Colonization history and introduction dynamics of Capsella bursa-pastoris (Brassicaceae) in North America: isozymes and quantitative traits

Multilocus isozyme genotypic composition for aspartate aminotransferase (AA T), leucine aminopeptidase (LAP) and glutamate dehydrogenase (GDH) was stud ied for Capsella in the source continent, Europe (9000 plants from 593 popu lations), and in the colonized continent, North America (2700 plants from 8 8 populations). North America was depauperate in the number of genotypes (b y approximate to 50%), but in terms of frequencies, a few genotypes were co mmon and shared by both continents. Although some, very rare, genotypes wer e, however, unique for North America, our data provided no evidence to indi cate that the introduced gene pools were reconstructed on a multilocus gene tic basis after introduction. Instead, they argued for a considerable numbe r of independent introduction events. Geographical distribution patterns of multilocus genotypes in Europe and North America were pronounced and enabl ed us to trace the colonization history of Californian Capsella back to Spa nish ancestral populations and those of temperate North America back to tem perate European gene pools. A random-block field experiment with 14 Califor nian populations from different climatic regions revealed that variation pa tterns of quantitative traits reflect ecotypic variation, and the ecologica l amplitude of Capsella in North America is similar to that in Europe, whic h can be traced back to the introduction of preadapted genotypes. It appear s that certain multilocus isozyme genotypes are associated with certain eco types. The variable European gene pool of Capsella was essentially introduc ed into North America without major genetic changes.