MORPHOTYPE AND CONFLICTING TAXONOMIES IN PTERIDIUM ( DENNSTAEDTIACEAE : PTERIDOPHYTA )

Conflicting taxonomic treatments of the diploid bracken ferns of the Laurasian lineage within Pteridium are briefly reviewed. The infraspecific entities recognised in this section of the genus can be separated on morphological, phenological and physiological criteria into two main groups referred to respectively as ‘aquilinum’ and ‘latiusculum’ morphotypes. Evidence of gene flow between morphotypes is discussed, including the finding from DNA studies that taxa of the ‘aquilinum’ and ‘latiusculum’ morphotypes in each major geographic region are more closely related to the contrasting morphotype from the same region than to the corresponding morphotype from another region. It is proposed here that the ‘aquilinum’ and ‘latiusculum’ morphotypes may be regarded as mediated by a simple genetic polymorphism involving alternative pleiotropic alleles at a single major morphogenetic locus comparable to those previously described in Athyrium and postulated in Asplenium and Pteridium, but other possible models are discussed. It is concluded that P. aquilinum is best treated as a single species containing at present 11 subspecies. INTRODUCTION A vigorous ongoing polemic continues to cloud taxonomic treatment of the diploid (2n = 104; Wolf et al., 1987,1988) bracken ferns [Pteridium aquilinum (L.) Kuhn] comprising a Laurasian lineage within the genus. Thomson and colleagues (Thomson, 2004; Thomson et al., 2005; Thomson et al., 2008; contra Thomson, 2000) regard P. aquilinum as a grouping of 11 subspecies (Table 1) that in general correspond to varieties within Tryon’s (1941) P. aquilinum subsp. aquilinum. In contrast, certain other contemporary classifications are based on separation of these taxa into two phenotypic groups designated here as the ‘aquilinum’ and ‘latiusculum’ morphotypes (Table 1). Page (1997) and coauthors (Page & Mill, 1995; Gureyeva & Page 2005) regard these ‘aquilinum’ and ‘latiusculum’ morphotypic groups as multi-species complexes represented in Europe by P. aquilinum and by P. pinetorum C.N. Page & R.R. Mill respectively, each with several subspecies. Fraser-Jenkins (1997), on the other hand, recognises P. latiusculum (Desv.) Hieron. ex R.E. Fr. at species level and accords subspecific rank to P. latiusculum subsp. pinetorum (C.N. Page & R.R. Mill) Fraser-Jenk. Stace (1997) and Karlsson (2000) group the European ‘aquilinum’ phenotypes as P. aquilinum subsp. aquilinum and those of ‘latiusculum’ as P. aquilinum subsp. latiusculum (Desv.) Hultén. Shorina & Perestoronina (2000) distinguish the European ‘latiusculum’ morphotype as P. aquilinum var. pinetorum (C.N. Page & R.R. FERN GAZ. 18(3): 101-109. 2008 101 102 FERN GAZ. 18(3): 101-109. 2008 1 Thomson, 2004 2 Also referred to as P. aquilinum subsp. latiusculum (Desv.) Hultén; P. aquilinum var. pinetorum (C.N. Page & R.R. Mill) Perestor. ; P. pinetorum C.N. Page & R.R. Mill; P. latiusculum subsp. pinetorum (C.N. Page & R.R. Mill) Fraser-Jenk. 3 Also referred to as P. revolutum (Blume) Nakai 4 Thomson et al., 2005 5 Thomson et al., 2008 Geographic region/ morphotype P. aquilinum subspecies P. aquilinum variety