Historical ecology of sub-Antarctic weevils (Coleoptera: Curculionidae): patterns and processes on isolated islands

Mitochondrial metagenomics reveal the independent colonization of the world's coasts by intertidal oribatid mites (Acari, Oribatida, Ameronothroidea)

Oribatid mites are an ancient group that already roamed terrestrial ecosystems in the early and middle Devonian. The superfamily of Ameronothroidea, a supposedly monophyletic lineage, represents the only group of oribatid mites that has successfully invaded the marine coastal environment. By using mitogenome data and nucleic ribosomal RNA genes (18S, 5.8S, 28S), we show that Ameronothroidea are a paraphyletic assemblage and that the land-to-sea transition happened three times independently. Common ancestors of the tropical Fortuyniidae and Selenoribatidae were the first to colonize the coasts and molecular calibration of our phylogeny dates this event to a period in the Triassic and Jurassic era (225-146 mya), whereas present-day distribution indicates that this event might have happened early in this period during the Triassic, when the supercontinent Pangaea still existed. The cold temperate northern hemispheric Ameronothridae colonized the marine littoral later in the late Jurassic-Early Cretaceous and had an ancient distribution on Laurasian coasts. The third and final land-to-sea transition happened in the same geological period, but approx. 30 my later when ancestors of Podacaridae invaded coastal marine environments of the Gondwanan landmasses.

Fifty million years of beetle evolution along the Antarctic Polar Front

The Antarctic environment is famously inhospitable to most terrestrial biodiversity, traditionally viewed as a driver of species extinction. Combining population- and species-level molecular data, we show that beetles on islands along the Antarctic Polar Front diversified in response to major climatic events over the last 50 Ma in surprising synchrony with the region’s marine organisms. Unique algae- and moss-feeding habits enabled beetles to capitalize on cooling conditions, which resulted in a decline in flowering plants—the typical hosts for beetles elsewhere. Antarctica’s cooling paleoclimate thus fostered the diversification of both terrestrial and marine life. Climatically driven evolutionary processes since the Miocene may underpin much of the region’s diversity, are still ongoing, and should be further investigated among Antarctic biota.

Global cooling and glacial–interglacial cycles since Antarctica’s isolation have been responsible for the diversification of the region’s marine fauna. By contrast, these same Earth system processes are thought to have played little role terrestrially, other than driving widespread extinctions. Here, we show that on islands along the Antarctic Polar Front, paleoclimatic processes have been key to diversification of one of the world’s most geographically isolated and unique groups of herbivorous beetles—Ectemnorhinini weevils. Combining phylogenomic, phylogenetic, and phylogeographic approaches, we demonstrate that these weevils colonized the sub-Antarctic islands from Africa at least 50 Ma ago and repeatedly dispersed among them. As the climate cooled from the mid-Miocene, diversification of the beetles accelerated, resulting in two species-rich clades. One of these clades specialized to feed on cryptogams, typical of the polar habitats that came to prevail under Miocene conditions yet remarkable as a food source for any beetle. This clade’s most unusual representative is a marine weevil currently undergoing further speciation. The other clade retained the more common weevil habit of feeding on angiosperms, which likely survived glaciation in isolated refugia. Diversification of Ectemnorhinini weevils occurred in synchrony with many other Antarctic radiations, including penguins and notothenioid fishes, and coincided with major environmental changes. Our results thus indicate that geo-climatically driven diversification has progressed similarly for Antarctic marine and terrestrial organisms since the Miocene, potentially constituting a general biodiversity paradigm that should be sought broadly for the region’s taxa.

Phenotypic plasticity in locomotor performance of a monophyletic group of weevils accords with the warmer is better hypothesis

Ectotherms may respond to variable environmental conditions by altering their phenotypes. Phenotypic plasticity was initially thought to be beneficial to an organism's physiological fitness. Several alternative hypotheses, have, however, been proposed with growing empirical support. In this study, we test the full suite of hypotheses by investigating acclimation responses of locomotor performance for nine populations of five species of sub-Antarctic weevils, using static and fluctuating temperatures. Species showed typical locomotion thermal performance curves with temperature of the maximum speed (Topt) ranging between 22.3±1.7°C (mean±s.e.) and 31.1±0.7°C. For most species Topt was not affected by acclimation. For maximum speed (Umax), significant, positive effects of acclimation were found for all species except a supralittoral one. Individuals acclimated to 0°C showed much lower values than the other two acclimation treatments (15°C and fluctuating 0-15°C). Performance breadth (the index of the breadth of the curve, Tbr) typically showed little response to acclimation. None of the traits of the supralittoral species were affected by acclimation treatment. Responses to stable and fluctuating temperature treatments were similar. Our findings also revealed that the mean estimated activation energy 0.40±0.015 eV (mean±s.e.) was lower than for other herbivores, the category to which these weevils belong, suggesting that some form of compensation in the rate-temperature relationship may be evident. Thus, we typically found support for the warmer is better hypothesis for acclimation of locomotor performance, though some compensation is evident.

