Population genetics of the invasive ascidian Botryllus schlosseri from South American coasts

The cosmopolitan colonial ascidian Botryllus schlosseri, most likely a Mediterranean Sea and European Atlantic species, is one of the known human-mediated invaders of coastal marine communities. Whereas numerous populations spreading along the Northern Hemisphere coasts have been intensively studied for various population genetics parameters, the data available on the Southern Hemisphere populations is sporadic, based on few and erratic field collections. By using 5 microsatellite loci, we studied gene diversity and possible introduction routes of 4 B. schlosseri populations on the east and west South American coasts. A Hardy-Weinberg exact test for all loci and all populations demonstrated a highly significant heterozygote deficiency. Analyses revealed high gene diversity in the Chilean populations of the west coast, whereas the maximal number of alleles per locus, the highest percentages of natural chimeras and private alleles and the highest levels of variability were observed in the Argentinean population of the east coast. Results further suggest that each of the Chilean populations was founded by a few genotypes. Comparing the genetic identities of South and North American B. schlosseri populations showed extensive dissimilarities, with hardly any common alleles shared, suggesting distinct B. schlosseri clades based on molecular biology data.