LARGE POPULATION SIZE PREDICTS THE DISTRIBUTION OF ASEXUALITY IN SCALE INSECTS

The coevolutionary consequences of biodiversity change

Coevolutionary selection is a powerful process shaping species interactions and biodiversity. Anthropogenic global environmental change is reshaping planetary biodiversity, including by altering the structure and intensity of interspecific interactions. However, remarkably little is understood of how coevolutionary selection is changing in the process. Here, we outline three interrelated pathways - change in evolutionary potential, change in community composition, and shifts in interaction trait distributions - that are expected to redirect coevolutionary selection under biodiversity change. Assessing how both ecological and evolutionary rules governing species interactions are disrupted under anthropogenic global change is of paramount importance to understand the past, present, and future of Earth's biodiversity.

The Fall Armyworm and Larger Grain Borer Pest Invasions in Africa: Drivers, Impacts and Implications for Food Systems

Invasive alien species (IAS) are a major biosecurity threat affecting globalisation and the international trade of agricultural products and natural ecosystems. In recent decades, for example, field crop and postharvest grain insect pests have independently accounted for a significant decline in food quantity and quality. Nevertheless, how their interaction and cumulative effects along the ever-evolving field production to postharvest continuum contribute towards food insecurity remain scant in the literature. To address this within the context of Africa, we focus on the fall armyworm, Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae), and the larger grain borer, Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae), two of the most important field and postharvest IAS, respectively, that have invaded Africa. Both insect pests have shown high invasion success, managing to establish themselves in >50% of the African continent within a decade post-introduction. The successive and summative nature of field and postharvest damage by invasive insect pests on the same crop along its value chain results in exacerbated food losses. This systematic review assesses the drivers, impacts and management of the fall armyworm and larger grain borer and their effects on food systems in Africa. Interrogating these issues is important in early warning systems, holistic management of IAS, maintenance of integral food systems in Africa and the development of effective management strategies.

True Parthenogenesis and Female-Biased Sex Ratios in Cicadomorpha and Fulgoromorpha (Hemiptera, Auchenorrhyncha)

Insects are renowned for their remarkable diversity of reproductive modes. Among these, the largest non-holometabolous order, Hemiptera, stands out with one of the most diversified arrays of parthenogenesis modes observed among insects. Although there are extensive reviews on reproduction without fertilization in some hemipteran higher taxa, no such analysis has been conducted for the large suborders Fulgoromorpha (planthoppers) and Cicadomorpha (leafhoppers). In both groups, there are species that reproduce by true parthenogenesis, specifically thelytoky, and in Fulgoromorpha, there are species that reproduce by pseudogamy or, more specifically, sperm-dependent parthenogenesis. In this review paper, we give and discuss the only currently known examples of true parthenogenesis in Fulgoromorpha and Cicadomorpha, mainly from the planthopper family Delphacidae and the leafhopper family Cicadellidae. We analyze patterns of distribution, ecology, mating behavior, acoustic communication, and cytogenetic and genetic diversity of parthenoforms and discuss hypotheses about the origin of parthenogenesis in each case. We also highlight examples in which natural populations show a shift in sex ratio toward females and discuss possible causes of this phenomenon, primarily the influence of endosymbiotic bacteria capable of altering the reproductive strategies of the hosts. Our review is mainly based on studies in which the authors have participated.

Sperm-dependent asexual species and their role in ecology and evolution

Sexual reproduction is the primary mode of reproduction in eukaryotes, but some organisms have evolved deviations from classical sex and switched to asexuality. These asexual lineages have sometimes been viewed as evolutionary dead ends, but recent research has revealed their importance in many areas of general biology. Our review explores the understudied, yet important mechanisms by which sperm-dependent asexuals that produce non-recombined gametes but rely on their fertilization, can have a significant impact on the evolution of coexisting sexual species and ecosystems. These impacts are concentrated around three major fields. Firstly, sperm-dependent asexuals can potentially impact the gene pool of coexisting sexual species by either restricting their population sizes or by providing bridges for interspecific gene flow whose type and consequences substantially differ from gene flow mechanisms expected under sexual reproduction. Secondly, they may impact on sexuals' diversification rates either directly, by serving as stepping-stones in speciation, or indirectly, by promoting the formation of pre- and postzygotic reproduction barriers among nascent species. Thirdly, they can potentially impact on spatial distribution of species, via direct or indirect (apparent) types of competition and Allee effects. For each such mechanism, we provide empirical examples of how natural sperm-dependent asexuals impact the evolution of their sexual counterparts. In particular, we highlight that these broad effects may last beyond the tenure of the individual asexual lineages causing them, which challenges the traditional perception that asexual lineages are short-lived evolutionary dead ends and minor sideshows. Our review also proposes new research directions to incorporate the aforementioned impacts of sperm-dependent asexuals. These research directions will ultimately enhance our understanding of the evolution of genomes and biological interactions in general.

