From hybridization to introgression between two closely related sympatric ant species

Patterns of evolution in MHC class II DQA and DQB exon 2 genes of Alpine mountain hares, Lepus timidus varronis, and sympatric and parapatric brown hares, L. europaeus, from Switzerland

In natural populations, hybridization is known to occur between a wide range of species. However, its evolutionary significance is less clear. Genes involved in fighting pathogens are considered excellent candidates for studying adaptive introgression, although both introgression and balancing selection can generate similar patterns of diversity and differentiation. Here, we compared DQA and DQB MHC class II and microsatellite allelic diversity of sympatric and parapatric mountain (Lepus timidus) and brown hare (L. europaeus) populations from Switzerland. We detected higher genetic diversity in brown hares compared to mountain hares at both MHC and microsatellite loci. We consider the observed patterns of microsatellite diversity both for L. europaeus and L. timidus as result of stochastic demographic processes while the pattern of MHC polymorphism of the studied hare populations can be explained by pathogen-driven selection. Rare bidirectional gene flow between both hare species seems to occur specifically for MHC alleles. However, the high number of shared alleles showing similar high frequency in both species suggests that reciprocally exchanged MHC alleles are being maintained via balancing selection. Adaptation to similar pathogen communities can also lead to parallel selection of MHC alleles. Positive selection, recombination and mutations have played different roles in shaping the patterns of MHC allelic diversity in and differentiation between both species. Results for the latter evolutionary forces do not show a better matching between the sympatric populations compared to the parapatric ones, suggesting a minor role of introgression for the observed evolutionary patterns of the studied hare species.

A community genomics approach to natural hybridization

Hybridization is a complicated, oft-misunderstood process. Once deemed unnatural and uncommon, hybridization is now recognized as ubiquitous among species. But hybridization rates within and among communities are poorly understood despite the relevance to ecology, evolution and conservation. To clarify, we examined hybridization across 75 freshwater fish communities within the Ozarks of the North American Interior Highlands (USA) by single nucleotide polymorphism (SNP) genotyping 33 species (N = 2865 individuals; double-digest restriction site-associated DNA sequencing (ddRAD)). We found evidence of hybridization (70 putative hybrids; 2.4% of individuals) among 18 species-pairs involving 73% (24/33) of study species, with the majority being concentrated within one family (Leuciscidae/minnows; 15 species; 66 hybrids). Interspecific genetic exchange-or introgression-was evident from 24 backcrossed individuals (10/18 species-pairs). Hybrids occurred within 42 of 75 communities (56%). Four selected environmental variables (species richness, protected area extent, precipitation (May and annually)) exhibited 73-78% accuracy in predicting hybrid occurrence via random forest classification. Our community-level assessment identified hybridization as spatially widespread and environmentally dependent (albeit predominantly within one diverse, omnipresent family). Our approach provides a more holistic survey of natural hybridization by testing a wide range of species-pairs, thus contrasting with more conventional evaluations.

Integrative characterization of genetic and phenotypic differentiation in an ant species complex with strong hierarchical population structure and low dispersal abilities

Low dispersal, occurrence of asexual reproduction and geographic discontinuity increase genetic differentiation between populations, which ultimately can lead to speciation. In this work, we used a multidisciplinary framework to characterize the genetic and phenotypic differentiation between and within two cryptic ant species with restricted dispersal, Cataglyphis cursor and C. piliscapa and used behavioral experiments to test for reproductive isolation. Their distribution is segregated by the Rhône River and they have been traditionally distinguished only by hair numbers, although a statistical assessment is still lacking. We found strong genetic (microsatellites, nuclear and mitochondrial sequences), morphological (number of hairs, tibia length, male genitalia) and chemical (cuticular hydrocarbons) differentiation not only between species but also among localities within species. However, inter-specific differentiation was slightly higher than intra-specific differentiation for most markers. Overall, this pattern could either reflect reproductive isolation or could result from a longer period of geographic isolation between species than among localities within species without necessarily involving reproductive isolation. Interestingly, our behavioral experiments showed an absence of mating between species associated to a higher aggressiveness of workers towards heterospecific males. This suggests that sexual selection may, at least partially, fuel reproductive isolation. We also showed that cuticular hydrocarbons, mtDNA sequences and number of hairs provide reliable criteria allowing species discrimination. Overall, this species complex offers a case study to further investigate varying stages of a speciation continuum by estimating reproductive isolation between pairs of localities varying by their level of genetic differentiation.

Appraising the Genetic Makeup of an Allochthonous Southern Pike Population: An Opportunity to Predict the Evolution of Introgressive Hybridization in Isolated Populations?

