Although most unicellular organisms reproduce asexually, most multicellular eukaryotes are obligately sexual. This implies that there are strong barriers that prevent the origin or maintenance of asexuality arising from an obligately sexual ancestor. By studying rare asexual animal species we can gain a better understanding of the circumstances that facilitate their evolution from a sexual ancestor. Of the known asexual animal species, many originated by hybridization between two ancestral sexual species. The balance hypothesis predicts that genetic incompatibilities between the divergent genomes in hybrids can modify meiosis and facilitate asexual reproduction, but there are few instances where this has been shown. Here we report that hybridizing two sexual Caenorhabditis nematode species (C. nouraguensis females and C. becei males) alters the normal inheritance of the maternal and paternal genomes during the formation of hybrid zygotes. Most offspring of this interspecies cross die during embryogenesis, exhibiting inheritance of a diploid C. nouraguensis maternal genome and incomplete inheritance of C. becei paternal DNA. However, a small fraction of offspring develop into viable adults that can be either fertile or sterile. Fertile offspring are produced asexually by sperm-dependent parthenogenesis (also called gynogenesis or pseudogamy); these progeny inherit a diploid maternal genome but fail to inherit a paternal genome. Sterile offspring are hybrids that inherit both a diploid maternal genome and a haploid paternal genome. Whole-genome sequencing of individual viable worms shows that diploid maternal inheritance in both fertile and sterile offspring results from an altered meiosis in C. nouraguensis oocytes and the inheritance of two randomly selected homologous chromatids. We hypothesize that hybrid incompatibility between C. nouraguensis and C. becei modifies maternal and paternal genome inheritance and indirectly induces gynogenetic reproduction. This system can be used to dissect the molecular mechanisms by which hybrid incompatibilities can facilitate the emergence of asexual reproduction.

Aspergillus nidulans has long-been used as a model organism to gain insights into the genetic basis of asexual and sexual developmental processes both in other members of the genus Aspergillus, and filamentous fungi in general. Paradigms have been established concerning the regulatory mechanisms of conidial development. However, recent studies have shown considerable genome divergence in the fungal kingdom, questioning the general applicability of findings from Aspergillus, and certain longstanding evolutionary theories have been questioned. The phylogenetic distribution of key regulatory elements of asexual reproduction in A. nidulans was investigated in a broad taxonomic range of fungi. This revealed that some proteins were well conserved in the Pezizomycotina (e.g. AbaA, FlbA, FluG, NsdD, MedA, and some velvet proteins), suggesting similar developmental roles. However, other elements (e.g. BrlA) had a more restricted distribution solely in the Eurotiomycetes, and it appears that the genetic control of sporulation seems to be more complex in the aspergilli than in some other taxonomic groups of the Pezizomycotina. The evolution of the velvet protein family is discussed based on the history of expansion and contraction events in the early divergent fungi. Heterologous expression of the A. nidulans abaA gene in Monascus ruber failed to induce development of complete conidiophores as seen in the aspergilli, but did result in increased conidial production. The absence of many components of the asexual developmental pathway from members of the Saccharomycotina supports the hypothesis that differences in the complexity of their spore formation is due in part to the increased diversity of the sporulation machinery evident in the Pezizomycotina. Investigations were also made into the evolution of sex and sexuality in the aspergilli. MAT loci were identified from the heterothallic Aspergillus (Emericella) heterothallicus and Aspergillus (Neosartorya) fennelliae and the homothallic Aspergillus pseudoglaucus (=Eurotium repens). A consistent architecture of the MAT locus was seen in these and other heterothallic aspergilli whereas much variation was seen in the arrangement of MAT loci in homothallic aspergilli. This suggested that it is most likely that the common ancestor of the aspergilli exhibited a heterothallic breeding system. Finally, the supposed prevalence of asexuality in the aspergilli was examined. Investigations were made using A. clavatus as a representative 'asexual' species. It was possible to induce a sexual cycle in A. clavatus given the correct MAT1-1 and MAT1-2 partners and environmental conditions, with recombination confirmed utilising molecular markers. This indicated that sexual reproduction might be possible in many supposedly asexual aspergilli and beyond, providing general insights into the nature of asexuality in fungi.

