Genetic diversity is positively associated with fine‐scale momentary abundance of an invasive ant

Many introduced species become invasive despite genetic bottlenecks that should, in theory, decrease the chances of invasion success. By contrast, population genetic bottlenecks have been hypothesized to increase the invasion success of unicolonial ants by increasing the genetic similarity between descendent populations, thus promoting co‐operation. We investigated these alternate hypotheses in the unicolonial yellow crazy ant, Anoplolepis gracilipes, which has invaded Arnhem Land in Australia's Northern Territory. We used momentary abundance as a surrogate measure of invasion success, and investigated the relationship between A. gracilipes genetic diversity and its abundance, and the effect of its abundance on species diversity and community structure. We also investigated whether selected habitat characteristics contributed to differences in A. gracilipes abundance, for which we found no evidence. Our results revealed a significant positive association between A. gracilipes genetic diversity and abundance. Invaded communities were less diverse and differed in structure from uninvaded communities, and these effects were stronger as A. gracilipes abundance increased. These results contradict the hypothesis that genetic bottlenecks may promote unicoloniality. However, our A. gracilipes study population has diverged since its introduction, which may have obscured evidence of the bottleneck that would likely have occurred on arrival. The relative importance of genetic diversity to invasion success may be context dependent, and the role of genetic diversity may be more obvious in the absence of highly favorable novel ecological conditions.     

Yellow crazy ant (<Emphasis Type="Italic">Anoplolepis gracilipes</Emphasis>) invasions within undisturbed mainland Australian habitats: no support for biotic resistance hypothesis

Ants are highly successful invaders, especially on islands, yet undisturbed mainland environments often do not contain invasive ants, and this observation is largely attributed to biotic resistance. An exception is the incursion of Yellow crazy ant Anoplolepis gracilipes within northeast Arnhem Land. The existence of A. gracilipes within this landscape’s intact environments containing highly competitive ant communities indicates that biotic resistance is not a terminally inhibitory factor mediating this ant’s distribution at the regional scale. We test whether biotic resistance may still operate at a more local scale by assessing whether ecological impacts are proportional to habitat suitability for A. gracilipes, as well as to the competitiveness of the invaded ant community. The abundance and species richness of native ants were consistently greater in uninfested than infested plots but the magnitude of the impacts did not differ between habitats. The abundance and ordinal richness of other macro-invertebrates were consistently lower in infested plots in all habitats. A significant negative relationship was found for native ant abundance and A. gracilipes abundance. No relationships were found between A. gracilipes abundance and any measure of other macro-invertebrates. The relative contribution of small ants (<2.5 mm) to total abundance and relative species richness was always greater in infested sites coinciding with a reduction of the contribution of the larger size classes. Differences in the relative abundance of ant functional groups between infested and uninfested sites reflected impacts according to ant size classes and ecology. The widespread scale of these incursions and non-differential level of impacts among the habitats, irrespective of native ant community competitiveness and abiotic suitability to A. gracilipes, does not support the biotic resistance hypothesis.     

The conundrum of the yellow crazy ant (Anoplolepis gracilipes) reproductive mode: no evidence for dependent lineage genetic caste determination

Asexual reproduction and hybridisation are often found among highly invasive plants and marine invertebrates. Recently, it has been suggested that clonality may enhance the success of invasive ants. In contrast, obligate hybridisation (dependent lineage genetic caste determination or DL GCD in ants) may decrease the chances of population persistence if one lineage is less prevalent than the other (asymmetry in lineage ratio). Genetic data available for the invasive yellow crazy ant (Anoplolepis gracilipes) suggest that it has an unconventional mode of reproduction that may involve asexual reproduction by workers or queens, or a form of genetic caste determination. Here, we investigated whether A. gracilipes reproduction involved DL GCD. The potential for worker reproduction was also assessed. We used microsatellite markers to assess the population structure of A. gracilipes workers, males, queens and sperm in queen spermathecae, from field collections in Arnhem Land. We found that a single queen lineage is present in Arnhem Land. The presence of a single lineage of queens discounts the possibility of DL GCD. Population structure separated queens and workers into different lineages, suggesting that these castes are determined genetically in A. gracilipes, or the mode of reproduction differs between workers and queens. Evidence for worker reproduction was weak. We conclude that the reproductive mode of A. gracilipes does not involve DL GCD. The resolution of the reproductive mode of A. gracilipes is complicated by a high prevalence of diploid males. The determination of the A. gracilipes reproductive mode remains a fascinating research question, and its resolution will improve our understanding of the contribution of the reproductive system to invasion success.     