Model selection in statistical historical biogeography of Neotropical insects-The Exophthalmus genus complex (Curculionidae: Entiminae)

Statistical historical biogeographic methods rely on models that represent various biogeographic processes. Until recently model selection in this domain was not widely used, and the impact of differential model selection on inferring biogeographic scenarios was not well understood. Focusing on Neotropical weevils in the Exophthalmus genus complex (EGC) (Insecta: Curculionidae: Entiminae), we compare three commonly used biogeographic models - DIVA (Dispersal-Vicariance Analysis), DEC (Dispersal-Extinction-Cladogenesis) and BayArea (Bayesian Analysis of Biogeography), and examine the impact of modeling founder-event jump dispersal on historical biogeographic reconstructions. We also investigate the biogeographic events that have shaped patterns of distribution, diversification, and endemism in this weevil lineage. We sample representatives of 65 species of the EGC and 26 outgroup terminals from the Neotropics, including Caribbean islands and the mainland. We reconstruct a molecular phylogeny based on six genes and apply molecular dating using a relaxed clock with three fossil calibration points. Historical biogeographic estimations and alternative biogeographic models are computed and compared with the R package BioGeoBEARS. Model selection strongly favors biogeographic models that include founder-event jump dispersal. Without modeling jump dispersal, estimations based on the three biogeographic models are dramatically different, especially for early-diverging nodes. When jump dispersal is included, the three biogeographic models perform similarly. Accordingly, we show that the Neotropical mainland was colonized by Caribbean species in the early Miocene, and that in situ diversification accounts for a majority (∼75%) of the biogeographic events in the EGC. Our study highlights the need to assess wide-ranging historical biogeographic processes - including founder-event jump dispersal - for best-fitting statistical Caribbean biogeographic reconstructions. Moreover, colonization of the Neotropical mainland from the Caribbean reinforces the notion that islands can be an important source of continental diversity.

Antarctic Entomology

The Antarctic region comprises the continent, the Maritime Antarctic, the sub-Antarctic islands, and the southern cold temperate islands. Continental Antarctica is devoid of insects, but elsewhere diversity varies from 2 to more than 200 species, of which flies and beetles constitute the majority. Much is known about the drivers of this diversity at local and regional scales; current climate and glacial history play important roles. Investigations of responses to low temperatures, dry conditions, and varying salinity have spanned the ecological to the genomic, revealing new insights into how insects respond to stressful conditions. Biological invasions are common across much of the region and are expected to increase as climates become warmer. The drivers of invasion are reasonably well understood, although less is known about the impacts of invasion. Antarctic entomology has advanced considerably over the past 50 years, but key areas, such as interspecific interactions, remain underexplored.

Mite dispersal among the Southern Ocean Islands and Antarctica before the last glacial maximum

It has long been maintained that the majority of terrestrial Antarctic species are relatively recent, post last glacial maximum, arrivals with perhaps a few microbial or protozoan taxa being substantially older. Recent studies have questioned this 'recolonization hypothesis', though the range of taxa examined has been limited. Here, we present the first large-scale study for mites, one of two dominant terrestrial arthropod groups in the region. Specifically, we provide a broad-scale molecular phylogeny of a biologically significant group of ameronothroid mites from across the maritime and sub-Antarctic regions. Applying different dating approaches, we show that divergences among the ameronothroid mite genera Podacarus, Alaskozetes and Halozetes significantly predate the Pleistocene and provide evidence of independent dispersals across the Antarctic Polar Front. Our data add to a growing body of evidence demonstrating that many taxa have survived glaciation of the Antarctic continent and the sub-Antarctic islands. Moreover, they also provide evidence of a relatively uncommon trend of dispersals from islands to continental mainlands. Within the ameronothroid mites, two distinct clades with specific habitat preferences (marine intertidal versus terrestrial/supralittoral) exist, supporting a model of within-habitat speciation rather than colonization from marine refugia to terrestrial habitats. The present results provide additional impetus for a search for terrestrial refugia in an area previously thought to have lacked ice-free ground during glacial maxima.

