Calonectria pseudonaviculata (Buxus blight).

C. pseudonaviculata is an asexual species in a genus of common ascomycete plant pathogens. It was identified relatively recently in the UK, as an introduced species causing a devastating shoot blight of boxwood [Buxus spp.] plants that are commonly used in gardens and landscaping. The full extent of its host range is not known, but Buxus spp. from different continents were found to be susceptible (Henricot et al., 2008). It was placed on the EPPO Alert list in 2004, as it appeared to be spreading to the mainland (EPPO, 2009a), and removed from the list in 2008. This pathogen has been reported from additional European countries in recent years, and may have been transported in asymptomatic infected plants or propagating materials. It survives well in plant debris and probably also in soil.

Genome evolution in a putatively asexual wasp

Asexual lineages are destined for extinction—a result predicted by theory and revealed in practice. Short-term benefits of asexuality are eventually outstripped by their fitness costs: losses of sex and recombination are together expected to reduce efficacy of selection, increase mutation load, and thus, lower fitness. We characterized genomic patterns of accumulating mutations in Diachasma muliebre, a parasitic wasp that has apparently lost sex, an inference supported by many decades of field collections of 1000s of individuals in which only females were found. The split between D. muliebre and its closest sexual relative, Diachasma ferrugineum, is quite recent, allowing us to observe initial events in the evolution of this putative asexual species. First, we find a faster rate of molecular evolution across the D. muliebre genome. In addition, we observed a marked excess of replacement nucleotide substitutions in orthologous genes in the putatively asexual D. muliebre when compared to D. ferrugineum. This pattern directly indicates genome-wide relaxed selection in this young, putatively asexual species, the resulting mutational load from which is expected to ultimately lead to extinction. However, these genomic effects occur in the presence of genomic recombination initially detected by a previous study and also supported by analyses of genome-wide substitution rates within codons. In addition, following completion of the genome sequence and its analysis, we discovered two D. muliebre males, suggesting the possibility of rare sex in this species. Haplodiploid animals, including the sexual ancestors of D. muliebre, bear small genetic loads, likely making their initial transitions to asexuality relatively benign. Paradoxically, an elevated rate of mutation accumulation resulting from asexuality, when accompanied by retention of recombination and/or rare sex, could actually be beneficial: we hypothesize that the novel variation introduced by mutation along with limited shuffling of genes may facilitate initial adaptation and extend persistence of such lineages.

Chromelosporium re-evaluated, with Chromelosporiopsis gen. nov. and Geohypha stat. nov.

Chromelosporium-like asexual morphs of the subterraneous Pezizaceae in the /Pachyphlodes clade are not congeneric with the type of Chromelosporium. Some Chromelosporium morphs are of species in the /Pezizaceae clade (the /Peziza clade), and the genus Chromelosporium as it has been defined is polyphyletic. A diagnostic character that distinguishes these two groups is the presence or absence of conidiophore fasciculation. Mononematous conidiophores characterize Chromelosporium and species related to the /Peziza clade. Synnematous conidiophores characterize Pachyphlodes and other asexual species and define the new genus Chromelosporiopsis, to be excluded from Chromelosporium. Hyphelia terrestris, long misapplied to Chromelosporium, is revaluated, lectotypified, redescribed and recombined as Geohypha terrestris.

Limited Mating Ability of a Wasp Strain with Rickettsia-Induced Thelytoky

Sexual reproduction is the dominant mode of reproduction in plants and animals; however, some species from various taxonomic groups reproduce asexually. Because some of these asexual species lack DNA recombination and so have low genetic variability, these asexual species are more likely to go extinct than sexual species. Neochrysocharis formosa (Westwood) (Hymenoptera: Eulophidae) is a solitary endoparasitoid in which both arrhenotokous and thelytokous strains occur in sympatry. The thelytokous strain is infected by a parthenogenesis-inducing Rickettsia bacterium. We investigated whether fertilized progeny can be produced between females and antibiotic-induced males of the thelytokous strain. The males produced by antibiotic treatment showed the same courtship behaviors as the arrhenotokous males, but at a lower rate, and did not produce fertilized progeny. The results confirm that the thelytokous strain has been maintained by a functional apomixis mechanism rather than by occasional sex, preserving a degree of heterozygosity.