Behaviourally and genetically distinct populations of an invasive ant provide insight into invasion history and impacts on a tropical ant community

Populations of invasive species often exhibit a high degree of spatial and temporal variability in abundance and hence their effects on resident communities. Here, we examine behavioural, genetic and environmental factors that influence variation in populations of the yellow crazy ant, Anoplolepis gracilipes, on the remote Nukunonu Atoll of Tokelau, Pacific Ocean. Behavioural assays revealed high levels of aggression between two groups of yellow crazy ants from different islands, and genetic analysis confirmed the presence of two distinct populations with unique mitochondrial (mt)DNA haplotypes, designated A and D. The two populations likely resulted from two separate invasion events. The populations exhibited significant differences in abundance of A. gracilipes, with a mean sevenfold difference in relative abundance between the two main haplotypes. The higher density haplotype D population coexisted with 50% fewer other ant species and altered ant community composition. Vegetation composition was also significantly different on islands harbouring the two populations. The results suggest genetic differences could play a role in the spatial and temporal variation in the effect of the yellow crazy ant on a small oceanic atoll. We could not differentiate between genetic effects and effects of vegetation. However, our results indicate that spatial variability in behaviour and impacts within populations of invasive species could be in part due to genetic differences, and play a substantial role in influencing the outcome of biological invasions.     

Phylogeography of the antilopine wallaroo (Macropus antilopinus) across tropical northern Australia

The distribution of antilopine wallaroo, Macropus antilopinus, is marked by a break in the species’ range between Queensland and the Northern Territory, coinciding with the Carpentarian barrier. Previous work on M. antilopinus revealed limited genetic differentiation between the Northern Territory and Queensland M. antilopinus populations across this barrier. The study also identified a number of divergent lineages in the Northern Territory, but was unable to elucidate any geographic structure. Here, we re‐examine these results to (1) determine phylogeographic patterns across the range of M. antilopinus and (2) infer the biogeographic barriers associated with these patterns. The tropical savannahs of northern Australia: from the Cape York Peninsula in the east, to the Kimberley in the west. We examined phylogeographic patterns in M. antilopinus using a larger number of samples and three mtDNA genes: NADH dehydrogenase subunit 2, cytochrome b, and the control region. Two datasets were generated and analyzed: (1) a subset of samples with all three mtDNA regions concatenated together and (2) all samples for just control region sequences that included samples from the previous study. Analysis included generating phylogenetic trees based on Bayesian analysis and intraspecific median‐joining networks. The contemporary spatial structure of M. antilopinus mtDNA lineages revealed five shallow clades and a sixth, divergent lineage. The genetic differences that we found between Queensland and Northern Territory M. antilopinus samples confirmed the split in the geographic distribution of the species. We also found weak genetic differentiation between Northern Territory samples and those from the Kimberley region of Western Australia, possibly due to the Kimberley Plateau–Arnhem Land barrier. Within the Northern Territory, two clades appear to be parapatric in the west, while another two clades are broadly sympatric across the Northern Territory. MtDNA diversity of M. antilopinus revealed an unexpectedly complex evolutionary history involving multiple sympatric and parapatric mtDNA clades across northern Australia. These phylogeographic patterns highlight the importance of investigating genetic variation across distributions of species and integrating this information into biodiversity conservation.     

Large scale unicoloniality: the population and colony structure of the invasive Argentine ant (<Emphasis Type="Italic">Linepithema humile</Emphasis>) in New Zealand

The Argentine ant is native to South America and has spread widely across the globe. In this study, we use genetic analyses and behavioural assays to examine the colony structure of Argentine ants in New Zealand. Diet modification studies were also carried out in order to help identify what factors influence these behavioural assays. There was no aggression observed between any pairings tested across the North Island of New Zealand, though we found that diet manipulations in the laboratory could lead to low levels of aggression between previously amiable Argentine ant nests. The New Zealand population of Argentine ants was characterized by low levels of genetic variation in six microsatellite loci from their nuclear genome. Additionally, the population also lacked significant genetic structuring with no patterns of regional differentiation or isolation by distance. An analysis of molecular variation(AMOVA) found that themajority of genetic variation was present at a nest level (93% of total genetic variance), with little genetic differentiation observed within or between regions (3 – 4% of total genetic variance). No correlation between aggression and genetic relatedness was observed. This evidence suggests that Argentine ants in New Zealand effectively form a unicolonial population, which is likely the result of colonization from a single source population. As far as we know, this is the first country to have an entirely unicolonial population of Argentine ants. Received 21 February 2007; revised 20 May 2007; accepted 22 May 2007.     

