Host shifts and evolutionary radiations of butterflies

Ecological divergence despite common mating sites: Genotypes and symbiotypes shed light on cryptic diversity in the black bean aphid species complex

Different host plants represent ecologically dissimilar environments for phytophagous insects. The resulting divergent selection can promote the evolution of specialized host races, provided that gene flow is reduced between populations feeding on different plants. In black bean aphids belonging to the Aphis fabae complex, several morphologically cryptic taxa have been described based on their distinct host plant preferences. However, host choice and mate choice are largely decoupled in these insects: they are host-alternating and migrate between specific summer host plants and shared winter hosts, with mating occurring on the shared hosts. This provides a yearly opportunity for gene flow among aphids using different summer hosts, and raises the question if and to what extent the ecologically defined taxa are reproductively isolated. Here, we analyzed a geographically and temporally structured dataset of microsatellite genotypes from A. fabae that were mostly collected from their main winter host Euonymus europaeus, and additionally from another winter host and fourteen summer hosts. The data reveals multiple, strongly differentiated genetic clusters, which differ in their association with different summer and winter hosts. The clusters also differ in the frequency of infection with two heritable, facultative endosymbionts, separately hinting at reproductive isolation and divergent ecological selection. Furthermore, we found evidence for occasional hybridization among genetic clusters, with putative hybrids collected more frequently in spring than in autumn. This suggests that similar to host races in other phytophagous insects, both prezygotic and postzygotic barriers including selection against hybrids maintain genetic differentiation among A. fabae taxa, despite a common mating habitat.

Elevated rates of dietary generalization in eusocial lineages of the secondarily herbivorous bees

BACKGROUND

Within the Hymenoptera, bees are notable for their relationship with flowering plants, being almost entirely dependent on plant pollen and nectar. Though functionally herbivorous, as a result of their role as pollinators, bees have received comparatively little attention as models for insect herbivory. Bees often display dietary specialization, but quantitative comparison against other herbivorous insects has not previously been conducted.

RESULTS

In the most comprehensive analysis to date for 860 bee species, dietary specialization amounted to 50.1% of studied species collecting pollen from between 1 and 2 botanical families with a relatively long tail of dietary generalists, with 11.1% of species collecting from more than 10 botanical families. This distribution deviated from the truncated Pareto distribution of dietary breadth seen in other herbivorous insect lineages. However, this deviation was predominantly due to eusocial bee lineages, which show a range of dietary breadths that conformed to a normal distribution, while solitary bees show a typical truncated distribution not strongly different from other herbivorous insects. We hypothesize that the relatively low level of dietary specialization in bees as a whole reflects the relaxation of the constraints typically observed in herbivorous insects with a comparatively reduced importance of plant chemistry and comparatively increased importance of phenology and foraging efficiency. The long flight periods of eusocial bees that are necessary to allow overlapping generations both allows and necessitates the use of multiple flowering resources, whereas solitary bees with short flight periods have more limited access to varied resources within a constrained activity period.

CONCLUSIONS

Collectively, solitary bees show slightly lower specialization compared to other herbivorous insects, possibly due to their balanced relationship with plants, rather than direct antagonism such as seen in the direct consumption of plant tissues. An additional factor may be the mediocre diversity of bees at low latitudes combined with low levels of dietary specialization, whereas these areas typically display a high rate of specialization by herbivorous insects in general. Though the most important factors structuring dietary specialization in bees appear to differ from many other herbivorous insects, solitary bees show a surprisingly similar overall pattern of dietary specialization.

Recent origin and diversification accompanied by repeated host shifts of thallus-mining flies (Diptera: Agromyzidae) on liverworts and hornworts

Despite the vast diversity of phytophagous insects that feed on vascular plants (tracheophytes), insects that feed on bryophytes remain understudied. Agromyzidae, one of the most species-rich phytophagous clades in Diptera, consists mainly of leaf-mining species that feed on tracheophytes. However, a recent discovery of thallus-mining species on liverworts and hornworts within the Liriomyza group of Phytomyzinae provides an opportunity to study host shifts between tracheophytes and bryophytes. This study aimed to explore the origin and diversification of thallus-miners and estimate the pattern and timing of host shifts. Phylogenetic analysis of Phytomyzinae has revealed that the thallus-mining agromyzids formed a separate clade, which was sister to a fern pinnule-miner. The diversification of bryophyte-associated agromyzids since the Oligocene involved multiple host shifts across various bryophyte taxa. The diversification of the thallus-mining Phytoliriomyza may have occurred at the same time as the leaf-mining agromyzid flies on herbaceous plants, indicating a dynamic history of interactions between bryophytes and herbivores in angiosperms-dominated ecosystems.

Post-dispersal astrobiological events: modelling macroevolutionary dynamics for lithopanspermia

Lithopanspermia is defined as dispersal of living extremophiles from one planetary body to another, through life-bearing rocks ejected by meteor impacts. If lithopanspermia proves concrete, it should be viewed as an eco-evolutionary phenomenon. Biogeographic/microevolutionary models have been proposed as analogues for lithopanspermia dynamics; however, extremophile arrival on a planetary body is not the end of story. Here, we suggest that eco-evolutionary (environment + organismal microevolution) dynamics can lead to distinct macroevolutionary scenarios after extremophile arrival on a planetary body. Speciation would be the most important factor in interplanetary dynamics due to the possibly long time and distance between dispersive events, similar to long-distance dispersal dynamics on Earth. In previously uninhabited planets, persistence of extremophiles and descendants depends almost only on evolvability of extremophiles against abiotic filters. Considering a previously inhabited planet, ecological interactions at local or global scales could drive persistence (speciation/extinction) of extremophiles in the new habitat. Thus, we might expect high extinction rates if negative interactions are dominant, or, high speciation, if positive interactions occur, with extremophile lineages overpower (or not) the native biota. If interplanetary dispersal is possible, theories about the evolution of life may be universal, leading to a general eco-evolutionary model for life in the Universe.