Theory predicts that sexual reproduction can either facilitate or restrain transposable element (TE) accumulation by providing TEs with a means of spreading to all individuals in a population, versus facilitating TE load reduction via purifying selection. By quantifying genomic TE loads over time in experimental sexual and asexual Saccharomyces cerevisiae populations, we provide direct evidence that TE loads decrease rapidly under asexual reproduction. We show, using simulations, that this reduction may occur via evolution of TE activity, most likely via increased excision rates. Thus, sex is a major driver of genomic TE loads and at the root of the success of TEs.

Explaining the overwhelming success of sex among eukaryotes is difficult given the obvious costs of sex relative to asexuality. Different studies have shown that sex can provide benefits in spatially heterogeneous environments under specific conditions, but whether spatial heterogeneity commonly contributes to the maintenance of sex in natural populations remains unknown. We experimentally manipulated habitat heterogeneity for sexual and asexual thrips lineages in natural populations and under seminatural mesocosm conditions by varying the number of hostplants available to these herbivorous insects. Asexual lineages rapidly replaced the sexual ones, independently of the level of habitat heterogeneity in mesocosms. In natural populations, the success of sexual thrips decreased with increasing habitat heterogeneity, with sexual thrips apparently only persisting in certain types of hostplant communities. Our results illustrate how genetic diversity-based mechanisms can favor asexuality instead of sex when sexual lineages co-occur with genetically variable asexual lineages.

Animals that can reproduce by both asexual agametic reproduction and sexual reproduction must transmit or re-establish their germ line post-embryonically. Although such a dual reproductive mode has evolved repeatedly among animals, how asexually produced individuals establish their germ line remains poorly understood in most groups. We investigated germ line development in the annelid Pristina leidyi, a species that typically reproduces asexually by paratomic fission, intercalating a new tail and head in the middle of the body followed by splitting. We found that in fissioning individuals, gonads occur in anterior segments in the anterior-most individual as well as in new heads forming within fission zones. Homologs of the germ line/multipotency genes piwi, vasa, and nanos are expressed in the gonads, as well as in proliferative tissues including the posterior growth zone, fission zone, and regeneration blastema. In fissioning animals, certain cells on the ventral nerve cord express a homolog of piwi, are abundant near fission zones, and sometimes make contact with gonads. Such cells are typically undetectable near the blastema and posterior growth zone. Time-lapse imaging provides direct evidence that cells on the ventral nerve cord migrate preferentially towards fission zones. Our findings indicate that gonads form routinely in fissioning individuals, that a population of piwi-positive cells on the ventral nerve cord is associated with fission and gonads, and that cells resembling these piwi-positive cells migrate along the ventral nerve cord. We suggest that the piwi-positive ventral cells are germ cells that transmit the germ line across asexually produced individuals via migration along the ventral nerve cord.

The unique mechanism by which leaf margin cells regain potency and then form a plantlet in Kalanchoë spp. remains elusive but involves organogenesis and embryogenesis in response to age, day length, nutrient availability, and drought stress. In light of this, we investigated whether TARGET OF RAPAMYCIN (TOR), a conserved protein kinase in eukaryotes that controls cell growth and metabolism in response to nutrient and energy availability, may regulate plantlet formation. Kalanchoë daigremontiana TOR (KdTOR) was expressed in the leaf margin at the site of plantlet initiation, in the early plantlet cotyledons, and in the root tip of the developed plantlet. Both chemical and genetic inhibition of TOR Kinase activity in Kalanchoë daigremontiana leaves disrupted plantlet formation. Furthermore, downregulation of KdTOR in transgenic plants led to wide-ranging transcriptional changes, including decreased K. daigremontiana SHOOTMERISTEMLESS and K. daigremontiana LEAFYCOTYLEDON1 expression, whereas auxin treatments induced KdTOR expression in the plantlet roots. These results suggest that the KdTOR pathway controls plantlet development in cooperation with auxin, organogenesis, and embryogenesis pathways. The ancient and highly conserved TOR Kinase therefore controls diverse and unique developmental pathways, such as asexual reproduction within the land plant lineage.