Invasive ants compete with and modify the trophic ecology of hermit crabs on tropical islands

Invasive species can dramatically alter trophic interactions. Predation is the predominant trophic interaction generally considered to be responsible for ecological change after invasion. In contrast, how frequently competition from invasive species contributes to the decline of native species remains controversial. Here, we demonstrate how the trophic ecology of the remote atoll nation of Tokelau is changing due to competition between invasive ants (Anoplolepis gracilipes) and native terrestrial hermit crabs (Coenobita spp.) for carrion. A significant negative correlation was observed between A. gracilipes and hermit crab abundance. On islands with A. gracilipes, crabs were generally restricted to the periphery of invaded islands. Very few hermit crabs were found in central areas of these islands where A. gracilipes abundances were highest. Ant exclusion experiments demonstrated that changes in the abundance and distribution of hermit crabs on Tokelau are a result of competition. The ants did not kill the hermit crabs. Rather, when highly abundant, A. gracilipes attacked crabs by spraying acid and drove crabs away from carrion resources. Analysis of naturally occurring N and C isotopes suggests that the ants are effectively lowering the trophic level of crabs. According to δ¹⁵ N values, hermit crabs have a relatively high trophic level on islands where A. gracilipes have not invaded. In contrast, where these ants have invaded we observed a significant decrease in δ¹⁵ N for all crab species. This result concurs with our experiment in suggesting long-term exclusion from carrion resources, driving co-occurring crabs towards a more herbivorous diet. Changes in hermit crab abundance or distribution may have major ramifications for the stability of plant communities. Because A. gracilipes have invaded many tropical islands where the predominant scavengers are hermit crabs, we consider that their competitive effects are likely to be more prominent in structuring communities than predation. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Are invasive ants better plant‐defense mutualists? A comparison of foliage patrolling and herbivory in sites with invasive yellow crazy ants and native weaver ants

Benefits arising from facultative mutualisms between ants and plants vary with the identity of the ant partner. Invasive and native ants are both attracted to plants that offer extrafloral nectar, but few studies have compared their abilities to displace herbivores and benefit plants. Yellow crazy ants Anoplolepis gracilipes have invaded eucalypt woodlands of Arnhem Land, northern Australia, where they displace the native dominant weaver ant Oecophylla smaragdina. We compared the plant defense services provided by A. gracilipes and O. smaragdina ants on trees with (Acacia lamprocarpa) and without (Eucalyptus tetrodonta) extrafloral nectar rewards through surrogate herbivore (termite) addition experiments and surveys of herbivore damage. Anoplolepis gracilipes were more likely than O. smaragdina to discover termites on A. lamprocarpa, but the likelihood of termite discovery on E. tetrodonta did not vary with ant species. Anoplolepis gracilipes were also more thorough in their attacks of termites, recruited 3.4–4 times more workers to termites, and were 3.4 times quicker at discovering termites on A. lamprocarpa than were O. smaragdina. Discovery of termites by other predators did not vary significantly between trees in A. gracilipes and O. smaragdina sites. Herbivory scores did not reflect the foliage patrolling pattern by the ants. Old A. lamprocarpa leaves and both new and old leaves and branches on E. tetrodonta in A. gracilipes sites had higher chewing herbivory scores than their counterparts in O. smaragdina sites. Our results reveal that the more aggressive and efficient foliar patrolling by A. gracilipes does not translate to increased plant protection. Ant invasions can disrupt native ant–plant mutualisms despite invasive ants possessing many traits associated with effective plant guarding.     