Spatio-Temporal Evolutionary Patterns of the Pieridae Butterflies (Lepidoptera: Papilionoidea) Inferred from Mitogenomic Data

Pieridae is one of the largest and almost cosmopolitan groups of butterflies, which plays an important role in natural ecosystems; however, to date, its phylogeny and evolutionary history have not been fully resolved. In this study, we obtained the complete or nearly complete mitochondrial genomes of 100 pierid taxa (six newly sequenced, sixty extracted from the whole-genome data, and thirty-four directly available from GenBank). At the same time, for the first time, we conducted comparative mitogenomic and phylogenetic analyses based on these mitogenomic data, to further clarify their spatio-temporal evolutionary patterns. Comparative mitogenomic analysis showed that, except for cox2, the GC content of each of the 13 protein-coding genes (PCGs) in the rapidly diverging subfamily Pierinae was higher than in its sister group Coliadinae. Moreover, the dN/dS values of nine genes (atp6, atp8, cox1, cox3, cob, nad1, nad3, nad5, and nad6) in Pierinae were also relatively higher than those in its sister group, Coliadinae. Phylogenetic analysis showed that all the resultant phylogenetic trees were generally in agreement with those of previous studies. The Pierinae family contained six clades in total with the relationship of (Leptosiaini + (((Nepheroniini + Arthocharidini) + Teracolini) + (Pierini + Elodini))). The Pieridae originated in the Palearctic region approximately 72.3 million years ago in the late Cretaceous, and the subfamily Pierinae diverged from this family around 57.9 million years ago in the Oriental region, shortly after the K-Pg mass extinction event; in addition, the spatio-temporal evolutionary patterns of Pierinae were closely correlated with geological events and environmental changes, as well as the host plant coevolutionary scenario in Earth's history. However, some incongruencies were observed between our results and those of previous studies in terms of shallow phylogenies for a few taxa, and should be further investigated.

Additive genetic effects in interacting species jointly determine the outcome of caterpillar herbivory

Plant-insect interactions are common and important in basic and applied biology. Trait and genetic variation can affect the outcome and evolution of these interactions, but the relative contributions of plant and insect genetic variation and how these interact remain unclear and are rarely subject to assessment in the same experimental context. Here, we address this knowledge gap using a recent host-range expansion onto alfalfa by the Melissa blue butterfly. Common garden rearing experiments and genomic data show that caterpillar performance depends on plant and insect genetic variation, with insect genetics contributing to performance earlier in development and plant genetics later. Our models of performance based on caterpillar genetics retained predictive power when applied to a second common garden. Much of the plant genetic effect could be explained by heritable variation in plant phytochemicals, especially saponins, peptides, and phosphatidyl cholines, providing a possible mechanistic understanding of variation in the species interaction. We find evidence of polygenic, mostly additive effects within and between species, with consistent effects of plant genotype on growth and development across multiple butterfly species. Our results inform theories of plant-insect coevolution and the evolution of diet breadth in herbivorous insects and other host-specific parasites.

Oviposition stimulants underlying different preferences between host races in the leaf-mining moth Acrocercops transecta (Lepidoptera: Gracillariidae)

The importance of plant chemistry in the host specialization of phytophagous insects has been emphasized. However, only a few chemicals associated with host shifting have been characterized. Herein, we focus on the leaf-mining moth Acrocercops transecta (Gracillariidae) consisting of ancestral Juglans (Juglandaceae)- and derived Lyonia (Ericaceae)-associated host races. The females of the Lyonia race laid eggs on a cover glass treated with an L. ovalifolia leaf extract; the extract was fractionated using silica gel and ODS column chromatography to isolate the oviposition stimulants. From a separated fraction, two analogous Lyonia-specific triterpenoid glycosides were characterized as oviposition stimulants. Furthermore, we observed probable contact chemosensilla on the distal portion of the female antennae. Lyonia race females laid their eggs on the non-host Juglans after the leaves were treated with a Lyonia-specific oviposition stimulant, although they do not lay eggs on Juglans. These results suggest that Lyonia race females do not lay eggs on Juglans leaves because the leaves do not contain specific oviposition stimulant(s). Otherwise, the activity of the oviposition stimulants overcomes oviposition deterrents contained in Juglans leaves. This paper describes the roles of plant chemicals in the different preferences between host races associated with distantly related plant taxa.

Geographic isolation drives speciation in Nearctic aphids

Across herbivorous insect clades, species richness and host-use diversity tend to positively covary. This could be because host-use divergence drives speciation, or because it raises the ecological limits on species richness. To evaluate these hypotheses, we performed phylogenetic path model analyses of the species diversity of Nearctic aphids. Here, we show that variation in the species richness of aphid clades is caused mainly by host-use divergence, whereas variation in speciation rates is caused more by divergence in non-host-related niche variables. Aphid speciation is affected by both the evolution of host and non-host-related niche components, but the former is largely caused by the latter. Thus, our analyses suggest that host-use divergence can both raise the ecological limits on species richness and drive speciation, although in the latter case, host-use divergence tends to be a step along the causal path leading from non-host-related niche evolution to speciation.

Using phylogenetic path modelling, geographic isolation is revealed to be the main driver of Nearctic aphid speciation, with subsequent host-use divergence raising the limits on clade species richness.

Genomics-based higher classification of the species-rich Hairstreaks (Lepidoptera: Lycaenidae: Eumaeini)

We propose a higher classification of the lycaenid hairstreak tribe Eumaeini - one of the youngest and most species-rich butterfly tribes - based on autosome, Lepidopteran Z sex chromosome, and mitochondrial protein-coding genes. The subtribe Neolycaenina Korb is a synonym of Callophryidina Tutt, and subtribe Tmolusina Bálint is a synonym of Strephonotina K. Johnson, Austin, Le Crom, & Salazar. Proposed names are Rhammina Prieto & Busby, new subtribe; Timaetina Busby & Prieto, new subtribe; Atlidina Martins & Duarte, new subtribe; Evenina Faynel & Grishin, new subtribe; Jantheclina Robbins & Faynel, new subtribe; Paiwarriina Lamas & Robbins, new subtribe; Cupatheclina Lamas & Grishin, new subtribe; Parrhasiina Busby & Robbins, new subtribe; Ipideclina Martins & Grishin, new subtribe; and Trichonidina Duarte & Faynel, new subtribe. Phylogenetic results from the autosome and Z sex chromosome analyses are similar. Future analyses of datasets with hundreds of terminal taxa may be more practical time-wise by focussing on the smaller number of sex chromosome sequences (2.6% of nuclear protein-coding sequences). The phylogenetic classification and biological summaries for each subtribe suggest that a variety of factors affected Eumaeini diversification. About a dozen kinds of male secondary sexual organs with frequent evolutionary gains and losses occur in Atlidina, Evenina, and Jantheclina (141 species combined). Females have been shown to use these organs to discriminate between conspecific and non-conspecific males, facilitating sympatry among close relatives. Eumaeina, Rhammina, and Timaetina (140 species combined) are overwhelmingly montane with some evidence for a higher incidence of sympatric diversification. Seven Neotropical lineages in five subtribes invaded the temperate parts of the Nearctic Region with a diversification increase in the Callophryidina (262 species). North American Satyrium and Callophrys then invaded the Palearctic at least once each, with a major species-richness increase in Satyrium. The evolution of litter feeding detritivores within Calycopidina (172 species) resulted in an increase in diversification rate compared with its flower-feeding sister lineage. Atlidina, Strephonotina, Parrhasiina, and Strymonina (562 species combined) each contain a mixture of genera that specialize on one or two caterpillar food plant families and genera that are polyphagous. These would be appropriate subtribes to assess how the breadth of caterpillar food plants and the frequency of host shifts affected diversification.

