The first demonstration of apparent allohomoploid speciation was in Pteris (Walker, 1958, 1962). By analyzing chromosome behavior and morphology, Walker showed that diploid species may be ecologically isolated, but not always reproductively isolated. Under some circumstances, hybrid swarms developed and apomictic species were frequent. Unfortunately, Walker's perceptive and seminal hypotheses have not been tested by the application of molecular methods such as isozymes or DNA. Nonetheless, Walker's work demonstrates the great evolutionary complexity that can be revealed through biosystematic studies of ferns, and current generations should consider testing his hypotheses.
More recent studies of a second instance of allohomoploid hybridization involve species of Polystichum in western North America. Using chloroplast DNA and isozyme characters, Mullenniex et al. (1998) demonstrated that when two morphologically distinct species of Polystichum (P. imbricans and P. munitum) co-occur, hybrid swarms can form. Typically, these swarms occur in partially shaded ecotonal habitats, and although there is not yet evidence of any stabilization of these interspecific but fully fertile hybrids, this could be the first stage of a process that may result in a new, ecologically distinct lineage.
The most extensively documented case of allohomoploidy involves tree fern species on Caribbean islands (Conant and Cooper-Driver, 1980). Tree ferns are distinctive among the ferns by lacking polyploidy and by containing numerous instances of interspecific hybrids that are fertile (Conant, 1990). In the allohomo-ploid model of speciation, the process is initiated through primary speciation that isolates species rapidly based on different ecological specificities. Isozyme studies have demonstrated that although these species are morphologically distinct, they have very high congeneric genetic identities (Barrington and Conant, 1989) and they are interfertile. Without disturbance, they might remain ecologically isolated from each other and eventually accumulate more genetic differentiation. Conant and Cooper-Driver (1980) used a combination of morphological and biochemical data to demonstrate, however, that the barriers to species isolation had broken down and fertile hybrids had formed. These hybrids were sufficiently morphologically distinct from their progenitors that they had been accorded species status and, at least in some circumstances, were ecologically isolated from their progenitors. Given that this is an island system, the most provocative component of this analysis is the possibility that the hybrid-derived species could migrate to a neighboring island that could contain only one or neither of the parental species.
In several groups of lycophytes, allohomoploid complexes have been reported. As summarized by Wagner (1992), there are apparently stabilized hybrids with normal meiosis and producing abundant, apparently viable spores in the genera Diphasiastrum, Lycopodiella, and Lycopodium. The Diphasiastrum complex includes three species that all cross with a fourth species, resulting in three fertile hybrids. Backcrosses have not been discovered and the stabilization of large clusters of hybrid shoots has been attributed to the considerable cloning ability of the species. Hybrids with the same chromosome complement as the parents, and producing apparently viable spores, have also been reported between two pairs of species in Lycopodiella and between two tropical species of Lycopodium (0llgaard, 1987). Thus, even among species that form subterranean gameto-phytes, hybridization occurs and can result in fertile progeny. Given the demonstration that random mating occurs in some populations of Lycopodiaceae species (Soltis and Soltis, 1988), discoveries of frequent hybridization should not be surprising. However, the indication that these hybrids have normal meioses and produce apparently viable spores suggests that post-zygotic, genetic barriers between species are lacking and that speciation via ecological specialization may be occurring.
Allohomoploidy, therefore, is a mechanism that emphasizes the role of ecology in promoting and maintaining diversity despite only minor genetic modification. If this mechanism is prominent, accurate characterization of species and speciation in some regions will be especially challenging. At the same time, demonstration that such a mechanism is operating provides important clues to the special complexity of some systems. In developing strategies for exploring species and speciation, the possibility of allohomoploid models must be entertained.
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