Cnidarians exhibit incredible reproductive diversity, with most capable of sexual and asexual reproduction. Here, we investigate factors that influence asexual reproduction in the burrowing sea anemone Nematostella vectensis, which can propagate asexually by transverse fission of the body column. By altering culture conditions, we demonstrate that the presence of a burrowing substrate strongly promotes transverse fission. In addition, we show that animal size does not affect fission rates, and that the plane of fission is fixed along the oral-aboral axis of the polyp. Homeobox transcription factors and components of the TGFβ, Notch, and FGF signalling pathways are differentially expressed in polyps undergoing physal pinching suggesting they are important regulators of transverse fission. Gene ontology analyses further suggest that during transverse fission the cell cycle is suppressed, and that cell adhesion and patterning mechanisms are downregulated to promote separation of the body column. Finally, we demonstrate that the rate of asexual reproduction is sensitive to population density. Collectively, these experiments provide a foundation for mechanistic studies of asexual reproduction in Nematostella, with implications for understanding the reproductive and regenerative biology of other cnidarian species.

Cnidarians exhibit incredible reproductive diversity, with most capable of sexual and asexual reproduction. Here we investigate factors that influence asexual reproduction in the burrowing sea anemone Nematostella vectensis , which can propagate asexually by transverse fission of the body column. By altering culture conditions, we demonstrate that the presence of a burrowing substrate strongly promotes transverse fission. In addition, we show that animal size does not affect fission rates, and that the plane of fission is fixed along the oral-aboral axis of the polyp. Homeobox transcription factors and components of the TGFβ, Notch, and FGF signaling pathways are differentially expressed in polyps undergoing physal pinching suggesting they are important regulators of transverse fission. Gene ontology analyses further suggest that during transverse fission the cell cycle is suppressed and that cell adhesion and patterning mechanisms are downregulated to promote separation of the body column. Finally, we demonstrate that the rate of asexual reproduction is sensitive to population density. Collectively, these experiments provide a foundation for mechanistic studies of asexual reproduction in Nematostella , with implications for understanding the reproductive and regenerative biology of other cnidarian species.

Trade-offs among traits influencing fitness are predicted by life history theory because resources allocated to one function are unavailable to another. Here we examine the relationship between two such traits, asexual reproduction and growth rate, in the filamentous fungus Neurospora crassa, where shared genetic and physiological factors and a source-sink energetic relationship between growth and reproduction may constrain the evolution of these traits. To test growth-reproduction relationships in this species, we independently selected on mycelial growth rate or asexual spore production in a heterogeneous lab-derived population and evaluated the response of the non-selected traits. Combined with phenotypes for the 20 wild strains used to produce the heterogeneous population and the genome-wide genotypes of 468 strains, these data show that growth and reproduction are highly plastic in N. crassa and do not trade off either among wild strains or after laboratory selection in two environments. Rather, we find no predictable growth-reproduction relationship in the environments tested, indicating an effective absence of genetic constraint between these traits. Our results suggest that growth rate and asexual reproduction may not respond predictably to environmental change and suggest that reliance on a single trait as a proxy for fitness in fungal studies may be inadvisable.

Sexual reproduction is a widely studied biological process because it is critically important to the genetics, evolution, and ecology of eukaryotes. Despite decades of study on this topic, no comprehensive explanation has been accepted that explains the evolutionary forces underlying its prevalence and persistence in nature. Monogonont rotifers offer a useful system for experimental studies relating to the evolution of sexual reproduction due to their rapid reproductive rate and close relationship to the putatively ancient asexual bdelloid rotifers. However, little is known about the molecular underpinnings of sex in any rotifer species.

