Sex, drugs and recombination: the wild life of Aspergillus

Throughout the eukaryotes, sexual reproduction is an almost universal phenomenon. However, within the Kingdom Fungi, this relationship is not so clear‐cut. Fungi exhibit a spectrum of reproductive modes and life‐cycles; amongst the better known species, sexual reproduction is often facultative, can be rare, and in over half of the known Ascomycota (the moulds) is unknown ( Taylor et al. 1999 ). However, over the last decade, it has become apparent that many of these asexual mitosporic taxa undergo cryptic recombination via unobserved mechanisms and that wholly asexual fungi are, in fact, a rarity ( Taylor et al. 1999, 2001 ; Heitman 2010 ). This revolution in our understanding of fungal sexuality has come about in two ways: Firstly, sexual reproduction leaves an imprint on fungal genomes by maintaining genes required for mating and by generating patterns of mutation and recombination restricted to meiotic processes. Secondly, scientists have become better at catching fungi in flagrante delicto. The genus Aspergillus is one such fungus where a combination of population genetics, genomics and taxonomy has been able to intuit the existence of sex, then to catch the fungus in the act and formally describe their sexual stages. So, why are sexy moulds exciting? One species in particular, Aspergillus flavus, is notorious for its ability to produce a diverse array of secondary metabolites, of which the polyketide aflatoxins (AF) are carcinogenic and others (such as cyclopiazonic acid) are toxigenic. Because of the predilection of A. flavus to grow on crops, such as peanuts, corn and cotton, biocontrol is widely used to mitigate infection by pre‐applying nonaflatoxigenic (AF?) strains to competitively exclude the wild‐type AF+ strains. However, the eventual fate in nature of these biocontrol strains is not known. In this issue of Molecular Ecology, Olarte et al. (2012) make an important contribution by using laboratory crosses of A. flavus to show that not only is AF highly heritable, but AF? strains can become AF+ via crossing over during meiosis. This observation has raised the spectre of cross‐breeding and non‐mendelian inheritance of AF between native and biocontrol strains of the fungus, leading to an increase in the natural diversity of the fungus with perhaps unanticipated consequences.     

Sexual Reproduction in Aspergillus flavus Sclerotia: Acquisition of Novel Alleles from Soil Populations and Uniparental Mitochondrial Inheritance

Aspergillus flavus colonizes agricultural commodities worldwide and contaminates them with carcinogenic aflatoxins. The high genetic diversity of A. flavus populations is largely due to sexual reproduction characterized by the formation of ascospore-bearing ascocarps embedded within sclerotia. A. flavus is heterothallic and laboratory crosses between strains of the opposite mating type produce progeny showing genetic recombination. Sclerotia formed in crops are dispersed onto the soil surface at harvest and are predominantly produced by single strains of one mating type. Less commonly, sclerotia may be fertilized during co-infection of crops with sexually compatible strains. In this study, laboratory and field experiments were performed to examine sexual reproduction in single-strain and fertilized sclerotia following exposure of sclerotia to natural fungal populations in soil. Female and male roles and mitochondrial inheritance in A. flavus were also examined through reciprocal crosses between sclerotia and conidia. Single-strain sclerotia produced ascospores on soil and progeny showed biparental inheritance that included novel alleles originating from fertilization by native soil strains. Sclerotia fertilized in the laboratory and applied to soil before ascocarp formation also produced ascospores with evidence of recombination in progeny, but only known parental alleles were detected. In reciprocal crosses, sclerotia and conidia from both strains functioned as female and male, respectively, indicating A. flavus is hermaphroditic, although the degree of fertility depended upon the parental sources of sclerotia and conidia. All progeny showed maternal inheritance of mitochondria from the sclerotia. Compared to A. flavus populations in crops, soil populations would provide a higher likelihood of exposure of sclerotia to sexually compatible strains and a more diverse source of genetic material for outcrossing.     