Reproductive plasticity in response to the changing cluster size during the breeding period: a case study in a spider mite

Animals living in clusters should adjust their reproductive strategies to adapt to the social environment. Theories predict that the benefits of cluster living would outweigh the costs of competition. Yet, it is largely unknown how animals optimize their reproductive fitness in response to the changing social environment during their breeding period. We used Tetranychus ludeni Zacher, a haplodiploid spider mite, to investigate how the ovipositing females modified their life-history traits in response to the change of cluster size (i.e., aggregation and dispersal) with a consistent population density (1 ♀/cm2). We demonstrate that (1) after females were shifted from a large cluster (16 ♀♀) to small ones (1 ♀, 5 and 10 ♀♀), they laid fewer and larger eggs with a higher female-biased sex ratio; (2) after females were shifted from small clusters to a large one, they laid fewer and smaller eggs, also with a higher female-biased sex ratio, and (3) increasing egg size significantly increased offspring sex ratio (% daughters), but did not increase immature survival. The results suggest that (1) females fertilize more larger eggs laid in a small population but lower the fertilization threshold and fertilize smaller eggs in a larger population, and (2) the reproductive adjustments in terms of egg number and size may contribute more to minimize the mate competition among sons but not to increase the number of inhabitants in the next generation. The current study provides evidence that spider mites can manipulate their reproductive output and adjust offspring sex ratio in response to dynamic social environments.

The thelytokous strain of the parasitoid Neochrysocharis formosa outperforms the arrhenotokous strain in reproductive capacity and biological control of agromyzid leafminers

BACKGROUND

Both arrhenotoky (sexual reproduction of females and asexual reproduction of males) and thelytoky (asexual reproduction of females) occur within the order Hymenoptera. The existence of both thelytokous and arrhenotokous strains within one species provides an opportunity to compare the biocontrol efficiency between two reproductive modes. The parasitoid Neochrysocharis formosa (Westwood) (Hymenoptera: Eulophidae) has thelytokous and arrhenotokous strains with sympatric distributions. This parasitoid is used to control invasive leafminers through feeding, stinging, and parasitization. To compare the biocontrol efficiency of the two strains, we analyzed life tables and host-killing parameters of these two strains reared on the leafminer Liriomyza sativae Blanchard using the age-stage, two-sex life table and the CONSUME-MSChart software.

RESULTS

Our results showed that the intrinsic rate of increase (r), finite rate of increase (λ), and net reproduction rate (R₀ ) of the thelytokous strain were significantly higher than those of the arrhenotokous strain. The thelytokous females also performed better than the arrhenotokous females for the net host-feeding rate, net host-stinging rate, and net host-killing rate, but not the finite parasitism rate. Conclusively, the finite host-killing rate of the thelytokous strain (0.8720 ± 0.0516) was significantly higher than that of the arrhenotokous strain (0.5914 ± 0.0832).

CONCLUSION

We concluded that thelytokous N. formosa is a better candidate as a biocontrol agent than arrhenotokous N. formosa to control leafminers. Our results shed light on how to choose a better biocontrol agent for integrated pest management (IPM) based on biological control, especially for co-occurring thelytokous and arrhenotokous parasitoids. © 2022 Society of Chemical Industry.

The clones are all right

Parthenogenetic grasshoppers confound predictions by showing no signs of decline.

Invasions of an obligate asexual daphnid species support the nearly neutral theory

To verify the "nearly neutral theory (NNT)," the ratio of nonsynonymous to synonymous substitutions (dN/dS) was compared among populations of different species. To determine the validity of NNT, however, populations that are genetically isolated from each other but share the same selection agents and differ in size should be compared. Genetically different lineages of obligate asexual Daphnia pulex invading Japan from North America are an ideal example as they satisfy these prerequisites. Therefore, we analyzed the whole-genome sequences of 18 genotypes, including those of the two independently invaded D. pulex lineages (JPN1 and JPN2) and compared the dN/dS ratio between the lineages. The base substitution rate of each genotype demonstrated that the JPN1 lineage having a larger distribution range diverged earlier and thus was older than the JPN2 lineage. Comparisons of the genotypes within lineages revealed that changes in dN/dS occurred after the divergence and were larger in the younger lineage, JPN2. These results imply that the JPN1 lineage has been more effectively subjected to purification selections, while slightly deteriorating mutations are less purged in JPN2 with smaller population size. Altogether, the lineage-specific difference in the dN/dS ratio for the obligate asexual D. pulex was well explained by the NNT.

Evidence from automixis with inverted meiosis for the maintenance of sex by loss of complementation

The adaptive value of sexual reproduction is still debated. A short-term disadvantage of asexual reproduction is loss of heterozygosity, which leads to the unmasking of recessive deleterious mutations. The cost of this loss of complementation is predicted to be higher than the twofold cost of meiosis for most types of asexual reproduction. Automixis with terminal fusion of sister nuclei is especially vulnerable to the effect of loss of complementation. It is found, however, in some taxa including oribatid mites, the most prominent group of ancient asexuals. Here, I show that automixis with terminal fusion is stable if it is associated with inverted meiosis and that this appears to be the case in nature, notably in oribatid mites. The existence of automixis with terminal fusion, and its co-occurrence with inverted meiosis, therefore, is consistent with the hypothesis that loss of complementation is important in the evolution of sexual reproduction.

