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asexual (adjective) – sexless, remaining sterile, not generative; i. e., lacking or not expressing modes of sexual reproduction; therefore propagating only by vegetative (= somatic) reproduction. It is often assumed that asexual reproduction represents an evolutionary disadvantage, limited by the lack of sexual recombination and thus genetic diversity. In theory, all asexual reproduction is clonal, i. e., it results in the formation of genetically identical offspring (clones). In fungi, however, the parasexual cycle as well as lateral gene transfer both provide means to increase genetic diversity even among clones. In lichens it has generally been argued that asexual propagation might be more successful than sexual reproduction. Somatic lichen propagules, like isidia or soredia, include both myco- and photobiont. In contrast, sexual spores are typically ejected without photobiont cells. These spores can establish a new thallus only if they encounter suitable photobionts. The species-pair hypothesis suggests that chemically identically lichen thalli differing in their mode of reproduction exist as distinct populations, the asexual thalli frequently being more widely distributed than their sexual counterparts.

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