Unravelling the paradox of loss of genetic variation during invasion: superclones may explain the success of a clonal invader

Phenotypic specialization of the pea aphid in its southern limit of distribution

The success of biological invasions ultimately relies on phenotypic traits of the invasive species. Aphids, which include many important pests worldwide, may have been successful invading new environments partly because they can maximize reproductive output by becoming parthenogenetic and losing the sexual phase of their reproductive cycle. However, invasive populations of aphids invading wide ranges can face contrasting environmental conditions and requiring different phenotypic strategies. Besides transitions in their reproductive cycle, it is only partially known which phenotypic traits might be associated to the invasion success of aphid populations in extended novel ranges. Here, we used four genotypes of the pea aphid Acyrthosiphon pisum from two localities in Chile to test for phenotypic specialization that might explain their establishment and spread in habitats exhibiting contrasting environmental conditions. We show that lineages living at a higher latitude with low temperatures show, in addition to facultative sexual reproduction, smaller body sizes, lower metabolic rates and a higher tolerance to the cold than the obligate asexual lineages living in a mild weather, at the expense of fecundity. Conversely, at higher temperatures only asexual lineages were found, which exhibit larger body sizes, higher reproductive outputs and consequently enhanced demographic ability. As a result, in conjunction with the reproductive mode, lineage specialization in physiological and life-history traits could be taken into account as an important strategy for populations of pea aphid to effectively invade extended novel ranges comprising different climatic conditions.

Genetic analysis reveals the putative native range and widespread double-clonal reproduction in the invasive longhorn crazy ant

Clonal reproduction can provide an advantage for invasive species to establish as it can circumvent inbreeding depression which often plagues introduced populations. The world's most widespread invasive ant, Paratrechina longicornis, was previously found to display a double-clonal reproduction system, whereby both males and queens are produced clonally, resulting in separate male and queen lineages, while workers are produced sexually. Under this unusual reproduction mode, inbreeding is avoided in workers as they carry hybrid interlineage genomes. Despite the ubiquitous distribution of P. longicornis, the significance of this reproductive system for the ant's remarkable success remains unclear, as its prevalence is still unknown. Further investigation into the controversial native origin of P. longicornis is also required to reconstruct the evolutionary histories of double-clonal lineages. Here, we examine genetic variation and characterize the reproduction mode of P. longicornis populations sampled worldwide using microsatellites and mitochondrial DNA sequences to infer the ant's putative native range and the distribution of the double-clonal reproductive system. Analyses of global genetic variations indicate that the Indian subcontinent is a genetic diversity hotspot of this species, suggesting that P. longicornis probably originates from this geographical area. Our analyses revealed that both the inferred native and introduced populations exhibit double-clonal reproduction, with queens and males around the globe belonging to two separate, nonrecombining clonal lineages. By contrast, workers are highly heterozygous because they are first-generation interlineage hybrids. Overall, these data indicate a worldwide prevalence of double clonality in P. longicornis and support the prediction that the unusual genetic system may have pre-adapted this ant for global colonization by maintaining heterozygosity in the worker force and alleviating genetic bottlenecks.

Lanza V, Rodríguez-Beltrán J, Galán JC, San Millán A, Cantón R, Coque TM. Evolutionary Pathways and Trajectories in Antibiotic Resistance

Evolution is the hallmark of life. Descriptions of the evolution of microorganisms have provided a wealth of information, but knowledge regarding "what happened" has precluded a deeper understanding of "how" evolution has proceeded, as in the case of antimicrobial resistance. The difficulty in answering the "how" question lies in the multihierarchical dimensions of evolutionary processes, nested in complex networks, encompassing all units of selection, from genes to communities and ecosystems. At the simplest ontological level (as resistance genes), evolution proceeds by random (mutation and drift) and directional (natural selection) processes; however, sequential pathways of adaptive variation can occasionally be observed, and under fixed circumstances (particular fitness landscapes), evolution is predictable. At the highest level (such as that of plasmids, clones, species, microbiotas), the systems' degrees of freedom increase dramatically, related to the variable dispersal, fragmentation, relatedness, or coalescence of bacterial populations, depending on heterogeneous and changing niches and selective gradients in complex environments. Evolutionary trajectories of antibiotic resistance find their way in these changing landscapes subjected to random variations, becoming highly entropic and therefore unpredictable. However, experimental, phylogenetic, and ecogenetic analyses reveal preferential frequented paths (highways) where antibiotic resistance flows and propagates, allowing some understanding of evolutionary dynamics, modeling and designing interventions. Studies on antibiotic resistance have an applied aspect in improving individual health, One Health, and Global Health, as well as an academic value for understanding evolution. Most importantly, they have a heuristic significance as a model to reduce the negative influence of anthropogenic effects on the environment.

