Although an increasing number of investigations have been made into the evolution of alien species once introduced, few studies have identified the invasion routes of these introduced species. Because multiple introductions are common in invasive species, failing to take into account the introduced lineages can be misleading when studying evolutionary change in alien species after they begin to extend their ranges. In Japan, diverse lineages of ryegrasses (Lolium spp.) were introduced as forage crops and contaminants in trading grain and have expanded to sandy coasts. We studied the expansion route of populations established along the coasts of three geographic regions within Japan by comparing variations in morphology and nuclear microsatellite and chloroplast DNA in the two habitats where ryegrasses were first introduced: croplands and international seaports. Chloroplast DNA haplotypes did not differ significantly among habitats and regions, but the coastal and seaport populations displayed similar microsatellite genetic compositions and morphological characteristics. Our results revealed that coastal populations originated from seaport populations derived from contaminants. Selective forces from the past, including domestication and naturalization, may have assisted the introduced lineages in colonizing new habitats.

Nomenclature: Italian ryegrass, Lolium multiflorum L.; rigid ryegrass, Lolium rigidum Gaudin.