Gametophyte Development of Karst- and Danixa-Specific Species of Adiantum Series Gravesiana (Pteridaceae) from China

Abstract. The ferns of Adiantum Series Gravesiana (Pteridaceae) consist of species that occur exclusively on Danxia (A. juxtapositum) and Karst (A. mariesii, A. longzhouensis, A. dentatum, A. obovatum, and A. gravesii) landforms. We cultured representatives of these six species from 20 different populations on both agar and soil to investigate and compare their gametophyte development. All species had the Vittaria-type pattern of spore germination except A. obovatum, for which we missed the best time to observe this developmental phase. However, two different patterns of subsequent development were observed: the Adiantum-type (i.e., naked, cordate mature gametophyte thallus) in A. mariesii, A. longzhouensis, A. juxtapositum, and A. gravesii and the Kaulinia-type (i.e., naked, ribbon-shaped mature gametophyte thallus) in A. dentatum and A. obovatum. All populations from A. juxtapositum and A. gravesii possess the same Adiantum-type, suggesting that Karst and Danxia soils were not the main factor causing the differences in gametophyte development. The Kaulinia-type adult thallus has not been observed previously in Adiantum and may be an adaptive strategy for survival within the relatively harsh micro-habitats experienced by A. dentatum and A. obovatum compared to other members of this series.