Differentiation in putative male sex pheromone components across and within populations of the African butterfly Bicyclus anynana as a potential driver of reproductive isolation

Sexual traits are often the most divergent characters among closely related species, suggesting an important role of sexual traits in speciation. However, to prove this, we need to show that sexual trait differences accumulate before or during the speciation process, rather than being a consequence of it. Here, we contrast patterns of divergence among putative male sex pheromone (pMSP) composition and the genetic structure inferred from variation in the mitochondrial cytochrome oxidase 1 and nuclear CAD loci in the African butterfly Bicyclus anynana (Butler, 1879) to determine whether the evolution of “pheromonal dialects” occurs before or after the differentiation process. We observed differences in abundance of some shared pMSP components as well as differences in the composition of the pMSP among B. anynana populations. In addition, B. anynana individuals from Kenya displayed differences in the pMSP composition within a single population that appeared not associated with genetic differences. These differences in pMSP composition both between and within B. anynana populations were as large as those found between different Bicyclus species. Our results suggest that “pheromonal dialects” evolved within and among populations of B. anynana and may therefore act as precursors of an ongoing speciation process.     

Elevated dominance of extrafloral nectary-bearing plants is associated with increased abundances of an invasive ant and reduced native ant richness

Aim Invasive ants can have substantial and detrimental effects on co‐occurring community members, especially other ants. However, the ecological factors that promote both their population growth and their negative influences remain elusive. Opportunistic associations between invasive ants and extrafloral nectary (EFN)‐bearing plants are common and may fuel population expansion and subsequent impacts of invasive ants on native communities. We examined three predictions of this hypothesis, compared ant assemblages between invaded and uninvaded sites and assessed the extent of this species in Samoa. Location The Samoan Archipelago (six islands and 35 sites). Methods We surveyed abundances of the invasive ant Anoplolepis gracilipes, other ant species and EFN‐bearing plants. Results Anoplolepis gracilipes was significantly more widely distributed in 2006 than in 1962, suggesting that the invasion of A. gracilipes in Samoa has progressed. Furthermore, (non‐A. gracilipes) ant assemblages differed significantly between invaded and uninvaded sites. Anoplolepis gracilipes workers were found more frequently at nectaries than other plant parts, suggesting that nectar resources were important to this species. There was a strong, positive relationship between the dominance of EFN‐bearing plants in the community and A. gracilipes abundance on plants, a relationship that co‐occurring ants did not display. High abundances of A. gracilipes at sites dominated by EFN‐bearing plants were associated with low species richness of native plant‐visiting ant species. Anoplolepis gracilipes did not display any significant relationships with the diversity of other non‐native ants. Main conclusions Together, these data suggest that EFN‐bearing plants may promote negative impacts of A. gracilipes on co‐occurring ants across broad spatial scales. This study underscores the potential importance of positive interactions in the dynamics of species invasions. Furthermore, they suggest that conservation managers may benefit from explicit considerations of potential positive interactions in predicting the identities of problematic invaders or the outcomes of species invasions.     

Genetic structure of introduced swamp buffalo subpopulations in tropical Australia

High densities of introduced herbivores can damage sensitive ecosystems, increase the risk of extinction of native biota, and host and spread disease. An essential step in managing large ‘feral’ animal populations is to quantify how they use habitats so that management interventions, such as culling, can be targeted to reduce densities and to minimize migration into areas from which animals have been removed. An effective method to quantify animal movements is by measuring landscape‐scale genetic population structure. We describe the genetic population structure of one of Australia's more destructive introduced mammals – the Asian swamp buffalo (Bubalus bubalis). We collected 524 skin samples from buffalo across their range in the Northern Territory of Australia. Allelic diversity in the Northern Territory population was low compared to those reported from populations in their native Asian habitats. The Australian population is tentatively made of three subpopulations; Melville Island, Eastern Arnhem and Central‐Western Arnhem populations. The Melville Island population is represented by a single cluster, while the Eastern Arnhem population has three clusters and the Central‐Western Arnhem population seven clusters. We found some support for isolation by distance across all the sampled populations, but little evidence for this relationship when comparing the two well‐mixed mainland meta‐populations. Despite their small founder populations and limited genetic variation, the persistence of buffalo in Australia has likely been aided by release from high predation, parasitism and disease typical of their native habitats.     

