Phylogenetic relationships of metazoan lamins. An unrooted phylogenetic tree using an alignment of 31 lamin coding sequences . Evolutionary history was inferred using the neighbor-joining method and evolutionary distances were computed using the maximum composite likelihood method [128, 129]. The tree is drawn to scale with branch lengths proportional to the evolution distances. In general, all invertebrate lamins are B-type lamins. The exception is Drosophila LamC, which is considered an A-type lamin . Vertebrate B-type lamins subdivide into three separate clades, B1, B2 and B3, the latter being specific to amphibians and fish. A-type lamins evolved from a Lamin B1-like ancestor and are unique to vertebrates. Major structural changes of lamins during evolution are indicated: the minus sign indicates a 90 amino acid deletion in the conserved Ig domain from tunicates (urochordate subphylum) ; the asterisk indicates the addition of 6 to 12 negatively charged amino acids within the tail domain of all lamins in the vertebrate lineage ; and the plus sign indicates the acquisition of an extra exon (exon 11), encoding 90 amino acids, in the tail domain of vertebrate A-type lamins .