Recombination and lineage‐specific gene loss in the aflatoxin gene cluster of Aspergillus flavus

Aflatoxins produced by Aspergillus flavus are potent carcinogens that contaminate agricultural crops. Recent efforts to reduce aflatoxin concentrations in crops have focused on biological control using nonaflatoxigenic A. flavus strains AF36 (=NRRL 18543) and NRRL 21882 (the active component of afla‐guard^®). However, the evolutionary potential of these strains to remain nonaflatoxigenic in nature is unknown. To elucidate the underlying population processes that influence aflatoxigenicity, we examined patterns of linkage disequilibrium (LD) spanning 21 regions in the aflatoxin gene cluster of A. flavus. We show that recombination events are unevenly distributed across the cluster in A. flavus. Six distinct LD blocks separate late pathway genes aflE, aflM, aflN, aflG, aflL, aflI and aflO, and there is no discernable evidence of recombination among early pathway genes aflA, aflB, aflC, aflD, aflR and aflS. The discordance in phylogenies inferred for the aflW/aflX intergenic region and two noncluster regions, tryptophan synthase and acetamidase, is indicative of trans‐species evolution in the cluster. Additionally, polymorphisms in aflW/aflX divide A. flavus strains into two distinct clades, each harbouring only one of the two approved biocontrol strains. The clade with AF36 includes both aflatoxigenic and nonaflatoxigenic strains, whereas the clade with NRRL 21882 comprises only nonaflatoxigenic strains and includes all strains of A. flavus missing the entire gene cluster or with partial gene clusters. Our detection of LD blocks in partial clusters indicates that recombination may have played an important role in cluster disassembly, and multilocus coalescent analyses of cluster and noncluster regions indicate lineage‐specific gene loss in A. flavus. These results have important implications in assessing the stability of biocontrol strains in nature.     

Genetic isolation among sympatric vegetative compatibility groups of the aflatoxin‐producing fungus Aspergillus flavus

Aspergillus flavus, a fungal pathogen of animals and both wild and economically important plants, is most recognized for producing aflatoxin, a cancer‐causing secondary metabolite that contaminates food and animal feed globally. Aspergillus flavus has two self/nonself recognition systems, a sexual compatibility system and a vegetative incompatibility system, and both play a role in directing gene flow in populations. Aspergillus flavus reproduces clonally in wild and agricultural settings, but whether a cryptic sexual stage exists in nature is currently unknown. We investigated the distribution of genetic variation in 243 samples collected over 4 years from three common vegetative compatibility groups (VCGs) in Arizona and Texas from cotton using 24 microsatellite loci and the mating type locus (MAT) to assess population structure and potential gene flow among A. flavus VCGs in sympatric populations. All isolates within a VCG had the same mating type with OD02 having MAT1‐2 and both CG136 and MR17 having MAT1‐1. Our results support the hypothesis that these three A. flavus VCGs are genetically isolated. We found high levels of genetic differentiation and no evidence of gene flow between VCGs, including VCGs of opposite mating‐type. Our results suggest that these VCGs diverged before domestication of agricultural hosts (>10 000 yr bp ).     

Sexuality Generates Diversity in the Aflatoxin Gene Cluster: Evidence on a Global Scale

Aflatoxins are produced by Aspergillus flavus and A. parasiticus in oil-rich seed and grain crops and are a serious problem in agriculture, with aflatoxin B₁ being the most carcinogenic natural compound known. Sexual reproduction in these species occurs between individuals belonging to different vegetative compatibility groups (VCGs). We examined natural genetic variation in 758 isolates of A. flavus, A. parasiticus and A. minisclerotigenes sampled from single peanut fields in the United States (Georgia), Africa (Benin), Argentina (Córdoba), Australia (Queensland) and India (Karnataka). Analysis of DNA sequence variation across multiple intergenic regions in the aflatoxin gene clusters of A. flavus, A. parasiticus and A. minisclerotigenes revealed significant linkage disequilibrium (LD) organized into distinct blocks that are conserved across different localities, suggesting that genetic recombination is nonrandom and a global occurrence. To assess the contributions of asexual and sexual reproduction to fixation and maintenance of toxin chemotype diversity in populations from each locality/species, we tested the null hypothesis of an equal number of MAT1-1 and MAT1-2 mating-type individuals, which is indicative of a sexually recombining population. All samples were clone-corrected using multi-locus sequence typing which associates closely with VCG. For both A. flavus and A. parasiticus, when the proportions of MAT1-1 and MAT1-2 were significantly different, there was more extensive LD in the aflatoxin cluster and populations were fixed for specific toxin chemotype classes, either the non-aflatoxigenic class in A. flavus or the B₁-dominant and G₁-dominant classes in A. parasiticus. A mating type ratio close to 1∶1 in A. flavus, A. parasiticus and A. minisclerotigenes was associated with higher recombination rates in the aflatoxin cluster and less pronounced chemotype differences in populations. This work shows that the reproductive nature of the population (more sexual versus more asexual) is predictive of aflatoxin chemotype diversity in these agriculturally important fungi.     

