Two allelic genes responsible for vegetative incompatibility in the fungus Podospora anserina are not essential for cell viability

Summary Vegetative incompatibility is a lethal reaction that destroys the heterokaryotic cells formed by the fusion of hyphae of non-isogenic strains in many fungi. That incompatibility is genetically determined is well known but the function of the genes triggering this rapid cell death is not. The two allelic incompatibility genes, s and S, of the fungus Podospora anserina were characterized. Both encode 30 kDa polypeptides, which differ by 14 amino acids between the two genes. These two proteins are responsible for the incompatibility reaction that results when cells containing s and S genes fuse. Inactivation of the s or S gene by disruption suppresses incompatibility but does not affect the growth or the sexual cycle of the mutant strains. This suggests that these incompatibility genes have no essential function in the life cycle of the fungus.     

EVOLUTION OF VEGETATIVE INCOMPATIBILITY IN FILAMENTOUS ASCOMYCETES. I. DETERMINISTIC MODELS

In ascomycetes vegetative incompatibility can prevent the somatic exchange of genetic material between conspecifics. It must occur frequently in natural populations, since in all species studied many vegetative compatibility groups (VCGs) are found. Using a population-genetic approach, this paper explores two possible selective explanations for the evolution of vegetative incompatibility in asexual fungi: selection by a nuclear parasitic gene, and selection by a harmful cytoplasmic element. In a deterministic model, assuming a random spatial distribution of VCGs in an infinitely sized population, it is found that neither of these forms of frequency-dependent selection can explain the large number of VCGs found in nature. The selective pressure for more VCGs disappears once a limited number of VCGs exist, because the frequency of compatible interactions decreases when the number of VCGs increases. In comparing the two selective explanations, selection by a cytoplasmic element seems a more plausible explanation than selection by a nuclear gene.     

Medial prefrontal cortex dissociation between self and others in a referential task: An fMRI study based on word traits

A number of recent neuroimaging studies using self referential tasks have investigated whether self referential processing depends on a unique neural basis that operates specifically in the medial prefrontal cortex. However, these studies have provided contradictory results despite the use of similar methodologies. We hypothesized that these discrepancies are partially related to the task-difficulty that presents dissociations reaction times in the self- and other-referential tasks. We therefore measured brain activity during self and other referential tasks to determine if such activity can be dissociated according to the reaction times (fast versus slow) for the trait words. Activation differed across self and other only in the slow word condition. The self referential condition with slow reaction time produced greater activation in the ventromedial prefrontal cortex, whereas the other referential condition with slow reaction time produced activation of the middle temporal gyrus. Results suggested that the task-difficulty might affect whether or not brain activities within MPFC would be dissociated between self- and other-referential processing.     

Cytoplasmic incompatibilities in the mosquito Culex pipiens: How to explain a cytotype polymorphism?*

Although cytoplasmic incompatibilities have been used as a means of eradicating the mosquito Culex pipiens, the population dynamics of these sterilities in relation to the coexistence of multiple incompatible cytotypes in a single area has not been investigated, except in the case of two unidirectionally incompatible cytotypes. An analytical model of the evolution of n cytotypes in an infinite panmictic population has been developed in order to investigate polymorphic equilibrium. A necessary criterion for the stability of such an equilibrium is established; it is shown that a stable polymorphism cannot exist between incompatible cytotypes. This result is discussed in the light of population dynamics and genetics of Culex pipiens, and of our present knowledge on incompatibilities. The consequences of a geographic structuring and of homogamy are considered. A careful reconsideration of previous experimental results disclosed probable nuclear effects and a serious experimental weakness: with the common procedure of backcrossing hybrid females to males of constant genotype it is not possible to rule out probable nuclear effects with paternal expression. It is concluded that incompatibilities in Culex pipiens may have a nuclear-cytoplasmic determinism.     

WOLBACHIA INFECTION IN DROSOPHILA SIMULANS: DOES THE FEMALE HOST BEAR A PHYSIOLOGICAL COST?