Spatial and temporal variability across life's hierarchies in the terrestrial Antarctic

Antarctica and its surrounding islands lie at one extreme of global variation in diversity. Typically, these regions are characterized as being species poor and having simple food webs. Here, we show that terrestrial systems in the region are nonetheless characterized by substantial spatial and temporal variations at virtually all of the levels of the genealogical and ecological hierarchies which have been thoroughly investigated. Spatial variation at the individual and population levels has been documented in a variety of genetic studies, and in mosses it appears that UV-B radiation might be responsible for within-clump mutagenesis. At the species level, modern molecular methods have revealed considerable endemism of the Antarctic biota, questioning ideas that small organisms are likely to be ubiquitous and the taxa to which they belong species poor. At the biogeographic level, much of the relatively small ice-free area of Antarctica remains unsurveyed making analyses difficult. Nonetheless, it is clear that a major biogeographic discontinuity separates the Antarctic Peninsula and continental Antarctica, here named the 'Gressitt Line'. Across the Southern Ocean islands, patterns are clearer, and energy availability is an important correlate of indigenous and exotic species richness, while human visitor numbers explain much of the variation in the latter too. Temporal variation at the individual level has much to do with phenotypic plasticity, and considerable life-history and physiological plasticity seems to be a characteristic of Antarctic terrestrial species. Environmental unpredictability is an important driver of this trait and has significantly influenced life histories across the region and probably throughout much of the temperate Southern Hemisphere. Rapid climate change-related alterations in the range and abundance of several Antarctic and sub-Antarctic populations have taken place over the past several decades. In many sub-Antarctic locations, these have been exacerbated by direct and indirect effects of invasive alien species. Interactions between climate change and invasion seem set to become one of the most significant conservation problems in the Antarctic. We conclude that despite the substantial body of work on the terrestrial biodiversity of the Antarctic, investigations of interactions between hierarchical levels remain scarce. Moreover, little of the available information is being integrated into terrestrial conservation planning, which lags far behind in this region by comparison with most others.

Environmental constraints on life histories in Antarctic ecosystems: tempos, timings and predictability

Knowledge of Antarctic biotas and environments has increased dramatically in recent years. There has also been a rapid increase in the use of novel technologies. Despite this, some fundamental aspects of environmental control that structure physiological, ecological and life-history traits in Antarctic organisms have received little attention. Possibly the most important of these is the timing and availability of resources, and the way in which this dictates the tempo or pace of life. The clearest view of this effect comes from comparisons of species living in different habitats. Here, we (i) show that the timing and extent of resource availability, from nutrients to colonisable space, differ across Antarctic marine, intertidal and terrestrial habitats, and (ii) illustrate that these differences affect the rate at which organisms function. Consequently, there are many dramatic biological differences between organisms that live as little as 10 m apart, but have gaping voids between them ecologically. Identifying the effects of environmental timing and predictability requires detailed analysis in a wide context, where Antarctic terrestrial and marine ecosystems are at one extreme of the continuum of available environments for many characteristics including temperature, ice cover and seasonality. Anthropocentrically, Antarctica is harsh and as might be expected terrestrial animal and plant diversity and biomass are restricted. By contrast, Antarctic marine biotas are rich and diverse, and several phyla are represented at levels greater than global averages. There has been much debate on the relative importance of various physical factors that structure the characteristics of Antarctic biotas. This is especially so for temperature and seasonality, and their effects on physiology, life history and biodiversity. More recently, habitat age and persistence through previous ice maxima have been identified as key factors dictating biodiversity and endemism. Modern molecular methods have also recently been incorporated into many traditional areas of polar biology. Environmental predictability dictates many of the biological characters seen in all of these areas of Antarctic research.

