Figure 1. Stem cells in Hydra express a single Lamin protein structurally similar to vertebrate B-type lamins. (A) Stem cells continuously proliferate in the middle body column of Hydra, and undergo terminal differentiation at the upper and lower body column ends. (B) Hydra body is made of ectodermal (ECT) and endodermal (END) epithelial layers, separated by the extracellular matrix (ECM) called mesoglea. (C) Three stem cell lineages are present in Hydra. Interstitial stem cells (i-cell) differentiate (diff) into somatic cells - nematocytes (nem), nerve (nv) and gland (gd) cells, and germline cells. Ectodermal and endodermal lineages represent unipotent stem cells. (D) Single HyLMN protein in Hydra shows typical structural features of nuclear Lamins: N-terminal motif for phosphorylation by cyclin-dependent kinases (CDK, red), alfa-helical rod domain (blue), putative nuclear localization signal (NLS, orange), immunoglobulin-like lamin terminal domain (LTD, green) and a C-terminal CaaX-like motif (CaaX, red). (E) Phylogenetic tree of Lamin homologs clusters HyLMN protein among Lamins from other cnidarians at the basis of Metazoan tree. Maximum-likelihood phylogram rooted using the Lamin-like sequence from a choanoflagellate Monosiga. Numbers at nodes are bootstrap support values calculated by 1000 iterations. See Methods for sequence accession numbers.