Four new species of Aspidiotini (Hemiptera, Diaspididae, Aspidiotinae) from Panama, with a key to Panamanian species

Four new species of armored scale insect, Clavaspisselvaticasp. nov., Clavaspisvirolaesp. nov., Davidsonaspistovomitaesp. nov., and Rungaspisneotropicalissp. nov., are described and illustrated from Panama. We also transfer two previously described species of Panamanian Aspidiotini to new genera, Hemiberlesiacrescentiae (Ferris) comb. nov. and Rungaspisrigida (Ferris) comb. nov., and report the first record of Selenaspidopsisbrowni Nakahara in Panama. A key to the species of Aspidiotini occurring in Panama is provided.

Extremely Widespread Parthenogenesis and a Trade-Off Between Alternative Forms of Reproduction in Mayflies (Ephemeroptera)

Studying alternative forms of reproduction in natural populations is of fundamental importance for understanding the costs and benefits of sex. Mayflies are one of the few animal groups where sexual reproduction co-occurs with different types of parthenogenesis, providing ideal conditions for identifying benefits of sex in natural populations. Here, we establish a catalog of all known mayfly species capable of reproducing by parthenogenesis, as well as species unable to do so. Overall, 1.8% of the described species reproduce parthenogenetically, which is an order of magnitude higher than reported in other animal groups. This frequency even reaches 47.8% if estimates are based on the number of studied rather than described mayfly species, as reproductive modes have thus far been studied in only 17 out of 42 families. We find that sex is a more successful strategy than parthenogenesis (associated with a higher hatching success of eggs), with a trade-off between the hatching success of parthenogenetic and sexual eggs. This means that improving the capacity for parthenogenesis may come at a cost for sexual reproduction. Such a trade-off can help explain why facultative parthenogenesis is extremely rare among animals despite its potential to combine the benefits of sexual and parthenogenetic reproduction. We argue that parthenogenesis is frequently selected in mayflies in spite of this probable trade-off because their typically low dispersal ability and short and fragile adult life may frequently generate situations of mate limitation in females. Mayflies are currently clearly underappreciated for understanding the benefits of sex under natural conditions.

A molecularphylogeny offorktail damselflies(genus Ischnura)revealsa dynamic macroevolutionary history of female colour polymorphisms

Colour polymorphisms are popular study systems among biologists interested in evolutionary dynamics, genomics, sexual selection and sexual conflict. In many damselfly groups, such as in the globally distributed genus Ischnura (forktails), sex-limited female colour polymorphisms occur in multiple species. Female-polymorphic species contain two or three female morphs, one of which phenotypically matches the male (androchrome or male mimic) and the other(s) which are phenotypically distinct from the male (heterochrome). These female colour polymorphisms are thought to be maintained by frequency-dependent sexual conflict, but their macroevolutionary histories are unknown, due to the lack of a robust molecular phylogeny. Here, we present the first time-calibrated phylogeny of Ischnura, using a multispecies coalescent approach (StarBEAST2) and incorporating both molecular and fossil data for 41 extant species (55% of the genus). We estimate the age of Ischnura to be between 13.8 and 23.4 millions of years, i.e. Miocene. We infer the ancestral state of this genus as female monomorphism with heterochrome females, with multiple gains and losses of female polymorphisms, evidence of trans-species female polymorphisms and a significant positive relationship between female polymorphism incidence and current geographic range size. Our study provides a robust phylogenetic framework for future research on the dynamic macroevolutionary history of this clade with its extraordinary diversity of sex-limited female polymorphisms.

What We Don't Know About Diet-Breadth Evolution in Herbivorous Insects

Half a million species of herbivorous insects have been described. Most of them are diet specialists, using only a few plant species as hosts. Biologists suspect that their specificity is key to their diversity. But why do herbivorous insects tend to be diet specialists? In this review, we catalog a broad range of explanations. We review the evidence for each and suggest lines of research to obtain the evidence we lack. We then draw attention to a second major question, namely how changes in diet breadth affect the rest of a species’ biology. In particular, we know little about how changes in diet breadth feed back on genetic architecture, the population genetic environment, and other aspects of a species’ ecology. Knowing more about how generalists and specialists differ should go a long way toward sorting out potential explanations of specificity, and yield a deeper understanding of herbivorous insect diversity.

The Evolutionary and Molecular Features of Broad Host-Range Necrotrophy in Plant Pathogenic Fungi

Necrotrophic fungal pathogens cause considerable disease on numerous economically important crops. Some of these pathogens are specialized to one or a few closely related plant species, whereas others are pathogenic on many unrelated hosts. The evolutionary and molecular bases of broad host-range necrotrophy in plant pathogens are not very well-defined and form an on-going area of research. In this review, we discuss what is known about broad host-range necrotrophic pathogens and compare them with their narrow host-range counterparts. We discuss the evolutionary processes associated with host generalism, and highlight common molecular features of the broad host-range necrotrophic lifestyle, such as fine-tuning of host pH, modulation of host reactive oxygen species and metabolic degradation of diverse host antimicrobials. We conclude that broad host-range necrotrophic plant pathogens have evolved a range of diverse and sometimes convergent responses to a similar selective regime governed by interactions with a highly heterogeneous host landscape.