Unraveling the Genetic Structure of the Coconut Scale Insect Pest (Aspidiotus rigidus Reyne) Outbreak Populations in the Philippines

Despite the fact that massive outbreaks of the coconut scale insect pest, Aspidiotus rigidus Reyne (Hemiptera: Diaspididae) are inflicting significant economic losses to the Philippines' coconut industry, little is known about the population and dispersal history of this invasive pest in the country. Here, we examined the genetic diversity, structure and demographic history of A. rigidus sampled from localities with reported outbreaks from 2014 to 2017. We analyzed the genetic structure of the outbreak populations using mitochondrial COI and nuclear EF-1α markers. Both markers and all methods of population genetic structure analyses indicate clear differentiation among the A. rigidus populations separating the north from the southern regions of the Philippines. Very low or no genetic differentiation was observed within and amongst the populations per geographic region indicating two unrelated outbreak events of the pest originating from two genetically uniform populations isolated in each respective region. Historical data supports the resurgence of an established A. rigidus population in the south which could have been driven by sudden climatic changes or human-induced habitat imbalance. Our study provides valuable information on the genetic differentiation of the two A. rigidus groups that would be useful for developing and implementing biological control strategies against this pest in the Philippines.

Multiple introductions and secondary dispersion of Tubastraea spp. in the Southwestern Atlantic

Accidental introduction through ballast water and biofouling are currently the main factors responsible for spreading non-indigenous species in the marine realm. In the Southwestern Atlantic, two scleractinian corals, Tubastraea coccinea and T. tagusensis, have been introduced by opportunistic colonization in 1980 and are now widespread along more than 3,500 km of coastline. To better understand the invasion process and the role of vectors in spreading these species, we sampled 306 and 173 colonies of T. coccinea and T. tagusensis from invaded sites, possible vectors and one native population. Analyses revealed a higher diversity of multi-locus genotypes (MLGs) on vectors, suggesting that they were contaminated prior to their arrival in the Southwestern Atlantic, and a high proportion of clones at invaded sites, with few genotypes spread over ~2,000 km. This broad distribution is most likely a result of secondary introductions through the transport of contaminated vectors. Results also suggest the occurrence of multiple invasions, mainly in the northernmost sites. In summary, clonality, secondary introductions, and multiple invasions are the main reasons for the broad spread and invasive success of Tubastraea spp. in the Southwestern Atlantic. Consequently, the correct control of vectors is the most effective approach for management and prevention of new invasions.

A cascade of biological invasions and parasite spillback in man-made Lake Kariba

Parasite spillback, the infection of a non-indigenous organism by a native parasite, is a highly important although understudied component of ecological invasion dynamics. Here, through the first analysis of the parasite fauna of lymnaeid gastropods of Lake Kariba (Zimbabwe). We illustrate how the creation of an artificial lake may lead to a cascade of biological invasions in which an invasive aquatic plant promotes the proliferation of invasive gastropods, which in turn alters the epidemiology of trematodiases of potential medical and veterinary importance. Using a new multiplex Rapid Diagnostic PCR assay, we assessed the prevalence of Fasciola sp. infections in the gastropod populations. Both gastropod hosts and trematode parasites were identified using DNA barcoding. We provide the first record of the invasive North-American gastropod Pseudosuccinea columella in Lake Kariba. This species was found at 14 out of 16 sampled sites and its abundance was strongly positively correlated with the abundance of the invasive South-American water hyacinth (Eichhornia crassipes). About 65% of the P. columella specimens analysed were infected with a hitherto unknown Fasciola species. Phylogenetic analyses indicate close affinity to Fasciola hepatica and F. gigantica, which cause fasciolosis, an important liver disease affecting both ruminants and humans. In addition, another non-native Lymnaeid species was found: a Radix sp. that clustered closely with a Vietnamese Radix species. Radix sp. hosted both amphistome and Fasciola trematodes. By linking an invasion cascade and parasite spillback, this study shows how both processes can act in combination to lead to potentially important epidemiological changes.

