Molecular identification and Morphological traits of the native and exotic Azolla species in Taiwan

Abstract

Azolla is a perennial aquatic fern and often forms symbiosis with nitrogen-fixing cyanobacteria. Flora of Taiwan records only one native Azolla species, A. pinnata R. Br., in the Island. Recently, an exotic Azolla species has been documented with naturalized populations in Taiwan and been reported as A. japonica or A. caroliniana. However, data supporting its identity have not been presented. The purposes of this study are: (1) to test the hypothesis that the exotic Azolla species is A. japonica or A. caroliniana, and (2) to illustrate the morphological differences between the exotic and the native Azolla species. The DNA sequences of four plastid regions, rbcL, atpB, rps4 and rps4-trnS, were generated from plants of the Azolla species growing in different regions of Taiwan. Our phylogenetic analyses reveal that the native species belongs to A. pinnata, while plants of the exotic species are phylogenetically nested within a clad, including A. microphylla. A. mexicana and A. craoliniana. The host-symbiont specificity between Azolla spp. and their cyanobionts was confirmed by the phylogeny of 16S rRNA genes, which also supports the grouping of the exotic species to the aforementioned clad. Thus, the phylogenetic results suggest that the exotic Azolla is more closely related to A. caroliniana than to A. japonica. A table summarizing the morphological characters of their sporophytes illustrates the differences in the gross form, epidermal structures and surface of rhizome between the native and exotic Azolla species which can be used to differentiate the two species in Taiwan.