Asexual parasites and their extraordinary host ranges

In diverse parasite taxa, from scale insects to root-knot nematodes, asexual lineages have exceptionally large host ranges, larger than those of their sexual relatives. Phylogenetic comparative studies of parasite taxa indicate that increases in host range and geographic range increase the probability of establishment of asexual lineages. At first pass, this convergence of traits appears counter-intuitive: intimate, antagonistic association with an enormous range of host taxa correlates with asexual reproduction, which should limit genetic variation within populations. Why would narrow host ranges favor sexual parasites and large host ranges favor asexual parasites? To take on this problem I link theory on ecological specialization to the two predominant hypotheses for the evolution of sex. I argue that both hypotheses predict a positive association between host range and the probability of invasion of asexual parasites, mediated either by variation in population size or in the strength of antagonistic coevolution. I also review hypotheses on colonization and the evolution of niche breadth in asexual lineages. I emphasize parasite taxa, with their diversity of reproductive modes and ecological strategies, as valuable assets in the hunt for solutions to the classic problems of the evolution of sex and geographic parthenogenesis.

No signal of deleterious mutation accumulation in conserved gene sequences of extant asexual hexapods

Loss of sex and recombination is generally assumed to impede the effectiveness of purifying selection and to result in the accumulation of slightly deleterious mutations. Empirical evidence for this has come from several studies investigating mutational load in a small number of individual genes. However, recent whole transcriptome based studies have yielded inconsistent results, hence questioning the validity of the assumption of mutational meltdown in asexual populations. Here, we study the effectiveness of purifying selection in eight asexual hexapod lineages and their sexual relatives, as present in the 1 K Insect Transcriptome Evolution (1KITE) project, covering eight hexapod groups. We analyse the accumulation of slightly deleterious nonsynonymous and synonymous point mutations in 99 single copy orthologue protein-coding loci shared among the investigated taxa. While accumulation rates of nonsynonymous mutations differed between genes and hexapod groups, we found no effect of reproductive mode on the effectiveness of purifying selection acting at nonsynonymous and synonymous sites. Although the setup of this study does not fully rule out nondetection of subtle effects, our data does not support the established consensus of asexual lineages undergoing ‘mutational meltdown’.

Sex loss in insects: causes of asexuality and consequences for genomes

Boasting a staggering diversity of reproductive strategies, insects provide attractive models for the comparative study of the causes and consequences of transitions to asexuality. We provide an overview of some contemporary studies of reproductive systems in insects and compile an initial database of asexual insect genome resources. Insect systems have already yielded some important insights into various mechanisms by which sex is lost, including genetic, endosymbiont-mediated, and hybridization. Studies of mutation and substitution after loss of sex provide the strongest empirical support for hypothesized effects of asexuality, whereas there is mixed evidence for ecological hypotheses such as increased parasite load and altered niche breadth in asexuals. Most hypotheses have been explored in a select few taxa (e.g. stick insects, aphids), such that much of the great taxonomic breadth of insects remain understudied. Given the variation in the proximate causes of asexuality in insects, we argue for expanding the taxonomic breadth of study systems. Despite some challenges for investigating sex in insects, the increasing cost-effectiveness of genomic sequencing makes data generation for closely-related asexual and sexual lineages increasingly feasible.

The evolution of caste-biasing symbionts in the social hymenoptera

The separation of individuals into reproductive and worker castes is the defining feature of insect societies. However, caste determination is itself a complex phenomenon, dependent on interacting genetic and environmental factors. It has been suggested by some authors that widespread maternally transmitted symbionts such as Wolbachia may be selected to interfere with caste determination, whilst others have discounted this possibility on theoretical grounds. We argue that there are in fact three distinct evolutionary scenarios in which maternally transmitted symbionts might be selected to influence the process of caste determination in a social hymenopteran host. Each of these scenarios generate testable predictions which we outline here. Given the increasing recognition of the complexity and multi-faceted nature of caste determination in social insects, we argue that maternally transmitted symbionts should also be considered as possible factors influencing the development of social hymenopterans.

The unusual reproductive system of head and body lice (Pediculus humanus)

Insect reproduction is extremely variable, but the implications of alternative genetic systems are often overlooked in studies on the evolution of insecticide resistance. Both ecotypes of Pediculus humanus (Phthiraptera: Pediculidae), the human head and body lice, are human ectoparasites, the control of which is challenged by the recent spread of resistance alleles. The present study conclusively establishes for the first time that both head and body lice reproduce through paternal genome elimination (PGE), an unusual genetic system in which males transmit only their maternally derived chromosomes. Here, we investigate inheritance patterns of parental genomes using a genotyping approach across families of both ecotypes and show that heterozygous males exclusively or preferentially pass on one allele only, whereas females transmit both in a Mendelian fashion. We do however observe occasional transmission of paternal chromosomes through males, representing the first known case of PGE in which whole-genome meiotic drive is incomplete. Finally, we discuss the potential implications of this finding for the evolution of resistance and invite the development of new theoretical models of how this knowledge might contribute to increasing the success of pediculicide-based management schemes.