Competition and geography underlie speciation and morphological evolution in Indo-Australasian monitor lizards

How biotic and abiotic factors act together to shape biological diversity is a major question in evolutionary biology. The recent availability of large datasets and development of new methodological approaches provide new tools to evaluate the predicted effects of ecological interactions and geography on lineage diversification and phenotypic evolution. Here, we use a near complete phylogenomic-scale phylogeny and a comprehensive morphological dataset comprising more than a thousand specimens to assess the role of biotic and abiotic processes in the diversification of monitor lizards (Varanidae). This charismatic group of lizards shows striking variation in species richness among its clades and multiple instances of endemic radiation in Indo-Australasia (i.e., the Indo-Australian Archipelago and Australia), one of Earth's most biogeographically complex regions. We found heterogeneity in diversification dynamics across the family. Idiosyncratic biotic and geographic conditions appear to have driven diversification and morphological evolution in three endemic Indo-Australasian radiations. Furthermore, incumbency effects partially explain patterns in the biotic exchange between Australia and New Guinea. Our results offer insight into the dynamic history of Indo-Australasia, the evolutionary significance of competition, and the long-term consequences of incumbency effects.

Mitogenomics and hidden-trait models reveal the role of phoresy and host shifts in the diversification of parasitoid blister beetles (Coleoptera: Meloidae)

Changes in life history traits are often considered speciation triggers and can have dramatic effects on the evolutionary history of a lineage. Here, we examine the consequences of changes in two life history traits, host‐type and phoresy, in the hypermetamorphic blister beetles, Meloidae. Subfamilies Nemognathinae and Meloinae exhibit a complex life cycle involving multiple metamorphoses and parasitoidism. Most genera and tribes are bee‐parasitoids, and include phoretic or nonphoretic species, while two tribes feed on grasshopper eggs. These different life strategies are coupled with striking differences in species richness among clades. We generated a mitogenomic phylogeny for Nemognathinae and Meloinae, confirming the monophyly of these two clades, and used the dated phylogeny to explore the association between diversification rates and changes in host specificity and phoresy, using state‐dependent speciation and extinction (SSE) models that include the effect of hidden traits. To account for the low taxon sampling, we implemented a phylogenetic‐taxonomic approach based on birth‐death simulations, and used a Bayesian framework to integrate parameter and phylogenetic uncertainty. Results show that the ancestral hypermetamorphic Meloidae was a nonphoretic bee‐parasitoid, and that transitions towards a phoretic bee‐parasitoid and grasshopper parasitoidism occurred multiple times. Nonphoretic bee‐parasitoid lineages exhibit significantly higher relative extinction and lower diversification rates than phoretic bee‐and grasshopper‐parasitoids, but no significant differences were found between the latter two strategies. This suggests that Orthopteran host shifts and phoresy contributed jointly to the evolutionary success of the parasitoid meloidae. We also demonstrate that SSE models can be used to identify hidden traits coevolving with the focal trait in driving a lineage's diversification dynamics.

Plastic Variation in the Phyletic Lineages of Cactophilic Drosophila meridionalis and Relation to Hosts as Potential for Diversification

The insect/plant interaction is known to be a trigger for diversification and even speciation. Experimental analyses on fitness traits and phenotypic variation using alternative host sites have been performed to understand the process of diversification relative to insect/plant interactions. For cactophilic species of Drosophila, the speciation process is considered an adaptive radiation in response to the exploration of species of the Cactaceae as breeding and feeding sites. In this work, we analyzed life history and morphological traits in individuals from two phyletic lineages (Evolutionarily Significant Units ESU) of the cactophilic species Drosophila meridionalis (Wasserman 1962) (Diptera: Drosophilidae) raised from media prepare. The characters analyzed corresponded to viability, developmental time, and four morphological measurements. The experiments were performed in a semi-natural medium prepared with fermenting tissues of the natural hosts, Cereus hildmaniannus and Opuntia monacantha. Viability, development time, and three morphological measurements were influenced by lineage, suggesting differentiation between the lineages. However, in O. monacantha, the mean viability was greater (~15%) and development time was longer (~336 h) than in C. hildmaniannus (~11% and ~301 h, respectively). Only the developmental time was significantly affected by the host cactus. In general, ESU group A had better values than ESU group BC for the evaluated traits. This finding suggested differentiation between the two lineages and different plastic responsiveness to the contrasting environments of the hosts, and that C. hildmaniannus may be a relatively stressful environment for the larvae, as for other Drosophila species.

Phylogenetic reconstruction of ancestral ecological networks through time for pierid butterflies and their host plants

The study of herbivorous insects underpins much of the theory that concerns the evolution of species interactions. In particular, Pieridae butterflies and their host plants have served as a model system for studying evolutionary arms races. To learn more about the coevolution of these two clades, we reconstructed ancestral ecological networks using stochastic mappings that were generated by a phylogenetic model of host-repertoire evolution. We then measured if, when, and how two ecologically important structural features of the ancestral networks (modularity and nestedness) evolved over time. Our study shows that as pierids gained new hosts and formed new modules, a subset of them retained or recolonised the ancestral host(s), preserving connectivity to the original modules. Together, host-range expansions and recolonisations promoted a phase transition in network structure. Our results demonstrate the power of combining network analysis with Bayesian inference of host-repertoire evolution to understand changes in complex species interactions over time.

The evolution of coevolution in the study of species interactions

The study of reciprocal adaptation in interacting species has been an active and inspiring area of evolutionary research for nearly 60 years. Perhaps owing to its great natural history and potential consequences spanning population divergence to species diversification, coevolution continues to capture the imagination of biologists. Here we trace developments following Ehrlich and Raven's classic paper, with a particular focus on the modern influence of two studies by Dr. May Berenbaum in the 1980s. This series of classic work presented a compelling example exhibiting the macroevolutionary patterns predicted by Ehrlich and Raven and also formalized a microevolutionary approach to measuring selection, functional traits, and understanding reciprocal adaptation between plants and their herbivores. Following this breakthrough was a wave of research focusing on diversifying macroevolutionary patterns, mechanistic chemical ecology, and natural selection on populations within and across community types. Accordingly, we breakdown coevolutionary theory into specific hypotheses at different scales: reciprocal adaptation between populations within a community, differential coevolution among communities, lineage divergence, and phylogenetic patterns. We highlight progress as well as persistent gaps, especially the link between reciprocal adaptation and diversification.

