Primary genetic linkage maps of the ascidian, Ciona intestinalis

For whole-genome analysis in a basal chordate (protochordate), we used F1 pseudo-testcross mapping strategy and amplified fragment length polymorphism (AFLP) markers to construct primary linkage maps of the ascidian tunicate Ciona intestinalis. Two genetic maps consisted of 14 linkage groups, in agreement with the haploid chromosome number, and contained 276 and 125 AFLP loci derived from crosses between British and Neapolitan individuals. The two maps covered 4218.9 and 2086.9 cM, respectively, with an average marker interval of 16.1 and 18.9 cM. We observed a high recombinant ratio, ranging from 25 to 49 kb/cM, which can explain the high degree of polymorphism in this species. Some AFLP markers were converted to sequence tagged sites (STSs) by sequence determination, in order to create anchor markers for the fragmental physical map. Our recombination tools provide basic knowledge of genetic status and whole genome organization, and genetic markers to assist positional cloning in C. intestinalis.