Sterile-fertile leaf dimorphy and evolution of soral types in Polybotrya (Dryopteridaceae)

The neotropical fern genus Polybotrya has strongly dimorphic sterile and fertile leaves (trophophylls and sporophylls). Sterile-fertile leaf dimorphy involves an ensemble of characteristics and not merely a single feature of the leaf. The reduction of green tissue is the most conspicuous aspect of d... Full description

The neotropical fern genus Polybotrya has strongly dimorphic sterile and fertile leaves (trophophylls and sporophylls). Sterile-fertile leaf dimorphy involves an ensemble of characteristics and not merely a single feature of the leaf. The reduction of green tissue is the most conspicuous aspect of dimorphy, but differences also occur in seasonality, duration, orientation, placement, size, color, texture, and venation. The sori of Polybotrya were previously described as acrostichoid, but three types of sori occur in the genus: botryoid, coenosoric, and a new type here termed "amphiacrostichoid." Botryoid sori, which are round and discrete, are considered primitive on the basis of outgroup comparison with other dryopteroid ferns. Coenosoric sori, which are oblong to linear, probably evolved from the botryoid type by basipetal fusion of the sori. Amphiacrostichoid sori developed from a coenosoric ancestor by the expansion of the margins of the receptacle beyond the original adaxial surface. As a result, the receptacle assumed its own adaxial surface and sporangia appeared on both surfaces of the leaf. Coenosoric and amphiacrostichoid sori contain diplodesmic veins that develop from the normal veins of the sterile leaf.