Great chemistry between us: The link between plant chemical defenses and butterfly evolution

Plants constantly cope with insect herbivory, which is thought to be the evolutionary driver for the immense diversity of plant chemical defenses. Herbivorous insects are in turn restricted in host choice by the presence of plant chemical defense barriers. In this study, we analyzed whether butterfly host-plant patterns are determined by the presence of shared plant chemical defenses rather than by shared plant evolutionary history. Using correlation and phylogenetic statistics, we assessed the impact of host-plant chemical defense traits on shaping northwestern European butterfly assemblages at a macroevolutionary scale. Shared chemical defenses between plant families showed stronger correlation with overlap in butterfly assemblages than phylogenetic relatedness, providing evidence that chemical defenses may determine the assemblage of butterflies per plant family rather than shared evolutionary history. Although global congruence between butterflies and host-plant families was detected across the studied herbivory interactions, cophylogenetic statistics showed varying levels of congruence between butterflies and host chemical defense traits. We attribute this to the existence of multiple antiherbivore traits across plant families and the diversity of insect herbivory associations per plant family. Our results highlight the importance of plant chemical defenses in community ecology through their influence on insect assemblages.

Why are there not more herbivorous insect species?

Insect species richness is estimated to exceed three million species, of which roughly half is herbivorous. Despite the vast number of species and varied life histories, the proportion of herbivorous species among plant-consuming organisms is lower than it could be due to constraints that impose limits to their diversification. These include ecological factors, such as vague interspecific competition; anatomical and physiological limits, such as neural limits and inability of handling a wide range of plant allelochemicals; phylogenetic constraints, like niche conservatism; and most importantly, a low level of concerted genetic variation necessary to a phyletic conversion. It is suggested that diversification ultimately depends on what we call the intrinsic trend of diversification of the insect genome. In support of the above, we survey the major types of host-specificity, the mechanisms and constraints of host specialization, possible pathways of speciation, and hypotheses concerning insect diversification.

Molecular phylogeny, classification, biogeography and diversification patterns of a diverse group of moths (Geometridae: Boarmiini)

Understanding how and why some groups have become more species-rich than others, and how past biogeography may have shaped their current distribution, are questions that evolutionary biologists have long attempted to answer. We investigated diversification patterns and historical biogeography of a hyperdiverse lineage of Lepidoptera, the geometrid moths, by studying its most species-rich tribe Boarmiini, which comprises ca. 200 genera and ca. known 3000 species. We inferred the evolutionary relationships of Boarmiini based on a dataset of 346 taxa, with up to eight genetic markers under a maximum likelihood approach. The monophyly of Boarmiini is strongly supported. However, the phylogenetic position of many taxa does not agree with current taxonomy, although the monophyly of most major genera within the tribe is supported after minor adjustments. Three genera are synonymized, one new combination is proposed, and four species are placed in incertae sedis within Boarmiini. Our results support the idea of a rapid initial diversification of Boarmiini, which also implies that no major taxonomic subdivisions of the group can currently be proposed. A time-calibrated tree and biogeographical analyses suggest that boarmiines appeared in Laurasia ca. 52 Mya, followed by dispersal events throughout the Australasian, African and Neotropical regions. Most of the transcontinental dispersal events occurred in the Eocene, a period of intense geological activity and rapid climate change. Diversification analyses showed a relatively constant diversification rate for all Boarmiini, except in one clade containing the species-rich genus Cleora. The present work represents a substantial contribution towards understanding the evolutionary origin of Boarmiini moths. Our results, inevitably biased by taxon sampling, highlight the difficulties with working on species-rich groups that have not received much attention outside of Europe. Specifically, poor knowledge of the natural history of geometrids (particularly in tropical clades) limits our ability to identify key innovations underlying the diversification of boarmiines.

Plant phylogeny drives arboreal caterpillar assemblages across the Holarctic

Assemblages of insect herbivores are structured by plant traits such as nutrient content, secondary metabolites, physical traits, and phenology. Many of these traits are phylogenetically conserved, implying a decrease in trait similarity with increasing phylogenetic distance of the host plant taxa. Thus, a metric of phylogenetic distances and relationships can be considered a proxy for phylogenetically conserved plant traits and used to predict variation in herbivorous insect assemblages among co-occurring plant species.Using a Holarctic dataset of exposed-feeding and shelter-building caterpillars, we aimed at showing how phylogenetic relationships among host plants explain compositional changes and characteristics of herbivore assemblages.Our plant-caterpillar network data derived from plot-based samplings at three different continents included >28,000 individual caterpillar-plant interactions. We tested whether increasing phylogenetic distance of the host plants leads to a decrease in caterpillar assemblage overlap. We further investigated to what degree phylogenetic isolation of a host tree species within the local community explains abundance, density, richness, and mean specialization of its associated caterpillar assemblage.The overlap of caterpillar assemblages decreased with increasing phylogenetic distance among the host tree species. Phylogenetic isolation of a host plant within the local plant community was correlated with lower richness and mean specialization of the associated caterpillar assemblages. Phylogenetic isolation had no effect on caterpillar abundance or density. The effects of plant phylogeny were consistent across exposed-feeding and shelter-building caterpillars.Our study reveals that distance metrics obtained from host plant phylogeny are useful predictors to explain compositional turnover among hosts and host-specific variations in richness and mean specialization of associated insect herbivore assemblages in temperate broadleaf forests. As phylogenetic information of plant communities is becoming increasingly available, further large-scale studies are needed to investigate to what degree plant phylogeny structures herbivore assemblages in other biomes and ecosystems.

Miocene Diversification and High-Altitude Adaptation of Parnassius Butterflies (Lepidoptera: Papilionidae) in Qinghai-Tibet Plateau Revealed by Large-Scale Transcriptomic Data

Simple Summary

Parnassius butterflies have contributed to fundamental studies in biogeography, insect–plant interactions, and other fields of conservation biology and ecology. However, the early evolutionary pattern and molecular adaptation mechanism of this alpine butterfly group to high altitudes in Qinghai–Tibet Plateau are poorly understood up to now. In this study, we report for the first time, a relatively large-scale transcriptomic dataset of eight Parnassius species and their two closely related papilionid species, a dated phylogeny based on hundreds of gene sequences, and potential genetic mechanisms underlying the high-altitude adaptation by investigating changes in evolutionary rates and positively selected genes. Overall, our findings indicate that the transcriptome data sets reported here can provide some new insights into the spatiotemporally evolutionary pattern and high altitude adaptation of Parnassius butterflies from the extrinsic and intrinsic view, and will support further expressional and functional studies that will help interested researchers to address evolution, biodiversity and conservation questions concerning Parnassius and other butterfly species.