Biological invasions are a major threat to the conservation of biodiversity, as invasive species affect native biota through competition, predation, pathogen introduction, habitat alteration, and hybridisation. The present study focuses on a southern pike population, Esox cisalpinus (Teleostei: Esocidae), that has been introduced outside the species' native range. Using microsatellite markers, this study's objective was to gather baseline genetic information and assess the presence of hybrids between this species and E. lucius in the introduced population. The resulting estimates of genetic diversity and effective population size are comparable to those observed in the species' native range. Although different methods yield contrasting and uncertain evidence regarding introgressive hybridization, the presence of late-generation hybrids cannot be completely ruled out. Large numbers of breeders as well as multiple introductions of genetically divergent cohorts and introgressive hybridisation may explain the high genetic diversity of this recently introduced southern pike population. The present study issues a warning that the conservation of southern pike' introgressive hybridisation between northern and southern pike might be underestimated. The genetic information gathered herein may unravel the origin, number of introduction events, and evolutionary trajectory of the introduced population. This information may help us understand the evolution of introgressive hybridisation in the southern pike's native areas.

Discrimination of conspecifics from heterospecifics in a hybrid zone: Behavioral and chemical cues in ants

Species and nestmate recognition in social insects occurs mostly through cuticular hydrocarbons acting as chemical cues. These compounds generate a colony-specific odor profile depending on genetic and environmental factors. Species and nestmate recognition results in specific behavioral responses, regulating the level of aggression toward other individuals during an interaction. Although species discrimination and recognition cues have been poorly studied in the context of interspecific hybridization, such systems offer an opportunity to further investigate the influence of heritable and environmental factors on recognition. We explored the strength of discrimination in a hybrid zone between two ant species-Tetramorium immigrans and T. caespitum-by comparing cuticular hydrocarbon profiles and measuring intra- and interspecific worker aggression in both areas of sympatry and areas of allopatry among species. Species cuticular hydrocarbon profiles were well-differentiated and interspecific aggression was high, revealing highly discriminating species recognition cues. Hybrids' cuticular hydrocarbon profiles consisted of a mixture of the parental bouquets, but also exhibited hybrid-specific patterns. Behavioral assays showed that T. immigrans is as aggressive toward hybrids as toward heterospecifics. Finally, aggression between heterospecific workers was lower when interacting individuals came from areas of sympatry among species than from areas of allopatry. Taken as a whole, these findings paint a particularly complex picture of the recognition system in T. immigrans, T. caespitum, and their hybrids, and highlight that hybrid zones afford a still underexplored opportunity for investigating recognition mechanisms and discrimination between species.

Impact of invasive ant species on native fauna across similar habitats under global environmental changes

Biotic invasions can predominantly alter the dynamics, composition, functions, and structure of natural ecosystems. Social insects, particularly ants, are among the most damaging invasive alien species. Invasive ant species are among the supreme threats to ecosystems. There are about 23 species of invasive ants recorded worldwide, according to the ant invasive databases. The ecological impacts of invasive ants comprise predation, hybridization, and competition with native species that changes the ecosystem processes with the biodiversity loss and upsurge of pests. The effects of invasion on native fauna in the same habitats might be catastrophic for the native community through various ecological mechanisms, e.g., habitat disturbance, resource competition, limiting the foraging activity of native species, and various other indirect mechanisms of invasive species. Invasive species may have harmful impacts on habitats and devastating effects on natural flora and fauna, and stopping these new species from being introduced is the most effective way to deter future invasions and maintain biodiversity. This paper reviews the literature to evaluate the effects of invasive ant species on the native species, including vertebrates, invertebrates, and plants sharing the same habitats as the non-native species under global environmental changes. We also highlighted the various management strategies that could be adopted in minimizing the adverse effects of these invasive ant species on the natural ecosystem. To this end, strategies that could regulate the mode and rate of invasion by these alien ant species are the most effective ways to deter future invasions and maintain biodiversity.

Hybridization and invasiveness in social insects - The good, the bad and the hybrid

Hybridization may help drive biological invasions by reducing Allee effects, increasing genetic variation, and generating novel adaptive genotypes/phenotypes. Social insects (ants, bees, wasps, and termites) are among the world's worst invasive species. In this review, we study the relationship between hybridization and invasiveness in social insects. We examine three types of hybridization based on the reproductive characteristics of first-generation hybrids. We discuss several examples of the association between hybridization and invasiveness, which are predominantly found in bees and termites. However, hybridization also occurs in several non-invasive species, and highly invasive species are not consistently associated with hybridization events, indicating that hybridization is not a main driver of invasiveness in social insects. We discuss why hybridization is not more commonly seen in invasive social insects.