Temperature- and body mass-related variation in cyclic gas exchange characteristics and metabolic rate of seven weevil species: Broader implications

The influence of temperature on metabolic rate and characteristics of the gas exchange patterns of flightless, sub-Antarctic Ectemnorhinus-group species from Heard and Marion islands was investigated. All of the species showed cyclic gas exchange with no Flutter period, indicating that these species are not characterized by discontinuous gas exchange cycles. Metabolic rate estimates were substantially lower in this study than in a previous one of a subset of the species, demonstrating that open-system respirometry methods provide more representative estimates of standard metabolic rate than do many closed-system methods. We recommend that the latter, and especially constant-pressure methods, either be abandoned for estimates of standard metabolic rate in insects, or have their outputs subject to careful scrutiny, given the wide availability of the former. V(.)CO(2) increase with an increase in temperature (range: 0-15 degrees C) was modulated by an increase in cycle frequency, but typically not by an increase in burst volume. Previous investigations of temperature-related changes in cyclic gas exchange (both cyclic and discontinuous) in several other insect species were therefore substantiated. Interspecific mass-scaling of metabolic rate (ca. 0.466-0.573, excluding and including phylogenetic non-independence, respectively) produced an exponent lower than 0.75 (but not distinguishable from it or from 0.67). The increase of metabolic rate with mass was modulated by an increase in burst volume and not by a change in cycle frequency, in keeping with investigations of species showing discontinuous gas exchange. These findings are discussed in the context of the emerging macrophysiological metabolic theory of ecology.

Water-balance characteristics respond to changes in body size in subantarctic weevils

Several environmental factors leading to size-dependent mortality influence insect body size. Few investigations have been concerned with the ways in which the mechanisms underlying variation in water-balance characteristics evolve in response to changes in body size that occur independently of water-balance requirements. Using an explicitly phylogenetic analysis, we show how body size has changed over time in the Ectemnorhinus group of weevils and how water-balance characteristics have evolved in response to this change and changes in habitat use. The basal species in the group are all large bodied and from moist environments. In response to a change in resource availability, there was a marked decline in size within the group. Despite the reduction in water content and dehydration tolerance that this meant, evolution of low whole-animal water-loss rates and high tolerance of dehydration resulted in conservation of desiccation resistance. The return to moist habitats in the group resulted in a reduction in dehydration tolerance and an increase in water-loss rate. Thus, dehydration tolerance and water-loss rate respond rapidly both when there is selection for water conservation and when this requirement is relaxed. Future laboratory selection experiments might usefully explore both directions of water-balance evolution.

Climatic variability and the evolution of insect freeze tolerance

Insects may survive subzero temperatures by two general strategies: Freeze-tolerant insects withstand the formation of internal ice, while freeze-avoiding insects die upon freezing. While it is widely recognized that these represent alternative strategies to survive low temperatures, and mechanistic understanding of the physical and molecular process of cold tolerance are becoming well elucidated, the reasons why one strategy or the other is adopted remain unclear. Freeze avoidance is clearly basal within the arthropod lineages, and it seems that freeze tolerance has evolved convergently at least six times among the insects (in the Blattaria, Orthoptera, Coleoptera, Hymenoptera, Diptera and Lepidoptera). Of the pterygote insect species whose cold-tolerance strategy has been reported in the literature, 29% (69 of 241 species studied) of those in the Northern Hemisphere, whereas 85 % (11 of 13 species) in the Southern Hemisphere exhibit freeze tolerance. A randomization test indicates that this predominance of freeze tolerance in the Southern Hemisphere is too great to be due to chance, and there is no evidence of a recent publication bias in favour of new reports of freeze-tolerant species. We conclude from this that the specific nature of cold insect habitats in the Southern Hemisphere, which are characterized by oceanic influence and climate variability must lead to strong selection in favour of freeze tolerance in this hemisphere. We envisage two main scenarios where it would prove advantageous for insects to be freeze tolerant. In the first, characteristic of cold continental habitats of the Northern Hemisphere, freeze tolerance allows insects to survive very low temperatures for long periods of time, and to avoid desiccation. These responses tend to be strongly seasonal, and insects in these habitats are only freeze tolerant for the overwintering period. By contrast, in mild and unpredictable environments, characteristic of habitats influenced by the Southern Ocean, freeze tolerance allows insects which habitually have ice nucleators in their guts to survive summer cold snaps, and to take advantage of mild winter periods without the need for extensive seasonal cold hardening. Thus, we conclude that the climates of the two hemispheres have led to the parallel evolution of freeze tolerance for very different reasons, and that this hemispheric difference is symptomatic of many wide-scale disparities in Northern and Southern ecological processes.