Plants of genus Azolla (Azollaceae) newly recorded from Lake Izunuma

Abstract

The genus Azolla (Azollaceae) is a floating water fern and two species, A. imbricata and A. japonica, are known from Japan. Recently, introduced species of the genus have been naturalized in Japan, but the difficulty of morphological characterization has made them overlooked. Here we report Azolla plants occurring in Lake Izunuma for the first time. Azolla plants were found on the southern coast of Lake Izunuma and covered an approximate 50×5 m area in high density. Morphological examination showed that the plants had both one-cell and two-cell leaf surface trichomes, and seemed to be a hybrid between A. microphyllum and A. filiculoides. Nucleotide sequence data of internal transcribed spacer (ITS) regions in nuclear rDNA showed a composite pattern of A. microphyllum and A. filiculoides sequences, also suggesting the hybrid nature of Azolla plants in Lake Izunuma. Although the hybrid is sterile, the fern’s ability of active vegetative reproduction may have some impacts on native organisms and environments in Lake Izunuma-Uchinuma if stable populations are established. Further studies are warranted.