in the Northern Hemisphere and were initially herb gallers. Inquilines originated once from gall inducers that lost the ability to initiate galls. Our results, albeit not conclusive, suggest a different scenario. The first gall wasps were more likely associated with woody host plants, and there must have been multiple origins of gall inducers, inquilines or both. One possibility is that gall inducers arose independently from inquilines in several lineages. Except for these surprising results, our analyses are largely consistent with previous studies. They confirm that gall wasps are conservative in their host-plant preferences, and that herb-galling lineages have radiated repeatedly onto the same set of unrelated host plants. We propose a revised classification of the family into twelve tribes, which are strongly supported as monophyletic across independent datasets. Four are new: Aulacideini, Phanacidini, Diastrophini and Ceroptresini. We present a key to the tribes and discuss their morphological and biological diversity. Until the relationships among the tribes are resolved, the origin and early evolution of gall wasps will remain elusive.

Importance of protein quality versus quantity in alternative host plants for a leaf-feeding insect

The nutritional value of alternative host plants for leaf-feeding insects such as caterpillars is commonly measured in terms of protein quantity. However, nutritional value might also depend on the quality of the foliar protein [i.e., the composition of essential amino acids (EAAs)]. A lack of comparative work on the EAA compositions of herbivores and their host plants has hampered the testing of this hypothesis. We tested the "protein quality hypothesis" using the tree-feeding caterpillars of Lymantria dispar (gypsy moth) and two taxonomically unrelated host plants, red oak (Quercus rubra) and sugar maple (Acer saccharum). Because L. dispar has higher fitness on oak than on maple, support for the hypothesis would be found if protein were of higher quality from oak than from maple. The whole-body EAA composition of L. dispar larvae was measured to estimate its optimum dietary protein composition, which was compared with the EAA compositions of oak and maple leaves. Contrary to the protein quality hypothesis, the EAA compositions of oak and maple were not significantly different in the spring. The growth-limiting EAAs in both tree species were histidine and methionine. Similar results were observed in the summer, with the exception that the histidine composition of oak was between 10 and 15% greater than in maple leaves. The two main factors that affected the nutritional value of protein from the tree species were the quantities of EAAs, which were consistently higher in oak, and the efficiency of EAA utilization, which decreased from 80% in May to <50% in August. We conclude that the relative nutritional value of red oak and sugar maple for L. dispar is more strongly affected by protein quantity than quality. Surveys of many wild herbaceous species also suggest that leaf-feeding insects would be unlikely to specialize on plants based on protein quality.

Host Plant Utilization Within Family Ericaceae by the Andromeda Lace Bug Stephanitis takeyai (Hemiptera: Tingidae)

This study examined host plant utilization by the Andromeda lace bug, Stephanitis takeyai Drake and Maa, within the family Ericaceae. The preferred host of S. takeyai is the Japanese pieris, Pieris japonica (Thunb.) D. Don ex G. Don, but it has been reported to exhibit host alternation and also to occur on other unrelated host plants. We examined the acceptability of ten landscape and fruit plants belonging to the family Ericaceae (Rhododendron calendulaceum, Rhododendron ‘Hampton Beauty’, Rhododendron ‘Autumn Empress’, Vaccinium arboreum, Vaccinium virgatum, Calluna vulgaris, Kalmia latifolia, Pieris floribunda, Pieris phillyreifolia and Pieris japonica ‘Temple Bells’) to S. takeyai. In no-choice tests adult survival did not vary significantly among the taxa. Highest leaf damage was recorded on P. japonica and R. calendulaceum, while slight damage was noted on V. arboreum and Rhododendron ‘Hampton Beauty’. Nymph emergence was observed in P. japonica, R. calendulaceum and Rhododendron ‘Hampton Beauty’. In multi-choice tests adult presence on leaves did not vary significantly except on day 9, when adults were more numerous on ‘Temple Bells’. Maximum leaf damage was recorded on P. japonica, ‘Temple Bells’. Several plants like Rhododendron and Vaccinium spp., which may not be favorable hosts, could serve as reservoirs for the pest.