Species and speciation without sex

Species and speciation have been called phenomena of sexual organisms, and many of the concepts developed with sexuals in mind. It is clear theoretically, however, that asexuals should be affected in similar ways by the diversifying processes that cause speciation in sexuals. This chapter investigates evidence for species and speciation in organisms with alternative lifestyles. Bdelloid rotifers are presented as a putative case of asexual species, before the theory and evidence for species in bacteria is discussed. Both theoretically and empirically, the notion that bacteria do not diversify into species can be dismissed. There are interesting differences from eukaryotes, including lower rates of recombination and greater frequency of gene transfer between distant relatives, but neither of these seems to prevent divergence into independently evolving species, at least at core genome regions. Experimental evolution is a useful but neglected avenue to test these ideas further.

Putatively asexual chrysophytes have meiotic genes: evidence from transcriptomic data

Chrysophytes are a large group of heterotrophic, phototrophic, or even mixotrophic protists that are abundant in aquatic as well as terrestrial environments. Although much is known about chrysophyte biology and ecology, it is unknown if they are sexual or not. Here we use available transcriptomes of 18 isolates of 15 putatively asexual species to inventory the presence of genes used in meiosis. Since we were able to detect a set of nine meiosis-specific and 29 meiosis-related genes shared by the chrysophytes, we conclude that they are secretively sexual and therefore should be investigated further using genome sequencing to uncover any missed genes from the transcriptomes.

Fundamental and realized feeding niche breadths of sexual and asexual stick insects

The factors contributing to the maintenance of sex over asexuality in natural populations remain unclear. Ecological divergences between sexual and asexual lineages could help to maintain reproductive polymorphisms, at least transiently, but the consequences of asexuality for the evolution of ecological niches are unknown. Here, we investigated how niche breadths change in transitions from sexual reproduction to asexuality. We used host plant ranges as a proxy to compare the realized feeding niche breadths of five independently derived asexual Timema stick insect species and their sexual relatives at both the species and population levels. Asexual species had systematically narrower realized niches than sexual species, though this pattern was not apparent at the population level. To investigate how the narrower realized niches of asexual species arise, we performed feeding experiments to estimate fundamental niche breadths but found no systematic differences between reproductive modes. The narrow realized niches found in asexual species are therefore probably a consequence of biotic interactions such as predation or competition, that constrain realized niche size in asexuals more strongly than in sexuals.

Fundamental and realized feeding niche breadths of sexual and asexual stick-insects

ABSTRACTThe factors contributing to the maintenance of sex over asexuality in natural populations remain largely unknown. Ecological divergences between lineages with different reproductive modes could help to maintain reproductive polymorphisms, at least transiently, but there is little empirical information on the consequences of asexuality for the evolution of ecological niches. Here, we investigated how niche breadths evolve following transitions from sexual reproduction to asexuality. We estimated and compared the realized feeding niche breadths of five independently derived asexual Timema stick insect species and their sexual relatives. We found that asexual species had a systematically narrower realized niche than sexual species. To investigate how the narrower realized niches of asexual versus sexual species come about, we quantified the breadth of their fundamental niches but found no systematic differences between reproductive modes. The narrow realized niches found in asexuals are therefore likely a consequence of biotic interactions that constrain realized niche size in asexuals more strongly than in sexuals. Interestingly, the fundamental niche was broader in the oldest asexual species compared to its sexual relative. This broad ecological tolerance may help explain how this species has persisted over more than a million years in absence of sex.

Dictyosporella hydei sp. nov., an asexual species from freshwater habitats in China

A new species Dictyosporella hydei was collected on submerged wood in Yunnan Province, China. The new species is characterized by globose to broadly cylindrical, muriform and brass coloured conidia consisting of multiple angular, subglobose cells. Its affinity to Dictyosporella was inferred from phylogenetic analysis based on LSU data. The new species together with the other two species of Dictyosporella formed a well-supported clade which was closely related to Distoseptisporaceae.