Sex in the wild: How and why field-based studies contribute to solving the problem of sex

Why and how sexual reproduction is maintained in natural populations, the so-called "queen of problems," is a key unanswered question in evolutionary biology. Recent efforts to solve the problem of sex have often emphasized results generated from laboratory settings. Here, we use a survey of representative "sex in the wild" literature to review and synthesize the outcomes of empirical studies focused on natural populations. Especially notable results included relatively strong support for mechanisms involving niche differentiation and a near absence of attention to adaptive evolution. Support for a major role of parasites is largely confined to a single study system, and only three systems contribute most of the support for mutation accumulation hypotheses. This evidence for taxon specificity suggests that outcomes of particular studies should not be more broadly extrapolated without extreme caution. We conclude by suggesting steps forward, highlighting tests of niche differentiation mechanisms in both laboratory and nature, and empirical evaluation of adaptive evolution-focused hypotheses in the wild. We also emphasize the value of leveraging the growing body of genomic resources for nonmodel taxa to address whether the clearance of harmful mutations and spread of beneficial variants in natural populations proceeds as expected under various hypotheses for sex.

Does a plant-eating insect's diet govern the evolution of insecticide resistance? Comparative tests of the pre-adaptation hypothesis

According to the pre‐adaptation hypothesis, the evolution of insecticide resistance in plant‐eating insects co‐opts adaptations that initially evolved during chemical warfare with their host plants. Here, we used comparative statistics to test two predictions of this hypothesis: (i) Insects with more diverse diets should evolve resistance to more diverse insecticides. (ii) Feeding on host plants with strong or diverse qualitative chemical defenses should prime an insect lineage to evolve insecticide resistance. Both predictions are supported by our tests. What makes this especially noteworthy is that differences in the diets of plant‐eating insect species are typically ignored by the population genetic models we use to make predictions about insecticide resistance evolution. Those models surely capture some of the differences between host‐use generalists and specialists, for example, differences in population size and migration rates into treated fields, but they miss other potentially important differences, for example, differences in metabolic diversity and gene expression plasticity. Ignoring these differences could be costly.

Evolution and comparative ecology of parthenogenesis in haplodiploid arthropods

Changes from sexual reproduction to female-producing parthenogenesis (thelytoky) have great evolutionary and ecological consequences, but how many times parthenogenesis evolved in different animal taxa is unknown. We present the first exhaustive database covering 765 cases of parthenogenesis in haplodiploid (arrhenotokous) arthropods, and estimate frequencies of parthenogenesis in different taxonomic groups. We show that the frequency of parthenogenetic lineages extensively varies among groups (0-38% among genera), that many species have both sexual and parthenogenetic lineages and that polyploidy is very rare. Parthenogens are characterized by broad ecological niches: parasitoid and phytophagous parthenogenetic species consistently use more host species, and have larger, polewards extended geographic distributions than their sexual relatives. These differences did not solely evolve after the transition to parthenogenesis. Extant parthenogens often derive from sexual ancestors with relatively broad ecological niches and distributions. As these ecological attributes are associated with large population sizes, our results strongly suggests that transitions to parthenogenesis are more frequent in large sexual populations and/or that the risk of extinction of parthenogens with large population sizes is reduced. The species database presented here provides insights into the maintenance of sex and parthenogenesis in natural populations that are not taxon specific and opens perspectives for future comparative studies.

Effective purifying selection in ancient asexual oribatid mites

Sex is beneficial in the long term because it can prevent mutational meltdown through increased effectiveness of selection. This idea is supported by empirical evidence of deleterious mutation accumulation in species with a recent transition to asexuality. Here, we study the effectiveness of purifying selection in oribatid mites which have lost sex millions of years ago and diversified into different families and species while reproducing asexually. We compare the accumulation of deleterious nonsynonymous and synonymous mutations between three asexual and three sexual lineages using transcriptome data. Contrasting studies of young asexual lineages, we find evidence for strong purifying selection that is more effective in asexual as compared to sexual oribatid mite lineages. Our results suggest that large populations likely sustain effective purifying selection and facilitate the escape of mutational meltdown in the absence of sex. Thus, sex per se is not a prerequisite for the long-term persistence of animal lineages.

Asexual reproduction is thought to be an evolutionary dead end in eukaryotes because deleterious mutations will not be purged effectively. Here, Brandt and colleagues show that anciently asexual oribatid mites in fact have reduced accumulation of deleterious mutations compared to their sexual relatives.

Can asexuality confer a short-term advantage? Investigating apparent biogeographic success in the apomictic triploid fern Myriopteris gracilis

PREMISE OF THE STUDY

Although asexual taxa are generally seen as evolutionary dead ends, asexuality appears to provide a short-term benefit in some taxa, including a wider geographic distribution compared to sexual relatives. However, this may be an illusion created by multiple, morphologically cryptic, asexual lineages, each occupying a relatively small area. In this study we investigate the role of multiple lineages in the biogeography of Myriopteris gracilis Fée (Pteridaceae), a North American apomictic triploid fern species with a particularly large range.

METHODS

Range-wide asexuality was assessed by counting spores/sporangium in 606 Myriopteris gracilis specimens from across the species range, and lineage structure was assessed with both plastid DNA sequence and Genotyping By Sequencing (GBS) SNP datasets.

