To sex, or not to sex ... Structures and strategies of reproduction in the family Umbilicariaceae (Lecanorales, Ascomycetes)

This is a study of the evolution and ecology of reproduction in the family of lichen-forming fungi U mbilicariaceae comprising the two genera Umbilicaria and Lasallia. Members of this family exhibit a variety of reproductive structures and strategies. The fungal partner of the symbiosis may reproduce by sexually generated ascospores, asexual thalloconidia, or both. Propagules reproducing the intact symbiosis include asexual isidia, soredia, thallyls, phyllidia and schizidia. The algal partner - unicellular Pseudotrebouxia (Chlorophyceae) - reproduces by asexual autospores when in symbiosis, and in addition zoospores and aplanospores in pure culture.

In about half of the species in Umbilicariaceae the fungal partner can only reproduce sexually - it is exclusively teleomorphic. In these species, apparently, the option of asexual reproduction has not been presented by evolution. In the taxa where this option has been presented, the forces of ecology and evolution seem to favour this option. Although no tax.on with asexual reproduction appears to have dispensed with sexuality altogether, the allocation to sexual reproduction tends to decrease and the resources are translocated to asexual reproduction. Different taxa represent progressive stages in this de-evolution of sex. In some of these cases sex is correlated with geographical distribution patterns or ecological factors such as high humidity or high population density and sib-competition. The latter observation supports the ‘Tangled Bank’ hypothesis for the maintenance of sex, as well as ESS models for reproductive allocations in heterocarpic plants. In some cases exclusively teleomorphic tax.a co-exist with morphologically virtually identical but mainly asexual (anamorphic) taxa, suggesting that a difference in reproductive mode may have led to speciation.

Associated with different propagule types are a number of other traits with consequences for the ecological performance of the species. These sets of correlated traits constitute strategies of reproduction, the core sets of specific life-history strategies. Three ‘pure’ reproductive strategies are tentatively distinguished in the Umbilicariaceae, depending on whether the propagule is sexually or asexually generated and whether it is symbiotic or not. The asexual propagules are close-dispersed while sexual propagules are more far-dispersed. A comparative study of the performance of representatives of the three strategy types in two glacier forelands shows that the sexually reproducing species are the fastest colonizers. The need to re-establish symbiosis at target site appears to be no great obstacle to sexual reproduction by ascospores.