Targeted Research to Improve Invasive Species Management: Yellow Crazy Ant Anoplolepis gracilipes in Samoa

Lack of biological knowledge of invasive species is recognised as a major factor contributing to eradication failure. Management needs to be informed by a site-specific understanding of the invasion system. Here, we describe targeted research designed to inform the potential eradication of the invasive yellow crazy ant Anoplolepis gracilipes on Nu’utele island, Samoa. First, we assessed the ant’s impacts on invertebrate biodiversity by comparing invertebrate communities between infested and uninfested sites. Second, we investigated the timing of production of sexuals and seasonal variation of worker abundance and nest density. Third, we investigated whether an association existed between A. gracilipes and carbohydrate sources. Within the infested area there were few other ants larger than A. gracilipes, as well as fewer spiders and crabs, indicating that A. gracilipes is indeed a significant conservation concern. The timing of male reproduction appears to be consistent with places elsewhere in the world, but queen reproduction was outside of the known reproductive period for this species in the region, indicating that the timing of treatment regimes used elsewhere are not appropriate for Samoa. Worker abundance and nest density were among the highest recorded in the world, being greater in May than in October. These abundance and nest density data form baselines for quantifying treatment efficacy and set sampling densities for post-treatment assessments. The number of plants and insects capable of providing a carbohydrate supply to ants were greatest where A. gracilipes was present, but it is not clear if this association is causal. Regardless, indirectly controlling ant abundance by controlling carbohydrate supply appears to be promising avenue for research. The type of targeted, site-specific research such as that described here should be an integral part of any eradication program for invasive species to design knowledge-based treatment protocols and determine assessment benchmarks to achieve eradication.     

Ecological divergence and speciation in common bottlenose dolphins in the western South Atlantic

Coastal and offshore ecotypes of common bottlenose dolphins have been recognized in the western South Atlantic, and it is possible that trophic niche divergence associated with social interactions is leading them to genetic and phenotypic differentiation. The significant morphological differentiation observed between these ecotypes suggests they represent two different subspecies. However, there is still a need to investigate whether there is congruence between morphological and genetic data to rule out the possibility of ecophenotypic variation accompanied by gene flow. Mitochondrial DNA (mtDNA) control region sequence data and 10 microsatellite loci collected from stranded and biopsied dolphins sampled in coastal and offshore waters of Brazil as well as 106 skulls for morphological analyses were used to determine whether the morphological differentiation was supported by genetic differentiation. There was congruence among the data sets, reinforcing the presence of two distinct ecotypes. The divergence may be relatively recent, however, given the moderate values of mtDNA nucleotide divergence (dA = 0.008), presence of one shared mtDNA haplotype and possibly low levels of gene flow (around 1% of migrants per generation). Results suggest the ecotypes may be in the process of speciation and reinforce they are best described as two different subspecies until the degree of nuclear genetic divergence is thoroughly evaluated: Tursiops truncatus gephyreus (coastal ecotype) and T. t. truncatus (offshore ecotype). The endemic distribution of T. t. gephyreus in the western South Atlantic and number of anthropogenic threats in the area reinforces the importance of protecting this ecotype and its habitat.     

Genetic divergence is more tightly related to call variation than landscape features in the Amazonian frogs Physalaemus petersi and P. freibergi

Behavioural isolation from divergence in male advertisement calls and female preferences is hypothesized to cause genetic divergence and speciation in the Amazonian frogs Physalaemus petersi and P. freibergi, yet the importance of call variation and landscape features in genetic divergence is unresolved. We tested for correlations between genetic divergence at microsatellite loci and (1) call variables; and (2) landscape variables among 10 populations of these frogs. Genetic divergence was not correlated with geographical distance, rivers or elevation. There was a strong positive relationship, however, between genetic divergence and inter‐population differences in one call variable, whine dominant frequency. Effective population sizes varied among sites (range = 15–846) and were often small, suggesting that genetic drift could influence call evolution. Evidence for fine‐scale genetic structure within sites was also found. Our results support the hypothesis that behavioural isolation from divergence in male calls and female preferences causes genetic divergence and speciation.     

Gene flow does not prevent personality and morphological differentiation between two blue tit populations

Understanding the causes and consequences of population phenotypic divergence is a central goal in ecology and evolution. Phenotypic divergence among populations can result from genetic divergence, phenotypic plasticity or a combination of the two. However, few studies have deciphered these mechanisms for populations geographically close and connected by gene flow, especially in the case of personality traits. In this study, we used a common garden experiment to explore the genetic basis of the phenotypic divergence observed between two blue tit (Cyanistes caeruleus) populations inhabiting contrasting habitats separated by 25 km, for two personality traits (exploration speed and handling aggression), one physiological trait (heart rate during restraint) and two morphological traits (tarsus length and body mass). Blue tit nestlings were removed from their population and raised in a common garden for up to 5 years. We then compared adult phenotypes between the two populations, as well as trait‐specific Q_st and F_st. Our results revealed differences between populations similar to those found in the wild, suggesting a genetic divergence for all traits. Q_st–F_st comparisons revealed that the trait divergences likely result from dissimilar selection patterns rather than from genetic drift. Our study is one of the first to report a Q_st–F_st comparison for personality traits and adds to the growing body of evidence that population genetic divergence is possible at a small scale for a variety of traits including behavioural traits.     

