Reproductive strategies in lichens

Lichens employ two divergent strategies of reproduction, a sexual method through spore propagation and a derivative asexual strategy based on systems perpetuating the symbiosis by the dissemination of specialized diaspores. The distribution of these pathways in 24 families in the order Lecanorales is presented. Fruticose and foliose life forms have a greater representation of vegetative diaspores than crustose forms. It is hypothesized that chemical evolution occurred prior to the development of secondary asexual reproductive mechanisms. The derived vegetative morphologies have apparently outlasted their sexual ancestors, although a number of closely related "species pairs" are known, indicating the joint survival of ancestral sexual lines with secondary asexual offspring. Species using a vegetative strategy often have a greater world distribution than their sexual counterparts in foliose and fruticose groups. No clear pattern is apparent in crustose groups. The advantages of vegetative strategies are a greater survival for propagules and rapid invasion of new habitats. The separation of these strategies and the evolution of soredia and isidia represents a gradient in the evolution of taxa.