Abstract

The early evolutionary pattern and molecular adaptation mechanism of alpine Parnassius butterflies to high altitudes in Qinghai–Tibet Plateau are poorly understood up to now, due to difficulties in sampling, limited sequence data, and time calibration issues. Here, we present large-scale transcriptomic datasets of eight representative Parnassius species to reveal the phylogenetic timescale and potential genetic basis for high-altitude adaptation with multiple analytic strategies using 476 orthologous genes. Our phylogenetic results strongly supported that the subgenus Parnassius formed a well-resolved basal clade, and the subgenera Tadumia and Kailasius were closely related in the phylogenetic trees. In addition, molecular dating analyses showed that the Parnassius began to diverge at about 13.0 to 14.3 million years ago (middle Miocene), correlated with their hostplant’s spatiotemporal distributions, as well as geological and palaeoenvironmental changes of the Qinghai–Tibet Plateau. Moreover, the accelerated evolutionary rate, candidate positively selected genes and their potentially functional changes were detected, probably contributed to the high-altitude adaptation of Parnassius species. Overall, our study provided some new insights into the spatiotemporally evolutionary pattern and high altitude adaptation of Parnassius butterflies from the extrinsic and intrinsic view, which will help to address evolution, biodiversity, and conservation questions concerning Parnassius and other butterfly species.

Is Sexual Conflict a Driver of Speciation? A Case Study With a Tribe of Brush-footed Butterflies

Understanding the evolutionary mechanisms governing the uneven distribution of species richness across the tree of life is a great challenge in biology. Scientists have long argued that sexual conflict is a key driver of speciation. This hypothesis, however, has been highly debated in light of empirical evidence. Recent advances in the study of macroevolution make it possible to test this hypothesis with more data and increased accuracy. In the present study, we use phylogenomics combined with four different diversification rate analytical approaches to test whether sexual conflict is a driver of speciation in brush-footed butterflies of the tribe Acraeini. The presence of a sphragis, an external mating plug found in most species among Acraeini, was used as a proxy for sexual conflict. Diversification analyses statistically rejected the hypothesis that sexual conflict is associated with shifts in diversification rates in Acraeini. This result contrasts with earlier studies and suggests that the underlying mechanisms driving diversification are more complex than previously considered. In the case of butterflies, natural history traits acting in concert with abiotic factors possibly play a stronger role in triggering speciation than does sexual conflict. [Acraeini butterflies; arms race; exon capture phylogenomics; Lepidoptera macroevolution; sexual selection; sphragis.].

Adaptive zones shape the magnitude of premating reproductive isolation in Timema stick insects

Simpson's fossil-record inspired model of 'adaptive zones' proposes that evolution is dominated by small fluctuations within adaptive zones, occasionally punctuated by larger shifts between zones. This model can help explain why the process of population divergence often results in weak or moderate reproductive isolation (RI), rather than strong RI and distinct species. Applied to the speciation process, the adaptive zones hypothesis makes two inter-related predictions: (i) large shifts between zones are relatively rare, (ii) when large shifts do occur they generate stronger RI than shifts within zones. Here, we use ecological, phylogenetic and behavioural data to test these predictions in Timema stick insects. We show that host use in Timema is dominated by moderate shifts within the systematic divisions of flowering plants and conifers, with only a few extreme shifts between these divisions. However, when extreme shifts occur, they generate greater RI than do more moderate shifts. Our results support the adaptive zones model, and suggest that the net contribution of ecological shifts to diversification is dependent on both their magnitude and frequency. We discuss the generality of our findings in the light of emerging evidence from diverse taxa that the evolution of RI is not always the only factor determining the origin of species diversity. This article is part of the theme issue 'Towards the completion of speciation: the evolution of reproductive isolation beyond the first barriers'.

Recent diversification of Chrysoritis butterflies in the South African Cape (Lepidoptera: Lycaenidae)

Although best known for its extraordinary radiations of endemic plant species, the South African fynbos is home to a great diversity of phytophagous insects, including butterflies in the genus Chrysoritis (Lepidoptera: Lycaenidae). These butterflies are remarkably uniform morphologically; nevertheless, they comprise 43 currently accepted species and 68 currently valid taxonomic names. While many species have highly restricted, dot-like distributions, others are widespread. Here, we investigate the phylogenetic and biogeographic history underlying their diversification by analyzing molecular markers from 406 representatives of all described species throughout their respective ranges. We recover monophyletic clades for both C. chrysaor and C. thysbe species-groups, and identify a set of lineages that fall between them. The estimated age of divergence for the genus is 32 Mya, and we document significantly rapid diversification of the thysbe species-group in the Pleistocene (~2 Mya). Using ancestral geographic range reconstruction, we show that West Fynbos is the most likely region of origin for the radiation of the thysbe species-group. The colonization of this region occurred 9 Mya and appears to have been followed by a long period of relative stasis before a recent increase in diversification. Thus, the thysbe radiation does not appear to have resulted from the colonization of new biogeographic areas. Rather, the impact of species interactions (with ants and plants), the appearance of key innovations, and/or the opening of new ecological niche space in the region might explain the sudden burst of speciation that occurred in this group 2 Mya. The biogeographic model suggests two different diversification processes with few historical cross-colonisations, one in eastern South Africa for the C. chrysaor group and the other in western South Africa for the remaining taxa. Distributional range assessments and ecological niche models for each species show important niche overlap, and in a few cases, complete overlap. However, these shared traits are not explained by phylogenetic history. Chrysoritis taxa frequently fly in sympatry and gene tree reticulation appears to be widespread at the species level, suggesting that several episodes of range shifts might have led to secondary sympatries, allowing limited gene flow that challenges species delimitation efforts. In addition, the unusually high diversification rate for the thysbe clade of 1.35 [0.91-1.81] lineages per million years also suggests the possibility of taxonomic oversplitting. The phylogeny presented here provides a framework for a taxonomic revision of the genus. We highlight cases of potential synonymy both in allopatry and sympatry, and stress the importance of dedicated studies to assess potential pre- and post-zygotic barriers giving rise to species delimitations of the thysbe group.

Do major host shifts spark diversification in butterflies?

The Escape and Radiate Hypothesis posits that herbivorous insects and their host plants diversify through antagonistic coevolutionary adaptive radiation. For more than 50 years, it has inspired predictions about herbivorous insect macro-evolution, but only recently have the resources begun to fall into place for rigorous testing of those predictions. Here, with comparative phylogenetic analyses of nymphalid butterflies, we test two of these predictions: that major host switches tend to increase species diversification and that such increases will be proportional to the scope of ecological opportunity afforded by a particular novel host association. We find that by and large the effect of major host-use changes on butterfly diversity is the opposite of what was predicted; although it appears that the evolution of a few novel host associations can cause short-term bursts of speciation, in general, major changes in host use tend to be linked to significant long-term decreases in butterfly species richness.