KEY RESULTS

Spore counting of >600 specimens identified no sexual populations, establishing that Myriopteris gracilis is exclusively asexual. The plastid data estimated the crown age of M. gracilis at ca. 2.5 mya and identified two lineages, each largely confined to the eastern or western portions of the range. These groups were further subdivided by the GBS data, revealing at least seven asexual lineages of varying geographic distributions, each occupying a relatively small portion of the total range of M. gracilis.

CONCLUSIONS

Although maintained exclusively through asexual reproduction, the broad distribution of Myriopteris gracilis is a compilation of numerous, independently formed asexual lineages. Since no single asexual lineage occupies the full extent of the species distribution, recurrent lineage formation should be considered when evaluating the short-term benefit of asexuality in this taxon and others.

Genetic diversity of armored scales (Hemiptera: Diaspididae) and soft scales (Hemiptera: Coccidae) in Chile

Scale insects (Sternorrhyncha: Coccoidea) are one of the most invasive and agriculturally damaging insect groups. Their management and the development of new control methods are currently jeopardized by the scarcity of identification data, in particular in regions where no large survey coupling morphological and DNA analyses have been performed. In this study, we sampled 116 populations of armored scales (Hemiptera: Diaspididae) and 112 populations of soft scales (Hemiptera: Coccidae) in Chile, over a latitudinal gradient ranging from 18°S to 41°S, on fruit crops, ornamental plants and trees. We sequenced the COI and 28S genes in each population. In total, 19 Diaspididae species and 11 Coccidae species were identified morphologically. From the 63 COI haplotypes and the 54 28S haplotypes uncovered, and using several DNA data analysis methods (Automatic Barcode Gap Discovery, K2P distance, NJ trees), up to 36 genetic clusters were detected. Morphological and DNA data were congruent, except for three species (Aspidiotus nerii, Hemiberlesia rapax and Coccus hesperidum) in which DNA data revealed highly differentiated lineages. More than 50% of the haplotypes obtained had no high-scoring matches with any of the sequences in the GenBank database. This study provides 63 COI and 54 28S barcode sequences for the identification of Coccoidea from Chile.

Gene expression plasticity across hosts of an invasive scale insect species

For plant-eating insects, we still have only a nascent understanding of the genetic basis of host-use promiscuity. Here, to improve that situation, we investigated host-induced gene expression plasticity in the invasive lobate lac scale insect, Paratachardina pseudolobata (Hemiptera: Keriidae). We were particularly interested in the differential expression of detoxification and effector genes, which are thought to be critical for overcoming a plant's chemical defenses. We collected RNA samples from P. pseudolobata on three different host plant species, assembled transcriptomes de novo, and identified transcripts with significant host-induced gene expression changes. Gene expression plasticity was pervasive, but the expression of most detoxification and effector genes was insensitive to the host environment. Nevertheless, some types of detoxification genes were more differentially expressed than expected by chance. Moreover, we found evidence of a trade-off between expression of genes involved in primary and secondary metabolism; hosts that induced lower expression of genes for detoxification induced higher expression of genes for growth. Our findings are largely consonant with those of several recently published studies of other plant-eating insect species. Thus, across plant-eating insect species, there may be a common set of gene expression changes that enable host-use promiscuity.

Sex Determination, Sex Chromosomes, and Karyotype Evolution in Insects

Insects harbor a tremendous diversity of sex determining mechanisms both within and between groups. For example, in some orders such as Hymenoptera, all members are haplodiploid, whereas Diptera contain species with homomorphic as well as male and female heterogametic sex chromosome systems or paternal genome elimination. We have established a large database on karyotypes and sex chromosomes in insects, containing information on over 13000 species covering 29 orders of insects. This database constitutes a unique starting point to report phylogenetic patterns on the distribution of sex determination mechanisms, sex chromosomes, and karyotypes among insects and allows us to test general theories on the evolutionary dynamics of karyotypes, sex chromosomes, and sex determination systems in a comparative framework. Phylogenetic analysis reveals that male heterogamety is the ancestral mode of sex determination in insects, and transitions to female heterogamety are extremely rare. Many insect orders harbor species with complex sex chromosomes, and gains and losses of the sex-limited chromosome are frequent in some groups. Haplodiploidy originated several times within insects, and parthenogenesis is rare but evolves frequently. Providing a single source to electronically access data previously distributed among more than 500 articles and books will not only accelerate analyses of the assembled data, but also provide a unique resource to guide research on which taxa are likely to be informative to address specific questions, for example, for genome sequencing projects or large-scale comparative studies.

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'.

Parallel evolution of highly conserved plastid genome architecture in red seaweeds and seed plants

Background

The red algae (Rhodophyta) diverged from the green algae and plants (Viridiplantae) over one billion years ago within the kingdom Archaeplastida. These photosynthetic lineages provide an ideal model to study plastid genome reduction in deep time. To this end, we assembled a large dataset of the plastid genomes that were available, including 48 from the red algae (17 complete and three partial genomes produced for this analysis) to elucidate the evolutionary history of these organelles.

