Evolution of the lamin protein family at the base of the vertebrate lineage

Lamin proteins are major constituents of the nuclear lamina. They are required for fundamental nuclear activities, as evidenced by the large number of laminopathies. Mutations in the human lamin A/C gene exhibit a broad spectrum of clinical manifestations. Most non-vertebrates including the nearest relatives of the vertebrates have only a single lamin gene. In jawed vertebrates (Gnathostomata), four lamin subtypes (B1, B2, LIII, and A) are found. Lampreys and hagfish form the two orders of jawless vertebrates, Agnatha, which represent the sister group of the Gnathostomata at the base of the vertebrate lineage. Lamin sequence information of lampreys and hagfish sheds light on the evolution of the lamin protein family at the base of the vertebrate lineage. In the genomes of the lamprey (Petromyzon marinus) and the hagfish (Eptatretus burgeri), only three lamin genes are present, a lamin A gene is lacking. The presence of an LIII gene in both, lampreys and hagfish, proves that the distinguishing features of this gene had been established before the agnathan/gnathostome split. The other two agnathan lamins, LmnI and LmnII, deviate strongly in their sequences from those of the gnathostome lamins. For none of these two agnathan lamins can orthology be established to one of the gnathostome lamin types. In the direct chromosomal neighbourhood of all three hagfish lamin genes, a MARCH3 paralog is found. This can be interpreted as further evidence that the vertebrate lamin genes have arisen in the course of the two rounds of whole genome duplication that took place at the base of the vertebrate lineage.