Minisatellite instability at the Adh locus reveals somatic polymorphism in amphioxus

Amphioxus (subphylum Cephalochordata) is the closest living relative to vertebrates and widely used for phylogenetic analyses of vertebrate gene evolution. Amphioxus genes are highly polymorphic, but the origin and nature of this variability is unknown. We have analyzed the alcohol dehydrogenase locus (Adh3) in two amphioxus species (Branchiostoma lanceolatum and Branchiostoma floridae) and found that genetic variation is related to repetitive DNA sequences, mainly minisatellites. Small pool-PCR assays indicated that allelic variants are generated by minisatellite instability. We conclude that the generation of new forms was not preferentially linked to germline processes but rather to somatic events leading to mosaic adult animals. Furthermore, most Adh minisatellites belong to a novel class, which we have named mirages. Their distinctive feature is that the repeat subunit spans the exon–intron boundaries and generates potential duplications of the splice sites. However, splicing may not be compromised as no aberrant mRNA variants were detected.