Contents lists available at ScienceDirect Molecular Phylogenetics and Evolution journal homepage: www.elsevier.com/locate/ympev A new evolutionary model for the vertebrate actin family including two novel groups ☆ Laura Witjes a , Marleen Van Troys a , Joël Vandekerckhove a , Klaas Vandepoele b,c , Christophe Ampe a, ⁎ a Department of Biomolecular Medicine, Ghent University, Albert Baertsoenkaai 3, B-9000 Ghent, Belgium b VIB Center for Plant Systems Biology, VIB, Technologiepark 927, B-9052 Ghent, Belgium c Department of Plant Biotechnology and Bioinformatics, Ghent University, Technologiepark 927, B-9052 Ghent, Belgium ARTICLE INFO Keywords: Vertebrate actin evolution Synteny Nomenclature Actin family Acte1 Acta4 ABSTRACT Database surveys in the vertebrate model organisms: chicken (Gallus gallus), western clawed frog (Xenopus tropicalis), anole lizard (Anolis carolinensis) and zebrafish (Danio rerio) indicate that in some of these species the number of actin paralogues differs from the well-established six paralogues in mouse (Mus musculus). To in- vestigate differential functions of actins and for establishing disease models it is important to know how actins in the different model organisms relate to each other and whether the vertebrate actin family is truly limited to six groups. Primarily through synteny analyses we discovered that the vertebrate actin family consists of eight instead of six orthologous actin groups for which we propose improved gene nomenclature. We also established that α-skeletal muscle, γ-enteric smooth muscle and γ-cytoplasmic actin genes originated prior to tetrapods contradicting an earlier and widely accepted model of actin evolution. Our findings allow a more reliable predictive classification of actin paralogues in (non-mammalian) vertebrates and contribute to a better under- standing of actin evolution as basis for biomedical research on actin-related diseases. 1. Introduction Actin is the central building block of the microfilament system in all eukaryotic cells. Many vertebrates have multiple actin genes with dis- tinct expression patterns that gave rise to their protein names. Human, mouse (Mus musculus) and other mammals have six characterized actin proteins: two non-muscle (β- and γ-cytoplasmic), two striated muscle (α-skeletal and α-cardiac) and two smooth muscle (α-vascular or aortic smooth and γ-enteric) actins (Vandekerckhove and Weber, 1984, 1978a), encoded by the genes Actb, Actg1, Acta1, Actc1, Acta2 and Actg2, respectively. We will refer to these as ‘canonical’ actins and to- gether these form the current mammalian actin family. Across species we will refer to all members of one type of actin as an orthologous actin group (for definitions see Glossary). Actins are characterized by a very high sequence conservation and on the protein level mainly differ in their N-terminal sequences, which has been used for actin typing (Vandekerckhove and Weber, 1984, 1981, 1978a, 1978b, 1978c). An evolutionary model for vertebrate actins has been proposed in which it was postulated that an ancestral actin gene duplicated prior to the vertebrate origin and gave rise to a cytoplasmic form and a muscle form (Kovilur et al., 1993; Kusakabe et al., 1997, 1999; Chiba et al., 2003). The cytoplasmic actin form duplicated and evolved into a β- and γ-form, events which were proposed to have occurred only recently in evolutionary time (Miwa et al., 1991), whereas the muscle form that duplicated resulted in a striated and a smooth muscle form in early vertebrate evolution (Miwa et al., 1991; Vandekerckhove and Weber, 1984). In this model, these two muscle forms underwent a second du- plication resulting in an α-skeletal and α-cardiac striated muscle actin, and an α-vascular and γ-enteric smooth muscle actin, respectively (Miwa et al., 1991; Vandekerckhove and Weber, 1984). Both events have been proposed to have occurred prior to or concomitant with the evolution from amphibians to reptiles or in primitive reptiles them- selves (Vandekerckhove and Weber, 1984). The above postulate is mainly derived from pre-genomic era data and, given the high sequence conservation, relied on several criteria for actin typing, including distinctive exon-intron structures of the genes, https://doi.org/10.1016/j.ympev.2019.106632 Received 29 April 2019; Received in revised form 19 September 2019; Accepted 23 September 2019 ☆ This paper is dedicated to the late professor Klaus Weber (Department of Biochemistry, Max Planck Institute for Biophysical Chemistry; Göttingen), a giant in the field of cytoskeletal research and a scientist keen on evolution. ⁎ Corresponding author. E-mail addresses: laura.witjes@ugent.be (L. Witjes), leen.vantroys@ugent.be (M. Van Troys), j.van.de.kerckhove@proximus.be (J. Vandekerckhove), Klaas.Vandepoele@UGent.be (K. Vandepoele), Christophe.ampe@ugent.be (C. Ampe). Molecular Phylogenetics and Evolution 141 (2019) 106632 Available online 24 September 2019 1055-7903/ © 2019 Elsevier Inc. All rights reserved. T