Plasmodial slime molds (Myxogastria or Myxomycetes) are common and widespread unicellular organisms that are commonly assumed to have a sexual life cycle culminating with the formation of often macroscopic fruiting bodies that efficiently disseminate spores. However, laboratory studies based on mating compatibility revealed the coexistence of asexual as well as sexual strains. To test this hypothesis in natural populations, we investigated the genetic variability of two species of the genus Lamproderma. Detailed ecological relevés were carried out in 2007 and 2009 in several deep ravines in the Elbsandsteingebirge (Saxony, south-eastern Germany). Morphological characters of 93 specimens of Lamproderma were recorded and genetic analyses, based on the small subunit ribosomal gene, the internal transcribed spacer 1 and partial elongation factor 1α sequences were carried out for 52 specimens. Genetic analyses showed the existence of two major clades, each composed of several discrete lineages. Most of these lineages were composed of several identical sequences (SSU, ITS 1 and EF-1α) which is explained best by an asexual mode of reproduction. Detrended Correspondence Analysis of morphological characters revealed two morphospecies that corresponded to the two major clades, except for one genotype (Lc6), thus challenging the morphospecies concept. Genetic patterns were not related to the geographical distribution: specimens belonging to the same genotype were found in distinct ravines, suggesting effective long-distance dispersal via spores, except for the Lc6 genotype which was found only in one ravine. Implications for the morphological and biological species concept are discussed.

Regeneration, the ability to replace lost body parts, is a widespread phenomenon in the animal kingdom often connected to asexual reproduction or fission, since the only difference between the two appears to be the stimulus that triggers them. Both developmental processes have largely been characterized; however, the molecular toolkit and genetic mechanisms underlying these events remain poorly unexplored. Annelids, in particular the oligochaete Pristina leidyi, provide a good model system to investigate these processes as they show diverse ways to regenerate, and can reproduce asexually through fission under laboratory conditions. Here, we used a comparative transcriptomics approach based on RNA-sequencing and differential gene expression analyses to understand the molecular mechanisms involved in anterior regeneration and asexual reproduction. We found 291 genes upregulated during anterior regeneration, including several regeneration-related genes previously reported in other annelids such as frizzled, paics, and vdra. On the other hand, during asexual reproduction, 130 genes were found upregulated, and unexpectedly, many of them were related to germline development during sexual reproduction. We also found important differences between anterior regeneration and asexual reproduction, with the latter showing a gene expression profile more similar to that of control individuals. Nevertheless, we identified 35 genes that were upregulated in both conditions, many of them related to cell pluripotency, stem cells, and cell proliferation. Overall, our results shed light on the molecular mechanisms that control anterior regeneration and asexual reproduction in annelids and reveal similarities with other animals, suggesting that the genetic machinery controlling these processes is conserved across metazoans.

Gametocytogenesis is fundamental for transmission of the malaria parasite Plasmodium falciparum from the human host to the mosquito vector, yet very little is understood about what triggers the switch between asexual reproduction and gametocytogenesis. Arresting the progression through the sexual cycle would block transmission of this disease. Here we identify a novel gene in P. falciparum that when genetically silenced reduces gametocyte production by a factor of 6, and when complemented up-regulates gametocyte-specific gene transcription.

A decade ago, the mixed reproductive strategy Asexual Queen Succession (AQS) was first described in termites. In AQS species, the workers, soldiers and dispersing reproductives are produced through sexual reproduction, while non-dispersing (neotenic) queens arise through automictic thelytokous parthenogenesis, replace the founding queen and mate with the founding king. As yet, AQS has been documented in six species from three lineages of lower (Rhinotermitidae) and higher (Termitinae: Termes group and Syntermitinae) termites. Independent evolution of the capacity of thelytoky as a preadaptation to AQS is supported by different mechanisms of automixis in each of the three clades. These pioneering discoveries prompt the question on the extent of thelytoky and AQS in the diversified family of higher termites.