Geographic variation in sex‐chromosome differentiation in the common frog (Rana temporaria)

In sharp contrast with birds and mammals, sex‐determination systems in ectothermic vertebrates are often highly dynamic and sometimes multifactorial. Both environmental and genetic effects have been documented in common frogs (Rana temporaria). One genetic linkage group, mapping to the largest pair of chromosomes and harbouring the candidate sex‐determining gene Dmrt1, associates with sex in several populations throughout Europe, but association varies both within and among populations. Here, we show that sex association at this linkage group differs among populations along a 1500‐km transect across Sweden. Genetic differentiation between sexes is strongest (F_ST = 0.152) in a northern‐boreal population, where male‐specific alleles and heterozygote excesses (F_IS = ?0.418 in males, +0.025 in females) testify to a male‐heterogametic system and lack of X‐Y recombination. In the southernmost population (nemoral climate), in contrast, sexes share the same alleles at the same frequencies (F_ST = 0.007 between sexes), suggesting unrestricted recombination. Other populations show intermediate levels of sex differentiation, with males falling in two categories: some cluster with females, while others display male‐specific Y haplotypes. This polymorphism may result from differences between populations in the patterns of X‐Y recombination, co‐option of an alternative sex‐chromosome pair, or a mixed sex‐determination system where maleness is controlled either by genes or by environment depending on populations or families. We propose approaches to test among these alternative models, to disentangle the effects of climate and phylogeography on the latitudinal trend, and to sort out how this polymorphism relates to the ‘sexual races’ described in common frogs in the 1930s.     

Method for monitoring deletions in the aflatoxin biosynthesis gene cluster of Aspergillus flavus with multiplex PCR

The report presents a rapid, inexpensive and simple method for monitoring indels with influence on aflatoxin biosynthesis within Aspergillus flavus populations. PCR primers were developed for 32 markers spaced approximately every 5 kb from 20 kb proximal to the aflatoxin biosynthesis gene cluster to the telomere repeat. This region includes gene clusters required for biosynthesis of aflatoxins and cyclopiazonic acid; the resulting data were named cluster amplification patterns (CAPs). CAP markers are amplified in four multiplex PCRs, greatly reducing the cost and time to monitor indels within this region across populations. The method also provides a practical tool for characterizing intraspecific variability in A. flavus not captured with other methods.

Significance and Impact of the Study

Aflatoxins, potent naturally‐occurring carcinogens, cause significant agricultural problems. The most effective method for preventing contamination of crops with aflatoxins is through use of atoxigenic strains of Aspergillus flavus to alter the population structure of this species and reduce incidences of aflatoxin producers. Cluster amplification pattern (CAP) is a rapid multiplex PCR method for identifying and monitoring indels associated with atoxigenicity in A. flavus. Compared to previous techniques, the reported method allows for increased resolution, reduced cost, and greater speed in monitoring the stability of atoxigenic strains, incidences of indel mediated atoxigenicity and the structure of A. flavus populations.     

Population genetics of Wolbachia‐infected,parthenogenetic and uninfected,sexual populations of Tetrastichus coeruleus (Hymenoptera: Eulophidae)

