High level of structural polymorphism driven by mobile elements in the Hox genomic region of the Chaetognath Spadella cephaloptera.

Little is known about the relationships between genome polymorphism, mobile element dynamics, and population size among animal populations. The chaetognath species Spadella cephaloptera offers a unique perspective to examine this issue because they display a high level of genetic polymorphism at the population level. Here, we have investigated in detail the extent of nucleotide and structural polymorphism in a region harboring Hox1 and several coding genes and presumptive functional elements. Sequencing of several bacterial artificial chromosome inserts representative of this nuclear region uncovered a high level of structural heterogeneity, which is mainly caused by the polymorphic insertion of a diversity of genetic mobile elements. By anchoring this variation through individual genotyping, we demonstrated that sequence diversity could be attributed to the allelic pool of a single population, which was confirmed by detection of extensive recombination within the genomic region studied. The high average level of nucleotide heterozygosity provides clues of selection in both coding and noncoding domains. This pattern stresses how selective processes remarkably cope with intense sequence turnover due to substitutions, mobile element insertions, and recombination to preserve the integrity of functional landscape. These findings suggest that genome polymorphism could provide pivotal information for future functional annotation of genomes.