Tolerance of Novel Toxins through Generalized Mechanisms: Simulating Gradual Host Shifts of Butterflies

Organisms encounter a wide range of toxic compounds in their environments, from chemicals that serve anticonsumption or anticompetition functions to pollutants and pesticides. Although we understand many detoxification mechanisms that allow organisms to consume toxins typical of their diet, we know little about why organisms vary in their ability to tolerate entirely novel toxins. We tested whether variation in generalized stress responses, such as antioxidant pathways, may underlie variation in reactions to novel toxins and, if so, their associated costs. We used an artificial diet to present cabbage white butterfly caterpillars (Pieris rapae) with plant material containing toxins not experienced in their evolutionary history. Families that maintained high performance (e.g., high survival, fast development time, large body size) on diets containing one novel toxic plant also performed well when exposed to two other novel toxic plants, consistent with a generalized response. Variation in constitutive (but not induced) expression of genes involved in oxidative stress responses was positively related to performance on the novel diets. While we did not detect reproductive trade-offs of this generalized response, there was a tendency to have less melanin investment in the wings, consistent with the role of melanin in oxidative stress responses. Taken together, our results support the hypothesis that variation in generalized stress responses, such as genes involved in oxidative stress responses, may explain the variation in tolerance to entirely novel toxins and may facilitate colonization of novel hosts and environments.

An Oviposition Stimulant for a Magnoliaceae-Feeding Swallowtail Butterfly, Graphium doson, from its Primary Host Plant, Michelia compressa

Chemical examination of plant constituents responsible for oviposition by a Magnoliaceae-feeding butterfly, Graphium doson, was conducted using its major host plant, Michelia compressa. A methanol extract prepared from young leaves of the plant elicited a strong oviposition response from females. The methanolic extract was then separated by solvent partition into three fractions: CHCl₃, i-BuOH, and aqueous fractions. Active substance(s) resided in both i-BuOH- and water-soluble fractions. Bioassay-guided further fractionation of the water-soluble substances by means of various chromatographic techniques led to the isolation of an oviposition stimulant. The stimulant was identified as D-(+)-pinitol on the basis of ¹³C NMR spectra and physicochemical properties. D-(+)-Pinitol singly exhibited a moderate oviposition-stimulatory activity at a dose of 150 μg/cm². This compound was present also in another host plant, Magnolia grandiflora, in a sufficient amount to induce oviposition behavior of G. doson females. Certain cyclitols including D-(+)-pinitol have been reported to be involved in stimulation of oviposition by some Aristolochiaceae- and Rutaceae-feeding papilionid butterflies. A possible pathway of phytochemical-mediated host shifts in the Papilionidae, in which certain cyclitols could enact important mediators, is discussed in relation to the evolution of cyclitol biosynthesis in plants.

Limited intrinsic postzygotic reproductive isolation despite chromosomal rearrangements between closely related sympatric species of small ermine moths (Lepidoptera: Yponomeutidae)

In evolutionarily young species and sympatric host races of phytophagous insects, postzygotic incompatibility is often not yet fully developed, but reduced fitness of hybrids is thought to facilitate further divergence. However, empirical evidence supporting this hypothesis is limited. To assess the role of reduced hybrid fitness, we studied meiosis and fertility in hybrids of two closely related small ermine moths, Yponomeuta padella and Yponomeuta cagnagella, and determined the extent of intrinsic postzygotic reproductive isolation. We found extensive rearrangements between the karyotypes of the two species and irregularities in meiotic chromosome pairing in their hybrids. The fertility of reciprocal F1 and, surprisingly, also of backcrosses with both parental species was not significantly decreased compared with intraspecific offspring. The results indicate that intrinsic postzygotic reproductive isolation between these closely related species is limited. We conclude that the observed chromosomal rearrangements are probably not the result of an accumulation of postzygotic incompatibilities preventing hybridization. Alternative explanations, such as adaptation to new host plants, are discussed.

Host defense triggers rapid adaptive radiation in experimentally evolving parasites

Adaptive radiation occurs when the members of a single lineage evolve different adaptive forms in response to selection imposed by competitors or predators. Iconic examples include Darwin's finches, Caribbean anoles, and Hawaiian silverswords, all of which live on islands. Although adaptive radiation is thought to be an important generator of biodiversity, most studies concern groups that have already diversified. Here, we take the opposite approach. We experimentally triggered diversification in the descendants of a single population of host-specific parasites confined to different host "islands." We show rapid adaptive divergence of experimentally evolving feather lice in response to preening, which is a bird's main defense against ectoparasites. We demonstrate that host defense exerts strong phenotypic selection for crypsis in lice transferred to different colored rock pigeons (Columba livia). During four years of experimental evolution (∼60 generations), the lice evolved heritable differences in color. Strikingly, the observed color differences spanned the range of phenotypes found among congeneric lice adapted to other species of birds. To our knowledge, this is the first real-time demonstration that microevolution is fast enough to simulate millions of years of macroevolutionary change. Our results further indicate that host-mediated selection triggers rapid divergence in the adaptive radiation of parasites, which are among the most diverse organisms on Earth.

The Role of N,N,N-Trimethylglycine in Oviposition of Eurema mandarina on Albizia julibrissin

The common grass yellow Eurema mandarina (Lepidoptera: Pieridae) uses the silk tree Albizia julibrissin (Fabaceae) as a primary host in Japan. We previously reported that D-pinitol, a cyclitol found in fresh leaves of A. julibrissin, solely elicits moderate oviposition responses from females. However, the aqueous neutral/amphoteric fraction of the fresh leaf extract containing D-pinitol weakly induces oviposition. Moreover, the aqueous neutral/amphoteric/basic fraction was significantly more active than the neutral/amphoteric fraction in eliciting responses, indicating that some basic compounds are involved in stimulating oviposition. High-resolution mass spectrometry and proton nuclear magnetic resonance measurements revealed that the aqueous basic faction contains N,N,N-trimethylglycine (trivial name: glycine betaine) in alkali metal salt form. The average concentration of this quaternary ammonium compound in fresh leaves was estimated to be 0.012% w/w in high performance liquid chromatography analyses. The authentic N,N,N-trimethylglycine induced oviposition at concentrations greater than 0.001% (w/v) and slightly enhanced female responses to the aqueous neutral fraction and authentic D-pinitol. However, its analogues, N,N-dimethylglycine, N-methylglycine, and glycine as well as its precursor choline were inactive. These results demonstrate that N,N,N-trimethylglycine, together with D-pinitol, serves as an stimulant of E. mandarina for oviposition on the leaves of A. julibrissin.

Unifying host-associated diversification processes using butterfly-plant networks

Explaining the exceptional diversity of herbivorous insects is an old problem in evolutionary ecology. Here we focus on the two prominent hypothesised drivers of their diversification, radiations after major host switch or variability in host use due to continuous probing of new hosts. Unfortunately, current methods cannot distinguish between these hypotheses, causing controversy in the literature. Here we present an approach combining network and phylogenetic analyses, which directly quantifies support for these opposing hypotheses. After demonstrating that each hypothesis produces divergent network structures, we then investigate the contribution of each to diversification in two butterfly families: Pieridae and Nymphalidae. Overall, we find that variability in host use is essential for butterfly diversification, while radiations following colonisation of a new host are rare but can produce high diversity. Beyond providing an important reconciliation of alternative hypotheses for butterfly diversification, our approach has potential to test many other hypotheses in evolutionary biology.