Wolbachia are endosymbiotic bacteria known to manipulate the reproduction of their hosts. These manipulations are expected to have consequences on the population genetics of the host, such as heterozygosity levels, genetic diversity and gene flow. The parasitoid wasp Tetrastichus coeruleus has populations that are infected with parthenogenesis‐inducing Wolbachia and populations that are not infected. We studied the population genetics of T. coeruleus between and within Wolbachia‐infected and uninfected populations, using nuclear microsatellites and mitochondrial DNA. We expected reduced genetic diversity in both DNA types in infected populations. However, migration and gene flow could introduce new DNA variants into populations. We therefore paid special attention to individuals with unexpected (genetic) characteristics. Based on nuclear and mitochondrial DNA, two genetic clusters were evident: a thelytokous cluster containing all Wolbachia‐infected, parthenogenetic populations and an arrhenotokous cluster containing all uninfected, sexual populations. Nuclear and mitochondrial DNA did not exhibit concordant patterns of variation, although there was reduced genetic diversity in infected populations for both DNA types. Within the thelytokous cluster, there was nuclear DNA variation, but no mitochondrial DNA variation. This nuclear DNA variation may be explained by occasional sex between infected females and males, by horizontal transmission of Wolbachia, and/or by novel mutations. Several females from thelytokous populations were uninfected and/or heterozygous for microsatellite loci. These unexpected characteristics may be explained by migration, by inefficient transmission of Wolbachia, by horizontal transmission of Wolbachia, and/or by novel mutations. However, migration has not prevented the build‐up of considerable genetic differentiation between thelytokous and arrhenotokous populations.     

Background selection and FST: Consequences for detecting local adaptation

Background selection is a process whereby recurrent deleterious mutations cause a decrease in the effective population size and genetic diversity at linked loci. Several authors have suggested that variation in the intensity of background selection could cause variation in F_ST across the genome, which could confound signals of local adaptation in genome scans. We performed realistic simulations of DNA sequences, using recombination maps from humans and sticklebacks, to investigate how variation in the intensity of background selection affects F_ST and other statistics of population differentiation in sexual, outcrossing species. We show that, in populations connected by gene flow, Weir and Cockerham's (1984; Evolution, 38 , 1358) estimator of F_ST is largely insensitive to locus‐to‐locus variation in the intensity of background selection. Unlike F_ST, however, d_XY is negatively correlated with background selection. Moreover, background selection does not greatly affect the false‐positive rate in F_ST outlier studies in populations connected by gene flow. Overall, our study indicates that background selection will not greatly interfere with finding the variants responsible for local adaptation.     

Molecular genetic variation and geographical structuring in Fusarium graminearum

Inter‐simple sequence repeat markers were employed to study the genetic structure of Fusarium graminearum populations collected from three Canadian provinces. Our study suggested high genetic diversity and frequent gene flow among population samples analysed. The analysis of molecular variance indicated that most of the gene diversity (91.78% to 97.23%) was distributed within populations. Frequent gene flow among the western prairie provinces and cluster analysis results indicated that the population sample from Alberta was closer to Saskatchewan than Manitoba, which could be the result of movement of the pathogen via infected grain or through wind‐borne ascospore dispersal. Analysis of multilocus associations showed that all populations were in linkage equilibrium, indicating that sexual recombination is a frequent phenomenon in F. graminearum populations from western Canada.     

Peanut Resistance Gene Expression in Response to Aspergillus flavus Infection During Seed Germination

Aspergillus flavus produces potent mutagenic and carcinogenic polyketide‐derived secondary metabolites known as aflatoxins. Development of host plant resistance in peanut and other crops is the most environmentally friendly and cost‐effective method to eliminate the serious problem of aflatoxin contamination in grains. To confirm that putative peanut genes identified in a previous microarray study were involved in peanut resistance to A. flavus infection, 14 genes were selected for further investigation through real‐time PCR. The results revealed diverse patterns of gene expression during seed germination after A. flavus inoculation. Based on the expression levels and the relative‐expression patterns over a 7‐day period, the 14 host genes could be classified into six different groups belonging to three main biochemical and genetic defence processes of lipid metabolism, oxidative signalling and cell‐wall synthesis during counter‐attack. A network of gene expression patterns was activated in sequential order in response to A. flavus invasion in both resistant and susceptible peanut lines during seed germination. Understanding gene expression patterns in peanut will be useful to breeders and other scientists interested in incorporating genetic resources of resistance against A. flavus into peanut germplasm and/or commercial cultivars via conventional and/or molecular methods.     

