Xeric environments impose significant constraints on plant life histories. In ferns, there is a strong correlation between the apomictic life cycle and drought-prone habitats. The life cycle of apomictic ferns involves modifications of both the sporophytic and gametophytic generations. Sporophytes have an altered sporogenetic process in which a premeiotic endomitosis results in chromosome duplication without cytokinesis. As such, spore mother cells have a doubled chromosome number, and meiosis produces spores with the same genetic composition as the original sporophyte. Gametophytes produced by these spores initiate sporophytes directly, without fertilization. The evolutionary origin of apomicts seems to be tied to forces imposed by stressful environments. Natural triploids are found at the geographical and/or physiological margins of diploid populations. Isozyme studies demonstrate that these triploids result from outcrossing between unreduced diploid gametes and “normal” haploid gametes. Such triploids are highly heterozygous, vigorous individuals, but cannot reproduce and disperse via spores. In lineages whose gametophytes seldom experience xeric conditions, these triploids occur as isolated sporophytes. However, when gametophytes regularly encounter stressful conditions, induction of sporophytes without fertilization may occur. In vitro studies show that intense insolation or high sucrose concentrations can cause gametophytes to spontaneously generate sporophytes. This process, coupled with consistent formation of unreduced gametes, has allowed ferns to proliferate in some of the driest habitats on earth. Taxon diversity is further enhanced by hybridization between apomicts and related diploid species, and this syndrome of life history features may drive adaptive radiation of desert ferns.

Key words: apomixis, life cycle, pteridophytes, xeric environments