Heterozygote deficiency and chimerism in remote populations of a colonial ascidian from New Zealand

Botryllus schlosseri, presumably a Mediterranean tunicate that became a worldwide distributed species, has colonized New Zealand islands during the last 2 centuries. Genetic diversity of 6 populations was tested by 5 polymorphic microsatellite loci (4 to 20 alleles per locus). Alleles distribution patterns at all loci are characterized by a few major and many rare alleles, suggesting a founder effect. B. schlosseri populations in the North and South Islands differ considerably in allele distribution, pointing to different founders and limited gene flow. A Hardy-Weinberg exact test for all loci and all New Zealand populations reveals a significant heterozygote deficiency. suggesting partial inbreeding as a result of an aggregated settlement of sibling B, schlosseri tadpoles. Natural chimeras (colonies expressing more than 2 alleles per individual at a locus) are found in most localities. Underestimated levels of chimerism are surprisingly high tan average of 8 %), reaching 14 % in the South Island. Comparisons with Californian and Israeli populations further support the generally accepted dogma that B. schlosseri is of a Mediterranean Sea origin, and suggest different founders for New Zealand and Californian coasts.