Hybridization between perennial ryegrass and Italian ryegrass in naturalized Japanese populations

Introduced Lolium species, including perennial ryegrass (Lolium perenne) and Italian ryegrass (Lolium multiflorum), have been widely utilized in Japan for forage, turf and soil conservation. These ryegrasses have escaped from cultivated areas and become naturalized, and this has become a serious issue in recent years. Interspecific hybrids between perennial ryegrass and Italian ryegrass have often been found in naturalized populations. It has also been suggested that hybridization between plant species might serve as a stimulus for the evolution of invasiveness. We surveyed the genetic structure of naturalized ryegrass populations in Japan using genetic markers that distinguished perennial ryegrass and Italian ryegrass. Of the 55 naturalized populations surveyed, 41 exhibited morphological traits of Italian ryegrass. DNA analysis using simple sequence repeat and chloroplast DNA markers characterized 20 of these 41 populations as Italian ryegrass, with the remaining populations as interspecific hybrid derivatives. Approximately half of the naturalized ryegrasses populations in Japan were inferred to include interspecific hybrids.