Patrick Abbot

Phytophagous Insect-Microbe Mutualisms and Adaptive Evolutionary Diversification

Abstract Adaptive diversification is a process intrinsically tied to species interactions. Yet, the influence of most types of interspecific interactions on adaptive evolutionary diversification remains poorly understood. In particular, the role of mutualistic interactions in shaping adaptive radiations has been largely unexplored, despite the ubiquity of mutualisms and increasing evidence of their ecological and evolutionary importance. Our aim here is to encourage empirical inquiry into the relationship between mutualism and evolutionary diversification, using herbivorous insects and their microbial mutualists as exemplars. Phytophagous insects have long been used to test theories of evolutionary diversification; moreover, the diversification of a number of phytophagous insect lineages has been linked to mutualisms with microbes. In this perspective, we examine microbial mutualist mediation of ecological opportunity and ecologically based divergent natural selection for their insect hosts. We also explore the conditions and mechanisms by which microbial mutualists may either facilitate or impede adaptive evolutionary diversification. These include effects on the availability of novel host plants or adaptive zones, modifying host-associated fitness trade-offs during host shifts, creating or reducing enemy-free space, and, overall, shaping the evolution of ecological (host plant) specialization. Although the conceptual framework presented here is built on phytophagous insect–microbe mutualisms, many of the processes and predictions are broadly applicable to other mutualisms in which host ecology is altered by mutualistic interactions.

Harnessing genomics for evolutionary insights

Next-generation DNA sequencing technologies can generate unprecedented amounts of genomic data, even for non-model organisms. Here we describe how these new technologies have facilitated recent key advances in ecology and evolutionary biology, and highlight several outstanding ecological and evolutionary questions that are distinctly suited to the innovations they provide. Importantly, using these technologies to their full potential requires careful experimental design and critical consideration of several caveats associated with them. Although several significant challenges remain to be resolved before the integration of next-generation sequencing technologies into single-investigator research programs, we argue that they will soon transform ecology and evolution by fundamentally changing the ranges and types of questions that can be addressed.

Carotenoids in unexpected places: gall midges, lateral gene transfer, and carotenoid biosynthesis in animals.

Carotenoids are conjugated isoprenoid molecules with many important physiological functions in organisms, including roles in photosynthesis, oxidative stress reduction, vision, diapause, photoperiodism, and immunity. Until recently, it was believed that only plants, microorganisms, and fungi were capable of synthesizing carotenoids and that animals acquired them from their diet, but recent studies have demonstrated that two arthropods (pea aphid and spider mite) possess a pair of genes homologous to those required for the first step of carotenoid biosynthesis. Absent in all other known animal genomes, these genes appear to have been acquired by aphids and spider mites in one or several lateral gene transfer events from a fungal donor. We report the third case of fungal carotenoid biosynthesis gene homologs in an arthropod: flies from the family Cecidomyiidae, commonly known as gall midges. Using phylogenetic analyses we show that it is unlikely that lycopene cyclase/phytoene synthase and phytoene desaturase homologs were transferred singly to an ancient arthropod ancestor; instead we propose that genes were transferred independently from related fungal donors after divergence of the major arthropod lineages. We also examine variation in intron placement and copy number of the carotenoid genes that may underlie function in the midges. This trans-kingdom transfer of carotenoid genes may represent a key innovation, underlying the evolution of phytophagy and plant-galling in gall midges and facilitating their extensive diversification across plant lineages.

Same Host-Plant, Different Sterols: Variation in Sterol Metabolism in an Insect Herbivore Community