Recent Speciation in Three Closely Related Sympatric Specialists: Inferences Using Multi-Locus Sequence, Post-Mating Isolation and Endosymbiont Data

Shifting between unrelated host plants is relatively rare for phytophagous insects, and distinct host specificity may play crucial roles in reproductive isolation. However, the isolation status and the relationship between parental divergence and post-mating isolation among closely related sympatric specialists are still poorly understood. Here, multi-locus sequence were used to estimate the relationship among three host plant–specific closely related flea beetles, Altica cirsicola, A. fragariae and A. viridicyanea (abbreviated as AC, AF and AV respectively). The tree topologies were inconsistent using different gene or different combinations of gene fragments. The relationship of AF+(AC+AV) was supported, however, by both gene tree and species tree based on concatenated data. Post-mating reproductive data on the results of crossing these three species are best interpreted in the light of a well established phylogeny. Nuclear-induced but not Wolbachia-induced unidirectional cytoplasmic incompatibility, which was detected in AC-AF and AF-AV but not in AC-AV, may also suggest more close genetic affinity between AC and AV. Prevalence of Wolbachia in these three beetles, and the endosymbiont in most individuals of AV and AC sharing a same wsp haplotype may give another evidence of AF+(AC+AV). Our study also suggested that these three flea beetles diverged in a relative short time (0.94 My), which may be the result of shifting between unrelated host plants and distinct host specificity. Incomplete post-mating isolation while almost complete lineage sorting indicated that effective pre-mating isolation among these three species should have evolved.

Phylogenetic analyses of the leaf beetle genus Galerucella: Evidence for host switching at speciation?

It is still the subject of lively debate whether sympatric speciation is a general mode of speciation as opposed to allopatric speciation. In herbivorous insects, host switching, i.e. colonization of, and adaptation to, a new host by a herbivore, has been proposed as one of the driving mechanisms of sympatric speciation. Evidence for host switching as a speciation driving mechanism can be inferred from phylogenies of herbivores and host plants: if the host plant phylogeny is randomly distributed over the herbivore phylogeny, this indicates host switching. The Chrysomelid beetle genus Galerucella is a good taxon to study for evidence of host switching, because several closely related Galerucella species form sympatric species complexes associated with various unrelated plant species. Here we present the phylogenetic relationships of 10 species in the genus Galerucella, based on the mitochondrial gene fragments of the NADH-2 (410 bp) and CO-I (659 bp) genes, and analyzed with Bayesian, Maximum Likelihood and Maximum Parsimony methods. The resulting molecular phylogenetic tree proved to be largely congruent with morphologically based taxonomy. The host-associated taxa of the Galerucella nymphaeae species complex are not defined as distinct gene pools under the phylogenetic species concept (PSC), however, the species complex as a whole is. Two results indicate the contribution of host switching to the speciation of Galerucella: (1) the host-associated taxa of the G. nymphaeae species complex have diverged very recently and (2) constrained ML analyses showed that host use constraints led to a significantly different Galerucella tree compared to unconstrained analyses. This evidence for host switching, together with the observation that several sister taxa using unrelated host plants live in sympatry, suggests that sympatric speciation by host race formation can be an important mode of speciation in this genus.

Genetic variation and covariation of aphid life-history traits across unrelated host plants