Fitness traits of three Drosophila simulans strains infected by endocellular bacteria belonging to the genus Wolbachia have been compared with those of replicate stocks previously cured from the infection by an antibiotic treatment. The traits measured were development time, egg-to-adult viability, egg hatch, productivity, fecundity, and the number of functional ovarioles. Individuals of the first strain were bi-infected by two Wolbachia variants, wHa and wNo. The second strain was infected by wHa, the third one by wNo. The Wolbachia studied here cause cytoplasmic incompatibility (CI), a high embryonic mortality (70% to > 90%) when an infected male is crossed with an uninfected female. Three generations after antibiotic treatment, we observed in all strains a significant drop in productivity in the cured stocks. This drop was not due to antibiotic toxicity and was associated with the loss of the Wolbachia. However the effect had disappeared in two of the three strains five generations after treatment, and could not be found in the third strain in a third measurement carried out 14 generations after treatment. The temporary nature of the productivity difference indicates that Wolbachia do not enhance productivity in infected strains. On the other hand, in all traits measured, our results show the absence of any negative effects of the Wolbachia on their host. This could be explained when considering Wolbachia evolution, as maternally transmitted parasites bear a strong selective pressure not to harm their female host. However, CI would allow the bacteria to be maintained even when harming the female. The apparent absence of deleterious effects caused by these Wolbachia might result from a trade-off, where a relatively low bacteria density would advantage the Wolbachia by suppressing any deleterious effects on the female host, at the cost of a weaker maternal transmission rate of the infection.     

Development of microsatellite markers in a clonal perennial herb,Convallaria keiskei

We developed eight polymorphic microsatellite simple sequence repeat (SSR) loci from genomic DNA of a clonal perennial herb, Convallaria keiskei, using a dual‐suppression‐polymerase chain reaction (PCR) technique and an improved technique. These markers with four to 10 alleles per locus identified 29 genotypes in 82 samples collected from a population in Hokkaido, Japan. The observed and expected heterozygosities ranged from 0.241 to 0.862 and from 0.639 to 0.825, respectively. These SSR markers will be available to identify genets and evaluate genetic diversity of C. keiskei.     

Introgression lines of Solanum sitiens,a wild nightshade of the Atacama Desert,in the genome of cultivated tomato

The wild tomato relative Solanum sitiens is a xerophyte endemic to the Atacama Desert of Chile and a potential source of genes for tolerance to drought, salinity and low‐temperature stresses. However, until recently, strong breeding barriers prevented its hybridization and introgression with cultivated tomato, Solanum lycopersicum L. We overcame these barriers using embryo rescue, bridging lines and allopolyploid hybrids, and synthesized a library of introgression lines (ILs) that captures the genome of S. sitiens in the background of cultivated tomato. The IL library consists of 56 overlapping introgressions that together represent about 93% of the S. sitiens genome: 65% in homozygous and 28% in heterozygous (segregating) ILs. The breakpoints of each segment and the gaps in genome coverage were mapped by single nucleotide polymorphism (SNP) genotyping using the SolCAP SNP array. Marker‐assisted selection was used to backcross selected introgressions into tomato, to recover a uniform genetic background, to isolate recombinant sub‐lines with shorter introgressions and to select homozygous genotypes. Each IL contains a single S. sitiens chromosome segment, defined by markers, in the genetic background of cv. NC 84173, a fresh market inbred line. Large differences were observed between the lines for both qualitative and quantitative morphological traits, suggesting that the ILs contain highly divergent allelic variation. Several loci contributing to unilateral incompatibility or hybrid necrosis were mapped with the lines. This IL population will facilitate studies of the S. sitiens genome and expands the range of genetic variation available for tomato breeding and research.     

Variation in Wolbachia cidB gene,but not cidA,is associated with cytoplasmic incompatibility mod phenotype diversity in Culex pipiens

Endosymbiotic Wolbachia bacteria are, to date, considered the most widespread symbionts in arthropods and are the cornerstone of major biological control strategies. Such a high prevalence is based on the ability of Wolbachia to manipulate their hosts' reproduction. One manipulation called cytoplasmic incompatibility (CI) is based on the death of the embryos generated by crosses between infected males and uninfected females or between individuals infected with incompatible Wolbachia strains. CI can be seen as a modification‐rescue system (or mod‐resc) in which paternal Wolbachia produce mod factors, inducing embryonic defects, unless the maternal Wolbachia produce compatible resc factors. Transgenic experiments in Drosophila melanogaster and Saccharomyces cerevisiae converged towards a model where the cidB Wolbachia gene is involved in the mod function while cidA is involved in the resc function. However, as cidA expression in Drosophila males was required to observe CI, it has been proposed that cidA could be involved in both resc and mod functions. A recent correlative study in natural Culex pipiens mosquito populations has revealed an association between specific cidA and cidB variations and changes in mod phenotype, also suggesting a role for both these genes in mod diversity. Here, by studying cidA and cidB genomic repertoires of individuals from newly sampled natural C. pipiens populations harbouring wPipIV strains from North Italy, we reinforce the link between cidB variation and mod phenotype variation fostering the involvement of cidB in the mod phenotype diversity. However, no association between any cidA variants or combination of cidA variants and mod phenotype variation was observed. Taken together our results in natural C. pipiens populations do not support the involvement of cidA in mod phenotype variation.