Results

We found extreme conservation of plastid genome architecture in the major lineages of the multicellular Florideophyceae red algae. Only three minor structural types were detected in this group, which are explained by recombination events of the duplicated rDNA operons. A similar high level of structural conservation (although with different gene content) was found in seed plants. Three major plastid genome architectures were identified in representatives of 46 orders of angiosperms and three orders of gymnosperms.

Conclusions

Our results provide a comprehensive account of plastid gene loss and rearrangement events involving genome architecture within Archaeplastida and lead to one over-arching conclusion: from an ancestral pool of highly rearranged plastid genomes in red and green algae, the aquatic (Florideophyceae) and terrestrial (seed plants) multicellular lineages display high conservation in plastid genome architecture. This phenomenon correlates with, and could be explained by, the independent and widely divergent (separated by >400 million years) origins of complex sexual cycles and reproductive structures that led to the rapid diversification of these lineages.

Electronic supplementary material

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Assortative mating and self-fertilization differ in their contributions to reinforcement, cascade speciation, and diversification

Cascade speciation and reinforcement can evolve rapidly when traits are pleiotropic and act as both signal/cue in nonrandom mating. Here, we examine the contribution of two key traits-assortative mating and self-fertilization-to reinforcement and (by extension) cascade speciation. First, using a population genetic model of reinforcement we find that both assortative mating and self-fertilization can make independent contributions to increased reproductive isolation, consistent with reinforcement. Self-fertilization primarily evolves due to its 2-fold transmission advantage when inbreeding depression (d) is lower (d < 0.45) but evolves as a function of the cost of hybridization under higher inbreeding depression (0.45 < d < 0.48). When both traits can evolve simultaneously, increased self-fertilization often prohibits the evolution of assortative mating. We infer that, under specific conditions, mating system transitions are more likely to lead to increased reproductive isolation and initiate cascade speciation, than assortative mating. Based on the results of our simulations, we hypothesized that transitions to self-fertilization could contribute to clade-wide diversification if reinforcement or cascade speciation is common. We tested this hypothesis with comparative data from two different groups. Consistent with our hypothesis, there was a trend towards uniparental reproduction being associated with increased diversification rate in the Nematode phylum. For the plant genus Mimulus, however, self-fertilization was associated with reduced diversification. Reinforcement driving speciation via transitions to self-fertilization might be short lived or unsustainable across macroevolutionary scales in some systems (some plants), but not others (such as nematodes), potentially due to differences in susceptibility to inbreeding depression and/or the ability to transition between reproductive modes.

Extreme genetic diversity in asexual grass thrips populations

The continuous generation of genetic variation has been proposed as one of the main factors explaining the maintenance of sexual reproduction in nature. However, populations of asexual individuals may attain high levels of genetic diversity through within-lineage diversification, replicate transitions to asexuality from sexual ancestors and migration. How these mechanisms affect genetic variation in populations of closely related sexual and asexual taxa can therefore provide insights into the role of genetic diversity for the maintenance of sexual reproduction. Here, we evaluate patterns of intra- and interpopulation genetic diversity in sexual and asexual populations of Aptinothrips rufus grass thrips. Asexual A. rufus populations are found throughout the world, whereas sexual populations appear to be confined to few locations in the Mediterranean region. We found that asexual A. rufus populations are characterized by extremely high levels of genetic diversity, both in comparison with their sexual relatives and in comparison with other asexual species. Migration is extensive among asexual populations over large geographic distances, whereas close sexual populations are strongly isolated from each other. The combination of extensive migration with replicate evolution of asexual lineages, and a past demographic expansion in at least one of them, generated high local clone diversities in A. rufus. These high clone diversities in asexual populations may mimic certain benefits conferred by sex via genetic diversity and could help explain the extreme success of asexual A. rufus populations.

The evolutionary dynamics of haplodiploidy: Genome architecture and haploid viability

Haplodiploid reproduction, in which males are haploid and females are diploid, is widespread among animals, yet we understand little about the forces responsible for its evolution. The current theory is that haplodiploidy has evolved through genetic conflicts, as it provides a transmission advantage to mothers. Male viability is thought to be a major limiting factor; diploid individuals tend to harbor many recessive lethal mutations. This theory predicts that the evolution of haplodiploidy is more likely in male heterogametic lineages with few chromosomes, as genes on the X chromosome are often expressed in a haploid environment, and the fewer the chromosome number, the greater the proportion of the total genome that is X-linked. We test this prediction with comparative phylogenetic analyses of mites, among which haplodiploidy has evolved repeatedly. We recover a negative correlation between chromosome number and haplodiploidy, find evidence that low chromosome number evolved prior to haplodiploidy, and that it is unlikely that diplodiploidy has reevolved from haplodiploid lineages of mites. These results are consistent with the predicted importance of haploid male viability.

