WINDHAM, MICHAEL D.1* and CHRISTOPHER H. HAUFLER2. 1Utah Museum of Natural History, University of Utah, Salt Lake City, UT 84112; 2Department of Ecology and Evolutionary Biology, University of Kansas, Lawrence, KS 66045. - Apomixis in ferns: A consequence of marginal habitats and outcrossing?
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