The interstitial environment of marine sandy bottoms is a nutrient-rich, sheltered habitat whilst at the same time also often a turbulent, space-limited, and ecologically challenging environment dominated by meiofauna. The interstitial fauna is one of the most diverse on earth and accommodates miniaturized representatives from many macrofaunal groups as well as several exclusively meiofaunal phyla. The colonization process of this environment, with the restrictions imposed by limited space and low Reynolds numbers, has selected for great morphological and behavioral changes as well as new life history strategies.Here we describe a new enteropneust species inhabiting the interstices among sand grains in shallow tropical waters of the West Atlantic. With a maximum body length of 0.6 mm, it is the first microscopic adult enteropneust known, a group otherwise ranging from 2 cm to 250 cm in adult size. Asexual reproduction by paratomy has been observed in this new species, a reproductive mode not previously reported among enteropneusts. Morphologically, Meioglossus psammophilus gen. et sp. nov. shows closest resemblance to an early juvenile stage of the acorn worm family Harrimaniidae, a result congruent with its phylogenetic placement based on molecular data. Its position, clearly nested within the larger macrofaunal hemichordates, suggests that this represents an extreme case of miniaturization. The evolutionary pathway to this simple or juvenile appearance, as chiefly demonstrated by its small size, dense ciliation, and single pair of gill pores, may be explained by progenesis. The finding of M. psammophilus gen. et sp. nov. underscores the notion that meiofauna may constitute a rich source of undiscovered metazoan diversity, possibly disguised as juveniles of other species.

Plant fitness is often a result of both sexual and asexual reproductive success and, in perennial plants, over several years. Folivory can affect both modes of reproduction. However, little is known about the effects of folivory on resource allocation to the two modes of reproduction simultaneously and across years. In a 2-year common garden experiment, we examined the effects of different levels of folivory by the strawberry leaf beetle, Galerucella tenella, on current growth, as well as current and future sexual and asexual reproduction (runners) of perennial woodland strawberry, Fragaria vesca. In addition, we measured the chlorophyll content in leaves in the year of experimental damage to determine whether there was increased photosynthetic activity, and, thus, a compensatory response to herbivory. Finally, we tested whether the previous year's folivory, as a result of its effect on plant fitness, affected the level of natural herbivory the plant experienced during the subsequent year. In the year of experimental damage, plants that were exposed to moderate and high levels of folivory (25% and 50% leaf area consumed, respectively) increased their photosynthetic activity compared to control plants. However, only plants exposed to high folivory exhibited negative effects, with a lower probability of flowering compared to control plants, indicating that plants exposed to low or moderate folivory were able to compensate for the damage. Negative effects of folivory were carried over to the subsequent year. Plants that were exposed to moderate folivory (25% leaf area consumed) during first year produced fewer flowers and fruits in the subsequent year. Runner production was consistently unaffected by folivory. The effects of experimental folivory on the level of natural herbivory were mediated via its effects on plant fitness. Our results show that the negative effects of folivory only influence sexual reproduction in woodland strawberry. Furthermore, even though woodland strawberry can tolerate moderate amounts of folivory in the short term, the negative effects on fitness appear later; this highlights the importance of studying the effects of herbivory over consecutive years in perennial plants.

Crabs of the genus Lybia have the remarkable habit of holding a sea anemone in each of their claws. This partnership appears to be obligate, at least on the part of the crab. The present study focuses on Lybia leptochelis from the Red Sea holding anemones of the genus Alicia (family Aliciidae). These anemones have not been found free living, only in association with L. leptochelis. In an attempt to understand how the crabs acquire them, we conducted a series of behavioral experiments and molecular analyses. Laboratory observations showed that the removal of one anemone from a crab induces a "splitting" behavior, whereby the crab tears the remaining anemone into two similar parts, resulting in a complete anemone in each claw after regeneration. Furthermore, when two crabs, one holding anemones and one lacking them, are confronted, the crabs fight, almost always leading to the "theft" of a complete anemone or anemone fragment by the crab without them. Following this, crabs "split" their lone anemone into two. Individuals of Alicia sp. removed from freshly collected L. leptochelis were used for DNA analysis. By employing AFLP (Fluorescence Amplified Fragments Length Polymorphism) it was shown that each pair of anemones from a given crab is genetically identical. Furthermore, there is genetic identity between most pairs of anemone held by different crabs, with the others showing slight genetic differences. This is a unique case in which one animal induces asexual reproduction of another, consequently also affecting its genetic diversity.

