Intraspecific hybridization of native and introduced subspecies of Phragmites australis in North America

Interspecific hybridization can lead to the extinction of native floral and faunal gene pools and may result in increased aggressiveness in some hybrid forms relative to their parental lineages. While hybridization between native and introduced species can dilute the native gene pool, interbreeding among subspecies is not currently recognized as a serious threat to native species. Phragmites australis offers the opportunity to investigate intraspecific hybridization because both native and introduced lineages occur in North America. Introduced Phragmites is arguably one of the most successful plant invaders in estuarine systems in North America, but native Phragmites populations are declining in the eastern U.S., in part because introduced Phragmites has replaced them. Despite range overlaps, hybridization has not been detected between the native and introduced lineages. One hypothesis holds that a phenological barrier precludes cross-pollination between native and introduced populations. However, hybridization should be possible given that native and introduced Phragmites strains are classified as subspecies. In a common garden experiment at the University of Rhode Island, we found substantial overlap in the timing of anthesis in paired populations of native and introduced Phragmites. Furthermore, in hand pollination studies, 87% of putative intraspecific hybrid crosses produced seed and 62% had rates of seed set of over 50%, a very high success rate for this species. Seeds produced from the crosses are currently being analyzed. Results of microsatellite analyses on putative hybrid seeds are presented and the implications for biological diversity and management of native and introduced Phragmites are discussed.