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A cytotaxonomic study of the Polypodium vulgare complex in northwestern North American Lang, Frank Alexander

Abstract

The circumboreal Polypodium vulgare complex consists of a series of closely related ferns of different ploidy levels. Two northwestern North American members of the complex, P. glycyrrhiza D. C. Eaton, and P. hesperium Maxon, have long been a source of taxonomic confusion since little has been known of their relationships within the complex in the Northern Hemisphere. The present cytotaxonomic investigation of these taxa has shown that they are composed of three cytotypes, two diploid and one tetraploid. P. glycyrrhiza has proved to be uniformly diploid (n = 37) and morphologically, ecologically and geographically distinct from P. hesperium. Investigation of P. hesperium has shown that this taxon, as usually treated by North American taxonomists, is composed of at least two distinct entities, one tetraploid (n = 74), the other diploid (n = 37). These two cytotypes are morphologically and ecologically as well as cytologically separable, and have independent geographical distributions. The type specimen of P. hesperium is morphologically comparable to tetraploid populations from the interior of British Columbia, and specimens from the type locality have proved to be tetraploid. It is recommended that P. hesperium be reserved for the tetraploid cytotype. The epithet montense is tentatively proposed for the diploid cytotype. Two morphologically distinct triploid hybrids were found in areas of sympatric occurrence of the three cytotypes. Morphologically these hybrids appear to be P. hesperium x P. glycyrrhiza and P. hes-perium x P. montense. At meiosis both hybrids showed n II • n I, which is interpreted to mean that the montense genome and the glycyrrhiza genome are both present in P. hesperium. P. hesperium sensu stricto appears to be of alloploid rather than autoploid origin since it forms only bivalents at meiosis. P. hesperium is also intermediate in morphology and ecology between P. glycyrrhiza and P. montense. It is postulated that P. hesperium is an allotetraploid derived from a pre-Pleistocene hybridization between r. glycyrrhiza and P. montense or their immediate progenitors. The hypotnesis is also made that P. hesperium originated largely because of climatic changes in the interior of the continent imposed by pre-Pleistocene orogenic activity. Morphology, ecology and geographical distribution indicate three main lines of differentiation among the diploid cytotypes. These diploids eventually gave rise to polyploid derivatives, probably in the late Tertiary before the advent of Pleistocene glaciation. The intergradation and morphological variability of these taxa are attributed to alloploidy, hybridization and phenotypic plasticity. The morphology, biochemistry, ecology and geographical distribution of the three species is circumscribed and discussed and a pragmatic key provided.

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