A central paradigm of life-history theory is the existence of resource mediated trade-offs among different traits that contribute to fitness, yet observations inconsistent with this tenet are not uncommon. We previously found a clonal population of the aphid Myzus persicae to exhibit positive genetic correlations among major components of fitness, resulting in strong heritable fitness differences on a common host. This raises the question of how this genetic variation is maintained. One hypothesis states that variation for resource acquisition on different hosts may override variation for allocation, predicting strong fitness differences within hosts as a rule, but changes in fitness hierarchies across hosts due to trade-offs. Therefore, we carried out a life-table experiment with 17 clones of M. persicae, reared on three unrelated host plants: radish, common lambsquarters and black nightshade. We estimated the broad-sense heritabilities of six life-history traits on each host, the genetic correlations among traits within hosts, and the genetic correlations among traits on different hosts (cross-environment genetic correlations). The three plants represented radically different environments with strong effects on performance of M. persicae, yet we detected little evidence for trade-offs. Fitness components were positively correlated within hosts but also between the two more benign hosts (radish and lambsquarters), as well as between those and another host tested earlier. The comparison with the most stressful host, nightshade, was hampered by low survival. Survival on nightshade also exhibited genetic variation but was unrelated to fitness on other hosts. Acknowledging that the number of environments was necessarily limited in a quantitative genetic experiment, we suggest that the rather consistent fitness hierarchies across very different plants provided little evidence to support the idea that the clonal variation for life-history traits and their covariance structure are maintained by strong genotypexenvironment interactions with respect to hosts. Alternative explanations are discussed.

Phylogenetic relationships and host‐plant evolution within the basal clade of Halictidae (Hymenoptera, Apoidea)

AbstractBees are among the most important pollinators of angiosperm plants. Many bee species show narrow host‐plant preferences, reflected both in behavioral and morphological adaptations to particular attributes of host‐plant pollen or floral morphology. Whether bee host‐plant associations reflect co‐cladogenesis of bees and their host plants or host‐switches to unrelated host plants is not clear. Rophitinae is a basal subfamily of Halictidae in which most species show narrow host‐plant preferences (oligolecty). We reconstructed the phylogenetic relationships among the rophitine genera using a combination of adult morphology (24 characters) and DNA sequence data (EF‐1α, LW rhodopsin, wingless; 2700 bp total). The data set was analyzed by parsimony, maximum likelihood and Bayesian methods. All methods yielded highly congruent results. Using the phylogeny, we investigated the pattern of host‐plant association as well as the historical biogeography of Rophitinae. Our biogeographical analysis suggests a number of dispersal/vicariance events: (1) a basal split between North America and South America (most likely a dispersal from South America to North America), and (2) at least two subsequent interchanges between North America and Eurasia (presumably via the northern hemisphere land bridges). Our analysis of host‐plant associations indicates that Rophitinae specialized on a closely related group of angiosperm orders in the Euasterid I clade (mainly Gentianales, Lamiales and Solanales). However, there is little evidence of cocladogenesis between bees and plants and strong evidence of host switches to unrelated host plants. Based on our phylogenetic results we describe two new tribes of Rophitinae: Conanthalictini new tribe (including the genus Conanthalictus) and Xeralictini new tribe (including Xeralictus and Protodufourea).© The Willi Hennig Society 2007.

Attributes and congruence of three molecular data sets: Inferring phylogenies among Septoria-related species from woody perennial plants

To improve our understanding of phylogenetic relationships within the anamorphic genus Septoria, three molecular data sets representing 2417 bp of nuclear and mitochondrial genes were evaluated. Separate gene analyses and combined analyses were performed using first, the maximum parsimony criterion and second, a Bayesian framework. The homogeneity of data partitions was evaluated via a combination of homogeneity partition tests and tree topology incongruence tests before conducting combined analyses. A last incongruence re-evaluation using partitioned Bremer support was performed on the combined tree, which corroborated the previous estimates. After each separate data set attributes were examined, simple explanations were advocated as the causes of the significant incongruences detected. The analysis of multiple gene partitions showed unprecedented phylogenetic resolution within the genus Septoria that supported the results from previously published single gene phylogenies. Specifically, we have delimited distinct but closely related species representing monophyletic groups that frequently correlated with their respective host families. Conversely, the occurrence of well-supported groups including closely related but distinct molecular taxa sampled on unrelated host-plants allowed us to reject, in these particular cases, the co-evolutionary concept expected between a parasite and its host and to discuss alternative evolutionary models recently proposed for these pathogens.

