Clinal Variation of Frond Morphology and its Adaptive Implication in the Fern Ceratopteris thalictroides in Japan

Frond morphology of the highly polymorphic, annual fern Ceratopteris thalictroides was examined for local populations in south and central Japan. Among the characters examined, frond length and frond dissection were most useful for recognizing several forms. Through the comparison of these characters among populations, a geographically clinal pattern of variation was recognized; plants in higher latitudes have smaller and less dissected fronds. Plants usually grow in rice paddies during a limited period from the harvest time to the beginning of the frost season. Concordance was observed between the degree of frond length and dissection of populations and the length of the viable period in their localities. The variability of cultivated progeny of selected populations from the south and the north suggests that there are at least two ecological lines; one is genetically restricted to large and much dissected fronds and the other to smaller and less dissected fronds. The latter may be an adaptation to the short viable period in rice paddies in northern regions. High phenotypic plasticity enables these lines to adjust well the degree of frond length and dissection to the length of the viable period in each locality.