The genetic architecture of genome‐wide recombination rate variation in allopolyploid wheat revealed by nested association mapping

Recombination affects the fate of alleles in populations by imposing constraints on the reshuffling of genetic information. Understanding the genetic basis of these constraints is critical for manipulating the recombination process to improve the resolution of genetic mapping, and reducing the negative effects of linkage drag and deleterious genetic load in breeding. Using sequence‐based genotyping of a wheat nested association mapping (NAM) population of 2,100 recombinant inbred lines created by crossing 29 diverse lines, we mapped QTL affecting the distribution and frequency of 102 000 crossovers (CO). Genome‐wide recombination rate variation was mostly defined by rare alleles with small effects together explaining up to 48.6% of variation. Most QTL were additive and showed predominantly trans‐acting effects. The QTL affecting the proximal COs also acted additively without increasing the frequency of distal COs. We showed that the regions with decreased recombination carry more single nucleotide polymorphisms (SNPs) with possible deleterious effects than the regions with a high recombination rate. Therefore, our study offers insights into the genetic basis of recombination rate variation in wheat and its effect on the distribution of deleterious SNPs across the genome. The identified trans‐acting additive QTL can be utilized to manipulate CO frequency and distribution in the large polyploid wheat genome opening the possibility to improve the efficiency of gene pyramiding and reducing the deleterious genetic load in the low‐recombining pericentromeric regions of chromosomes.     

Mating system and environmental variation drive patterns of adaptation in Boechera spatifolia (Brassicaceae)

Determining the relative contribution of population genetic processes to the distribution of natural variation is a major goal of evolutionary biology. Here, we take advantage of variation in mating system to test the hypothesis that local adaptation is constrained by asexual reproduction. We explored patterns of variation in ecological traits and genome‐wide molecular markers in Boechera spatifolia (Brassicaceae), a species that contains both apomictic (asexual) and sexual individuals. Using a combination of quantitative genetics, neutral genetic (SSR) and genome‐wide single nucleotide polymorphism, we assessed the hypothesis that asexual lineages should have reduced signatures of adaptation relative to sexual conspecifics. All three measures (traits, SSRs, SNPs) demonstrated that apomicts are genetically distinct from sexuals, regardless of population location. Additionally, phylogenetic clustering revealed that the apomictic group shared a single common ancestor. Across the landscape, sexual genome‐wide SNP variation was strongly associated with latitude (r² > 0.9), indicating that sexual populations have differentiated across an environmental gradient. Furthermore, flowering time and growth rate, as assessed in a common garden, strongly covary with the elevation and latitude of the source population. Despite a wide geographic distribution that largely overlaps with sexual populations, there was little evidence for differentiation in molecular markers or quantitative characters among apomictic populations. Combined, these data indicated that, in contrast to asexual populations, sexual populations show evidence of local adaptation.     

Enhanced diversity and aflatoxigenicity in interspecific hybrids of Aspergillus flavus and Aspergillus parasiticus

Aspergillus flavus and A. parasiticus are the two most important aflatoxin‐producing fungi responsible for the contamination of agricultural commodities worldwide. Both species are heterothallic and undergo sexual reproduction in laboratory crosses. Here we examine the possibility of interspecific matings between A. flavus and A. parasiticus. These species can be distinguished morphologically and genetically, as well as by their mycotoxin profiles. Aspergillus flavus produces both B aflatoxins and cyclopiazonic acid (CPA), B aflatoxins or CPA alone, or neither mycotoxin; Aspergillus parasiticus produces B and G aflatoxins or the aflatoxin precursor O‐methylsterigmatocystin, but not CPA. Only four of forty‐five attempted interspecific crosses between opposite mating types of A. flavus and A. parasiticus were fertile and produced viable ascospores. Single ascospore strains from each cross were shown to be recombinant hybrids using multilocus genotyping and array comparative genome hybridization. Conidia of parents and their hybrid progeny were haploid and predominantly monokaryons and dikaryons based on flow cytometry. Multilocus phylogenetic inference showed that experimental hybrid progeny were grouped with naturally occurring A. flavus L strain and A. parasiticus. Higher total aflatoxin concentrations in some F1 progeny strains compared to midpoint parent aflatoxin levels indicate synergism in aflatoxin production; moreover, three progeny strains synthesized G aflatoxins that were not produced by the parents, and there was evidence of allopolyploidization in one strain. These results suggest that hybridization is an important diversifying force resulting in the genesis of novel toxin profiles in these agriculturally important fungi.     