Public Engagement Provides First Insights on Po Plain Ant Communities and Reveals the Ubiquity of the Cryptic Species Tetramorium immigrans (Hymenoptera, Formicidae)

Simple Summary

Public involvement in biodiversity research in the form of Citizen Science is a powerful tool to improve our understanding of the natural world, and it is especially suitable for the study of heavily populated environments. Ants’ ubiquity and diversity, their role as ecological bioindicators, and the fact that most species can easily be sampled makes them ideal candidates for this kind of studies. In the framework of the international School of Ants citizen science project, we joined the “BioBlitz Lombardia” in which citizens are invited to collect biodiversity data on several parks from Lombardy (Po Plain, Italy). As a result, we recorded 30 ant species and obtained a first characterization of the region’s ant assemblages. We studied their patterns of variation in relation with the ecological difference between the studies sites, which ranged from urban to subalpine areas. In addition, we detected the presence of a cryptic species (Tetramorium immigrans) whose distribution and identity were only recently clarified. It likely represents an under-recorded introduced species in the region. Advantages and critical aspects of using CS methodology for the study of biodiversity are discussed in light of our experience.

Abstract

Ants are considered a useful model for biodiversity monitoring and several of their characteristics make them promising for citizen science (CS) projects. Involving a wide range of public figures into collecting valuable data on the effect of human impact on ant biodiversity, the School of Ants (SoA) project represents one of the very few attempts to explore the potential of these insects in CS. Through the collaboration with the “BioBlitz Lombardia” project, we tested the SoA protocol on 12 Northern Italy parks, ranging from urban green to subalpine protected sites. As a result, we obtained some of the very first quantitative data characterizing the ants of this region, recording 30 species and highlighting some interesting ecological patterns. These data revealed the ubiquitous presence of the recently taxonomically defined cryptic species Tetramorium immigrans, which appears to be probably introduced in the region. We also discuss advantages and criticisms encountered applying the SoA protocol, originally intended for schools, to new categories of volunteers, from BioBlitz participants to park operators, suggesting best practices based on our experience.

Monitoring the influx of new species through citizen science: the first introduced ant in Denmark

Climate change and invasive species threaten biodiversity, yet rigorous monitoring of their impact can be costly. Citizen science is increasingly used as a tool for monitoring exotic species, because citizens are geographically and temporally dispersed, whereas scientists tend to cluster in museums and at universities. Here we report on the establishment of the first exotic ant taxon (Tetramorium immigrans) in Denmark, which was discovered by children participating in The Ant Hunt. The Ant Hunt is a citizen science project for children that we ran in 2017 and 2018, with a pilot study in 2015. T. immigrans was discovered in the Botanical Garden of the Natural History Museum of Denmark in 2015 and confirmed as established in 2018. This finding extends the northern range boundary of T. immigrans by almost 460 km. Using climatic niche modelling, we compared the climatic niche of T. immigrans in Europe with that of T. caespitum based on confirmed observations from 2006 to 2019. T. immigrans and T. caespitum had a 13% niche overlap, with T. immigrans showing stronger occurrence in warmer and drier areas compared to T. caespitum. Mapping the environmental niches onto geographic space identified several, currently uninhabited, areas as climatically suitable for the establishment of T. immigrans. Tetramorium immigrans was sampled almost three times as often in areas with artificial surfaces compared to T. caespitum, suggesting that T. immigrans may not be native to all of Europe and is being accidentally introduced by humans. Overall, citizen scientists collected data on ants closer to cities and harbours than scientists did and had a stronger bias towards areas of human disturbance. This increased sampling effort in areas of likely introduction of exotic species naturally increases the likelihood of discovering species sooner, making citizen science an excellent tool for exotic species monitoring, as long as trained scientists are involved in the identification process.

Multiple mating in the context of interspecific hybridization between two Tetramorium ant species

In eusocial Hymenoptera, haplodiploidy and polyandry may facilitate selection for hybridization. Interspecific hybridization is widespread in ants and can lead to hybrid inviability as well as the formation of new species through hybrid speciation. However, in ants, polyandry is uncommon. By analyzing microsatellite markers on 15 ant workers per colony, we show that the mating system of 28 pure colonies of Tetramorium immigrans, 15 pure colonies of Tetramorium caespitum, and 27 hybrid colonies is a monogyne/polyandrous mating system, with a higher mating rate in T. caespitum (mean = 2.4 males vs. 1.7 in T. immigrans). Hybrid queens, but no hybrid fathers, were deduced from workers' genotypes, in accordance with Haldane's rule extended to haplodiploid organisms, which states that the haploid sex should more often be sterile or inviable. In five colonies, hybridization and multiple mating allowed the simultaneous production of both hybrid and nonhybrid offspring. Although rare, these situations hinted at asymmetrical, larger contributions of T. immigrans vs. T. caespitum males to offspring production. Together, these findings point toward a complex and dynamic mating system in T. immigrans and T. caespitum, and contribute to better understand interspecific hybridization mechanisms and their consequences on genetic and taxonomic diversity. The study of polyandry within a hybrid zone is unprecedented and opens new opportunities to better understand interspecific hybridization mechanisms and their short- to long-term consequences.