Evolutionary problems in centrosome and centriole biology

Centrosomes have been an enigma to evolutionary biologists. Either they have been the subject of ill-founded speculation or they have been ignored. Here, we highlight evolutionary paradoxes and problems of centrosome and centriole evolution and seek to understand them in the light of recent advances in centrosome biology. Most evolutionary accounts of centrosome evolution have been based on the hypothesis that centrosomes are replicators, independent of the nucleus and cytoplasm. It is now clear, however, that this hypothesis is not tenable. Instead, centrosomes are formed de novo each cell division, with the presence of an old centrosome regulating, but not essential for, the assembly of a new one. Centrosomes are the microtubule-organizing centres of cells. They can potentially affect sensory and motor characters (as the basal body of cilia), as well as the movements of chromosomes during cell division. This latter role does not seem essential, however, except in male meiosis, and the reasons for this remain unclear. Although the centrosome is absent in some taxa, when it is present, its structure is extraordinarily conserved: in most taxa across eukaryotes, it does not appear to evolve at all. And yet a few insect groups display spectacular hypertrophy of the centrioles. We discuss how this might relate to the unusual reproductive system found in these insects. Finally, we discuss why the fate of centrosomes in sperm and early embryos might differ between different groups of animals.

The sperm of Matsucoccus feytaudi (Insecta, Coccoidea): Can the microtubular bundle be considered as a true flagellum?

In the present work the spermiogenesis and sperm structure of Matsucoccus feytaudi, a primary pest of the maritime pine in southern eastern Europe, is studied. In addition to the already known characteristics of coccid sperm, such as the absence of the acrosome and mitochondria, and the presence of a bundle of microtubules responsible for sperm motility, a peculiar structure from which the microtubule bundle takes origin is described. Such a structure--a short cylinder provided with a central hub surrounded by several microtubules with a dense wall--is regarded as a Microtubule Organizing Centre (MTOC). During spermiogenesis, quartets of fused spermatids are formed; from each spermatid, a bundle of microtubules, generated by the MTOC, projects from the cell surface. Each cell has two centrioles, suggesting the lack of a meiotic process and the occurrence of parthenogenesis. At the end of the spermiogenesis, when the cysts containing bundles of sperm are formed, part of the nuclear material together with the MTOC structure is eliminated. Based on the origin of the microtubular bundle from the MTOC, the nature of the bundle as a flagellum is discussed.

A phylogenetic test of the Red Queen Hypothesis: outcrossing and parasitism in the Nematode phylum

Sexual outcrossing is costly relative to selfing and asexuality, yet it is ubiquitous in nature, a paradox that has long puzzled evolutionary biologists. The Red Queen Hypothesis argues that outcrossing is maintained by antagonistic interactions between host and parasites. Most tests of this hypothesis focus on the maintenance of outcrossing in hosts. The Red Queen makes an additional prediction that parasitic taxa are more likely to be outcrossing than their free-living relatives. We test this prediction in the diverse Nematode phylum using phylogenetic comparative methods to evaluate trait correlations. In support of the Red Queen, we demonstrate a significant correlation between parasitism and outcrossing in this clade. We find that this correlation is driven by animal parasites, for which outcrossing is significantly enriched relative to both free-living and plant parasitic taxa. Finally, we test hypotheses for the evolutionary history underlying the correlation of outcrossing and animal parasitism. Our results demonstrate that selfing and asexuality are significantly less likely to arise on parasitic lineages than on free-living ones. The findings of this study are consistent with the Red Queen Hypothesis. Moreover, they suggest that the maintenance of genetic variation is an important factor in the persistence of parasitic lineages.

Molecular Identification of Diaspididae and Elucidation of Non-Native Species Using the Genes 28s and 16s

Armored scale insects pose a serious threat to habitat conservation across the globe because they include some of the most potent invasive species in the world. They are such a serious concern because their basic morphology, small size, and polyphagous feeding habits often allow them to exist undetected by growers and quarantine experts. In order to provide a potential solution to the problem, we have attempted to elucidate the effectiveness of molecular identification techniques using ribosomal 28s and endosymbiotic 16s rRNA. Sequence data was obtained from many field-collected insects to test the feasibility of identification techniques. A protocol for quick species determination based on sequence data is provided.

The exact distributions of F(IS) under partial asexuality in small finite populations with mutation

Reproductive systems like partial asexuality participate to shape the evolution of genetic diversity within populations, which is often quantified by the inbreeding coefficient F_IS. Understanding how those mating systems impact the possible distributions of F_IS values in theoretical populations helps to unravel forces shaping the evolution of real populations. We proposed a population genetics model based on genotypic states in a finite population with mutation. For populations with less than 400 individuals, we assessed the impact of the rates of asexuality on the full exact distributions of F_IS, the probabilities of positive and negative F_IS, the probabilities of fixation and the probabilities to observe changes in the sign of F_IS over one generation. After an infinite number of generations, we distinguished three main patterns of effects of the rates of asexuality on genetic diversity that also varied according to the interactions of mutation and genetic drift. Even rare asexual events in mainly sexual populations impacted the balance between negative and positive F_IS and the occurrence of extreme values. It also drastically modified the probability to change the sign of F_IS value at one locus over one generation. When mutation prevailed over genetic drift, increasing rates of asexuality continuously increased the variance of F_IS that reached its highest value in fully asexual populations. In consequence, even ancient asexual populations showed the entire F_IS spectrum, including strong positive F_IS. The prevalence of heterozygous loci only occurred in full asexual populations when genetic drift dominated.