Abstracts (first author)

Poster 

A completely unknown lifecycle in mushrooms: cyclical inbreeding and haplo-diploidy

Author(s): Aanen DK

Summary:

Mycena galericulata (Basidiomycota, Agaricales) occurs in two forms, a clampless with two-spored basidia and a clamped with four-spored basidia. It is generally accepted that the two-spored form is haploid asexual (apomictic), and the four-spored form sexual (dikaryotic and heterothallic). In order to study the interrelationship between both forms, we performed mating tests and phylogenetic and genetic analyses of a sample of both forms. Surprisingly, our results are inconsistent with any currently known life-cycle. While the four-spored form is heterothallic indeed, we show that the two-spored form is diploid, and produces diploid spores via intra-tetrad selfing. However, the absence of genetic differentiation between both forms, and the high degree of heterozygosity in the two-spored form, indicate that the two-spored form frequently arises from the four-spored. We hypothesise that the two-spored form can again give rise to four-spored forms. Consistent with this, we discovered that a small percentage of fruiting bodies has both two-spored and four-spored basidia.


Talk 

Allorecognition stabilizes multicellularity

Author(s): Aanen DK, Czaran T, Hoekstra RF

Summary:

The cells of a multicellular individual face the social dilemma of potentially increasing their personal fitness by increased reproduction at the cost of fitness of the multicellular individual. Organisms capable of somatic fusion are most sensitive to this somatic parasitism, since parasitic mutant cells can infect other individuals. Allorecognition, found in many multicellular organisms, limits the spread of somatic parasites. However, previous models have not satisfactorily demonstrated that this long-term benefit is sufficient to offset immediate disadvantages of reduced fusion experienced by new, initially rare, allorecognition types. Using a cellular automaton approach, we model the joint evolution of allorecognition and somatic parasitism in a multicellular organism resembling an asexual ascomycete fungus. Individuals can fuse with neighboring individuals, but only if they have the same allotype. Fusion with a parasite decreases the total reproductive output of the individual, but the parasite compensates for this individual fitness reduction by a disproportional share of the offspring. Our study shows that the mere threat of parasitism can select for high allorecognition diversity, which on its turn provides efficient protection against invasion of somatic parasites. Moderate population viscosity combined with weak global dispersal provided the best conditions for the joint evolution of allorecognition and stable multicellularity.


Video


Abstracts (coauthor)

Culture-independent characterisation of the core gut microbiome of fungus-growing termites

Author(s): Otani, S, Mikaelyan A, Nobre T, Hansen LH, Sørensen SJ, Aanen DK, Boomsma JJ, Brune A, Poulsen M

Summary:

Fungus-growing termites (subfamily Macrotermitinae, family Termitidae) live in an obligate mutualistic symbiosis with the fungus Termitomyces. All other termites rely on gut microbes for the breakdown of plant material and other forage, and it has been generally assumed that the association with Termitomyces has reduced the need for fermentative gut microbes after the Macrotermitinae became fungus-farmers. Only few studies have explored this in any detail and the identities, levels of interaction-specificity with the termite host, and consistency in bacterial communities between host species have remained largely unknown. Here, we employ bacterial 16S rRNA 454 high-throughput pyrosequencing to identify a potential core microbiome in the fungus-growing termites - i.e., a distinct set of bacteria present across lineages in the termite phylogeny. Comparative analysis of 9 fungus-growing termite species from 5 genera suggests that a core gut microbiome indeed exists, as all bacterial taxa of high abundance were present in all termite species examined. However, quantitative differences in microbiome composition between termite species and genera were also noticed, possibly associated with differences in substrate use and Termitomyces lineage reared. Our results are consistent with major changes in gut microbiomes having occurred when fungus farming evolved 30 MYA, followed by relatively modest elaborations in response to ecological conditions. This might help explain why neither the termites nor Termitomyces ever abandoned the symbiosis or teamed-up with another termite of fungal partner lineage.

Summary:

Multicellularity is a highly cooperative state prone to invasion by cheating genotypes that use the resources provided by the multicellular organism without contributing their fair share to non-reproductive functions of the organism (e.g. cancers). Kin selection, often realised through regular single -celled bottlenecks (and in some organisms by an early germline separation), is a solution to prevent selection for cheating. In fungi, the lack of an early germline separation and the potential to fuse with other individuals make cheating a realistic threat. However a genetic allorecognition mechanism that limits fusion to almost only clonally related individuals, seems to effectively protect fungi against cheating genotypes. In order to test the hypothesis that cheating is a realistic threat to multicellular growth in fungi, we used an experimental evolution approach with Neurospora crassa, that maximised the potential for cheating genotypes by selecting under low relatedness and completely local competition (i.e. under a high inoculation density of spores, in the absence of genetic allorecognition). Within less than 300 generations all eight replicate lines contained genotypes that matched our criteria for cheating: they had increased relative fitness (measured as proportion of spores produced) when in competition with a cooperative ancestral type, but spore production in monoculture was significantly decreased. So there is a clear trade-off between competitive fitness and production of asexual spores when grown alone. Contrary to predictions about the evolution of social behaviour that cheating genotypes will completely eradicate the social behaviour (the tragedy of the commons), we found a stable polymorphism in all evolved lines: a relatively cooperative type producing many spores when grown in monoculture, and the cheating type described above. We are currently studying the conditions leading to this apparently balanced polymorphism in our evolving lines.

Summary:

Multicellularity is a highly cooperative state prone to invasion by cheating genotypes that use the resources provided by the multicellular organism without contributing their fair share to non-reproductive functions of the organism (e.g. cancers). Kin selection, often realised through regular single-celled bottlenecks (and in some organisms by an early germline separation), is a solution to protect against cheating. In fungi, the lack of an early germline separation and the potential to fuse with other individuals make cheating a realistic threat. However a genetic allorecognition mechanism that limits fusion to almost only clonally related individuals, seems to effectively protect fungi against cheating genotypes. We have shown earlier that in the absence of cheating genotypes, the fitness advantage of fusion selects against allotype diversity in the fungus Neurospora crassa. Individuals with more common allotypes have a higher fitness because they fuse more frequently and gain a larger average size. Studies that model evolution of allorecognition in fungi show that cheating can cause a stable polymorphism for genetic allorecognition loci. Using cheating genotypes generated during an experimental evolution experiment, we empirically test the hypothesis that genetic allorecognition in fungi can be stabilized by the presence of cheating.

Contacts

Chairman: Octávio S. Paulo
Tel: 00 351 217500614 direct
Tel: 00 351 217500000 ext22359
Fax: 00 351 217500028
email: mail@eseb2013.com

Address

XIV Congress of the European Society for Evolutionary Biology

Organization Team
Department of Animal Biology (DBA)
Faculty of Sciences of the University of Lisbon
P-1749-016 Lisbon
Portugal

Website

Computational Biology & Population Genomics Group 
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