Genetic structure and differentiation of Plantago major reveals a pair of sympatric sister species

Seeds of the widespread weed Plantago major were collected from 10 European countries, as well as Trinidad and North America. The seed collections wer e from populations of two taxa which are ecologically rather than geographi cally separated and formally recognized as the subspecies Plantago major ss p. major and P.m. ssp. intermedia (also called Pm. ssp. pleiosperma). Eight polymorphic allozyme loci and 73 random-primed DNA fragments were scored, as well as 11 morphological characters. Complete concordance between morpho logical traits and genetic data provides evidence that these two taxa, alth ough very similar, are distinct species. They are both widespread, they are broadly sympatric and capable of interbreeding. However, slight morphologi cal and ecological differences coincide with genetic clustering of populati ons from widely separated locations. In addition, P. major and I? intermedi a differ in their population structure: P. intermedin has greater genetic d iversity among populations and less genetic variance within populations tha n P. major. We suggest that differences between the two species in their le vels of selfing may explain the distinctive genetic structure of each speci es. We hypothesize a link between selfing rate and lifespan of the two taxa . P. major is characterized by lower genetic variation among populations, a higher rate of outcrossing, longer lifespan and production of fewer seeds per seed capsule. P. intermedia is more highly structured with much differe ntiation among populations, a higher rate of inbreeding and it often grows as an annual.