A worldwide phylogeography of the whiteworm lichens Thamnolia reveals three lineages with distinct habitats and evolutionary histories

Thamnolia is a lichenized fungus with an extremely wide distribution, being encountered in arctic and alpine environments in most continents. In this study, we used molecular markers to investigate the population structure of the fungal symbiont and the associated photosynthetic partner of Thamnolia. By analyzing molecular, morphological, and chemical variation among 253 specimens covering the species distribution range, we revealed the existence of three mycobiont lineages. One lineage (Lineage A) is confined to the tundra region of Siberia and the Aleutian Islands, a second (Lineage B) is found in the high alpine region of the Alps and the Carpathians Mountains, and a third (Lineage C) has a worldwide distribution and covers both the aforementioned ecosystems. Molecular dating analysis indicated that the split of the three lineages is older than the last glacial maximum, but the distribution ranges and the population genetic analyses suggest an influence of last glacial period on the present‐day population structure of each lineage. We found a very low diversity of Lineage B, but a higher and similar one in Lineages A and C. Demographic analyses suggested that Lineage C has its origin in the Northern Hemisphere, possibly Scandinavia, and that it has passed through a bottleneck followed by a recent population expansion. While all three lineages reproduce clonally, recombination tests suggest rare or past recombination in both Lineages A and C. Moreover, our data showed that Lineage C has a comparatively low photobiont specificity, being found associated with four widespread Trebouxia lineages (three of them also shared with other lichens), while Lineages A and B exclusively harbor T. simplex s. lat. Finally, we did not find support for the recognition of taxa in Thamnolia based on either morphological or chemical characters.

What was old is new again: thermal adaptation within clonal lineages during range expansion in a fungal pathogen

Range-expanding species are expected to gain an increasing importance in the context of global change. They provide a great opportunity to study contemporary evolutionary changes and to unravel the mechanisms of evolution. Cryphonectria parasitica, the causal agent of chestnut blight, originating from Asia, has been spread since the beginning of the 20th century into different continents. We took advantage of the C. parasitica recent emergence in northern France to study the changes in population genetic structure and in phenotypic traits along this colonization and climatic gradient. Four hundred twenty-seven C. parasitica isolates were sampled in 47 chestnut sites in northern France. The C. parasitica outbreak in the north was found to be due to the expansion of five dominant clonal groups from southern France and to the emergence of a few rare recombined genotypes. The evolutionary changes during C. parasitica range expansion were studied by analysing phenotypic changes in isolates from the same clonal lineage, with or without a geographic shift. Growth rates were assessed in vitro, at four temperatures. The northern isolates grew faster at 12 and 15 °C and more slowly at 28 and 32 °C than the southern isolates. These results strongly suggest local adaptation to low temperatures in C. parasitica, with a trade-off of slower growth at high temperatures. They also reflect the high evolutionary potential of C. parasitica along a colonization gradient and show that clonal evolution is not a limitation for the rapid thermal adaptation of this invasive fungal species.

Superclone Expansion, Long-Distance Clonal Dispersal and Local Genetic Structuring in the Coral Pocillopora damicornis Type β in Reunion Island, South Western Indian Ocean

The scleractinian coral Pocillopora damicornis type β is known to present a mixed reproduction mode: through sexual reproduction, new genotypes are created, while asexual reproduction insures their propagation. In order to investigate the relative proportion of each reproduction mode in P. damicornis type β populations from Reunion Island, Indian Ocean, clonal propagation along the west coast was assessed through four sampling sites with increasing geographical distance between sites. Coral colonies were sampled either exhaustively, randomly or haphazardly within each site, and genotypic diversity was assessed using 13 microsatellite loci over a total of 510 P. damicornis type β determined a posteriori from their mtDNA haplotype (a 840 bp sequenced fragment of the Open Reading Frame). Overall, 47% of all the sampled colonies presented the same multi-locus genotype (MLG), a superclone, suggesting that asexual propagation is extremely important in Reunion Island. Within each site, numerous MLGs were shared by several colonies, suggesting local clonal propagation through fragmentation. Moreover, some of these MLGs were found to be shared among several sites located 40 km apart. While asexual reproduction by fragmentation seems unlikely over long distances, our results suggest a production of parthenogenetic larvae. Despite shared MLGs, two differentiated clusters were enclosed among populations of the west coast of Reunion Island, revealing the necessity to set up appropriate managing strategies at a local scale.

