Recombinant haplotypes reflect sexual reproduction in symbiotic arbuscular mycorrhizal fungi

Arbuscular mycorrhizal fungi (AMF) are part of the most widespread fungal-plant symbiosis. They colonize at least 80% of plant species, promote plant growth and plant diversity. These fungi are multinucleated and display either one or two nucleus genotypes (monokaryon and dikaryon) determined by a putative mating-type locus. This taxon has been considered as an ancient asexual scandal because of the lack of observable sexual structures. Despite the identification of a putative mating-type (MAT-type) locus and the functional activation of genes related to mating when two isolates co-exist, it is still unknown if AMF display a sexual or a parasexual life cycle.To test if AMF genomes display signatures of a sexual life-cycle involving the putative MAT-locus, we used publicly available genome sequences to test if recombining nucleotype-specific haplotypes could be identified using short-read Illumina sequences. We identified nucleus genotype-specific haplotypes within dikaryons and compared them to orthologous gene sequences from related monokaryon isolates displaying similar putative MAT-types. We show that haplotypes within a dikaryon isolate are more similar to homologue sequences of isolates having the same MAT-type than among them. We demonstrate that these genotype-specific haplotypes are recombinant, and are not consistently most similar to the monokaryon isolate sharing the same mating-type allele.These results are consistent with a sexual origin of the dikaryon rather than a parasexual origin and provides an important step to understand the life cycle of these globally important symbiotic fungi.

Two indigenous Berberis species from Spain were confirmed as alternate hosts of the yellow rust fungus Puccinia striiformis f.sp. tritici

Puccinia striiformis f.sp. tritici (Pst), which causes yellow (or stripe) rust on wheat, is a macrocyclic and heteroecious fungus. In this study, we investigated whether Berberis vulgaris subsp. seroi and B. vulgaris subsp. australis, which are indigenous in Spain, may serve as alternate hosts for Pst. Wheat leaves bearing telia of an isolate of Pst were harvested and used to inoculate plants of both barberry subspecies. Pycnia were observed on the adaxial side of the leaves from 10 days after inoculation (dai). Following successful fertilisation, aecia were observed on the abaxial side of the leaves from 16 dai. At 27 dai, barberry leaves bearing aecia were detached and used to inoculate susceptible wheat seedlings of cultivar ‘Morocco’. Uredinia were observed on wheat seedlings from 12 days after aeciospore exposure. Eighty-three single lesions were recovered from individual wheat leaves, of which 43 were genotyped using 19 Pst simple sequence repeat markers (SSR). In total, 19 multilocus genotypes (MLGs) were identified among the 43 progeny isolates. The SSR genotyping confirmed that all 43 isolates were derived from the parental isolate. Seven heterozygous SSR markers showed segregation among the progenies, whereas none of the 12 homozygous markers resulted in segregation. These results demonstrated that B. vulgaris subspp. seroi and australis can serve as alternate hosts for the yellow rust fungus, which may result in novel virulence combinations that can have a detrimental impact on wheat production. Although Pst has not been detected on these barberry species in nature, this study highlights the importance of rust surveillance in barberry areas where suitable conditions for completion of the sexual life cycle may be present.

Chromosome loss rate in cells of different ploidy can be explained by spindle self-organization

SummaryThe mitotic spindle segregates chromosomes and minimizes chromosome loss for the specific number of chromosomes present in an organism. In Saccharomyces cerevisiae, for example, haploid and diploid cells are part of the sexual life cycle and have a thousand times lower rate of chromosome loss than tetraploid cells. Currently it is unclear what constrains the number of chromosomes that can be segregated with high fidelity in an organism. Here we developed a mathematical model to study if different rates of chromosome loss in cells with different ploidy can arise from changes in (1) spindle dynamics and (2) a maximum duration of mitotic arrest, after which cells enter anaphase. Our model reveals how small increases in spindle assembly time can result in exponential differences in rate of chromosomes loss between cells of increasing ploidy and predicts the maximum duration of mitotic arrest.

Sexual conflict and the evolution of asexuality at low population densities

Theories for the evolution of sex rarely include facultatively sexual reproduction. Sexual harassment by males is an underappreciated factor: it should at first sight increase the relative advantage of asexual reproduction by increasing the cost of sex. However, if the same females can perform either sexual or asexual life cycles, then females trying to reproduce asexually may not escape harassment. If resisting male harassment is costly, it might be beneficial for a female to accept a mating and undertake a sexual life cycle rather than ‘insist’ on an asexual one. We investigate the effects of sexual harassment on the maintenance of sex under different population densities. Our model shows that resisting matings pays off at low population densities, which leads to the complete extinction of males, and thus to the evolution of completely asexual populations. Facultative sex persists in a narrow range of slightly higher densities. At high densities, selection favours giving up resisting male mating attempts and thus sexual reproduction takes over. These interactions between the outcomes of sexual conflict and population density suggest an explanation for the rarity of facultative sex and also patterns of geographical parthenogenesis, where marginal environments with potentially low densities are associated with asexuality.