Toxicity and population structure of the Rough‐Skinned Newt (Taricha granulosa) outside the range of an arms race with resistant predators

Species interactions, and their fitness consequences, vary across the geographic range of a coevolutionary relationship. This spatial heterogeneity in reciprocal selection is predicted to generate a geographic mosaic of local adaptation, wherein coevolutionary traits are phenotypically variable from one location to the next. Under this framework, allopatric populations should lack variation in coevolutionary traits due to the absence of reciprocal selection. We examine phenotypic variation in tetrodotoxin (TTX) toxicity of the Rough‐Skinned Newt (Taricha granulosa) in regions of allopatry with its TTX‐resistant predator, the Common Garter Snake (Thamnophis sirtalis). In sympatry, geographic patterns of phenotypic exaggeration in toxicity and toxin‐resistance are closely correlated in prey and predator, implying that reciprocal selection drives phenotypic variation in coevolutionary traits. Therefore, in allopatry with TTX‐resistant predators, we expect to find uniformly low levels of newt toxicity. We characterized TTX toxicity in northwestern North America, including the Alaskan panhandle where Ta. granulosa occur in allopatry with Th. sirtalis. First, we used microsatellite markers to estimate population genetic structure and determine if any phenotypic variation in toxicity might be explained by historical divergence. We found northern populations of Ta. granulosa generally lacked population structure in a pattern consistent with northern range expansion after the Pleistocene. Next, we chose a cluster of sites in Alaska, which uniformly lacked genetic divergence, to test for phenotypic divergence in toxicity. As predicted, overall levels of newt toxicity were low; however, we also detected unexpected among‐ and within‐population variation in toxicity. Most notably, a small number of individuals contained large doses of TTX that rival means of toxic populations in sympatry with Th. sirtalis. Phenotypic variation in toxicity, despite limited neutral genetic divergence, suggests that factors other than reciprocal selection with Th. sirtalis likely contribute to geographic patterns of toxicity in Ta. granulosa.     

Closely related octopus species show different spatial genetic structures in response to the Antarctic seascape

Determining whether comparable processes drive genetic divergence among marine species is relevant to molecular ecologists and managers alike. Sympatric species with similar life histories might be expected to show comparable patterns of genetic differentiation and a consistent influence of environmental factors in shaping divergence. We used microsatellite loci to quantify genetic differentiation across the Scotia Arc in three species of closely related benthic octopods, Pareledone turqueti, P. charcoti, and Adelieledone polymorpha. The relative importance of environmental factors (latitude, longitude, depth, and temperature) in shaping genetic structure was investigated when significant spatial genetic structure was uncovered. Isolated populations of P. turqueti and A. polymorpha at these species’ range margins were genetically different to samples close to mainland Antarctica; however, these species showed different genetic structures at a regional scale. Samples of P. turqueti from the Antarctic Peninsula, Elephant Island, and Signy Island were genetically different, and this divergence was associated primarily with sample collection depth. By contrast, weak or nonsignificant spatial genetic structure was evident across the Antarctic Peninsula, Elephant Island, and Signy Island region for A. polymorpha, and slight associations between population divergence and temperature or depth (and/or longitude) were detected. Pareledone charcoti has a limited geographic range, but exhibited no genetic differentiation between samples from a small region of the Scotia Arc (Elephant Island and the Antarctic Peninsula). Thus, closely related species with similar life history strategies can display contrasting patterns of genetic differentiation depending on spatial scale; moreover, depth may drive genetic divergence in Southern Ocean benthos.     