Insects lack the ability to synthesize sterols de novo, which are required as cell membrane inserts and as precursors for steroid hormones. Herbivorous insects typically utilize cholesterol as their primary sterol. However, plants rarely contain cholesterol, and herbivorous insects must, therefore, produce cholesterol by metabolizing plant sterols. Previous studies have shown that insects generally display diversity in phytosterol metabolism. Despite the biological importance of sterols, there has been no investigation of their metabolism in a naturally occurring herbivorous insect community. Therefore, we determined the neutral sterol profile of Solidago altissima L., six taxonomically and ecologically diverse herbivorous insect associates, and the fungal symbiont of one herbivore. Our results demonstrated that S. altissima contained Δ7-sterols (spinasterol, 22-dihydrospinasterol, avenasterol, and 24-epifungisterol), and that 85% of the sterol pool existed in a conjugated form. Despite feeding on a shared host plant, we observed significant variation among herbivores in terms of their qualitative tissue sterol profiles and significant variation in the cholesterol content. Cholesterol was absent in two dipteran gall-formers and present at extremely low levels in a beetle. Cholesterol content was highly variable in three hemipteran phloem feeders; even species of the same genus showed substantial differences in their cholesterol contents. The fungal ectosymbiont of a dipteran gall former contained primarily ergosterol and two ergosterol precursors. The larvae and pupae of the symbiotic gall-former lacked phytosterols, phytosterol metabolites, or cholesterol, instead containing an ergosterol metabolite in addition to unmetabolized ergosterol and erogsterol precursors, thus demonstrating the crucial role that a fungal symbiont plays in their nutritional ecology. These data are discussed in the context of sterol physiology and metabolism in insects, and the potential ecological and evolutionary implications.

Ant-Aphid Interactions: Are Ants Friends, Enemies, or Both?

Abstract Interactions between ants and aphids range from mutualistic to antagonistic. Understanding the ecological basis for such interactions requires understanding the costs and benefits to the aphids of ant-tending. Such an analysis is not simple, because ants can simultaneously have positive and negative effects upon aphids. The aphids Pleotrichophorus utensis Pack & Knowlton and Uroleucon escalantii Knowlton (both Hemiptera: Aphididae) are occasionally tended by Formica obscuripes Forel (Hymenoptera: Formicidae) at field sites in central Colorado. To compare the relative effects of protection and predation by ants on aphid abundance, we experimentally crossed the presence of the ants and other predators on host plants on which one or both aphids occur. Within a week of the start of the experiment, ants had a strong negative impact on aphid numbers that lasted the course of the experiment. Nonant predators initially had a weak negative effect on aphids, but by the end of the experiment, the negative effect of nonant predators was similar in magnitude to the effect of the ants. The negative effect of ants and other enemies on aphids was nonadditive; simultaneous predation by ants and other enemies was not as strong as expected from estimates of predation rates by only ants or only other enemies. This study suggests that ants simultaneously protect and prey upon aphids. We suggest selection to appease ants and to gain protection from ants can both be important forces generating ant–aphid mutualisms.

Evolutionary diversification of the gall midge genus Asteromyia (Cecidomyiidae) in a multitrophic ecological context

Gall-forming insects provide ideal systems to analyze the evolution of host-parasite interactions and understand the ecological interactions that contribute to evolutionary diversification. Flies in the family Cecidomyiidae represent the largest radiation of gall-forming insects and are characterized by complex trophic interactions with plants, fungal symbionts, and predators. We analyzed the phylogenetic history and evolutionary associations of the North American cecidomyiid genus Asteromyia, which is engaged in a complex and perhaps co-evolving community of interactions with host-plants, fungi, and parasitoids. Mitochondrial gene trees generally support current classifications, but reveal extensive cryptic diversity within the eight named species. Asteromyia likely radiated after their associated host-plants in the Astereae, but species groups exhibit strong associations with specific lineages of Astereae. Evolutionary associations with fungal mutualists are dynamic, however, and suggest rapid and perhaps coordinated changes across trophic levels.

ON THE EVOLUTION OF DISPERSAL AND ALTRUISM IN APHIDS

How competitive interactions and population structure promote or inhibit cooperation in animal groups remains a key challenge in social evolution. In eusocial aphids, there is no single explanation for what predisposes some lineages of aphids to sociality, and not others. Because the assumption has been that most aphid species occur in essentially clonal groups, the roles of intra- and interspecific competition and population structure in aphid sociality have been given little consideration. Here, I used microsatellites to evaluate the patterns of variation in the clonal group structure of both social and nonsocial aphid species. Multiclonal groups are consistent features across sites and host plants, and all species—social or not—can be found in groups composed of large fractions of multiple clones, and even multiple species. Between-group dispersal in gall-forming aphids is ubiquitous, implying that factors acting ultimately to increase between-done interactions and decrease within-group relatedness were present in aphids prior to the origins of sociality. By demonstrating that between-group dispersal is common in aphids, and thus interactions between clones are also common, these results suggest that understanding the ecological dynamics of dispersal and competition may offer unique insights into the evolutionary puzzle of sociality in aphids.

