Allozymes were used to study the spatial attributes of clones (genets) comprising a population of Pteridium aquilinum (L.) Kuhn var. latiusculum (Desv.) Underw. ex Heller (bracken fern) in the Appalachian Mountains of Virginia. Ramets (individual leaves) were sampled at intervals of 165 m (or less in some cases) and genotyped for six polymorphic isozyme loci to produce a map depicting the spatial patterning of genets. Forty-five distinct genotypes were detected, 14 of which were sampled more than once, five of these more than four times. Genotype proportions at all loci except Pgm-1 conformed to Hardy-Weinberg expectations. Estimation of allele frequencies in the population used a "round-robin" approach that removed any upward bias for rare alleles that distinguish genets. Based on these allele frequencies, the probability that each genotype could arise independently and be sampled was calculated. Some genotypes represented by widely separated ramets had very low probabilities of re-encounter, documenting fragmentation of widespread genets. Coarse-scale mapping indicated a population consisting of many small genets and a few very large ones (up to 1,015 m across). The larger genets tended to be irregular in shape, fragmented, and overlapping. Fine scale mapping of individual fronds in spatially discrete patches of ramets revealed extensive intergrowth of genets, indicating that P. aquilinum exhibits a "guerrilla-type" clonal morphology.

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A large proportion of plant species has originated through allopolyploidy: interspecific hybridization followed by chromosome doubling. Heterozygosity remains fixed in allopolyploids because of nonsegregation of parental chromosomes. Two allotetraploid species of the fern genus Asplenium show allozyme polymorphisms at loci that are polymorphic in their diploid progenitors, indicating that each has originated more than once and implicating continued gene flow from diploids to tetraploids.

Hybrids and hybrid species are common among ferns, and they account for many of the problems in species definition in the group. Most systematic inquiry into the evolutionary process in ferns has addressed hybrid species, because meaningful explanations of their origins are feasible (Manton, 1950). As a result, complexes of hybrids, hybrid species, and their progenitor species have been popular subjects for experimental work. Here, we address the definition and changing perception of these hybrid species in the light of improvements in the data available to systematists. Once we have established basic definitions, we demonstrate the utility of recent advances in defining hybrid species of ferns. With this orientation, we investigate the status of hybrid species in the context of reigning species concepts. Renewed reproductive interaction between populations or species following a period of isolation characterizes all hybrids; hence hybrids are often spoken of as the products of secondary contact. Hybrids are unique in that they arise when isolating mechanisms fail; thus they are evolutionarily a consequence of the disruption of the divergence process that leads to ordinary (primary) species. Consequently, the hybrid is at once a novelty and a rehash: it is a novel combination of genetic and morphological features already present in its progenitors. These features need not be intermediate: see Grant (1975) on transgressive segregation and Barrington, 1986a. Fern hybrids are predominantly sterile (Knobloch, 1976), though there is a small, disparate set of variously fertile hybrids (in Pteris, Walker, 1958; in Dryopteris, Whittier & Wagner, 1961; in the Cyatheaceae, Conant & Cooper-Driver, 1980). The origin and evolutionary significance of sterile hybrids have been the subject of most studies relevant to a discussion of species concepts in pteridophytes (Lovis, 1977). Many fern species are thought to be derived from hybrids. Traditionally, these taxa are argued to be 1) species because they breed true and they are autonomous and 2) hybrid because comparison with allied taxa yields evidence of intermediacy. Both allopolyploid and allohomoploid species have been reported in the ferns. The allopolyploid species is an old concept with much experimental support (for a historical synopsis see Manton, 1950). New allopolyploid species ordinarily arise from sterile hybrids via doubling of chromosome sets and consequent restoration of fertility. The typical angiosperm

As has been shown for many kinds of organisms, barriers to interspecific hybridization may differ in strength between reciprocal crosses, resulting in a bias in the probability that one or the other species may be the maternal or paternal parent of hybrids. The fern Dryopteris Xtriploidea, the "backcross" hybrid between the diploid D. intermedia and the tetraploid D. carthusiana, occurs in large numbers in nature, providing an opportunity to investigate whether such a bias exists. Differences in the chloroplast genome distinguishing the two parental species were discovered in the sequence of the trnL region following amplification by polymerase chain reaction (PCR), including a Mse I restriction site. This allowed rapid identification of the donor of the chloroplast genome, and therefore the maternal parent of each hybrid, assuming chloroplast DNA to be maternally inherited in Dryopteris. Analysis was carried out on 127 hybrids, shown to be of independent origin using allozymes, occurring at three localities in Virginia and West Virginia. When samples from all localities were pooled, 91 possessed the D. carthusiana trnL genotype and 36 possessed the D. intermedia genotype, a ratio that is significantly different (P < 0.001) from the null hypothesis of no gender bias. The strength of the bias differed significantly among the three sites, however, with bias at the West Virginia site much stronger (5.6:1 carthusiana:intermedia; P < 0.001) than at either Virginia site (1.55:1 and 1.43:1 carthusiana:intermedia, respectively; P > 0.05 in both cases). The cause of the strong bias in the West Virginia sample is unknown, as is the cause of the population differences. Causes of bias could include differences between the parental species related to their ploidy difference, including sizes of gametes and/or gametangia, sperm motility, breeding system (D. intermedia is outcrossing while D. carthusiana is selfing), or the nature and strength of interspecific isolating mechanisms.

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