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

Botryllus schlosseri, presumably a Mediterranean tunicate that became a wor ldwide distributed species, has colonized New Zealand islands during the la st 2 centuries. Genetic diversity of 6 populations was tested by 5 polymorp hic microsatellite loci (4 to 20 alleles per locus). Alleles distribution p atterns at all loci are characterized by a few major and many rare alleles, suggesting a founder effect. B. schlosseri populations in the North and So uth Islands differ considerably in allele distribution, pointing to differe nt founders and limited gene flow. A Hardy-Weinberg exact test for all loci and all New Zealand populations reveals a significant heterozygote deficie ncy. suggesting partial inbreeding as a result of an aggregated settlement of sibling B, schlosseri tadpoles. Natural chimeras (colonies expressing mo re 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 Isra eli populations further support the generally accepted dogma that B. schlos seri is of a Mediterranean Sea origin, and suggest different founders for N ew Zealand and Californian coasts.