Molecular characterization of toxigenic and atoxigenic Aspergillus flavus isolates,collected from peanut fields in China

Aims: The objectives of this study were to assess the genetic relationships between toxigenic and atoxigenic isolates of Aspergillus flavus collected from peanut fields in China, and to analyse deletions within the aflatoxin biosynthetic gene cluster for the atoxigenic isolates. Methods and Results: Analysis of random‐amplified polymorphic DNA and microsatellite‐primed PCR data showed that the toxigenic and atoxigenic isolates of A. flavus were not clustered based on their regions and their ability of aflatoxin and sclerotial production. These results were further supported by DNA sequence of ITS, pksA and omtA genes. PCR assays showed that 24 of 35 isolates containing no detectable aflatoxins had the entire aflatoxin gene cluster. Eleven atoxigenic isolates had five different deletion patterns in the cluster. Conclusions: Toxigenic and atoxigenic isolates of A. flavus are genetically similar, but some atoxigenic isolates having deletions within the aflatoxin gene cluster can be identified readily by PCR assays. Significance and Impact of the Study: Because the extensive deletions within the aflatoxin gene cluster are not rare in the atoxigenic isolates, analysis of deletion within the cluster would be an effective method for the rapid screening of atoxigenic isolates for developing biocontrol agents.     

Epigenetic variation reflects dynamic habitat conditions in a rare floodplain herb

Variation of DNA methylation is thought to play an important role for rapid adjustments of plant populations to dynamic environmental conditions, thus compensating for the relatively slow response time of genetic adaptations. However, genetic and epigenetic variation of wild plant populations has not yet been directly compared in fast changing environments. Here, we surveyed populations of Viola elatior from two adjacent habitat types along a successional gradient characterized by strong differences in light availability. Using amplified fragment length polymorphisms (AFLP) and methylation‐sensitive amplification polymorphisms (MSAP) analyses, we found relatively low levels of genetic (H′_gen = 0.19) and epigenetic (H′_epi = 0.23) diversity and high genetic (?_ST = 0.72) and epigenetic (?_ST = 0.51) population differentiation. Diversity and differentiation were significantly correlated, suggesting that epigenetic variation partly depends on the same driving forces as genetic variation. Correlation‐based genome scans detected comparable levels of genetic (17.0%) and epigenetic (14.2%) outlier markers associated with site specific light availability. However, as revealed by separate differentiation‐based genome scans for AFLP, only few genetic markers seemed to be actually under positive selection (0–4.5%). Moreover, principal coordinates analyses and Mantel tests showed that overall epigenetic variation was more closely related to habitat conditions, indicating that environmentally induced methylation changes may lead to convergence of populations experiencing similar habitat conditions and thus may play a major role for the transient and/or heritable adjustment to changing environments. Additionally, using a new MSAP‐scoring approach, we found that mainly the unmethylated (?_ST = 0.60) and CG‐methylated states (?_ST = 0.46) of epiloci contributed to population differentiation and putative habitat‐related adaptation, whereas CHG‐hemimethylated states (?_ST = 0.21) only played a marginal role.     

Reproduction and dispersal in an ant-associated root aphid community

Clonal organisms with occasional sex are important for our general understanding of the costs and benefits that maintain sexual reproduction. Cyclically parthenogenetic aphids are highly variable in their frequency of sexual reproduction. However, studies have mostly focused on free‐living aphids above ground, whereas dispersal constraints and dependence on ant‐tending may differentially affect the costs and benefits of sex in subterranean aphids. Here, we studied reproductive mode and dispersal in a community of root aphids that are obligately associated with the ant Lasius flavus. We assessed the genetic population structure of four species (Geoica utricularia, Tetraneura ulmi, Forda marginata and Forda formicaria) in a Dutch population and found that all species reproduce predominantly if not exclusively asexually, so that populations consist of multiple clonal lineages. We show that population viscosity is high and winged aphids rare, consistent with infrequent horizontal transmission between ant host colonies. The absence of the primary host shrub (Pistacia) may explain the absence of sex in three of the studied species, but elm trees (Ulmus) that are primary hosts of the fourth species (T. ulmi) occurred within a few km of the study population. We discuss the extent to which obligate ant‐tending and absence of primary hosts may have affected selection for permanent parthenogenesis, and we highlight the need for further study of these aphids in Southern Europe where primary hosts may occur close to L. flavus populations, so that all four root aphid species would have realistic opportunities for completing their sexual life cycle.     

