Chromosome study of the fern genus Cyrtomium (Dryopteridaceae)

Cyrtomium (Dryopteridaceae) is an Asiatic genus comprising about 40 species characterized by having anastomosing veins that form areolae with included veinlets. Twelve chromosome counts representing 11 taxa were made in the present study. The chromosome numbers of six species endemic to China are reported for the first time: C. uniseriale, 2n = 164; C. guizhouense, 2n = 82; C. shingianum, n = 82, 2n = c.164; C. chingianum, 2n = 164; C. urophyllum, 2n = 82 and C. aequibasis, 2n = 123. The results were used to show that (1) C. uniseriale should be reduced to varietal status under C. balansae; (2) subseries Balansana is not closely related to any other Cyrtomium species and should be separated from Cyrtomium; and (3) C. guizhouense and C. lonchitoides are basal groups when subseries Balansana is not considered. These results were also supported by morphology. A high level of differentiation is a distinct characteristic in Cyrtomium. The genus has two reproductive types: sexual and apogamous. Sexual species are distributed mostly in China. Southwestern China is a modern diversity centre of Cyrtomium, with 80.6% of known species occurring there. Many species are endemic, most of which are sexual tetraploids or apogamous triploids. Both the apogamous reproductive type and polyploid species are adaptions to limestone habitats. (c) 2006 The Linnean Society of London.