Ant–aphid interactions on Asclepias syriaca are mediated by plant genotype and caterpillar damage

The means by which plant genotypes influence species interactions and arthropod community structure remain poorly understood. One potential, but largely unstudied mechanism is that occurring through plant genetic variation in induced responses to herbivory. Here we test whether induced responses to leaf damage and genotypic variation for induction in Asclepias syriaca influence interactions among Formica podzolica ants, the ant‐tended aphid Aphis asclepiadis, and the untended aphid Myzocallis asclepiadis. In so doing, we assess genetic variation in plant‐mediated interactions among different herbivore guilds. We conducted a three‐way factorial field experiment manipulating plant genotype, leaf damage by specialist monarch caterpillars Danaus plexippus, and ant presence, and documented effects on aphid and ant abundances. Leaf damage increased Aphis abundance in both the presence and absence of ants and Myzocallis abundance under ant exclusion. In the presence of ants, leaf damage decreased Myzocallis abundance, likely due to effects on ant–Myzocallis interactions; ants showed a positive association with Myzocallis, leaf damage increased the strength of this association (425% more ants per aphid), and this in turn fed back to suppress Myzocallis abundance. Yet, these aggregate effects of leaf damage on Myzocallis and ants were underlain by substantial variation among milkweed geno types, with leaf damage inducing lower aphid and ant abundances on some genotypes, but higher abundances on others. As a consequence, a substantial fraction of the variation in leaf damage effects on ants (R² =0.42) was explained by milkweed genetic variation in the strength and sign of leaf damage effects on Myzocallis. Although plant genetic variation influenced Aphis abundance, this did not translate into genetic variation in ant abundance, and leaf damage did not influence Aphis–ant interactions. Overall, we show that variation in induced responses to herbivory is a relevant condition by which plant genotype influences interactions in plant‐centered arthropod communities and provide novel results of effects on the third trophic level.     

Divergent population structure and climate associations of a chromosomal inversion polymorphism across the Mimulus guttatus species complex

Chromosomal rearrangement polymorphisms are common and increasingly found to be associated with adaptive ecological divergence and speciation. Rearrangements, such as inversions, reduce recombination in heterozygous individuals and thus can protect favourable allelic combinations at linked loci, facilitating their spread in the presence of gene flow. Recently, we identified a chromosomal inversion polymorphism that contributes to ecological adaptation and reproductive isolation between annual and perennial ecotypes of the yellow monkeyflower, Mimulus guttatus. Here we evaluate the population genetic structure of this inverted region in comparison with the collinear regions of the genome across the M. guttatus species complex. We tested whether annual and perennial M. guttatus exhibit different patterns of divergence for loci in the inverted and noninverted regions of the genome. We then evaluated whether there are contrasting climate associations with these genomic regions through redundancy analysis. We found that the inversion exhibits broadly different patterns of divergence among annual and perennial M. guttatus and is associated with environmental variation across population accessions. This study is the first widespread population genetic survey of the diversity of the M. guttatus species complex. Our findings contribute to a greater understanding of morphological, ecological, and genetic evolutionary divergence across this highly diverse group of closely related ecotypes and species. Finally, understanding species relationships among M. guttatus sp. has hitherto been stymied by accumulated evidence of substantial gene flow among populations as well as designated species. Nevertheless, our results shed light on these relationships and provide insight into adaptation in life history traits within the complex.     

Adaptive divergence despite strong genetic drift: genomic analysis of the evolutionary mechanisms causing genetic differentiation in the island fox (Urocyon littoralis)

The evolutionary mechanisms generating the tremendous biodiversity of islands have long fascinated evolutionary biologists. Genetic drift and divergent selection are predicted to be strong on islands and both could drive population divergence and speciation. Alternatively, strong genetic drift may preclude adaptation. We conducted a genomic analysis to test the roles of genetic drift and divergent selection in causing genetic differentiation among populations of the island fox (Urocyon littoralis). This species consists of six subspecies, each of which occupies a different California Channel Island. Analysis of 5293 SNP loci generated using Restriction‐site Associated DNA (RAD) sequencing found support for genetic drift as the dominant evolutionary mechanism driving population divergence among island fox populations. In particular, populations had exceptionally low genetic variation, small N_e (range = 2.1–89.7; median = 19.4), and significant genetic signatures of bottlenecks. Moreover, islands with the lowest genetic variation (and, by inference, the strongest historical genetic drift) were most genetically differentiated from mainland grey foxes, and vice versa, indicating genetic drift drives genome‐wide divergence. Nonetheless, outlier tests identified 3.6–6.6% of loci as high F_ST outliers, suggesting that despite strong genetic drift, divergent selection contributes to population divergence. Patterns of similarity among populations based on high F_ST outliers mirrored patterns based on morphology, providing additional evidence that outliers reflect adaptive divergence. Extremely low genetic variation and small N_e in some island fox populations, particularly on San Nicolas Island, suggest that they may be vulnerable to fixation of deleterious alleles, decreased fitness and reduced adaptive potential.