Zymoseptoria tritici, the causal agent of Septoria tritici blotch, is a fungal wheat pathogen that causes significant global yield losses. Within Z. tritici populations, quantitative differences in virulence among different isolates are commonly observed; however, the genetic components that underpin these differences remain elusive. In this study, intraspecific comparative transcriptomic analysis was used to identify candidate genes that contribute to differences in virulence on the wheat cultivar WW2449. This led to the identification of a multicopy gene that was not expressed in the high-virulence isolate when compared to the medium- and low-virulence isolates. Further investigation suggested this gene resides in a 7.9-kb transposon. Subsequent long-read sequencing of the isolates used in the transcriptomic analysis confirmed that this gene did reside in an active Class II transposon, which is composed of four genes named REP9-1 to -4. Silencing and overexpression of REP9-1 in two distinct genetic backgrounds demonstrated that its expression alone reduces the number of pycnidia produced by Z. tritici during infection. The REP9-1 gene identified within a Class II transposon is the first discovery of a gene in a transposable element that influences the virulence of Z. tritici. This discovery adds further complexity to genetic loci that contribute to quantitative virulence in this important pathogen.

We analysed the patterns of genetic variability of eastern Mediterranean populations of the scleractinian coral Cladocora caespitosa, from the Aegean and Levantine seas, using 19 polymorphic microsatellite loci, 11 of which were newly characterized. The observed genetic pattern reflects a scenario of isolation by environment: F ST comparisons showed a higher degree of genetic differentiation between the two Cypriot populations that are separated by only 11 km than between these two Levantine populations and the Aegean population in Greece, which are separated by 1300 km. We hypothesize that local-scale oceanographic factors influenced the dispersal of planulae between the geographically close populations, playing a crucial role in the genetic structure of this coastal coral. Yet, despite being characterized as a species with limited dispersal and high self-recruitment, large-scale migration does eventually occur as first-generation migrants were identified between the most distant populations. In line with previous findings of reproductive plasticity in C. caespitosa, we also found localized differences in reproduction mode (sexual vs. asexual) within a geographically limited context. Several individuals were identified as clones, indicating the predominance of asexual reproduction in one of the Cypriot populations. We interpret this predominance either as a direct response to or as an indirect consequence of perturbations suffered by this C. caespitosa population. These perturbations are caused by unfavourable environmental conditions that threatened local survival, in particular water temperature changes and windstorm swells. Asexual reproduction may be a mechanism used by C. caespitosa to counteract mortality events and recolonize devastated areas, and likely accounts for the occasional high levels of clonality and low levels of genetic diversity. Local adaptations such as these should therefore be considered in conservation and management strategies to maintain and preserve the gene pool of this endangered species.

Fusarium fujikuroi, a causal agent of Rice Bakanae Disease, produces secondary metabolites such as gibberellin, pigments bikaverin, and mycotoxins fumonisins. Fumonisins produced by F. fujikuroi pose a severe threat to human and animal health. The copper chaperone protein plays a critical role in different growth stages of plants, fungi, and yeasts, but their functions and regulation in fumonisin biosynthesis are still unclear. Here, a copper chaperone protein, FfCOX17, was identified in F. fujikuroi. The FfCOX17 deletion mutant (∆FfCOX17) exhibited decreased vegetative growth and asexual reproduction. The transcriptional level of the FfFUM2 gene was significantly induced in ∆FfCOX17, and the fumonisin production in ∆FfCOX17 mutants was significantly increased compared to wild-type F. fujikuroi, but the pathogenicity of ∆FfCOX17 mutants was unaffected, which may be caused by the no significantly changed gibberellin content. ∆FfCOX17 showed decreased sensitivity to oxidative stress, osmotic stress, and increased sensitivity to cell wall stress, heat shock stress, and high concentration glucose. In addition, ∆FfCOX17 also showed increased sensitivity to fungicide fluazinam and fludioxonil, and decreased sensitivity to phenamacril and prochloraz. Taken together, this study suggested that FfCOX17 is critical for fumonisin production, vegetative growth, asexual reproduction, and fungicide sensitivity, but is not required for the virulence function of F. fujikuroi on rice.