Species and Evolution in Asexually Reproducing Independent Fern Gametophytes

Vegetative reproduction by the gametophyte generation has allowed a number of fern species to persist beyond their normal geographic range. In at least two of these, Trichomanes and Vittaria in the eastern United States, the sporophyte stage of the life cycle has been eliminated completely, perhaps for ten million years or more. Despite this long period of reliance on vegetative reproduction, genetic diversity in the Appalachian Vittaria gametophytes, as measured by starch gel enzyme electrophoresis, remains comparable to that of sexual plants, although it is largely partitioned into monomorphic populations. Widely separated populations are not particularly di- vergent; the taxon as a whole appears equally as cohesive as sexual species, suggesting that factors other than gene flow are responsible for long term maintenance of species integrity. The Appalachian Vittaria gametophytes and similarly derived taxa merit recognition as distinct species. Sexually reproducing pteridophytes alter- nate a diploid, spore-producing plant, the spo- rophyte, with a haploid, gamete-producing plant, the gametophyte. In most taxa, the ga- metophyte generation is short-lived and incon- spicuous relative to the sporophyte plants, which are generally perennial. Life spans of