Facilitation of environmental adaptation and evolution by epigenetic phenotype variation: insights from clonal, invasive, polyploid, and domesticated animals

There is increasing evidence, particularly from plants, that epigenetic mechanisms can contribute to environmental adaptation and evolution. The present article provides an overview on this topic for animals and highlights the special suitability of clonal, invasive, hybrid, polyploid, and domesticated species for environmental and evolutionary epigenetics. Laboratory and field studies with asexually reproducing animals have shown that epigenetically diverse phenotypes can be produced from the same genome either by developmental stochasticity or environmental induction. The analysis of invasions revealed that epigenetic phenotype variation may help to overcome genetic barriers typically associated with invasions such as bottlenecks and inbreeding. Research with hybrids and polyploids established that epigenetic mechanisms are involved in consolidation of speciation by contributing to reproductive isolation and restructuring of the genome in the neo-species. Epigenetic mechanisms may even have the potential to trigger speciation but evidence is still meager. The comparison of domesticated animals and their wild ancestors demonstrated heritability and selectability of phenotype modulating DNA methylation patterns. Hypotheses, model predictions, and empirical results are presented to explain how epigenetic phenotype variation could facilitate adaptation and speciation. Clonal laboratory lineages, monoclonal invaders, and adaptive radiations of different evolutionary age seem particularly suitable to empirically test the proposed ideas. A respective research agenda is presented.

Evidence of plastic probing behavior in a 'superclone' of the grain aphid Sitobion avenae

Asexual reproduction is very common in invasive insect pest. In the recent years, increasing evidences have shown that some invasive asexual lineages display an outstanding capacity to predominate in space and persist on time (superclones). However, little is known about the host-use behavior of these superclones. The English grain aphid Sitobion avenae (Fabricius) (Hemiptera: Aphididae) is one of the major pests of cereals worldwide. Chilean populations of the grain aphid are characterized by a high degree of heterozygosity and low genotypic variability across regions and years, with only four predominant superclone genotypes representing nearly 90% of populations. In this study, (1) the reproductive performance and (2) the probing behavior followed a host shift of one superclone and one non-superclone of S. avenae, were compared. The host plant in the superclone did not affect the reproductive performance, while in the non-superclone was lower on highly defended wheat seedling. The experimental switching of the host plants from barley (without chemical defenses) to two wheat species with low and high levels of chemical defenses, revealed that superclone exhibited a flexible probing activities related to access of sieve elements, while the non-superclone exhibited rigid responses. These findings are consistent with the pattern of occurrence of these genotypes in the field on cereals with different plant defenses (e.g. benzoxazinoids). These responses are discussed on the view of developing new strategies for the management in invasive populations of aphid pest species.

What does the geography of parthenogenesis teach us about sex?

Theory predicts that sexual reproduction is difficult to maintain if asexuality is an option, yet sex is very common. To understand why, it is important to pay attention to repeatably occurring conditions that favour transitions to, or persistence of, asexuality. Geographic parthenogenesis is a term that has been applied to describe a large variety of patterns where sexual and related asexual forms differ in their geographic distribution. Often asexuality is stated to occur in a habitat that is, in some sense, marginal, but the interpretation differs across studies: parthenogens might not only predominate near the margin of the sexuals' distribution, but might also extend far beyond the sexual range; they may be disproportionately found in newly colonizable areas (e.g. areas previously glaciated), or in habitats where abiotic selection pressures are relatively stronger than biotic ones (e.g. cold, dry). Here, we review the various patterns proposed in the literature, the hypotheses put forward to explain them, and the assumptions they rely on. Surprisingly, few mathematical models consider geographic parthenogenesis as their focal question, but all models for the evolution of sex could be evaluated in this framework if the (often ecological) causal factors vary predictably with geography. We also recommend broadening the taxa studied beyond the traditional favourites.This article is part of the themed issue 'Weird sex: the underappreciated diversity of sexual reproduction'.