THE EVOLUTIONARY SIGNIFICANCE OF ASEXUAL REPRODUCTION IN MOSSES

Abstract

  The establishment of mosses in the field and the effects of mature plants on germination have received attention recently. Experimental data and theoretical expectations suggest that sexual and asexual reproduction may play different roles in moss ecology. Spores require fertilization (and thus abundant free water), disperse farther, and germinate best in previously uncolonized substrates; vegetative diaspores can be produced under more stressful conditions, disperse locally, and germinate relatively better than spores in contact with existing colonies. These ecological differences may be of greater importance than any difference between these two types of diaspores in amount of genetic diversity. The evolutionary significance of asexual reproduction has been downplayed relative to sexual reproduction, which we argue is unwarranted. Given the apical cell mode of growth, somatic mutation in asexual lineages can provide levels of genetic variation equivalent to those of purely sexual lineages. Many different types of asexual diaspores have evolved in mosses, usually via heterochrony (neoteny). The terminology for these is confused; we present and discuss a classification of asexual diaspore types based primarily on ontogeny and iterative homology. To study the possible adaptive roles of asexual reproduction and the phylogenetic homologies of asexual diaspore types, it will be necessary to examine specific cases within a phylogenetic framework .