Unifying host-associated diversification processes using butterfly-plant networks

Explaining the exceptional diversity of herbivorous insects is an old problem in evolutionary ecology. Here we focus on the two prominent hypothesised drivers of their diversification, radiations after major host switch or variability in host use due to continuous probing of new hosts. Unfortunately, current methods cannot distinguish between these hypotheses, causing controversy in the literature. Here we present an approach combining network and phylogenetic analyses, which directly quantifies support for these opposing hypotheses. After demonstrating that each hypothesis produces divergent network structures, we then investigate the contribution of each to diversification in two butterfly families: Pieridae and Nymphalidae. Overall, we find that variability in host use is essential for butterfly diversification, while radiations following colonisation of a new host are rare but can produce high diversity. Beyond providing an important reconciliation of alternative hypotheses for butterfly diversification, our approach has potential to test many other hypotheses in evolutionary biology.

Herbivorous insects could diversify through radiations after major host switches or through constant variability in new host use. With phylogenetic and network analyses, Braga et al. show that variability in host use supports most butterfly diversification, while rare radiations can further boost diversity.

Evolutionary Conflict

Evolutionary conflict occurs when two parties can each affect a joint phenotype, but they gain from pushing it in opposite directions. Conflicts occur across many biological levels and domains but share many features. They are a major source of biological maladaptation. They affect biological diversity, often increasing it, at almost every level. Because opponents create selection that can be strong, persistent, and malevolent, conflict often leads to accelerated evolution and arms races. Conflicts might even drive the majority of adaptation, with pathogens leading the way as selective forces. The evolution of conflicts is complex, with outcomes determined partly by the relative evolvability of each party and partly by the kinds of power that each evolves. Power is a central issue in biology. In addition to physical strength and weapons, it includes strength from numbers and complexity; abilities to bind and block; advantageous timing; and abilities to acquire, use, and distort information.

Evolution of ontogenic change in color defenses of swallowtail butterflies

Natural selection by visually hunting predators has led to the evolution of color defense strategies such as masquerade, crypsis, and aposematism that reduce the risk of predation in prey species. These color defenses are not mutually exclusive, and switches between strategies with ontogenic development are widespread across taxa. However, the evolutionary dynamics of ontogenic color change are poorly understood. Using comparative phylogenetics, we studied the evolution of color defenses in the complex life cycles of swallowtail butterflies (family Papilionidae). We also tested the relative importance of life history traits, chemical and visual backgrounds, and ancestry on the evolution of protective coloration. We found that vulnerable early- and late-instar caterpillars of species that feed on sparsely vegetated, toxic plants were aposematic, whereas species that feed on densely vegetated, nontoxic plants had masquerading and cryptic caterpillars. Masquerading caterpillars resembled bird droppings at early instars and transitioned to crypsis with an increase in body size at late instars. The immobile pupae-safe from motion-detecting, visually hunting predators-retained the ancestral cryptic coloration in all lineages, irrespective of the toxic nature of the host plant. Thus, color defense strategy (masquerade, crypsis, or aposematism) at a particular lifestage in the life cycle of swallowtail butterflies was determined by the interaction between life history traits such as body size and motion levels, phytochemical and visual backgrounds, and ancestry. We show that ontogenic color change in swallowtail butterflies is an adaptive response to age-dependent vulnerability to predation.

Assessing species saturation: conceptual and methodological challenges

Is there a maximum number of species that can coexist? Intuitively, we assume an upper limit to the number of species in a given assemblage, or that a lineage can produce, but defining and testing this limit has proven problematic. Herein, we first outline seven general challenges of studies on species saturation, most of which are independent of the actual method used to assess saturation. Among these are the challenge of defining saturation conceptually and operationally, the importance of setting an appropriate referential system, and the need to discriminate among patterns, processes and mechanisms. Second, we list and discuss the methodological approaches that have been used to study species saturation. These approaches vary in time and spatial scales, and in the variables and assumptions needed to assess saturation. We argue that assessing species saturation is possible, but that many studies conducted to date have conceptual and methodological flaws that prevent us from currently attaining a good idea of the occurrence of species saturation.

Diversification rates, host plant shifts and an updated molecular phylogeny of Andean Eois moths (Lepidoptera: Geometridae)

Eois is one of the best-investigated genera of tropical moths. Its close association with Piper plants has inspired numerous studies on life histories, phylogeny and evolutionary biology. This study provides an updated view on phylogeny, host plant use and temporal patterns of speciation in Eois. Using sequence data (2776 bp) from one mitochondrial (COI) and one nuclear gene (Ef1-alpha) for 221 Eois species, we confirm and reinforce previous findings regarding temporal patterns of diversification. Deep diversification within Andean Eois took place in the Miocene followed by a sustained high rate of diversification until the Pleistocene when a pronounced slowdown of speciation is evident. In South America, Eois diversification is very likely to be primarily driven by the Andean uplift which occurred concurrently with the entire evolutionary history of Eois. A massively expanded dataset enabled an in-depth look into the phylogenetic signal contained in host plant usage. This revealed several independent shifts from Piper to other host plant genera and families. Seven shifts to Peperomia, the sister genus of Piper were detected, indicating that the shift to Peperomia was an easy one compared to the singular shifts to the Chloranthaceae, Siparunaceae and the Piperacean genus Manekia. The potential for close co-evolution of Eois with Piper host plants is therefore bound to be limited to smaller subsets within Neotropical Eois instead of a frequently proposed genus-wide co-evolutionary scenario. In regards to Eois systematics we confirm the monophyly of Neotropical Eois in relation to their Old World counterparts. A tentative biogeographical hypothesis is presented suggesting that Eois originated in tropical Asia and subsequently colonized the Neotropics and Africa. Within Neotropical Eois we were able to identify the existence of six clades not recognized in previous studies and confirm and reinforce the monophyly of all 9 previously delimited infrageneric clades.

Polyphagy and diversification in tussock moths: Support for the oscillation hypothesis from extreme generalists

Theory on plasticity driving speciation, as applied to insect-plant interactions (the oscillation hypothesis), predicts more species in clades with higher diversity of host use, all else being equal. Previous support comes mainly from specialized herbivores such as butterflies, and plasticity theory suggests that there may be an upper host range limit where host diversity no longer promotes diversification. The tussock moths (Erebidae: Lymantriinae) are known for extreme levels of polyphagy. We demonstrate that this system is also very different from butterflies in terms of phylogenetic signal for polyphagy and for use of specific host orders. Yet we found support for the generality of the oscillation hypothesis, in that clades with higher diversity of host use were found to contain more species. These clades also consistently contained the most polyphagous single species. Comparing host use in Lymantriinae with related taxa shows that the taxon indeed stands out in terms of the frequency of polyphagous species. Comparative evidence suggests that this is most probably due to its nonfeeding adults, with polyphagy being part of a resulting life history syndrome. Our results indicate that even high levels of plasticity can drive diversification, at least when the levels oscillate over time.