Symbiont-mediated phenotypic variation without co-evolution in an insect-fungus association.

Recent studies have shown that symbionts can be a source of adaptive phenotypic variation for their hosts. It is assumed that co‐evolution between hosts and symbionts underlies these ecologically significant phenotypic traits. We tested this assumption in the ectosymbiotic fungal associate of the gall midge Asteromyia carbonifera. Phylogenetic analysis placed the fungal symbiont within a monophyletic clade formed by Botryosphaeria dothidea, a typically free‐living (i.e. not associated with an insect host) plant pathogen. Symbiont isolates from four divergent midge lineages demonstrated none of the patterns common to heritable microbial symbioses, including parallel diversification with their hosts, substitution rate acceleration, or A+T nucleotide bias. Amplified fragment length polymorphism genotyping of the symbiont revealed that within‐lineage genetic diversity was not clustered along host population lines. Culture‐based experiments demonstrated that the symbiont‐mediated variation in gall phenotype is not borne out in the absence of the midge. This study shows that symbionts can be important players in phenotypic variation for their hosts, even in the absence of a co‐evolutionary association.

Comparative analysis of the biodiversity of fungal endophytes in insect-induced galls and surrounding foliar tissue

Insect-induced galls are abnormal plant growths that can provide food and shelter to their inhabitants, resulting in stressed plant tissue that may alter the conditions for the colonization or proliferation of endophytic fungi. We investigated the effect gall formation has on fungal endophyte communities and diversity. Using three closely-related gall-forming aphid species that specialize on poplars, we characterized fungal endophyte diversity in galls and surrounding petiole and leaf lamina tissue. A total of 516 fungal endophyte samples were isolated from 272 tissue samples (32 leaves, 31 petioles, and 209 galls), resulting in 23 distinct morphotypes. Despite sharing a common host plant and often forming spatially contiguous galls, the endophyte profiles within the galls of each aphid species were distinct, not only from the galls of the other species, but also from surrounding plant tissue. These results suggest that insect galls can affect the composition of fungal endophyte species in plant tissues, by altering either the colonization or proliferation of their endophytic mycobiota. Likewise, fungal endophytes may be important in the ecology and evolution of insect galls.

Rampant Host and Defensive Phenotype Associated Diversification in a Goldenrod Gall Midge

Natural selection can play an important role in the genetic divergence of populations and their subsequent speciation. Such adaptive diversification, or ecological speciation, might underlie the enormous diversity of plant‐feeding insects that frequently experience strong selection pressures associated with host plant use as well as from natural enemies. This view is supported by increasing documentation of host‐associated (genetic) differentiation in populations of plant‐feeding insects using alternate hosts. Here, we examine evolutionary diversification in a single nominal taxon, the gall midge Asteromyia carbonifera (O.S.), with respect to host plant use and gall phenotype. Because galls can be viewed as extended defensive phenotypes of the midges, gall morphology is likely to be a reflection of selective pressures by enemies. Using phylogenetic and comparative analyses of mtDNA and nuclear sequence data, we find evidence that A. carbonifera populations are rapidly diversifying along host plant and gall morphological lines. At a broad scale, geography explains surprisingly little genetic variation, and there is little evidence of strict co‐cladogenesis with their Solidago hosts. Gall morphology is relatively labile, distinct gall morphs have evolved repeatedly and colonized multiple hosts, and multiple genetically and morphologically distinct morphs frequently coexist on a single host plant species. These results suggest that Asteromyia carbonifera is in the midst of an adaptive radiation driven by multitrophic selective pressures. Similar complex community pressures are likely to play a role in the diversification of other herbivorous insect groups.