Genetic rescue: a safe or risky bet?

Small and isolated populations face threats from genetic drift and inbreeding. To rescue populations from these threats, conservation biologists can augment gene flow into small populations to increase variation and reduce inbreeding depression. Spectacular success stories include greater prairie chickens in Illinois (Westermeier et al. 1998 ), adders in Sweden (Madsen et al. 1999 ) and panthers in Florida (Johnson et al. 2010 ). However, we also know that performing such crosses risks introducing genes that may be poorly adapted to local conditions or genetic backgrounds. A classic example of such ‘outbreeding depression’ occurred when different subspecies of ibex from Turkey and the Sinai were introduced to assist recovery of an ibex population in Czechoslovakia (Templeton 1986 ). Despite being fertile, the hybrids birthed calves too early, causing the whole population to disappear. In the face of uncertainty, conservation biologists have tended to respect genetic identity, shying away from routinely crossing populations. In this issue of Molecular Ecology, Frankham ( 2015 ) compiles empirical data from experimental studies to assess the costs and benefits of between‐population crosses (Fig.  1 ). Crosses screened to exclude those involving highly divergent populations or distinct habitats show large heterosis with few apparent risks of outbreeding depression. This leads Frankham to advocate for using assisted gene flow more widely. But do the studies analysed in this meta‐analysis adequately test for latent outcrossing depression?     

Sexual reproduction and recombination in the aflatoxin-producing fungus Aspergillus parasiticus

The fungal phylum Ascomycota comprises a large proportion of species with no known sexual stage, despite high genetic variability in field populations. One such asexual species, Aspergillus parasiticus, is a potent producer of carcinogenic and hepatotoxic aflatoxins, polyketide-derived secondary metabolites that contaminate a wide variety of agricultural crops. In this study, individuals of A. parasiticus from a population showing an evolutionary history of recombination were examined for sexual reproduction. Crosses between strains with opposite mating-type genes MAT1-1 and MAT1-2 resulted in the development of ascospore-bearing ascocarps embedded within stromata. Sexually compatible strains belonged to different vegetative compatibility groups. Recombination through the independent assortment of chromosomes 3 and 6 was detected using loci for mating type, aflatoxin gene cluster, and a protein-encoding gene. Our discovery of the sexual stage in A. parasiticus has important implications for current biological control strategies using nontoxigenic strains to reduce aflatoxin contamination in crops.     

Sex at the origin: an Asian population of the rice blast fungus Magnaporthe oryzae reproduces sexually

Sexual reproduction may be cryptic or facultative in fungi and therefore difficult to detect. Magnaporthe oryzae, which causes blast, the most damaging fungal disease of rice, is thought to originate from southeast Asia. It reproduces asexually in all rice‐growing regions. Sexual reproduction has been suspected in limited areas of southeast Asia, but has never been demonstrated in contemporary populations. We characterized several M. oryzae populations worldwide both biologically and genetically, to identify candidate populations for sexual reproduction. The sexual cycle of M. oryzae requires two strains of opposite mating types, at least one of which is female‐fertile, to come into contact. In one Chinese population, the two mating types were found to be present at similar frequencies and almost all strains were female‐fertile. Compatible strains from this population completed the sexual cycle in vitro and produced viable progenies. Genotypic richness and linkage disequilibrium data also supported the existence of sexual reproduction in this population. We resampled this population the following year, and the data obtained confirmed the presence of all the biological and genetic characteristics of sexual reproduction. In particular, a considerable genetic reshuffling of alleles was observed between the 2 years. Computer simulations confirmed that the observed genetic characteristics were unlikely to have arisen in the absence of recombination. We therefore concluded that a contemporary population of M. oryzae, pathogenic on rice, reproduces sexually in natura in southeast Asia. Our findings provide evidence for the loss of sexual reproduction by a fungal plant pathogen outside its centre of origin.