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Geographical, ecological, morphological, and cytological observations indicate that the peculiar Central American fern, Pleuroderris michleriana (D. C. Eaton) Maxon arises by hybridization between Tectaria incisa Cav. and Dictyoxiphium panamense Hooker where they occur together or near each other. Although crossing is well known among ferns, Pleuroderris is singular among hybrids because of the very profound differences in leaf and soral structures between its parents. In spite of being known from seven localities, in some of which it is rather common, Pleuroderris is sexually sterile with irregular meiosis. The chromosomes show unexpectedly extensive pairing. WHEN HE NAMED THE FERN GENUS Pleuroderris, Maxon (1934) noted its curious taxonomic history. It was redescribed no less than six times under six different specific epithets: Lindsaea michleriana Eaton, Hypoderris seemanni Prentice, H. marginalis Fourn., H. adnata Fourn., Polypodium tatei Baker, and Hypoderris heteroneuroides Christ. Adding to this chaotic state of affairs, Baker (1874) included the single known species of Pleuroderris under three different names in the same book! Further, he placed them (Lindsaea michleriana, Hypoderris seemanni, and Polypodium tatei) in three distinct taxonomic tribes. We are not concerned with mere vagaries of taxonomic nomendature. The problems are more basic: What makes Pleuroderris such a curiosity? Maxon suggested that this fern might actually have originated by the crossing of two very distinctive genera. He noted the asymmetrical shapes of many of the leaf blades, the diversity in soral form, and the seeming lack of normal spore content in the sporangia, characteristics that would tend to support the hypothesis that Pleuroderris was a hybrid. He considered hybridization between Dictyoxiphium panamense Hooker and Tectaria martinicensis [= T. incisa Cav.:l, even though these species belong to the tribes Davalliae and Dryopterideae respectively. Copeland (1947) wrote, "I leave this conjecture where Maxon did, and will not question the status of the genus because of its possible or probable hybrid origin." |
