Fig. 9

Overview of metazoan tektin gene family evolution. a Evidence-based parsimonious hypothesis of the evolution of the tektin gene family in metazoans. Results from this study are displayed within a mostly accepted phylogeny of metazoan taxa. The branching pattern of nonbilaterian taxa especially the ctenophores are still debated. However, the proposed scenario is consistent with either ctenophores or poriferans as the earliest branching metazoan taxon. The number of hypothesized ancestral tektin genes is given at each major node (blue circles) with a suggested nomenclature that reflects the evolutionary origin on the Tektin classes is given at the right margin. Note that according to our analysis the four Tektin classes, Tektin-1, − 2, − 4, and 3/5 exist only in bilaterians, while the extant bilaterian and nonbilaterian tektin gene complements originated from two ancestral metazoan tektins, tektin-2 and tektin-1/4/3/5. Major duplications (blue squares) and losses (blue crosses) of tektin genes that affect every species within a phylum are shown. Metazoan phyla and in parenthesis the range of tektin genes found within species of each phylum are shown in the top row. Number of tektin genes range from complete absence in placozoans (0) by gene loss, to ten tektin genes in the annelid leech (10) by multiple gene duplications. Note that three spiralian taxa, the platyhelminthes (5–10), the annelids (5–10), and the mollusks (5–8) contain each species with the ancestral spiralian tektin gene complement of five, and species that have expanded their complement up to ten tektin genes by independent gene duplications. Uncertainty of the placement of the Xenacoelomorpha either before or after the duplication of tektin-4/3/5 leading to tektin-4 and tektin-3/5 is indicated by dotted lines. b Hypothesized scenario for the evolution of the Tektin filament. Pioneering biochemical and structural studies on tektins in motile cilia of sea urchins by Linck and colleagues have provided evidence for axonemal Tektin filaments constructed by multiples of two heterodimers built with Tektin-2 (green circle) and Tektin-4 (red circle), and one homodimer built with Tektin-1 (blue circle) protein units (compare to Fig. 1c). This phylogenetic study provided evidence that these three Tektins existed already in a bilaterian ancestor including a fourth Tektin-3/5 (yellow circle) with unknown filamentous Tektin functions (shown in right column), and enables predictions about the composition for earlier stages in Tektin filament evolution with the ancestral tektin gene complements comprising of one (tektin-2/1/4/3/5, black circle), two (tektin-2, green circle; tektin-1/4/3/5, purple circle), and three tektin genes in the unicellular, metazoan, and early bilaterian ancestor, respectively (shown in three left columns). Upper row shows which predicted Tektin protein makes up the filamentous Tektin units at discreet steps in metazoan evolution, and lists extant organisms in which this prediction could be tested. Lower row shows diagrams of the predicted composition of the hypothetical Tektin filament at discreet steps in evolution