Sources of Variation in the Gut Microbial Community of Lycaeides melissa Caterpillars

Microbes can mediate insect-plant interactions and have been implicated in major evolutionary transitions to herbivory. Whether microbes also play a role in more modest host shifts or expansions in herbivorous insects is less clear. Here we evaluate the potential for gut microbial communities to constrain or facilitate host plant use in the Melissa blue butterfly (Lycaeides melissa). We conducted a larval rearing experiment where caterpillars from two populations were fed plant tissue from two hosts. We used 16S rRNA sequencing to quantify the relative effects of sample type (frass versus whole caterpillar), diet (plant species), butterfly population and development (caterpillar age) on the composition and diversity of the caterpillar gut microbial communities, and secondly, to test for a relationship between microbial community and larval performance. Gut microbial communities varied over time (that is, with caterpillar age) and differed between frass and whole caterpillar samples. Diet (host plant) and butterfly population had much more limited effects on microbial communities. We found no evidence that gut microbe community composition was associated with caterpillar weight, and thus, our results provide no support for the hypothesis that variation in microbial community affects performance in L. melissa.

Dated phylogeny and dispersal history of the butterfly subfamily Nymphalinae (Lepidoptera: Nymphalidae)

The origin and dispersal history of the large butterfly subfamily Nymphalinae are not fully understood, due to internal phylogenetic and time calibration issues. We conducted phylogenetic and dating analyses using mitochondrial and nuclear genes of biogeographically diverse groups of the Nymphalinae in order to resolve some controversial relationships and the paleobiogeographic pattern of the subfamily. Our results support the sister relationship of Vanessa (Tribe Nymphalini) and the Nymphalis-group, and the grouping of the three old-world genera (Rhinopalpa, Kallimoides and Vanessula) within Tribe Victorinini. Molecular dating analyses invoking two additional calibrations under the butterfly-host plant coevolutionary scenarios result in a relatively deeper divergence of the subfamily's two major clades (Nymphalini and the Kallimoids), compatible with the Cretaceous floral turnover scenario during the so-called Cretaceous Terrestrial Revolution. Phylobiogeographic analyses reveal that the Oriental region is probably the center of early divergences for Nymphalinae after the Cretaceous-Paleogene (K-Pg) mass extinction, followed by repeated dispersals into the rest of the Old World and the New World during various periods beginning in Eocene. The biogeographic history indicates that temperature changes and host-plant diversification may have facilitated the dispersals of this butterfly subfamily, with accelerated global colonization during the middle to late Miocene.

Hostplant change and paleoclimatic events explain diversification shifts in skipper butterflies (Family: Hesperiidae)

BACKGROUND

Skippers (Family: Hesperiidae) are a large group of butterflies with ca. 4000 species under 567 genera. The lack of a time-calibrated higher-level phylogeny of the group has precluded understanding of its evolutionary past. We here use a 10-gene dataset to reconstruct the most comprehensive time-calibrated phylogeny of the group, and explore factors that affected the diversification of these butterflies.

RESULTS

Ancestral state reconstructions show that the early hesperiid lineages utilized dicots as larval hostplants. The ability to feed on monocots evolved once at the K-Pg boundary (ca. 65 million years ago (Mya)), and allowed monocot-feeders to diversify much faster on average than dicot-feeders. The increased diversification rate of the monocot-feeding clade is specifically attributed to rate shifts in two of its descendant lineages. The first rate shift, a four-fold increase compared to background rates, happened ca. 50 Mya, soon after the Paleocene-Eocene thermal maximum, in a lineage of the subfamily Hesperiinae that mostly fed on forest monocots. The second rate shift happened ca. 40 Mya in a grass-feeding lineage of Hesperiinae when open-habitat grasslands appeared in the Neotropics owing to gradual cooling of the atmospheric temperature.

CONCLUSIONS

The evolution of monocot feeding strongly influenced diversification of skippers. We hypothesize that although monocot feeding was an intrinsic trait that allowed exploration of novel niches, the lack of extensive availability of monocots comprised an extrinsic limitation for niche exploration. The shifts in diversification rate coincided with paleoclimatic events during which grasses and forest monocots were diversified.

Landscape simplification weakens the association between terrestrial producer and consumer diversity in Europe

Land-use change is one of the primary drivers of species loss, yet little is known about its effect on other components of biodiversity that may be at risk. Here, we ask whether, and to what extent, landscape simplification, measured as the percentage of arable land in the landscape, disrupts the functional and phylogenetic association between primary producers and consumers. Across seven European regions, we inferred the potential associations (functional and phylogenetic) between host plants and butterflies in 561 seminatural grasslands. Local plant diversity showed a strong bottom-up effect on butterfly diversity in the most complex landscapes, but this effect disappeared in simple landscapes. The functional associations between plant and butterflies are, therefore, the results of processes that act not only locally but are also dependent on the surrounding landscape context. Similarly, landscape simplification reduced the phylogenetic congruence among host plants and butterflies indicating that closely related butterflies become more generalist in the resources used. These processes occurred without any detectable change in species richness of plants or butterflies along the gradient of arable land. The structural properties of ecosystems are experiencing substantial erosion, with potentially pervasive effects on ecosystem functions and future evolutionary trajectories. Loss of interacting species might trigger cascading extinction events and reduce the stability of trophic interactions, as well as influence the longer term resilience of ecosystem functions. This underscores a growing realization that species richness is a crude and insensitive metric and that both functional and phylogenetic associations, measured across multiple trophic levels, are likely to provide additional and deeper insights into the resilience of ecosystems and the functions they provide.

Plant feeding promotes diversification in the Crustacea

About half of the world's animal species are arthropods associated with plants, and the ability to consume plant material has been proposed to be an important trait associated with the spectacular diversification of terrestrial insects. We review the phylogenetic distribution of plant feeding in the Crustacea, the other major group of arthropods that commonly consume plants, to estimate how often plant feeding has arisen and to test whether this dietary transition is associated with higher species numbers in extant clades. We present evidence that at least 31 lineages of marine, freshwater, and terrestrial crustaceans (including 64 families and 185 genera) have independently overcome the challenges of consuming plant material. These plant-feeding clades are, on average, 21-fold more speciose than their sister taxa, indicating that a shift in diet is associated with increased net rates of diversification. In contrast to herbivorous insects, most crustaceans have very broad diets, and the increased richness of taxa that include plants in their diet likely results from access to a novel resource base rather than host-associated divergence.