COMPARATIVE POPULATION GENETIC STRUCTURES AND LOCAL ADAPTATION OF TWO MUTUALISTS

Similar patterns of dispersal and gene flow between closely associated organisms may promote local adaptation and coevolutionary processes. We compare the genetic structures of the two species of a plant genus (Roridula gorgonias and R. dentata) and their respective obligately associated hemipteran mutualists (Pameridea roridulae and P. marlothi) using allozymes. In addition, we determine whether genetic structure is related to differences in host choice by Pameridea. Allozyme variation was found to be very structured among plant populations but less so among hemipteran populations. Strong genetic structuring among hemipteran populations was only evident when large distances isolated the plant populations on which they live. Although genetic distances among plant populations were correlated with genetic distances among hemipteran populations, genetic distances of both plants and hemipterans were better correlated with geographic distance. Because Roridula and Pameridea have different scales of gene flow, adaptation at the local population level is unlikely. However, the restricted gene flow of both plants and hemipterans could enable adaptation to occur at a regional level. In choice experiments, the hemipteran (Pameridea) has a strong preference for its carnivorous host plant (Roridula) above unrelated host plants. Pameridea also prefers its host species to its closely related sister species. Specialization at the specific level is likely to reinforce cospeciation processes in this mutualism. However, Pameridea does not exhibit intraspecific preferences toward plants from their natal populations above plants from isolated, non-natal populations.

Explaining the coexistence of asexuals with their sexual progenitors: no evidence for general‐purpose genotypes in obligate parthenogens of the peach‐potato aphid, Myzus persicae

AbstractIn parthenogens, selection acts on entire genotypes rather than individual alleles. The general‐purpose genotype hypothesis (GPG) predicts that temporally variable environments select for clones with broad ecological tolerances. These general‐purpose genotypes should exhibit low fitness variance and high geometric mean fitness across environments. We tested this hypothesis by comparing the fitness of obligately and cyclically parthenogenetic genotypes of the peach‐potato aphid, Myzus persicae, on three unrelated host plants. We found genetic variation for the relative performance on different hosts, but no difference in geometric mean fitness between obligate and cyclical parthenogens. Thus, for an environmental variable of major importance to aphids, the GPG hypothesis was not supported. In addition, the lack of an overall fitness difference between reproductive modes suggests that cyclical parthenogens incur no cost of polyphenism, but neither can they compensate for the cost of sex during the parthenogenetic phase of their life cycle.

RADIATION, DIVERSITY, AND HOST-PLANT INTERACTIONS AMONG ISLAND AND CONTINENTAL LEGUME-FEEDING PSYLLIDS

Island archipelagos and insect-plant associations have both independently provided many useful systems for evolutionary study. The arytainine psyllid (Sternorrhyncha: Hemiptera) radiation on broom (Fabaceae: Genisteae) in the Canary Island archipelago provides a discrete system for examining the speciation of highly host-specific phytophagous insects in an island context. Phylogenetic reconstructions based on three datasets (adult and nymph morphological characters, and two mitochondrial DNA regions: part of the small subunit rRNA, and part of cytochrome oxidase I, cytochrome oxidase II and the intervening tRNA leucine) are generally consistent. The combined molecular tree provides a well-supported estimate of psyllid relationships and shows that there have been several colonizations of the Macaronesian islands but that only one has resulted in a significant radiation. Psyllid diversification has apparently been constrained by the presence of suitable host groups within the genistoid legumes, and the diversity, distribution, and abundance of those groups. The phylogeny, by indicating pairs of sister species, allows putative mechanisms of speciation to be assessed. The most common conditions associated with psyllid speciation are geographical allopatry with a host switch to closely related hosts (six examples), or geographical allopatry on the same host (four examples). Where allopatric speciation involves a host switch, these have all been to related hosts. There is some evidence that switches between unrelated host plants may be more likely in sympatry. Only one sister pair (Aryrtainilla cytisi and A. telonicola) and the putative host races of Arytinnis modica are sympatric but on unrelated hosts, which may be a necessary condition for sympatric speciation in these insects. Where several psyllids share the same host, resources appear to be partitioned by ecological specialization and differing psyllid phenology.