Morphological and genomic diversity in the genus Pteridium (Dennstaedtiaceae)

Morphometric comparisons of frond material grown under standardized garden conditions and DNA fingerprinting by arbitrarily-primed PCR were used to assess taxonomic groupings and relationships in the cosmopolitan bracken ferns of the genus Pteridium (Dennstaedtiaceae). The genus comprises a limited number of relatively stable, generally at least partially interfertile, morphotypes which were placed by R. M. Tryon (Rhodora 43: 1-31, 37-67, 1941) in a single species containing 12 varieties. Both morphometric analysis and DNA fingerprinting of 72 accessions including 11 of these varieties (excluded is var. feel from Central America) resolves groupings corresponding to vars africanum, arachnoideum, esculentum, latiusculum and revolutum from each other. DNA fingerprinting further (a) distinguishes an additional grouping of Atlantic Island (Azores, Madeira) and European brackens as an 'aquilinum complex' including var, aquilinum and a number of morphotypes recognised by C. N. Page (The ferns of Britain and Ireland 2nd edn. Cambridge: Cambridge University Press, 1997) and others at various taxonomic levels (near atlanticum, fulvum. pinetorum, osmundaceum); (b) confirms var. yarrabense as a tetraploid hybrid (4n = 208) of var. esculentum and var, revolutum; (c) establishes that at least those accessions of var, caudatum examined here are tetraploid hybrids involving var. arachnoideum as one progenitor; (d) indicates that the closest relatives of var. decompositum are var. latiusculum and var, revolutum: and (e) provides evidence of close genomic relationships between var. latiusculum, var. pseudocaudatum and var. pubescens in North America. These conclusions are consistent with the results of the morphometric analysis. The DNA evidence suggests that morphotypes in Pteridium are determined by specific qualitative and quantitative combinations of a limited number of highly conserved, additively assorted, genomic elements. A general model based on allopolyploidy followed by one or more rounds of autogamous allohomoploidy is proposed to account for the origin, maintenance and interrelationships of morphotypes in Pteridium. It is suggested that Tryon's varieties africanum, aquilinum: arachnoideum, decompositum, esculentum, latiusculum and revolutum might best be treated as species. pseudocaudatum and pubescens as varieties within latiusculum; yarrabense and caudatum (at least in part) as hybrids. (C) 2000 Annals of Botany Company.