Independent Evolution of Striated Muscles in Cnidarians and Bilaterians

Abstract

Striated muscles are present in bilaterian animals (for example, vertebrates, insects and annelids) and some non-bilaterian eumetazoans (that is, cnidarians and ctenophores). The considerable ultrastructural similarity of striated muscles between these animal groups is thought to reflect a common evolutionary origin. Here we show that a muscle protein core set, including a type II myosin heavy chain (MyHC) motor protein characteristic of striated muscles in vertebrates, was already present in unicellular organisms before the origin of multicellular animals. Furthermore, 'striated muscle' and 'non-muscle' myhc orthologues are expressed differentially in two sponges, compatible with a functional diversification before the origin of true muscles and the subsequent use of striated muscle MyHC in fast-contracting smooth and striated muscle. Cnidarians and ctenophores possess striated muscle myhc orthologues but lack crucial components of bilaterian striated muscles, such as genes that code for titin and the troponin complex, suggesting the convergent evolution of striated muscles. Consistently, jellyfish orthologues of a shared set of bilaterian Z-disc proteins are not associated with striated muscles, but are instead expressed elsewhere or ubiquitously. The independent evolution of eumetazoan striated muscles through the addition of new proteins to a pre-existing, ancestral contractile apparatus may serve as a model for the evolution of complex animal cell types.

Figure 1. Complex phylogenomic distribution of contractile machinery (a) and z-disc interactome (b) components

Rows: gene names of vertebrate and/or D. melanogaster contractile machinery (a) or z-disc complex (b) components. Columns: species and their phylogenetic relationship^, . Asterisk: only a preliminary assembly without gene predictions was available for M. leidyi. Row labels in (a): site of predominant gene expression; in (b): species with reported z-disc localization of the gene product. Multifamily protein and uncertain orthologies supported by further molecular phylogenetic and protein domain analyses (Supplementary Figs. 2, 6, 7). All abbreviations in Supplementary Table 1.

Figure 2. Ancient myhc gene duplication predated animal radiation

Maximum likelihood phylogenetic tree of MyHC type II proteins with nodes collapsed if they diverged between neighbour-joining, maximum likelihood, or Bayesian inference. The nesting of protist MyHCs within the MyHC-nm orthology group supports a myhc duplication event in the common ancestor of Metazoa, Choanoflagellata, Filasterea and Ichthyosporea, but also assumes secondary losses of myhc-st genes in protist phyla. Diagrams: MyHC domain structures. Final alignment length: 1730 a.a. Scale bar: 0.2 changes per site. Coloured numbers: positions of non-canonical coiled-coil domains. a.a.: amino acid. Species abbreviations, sequence accession and protein model numbers in Supplementary Table 1.

Figure 3. Expression of myhc-st in a demosponge, and in anthozoan and hydrozoan cnidarians

In situ hybridisations (a, d-g, l-o) and schematic representations (c, h-k, p-r) of myhc-st expression in the adult demosponge Tethya wilhelma (a,c), the anthozoan Nematostella vectensis (d-k), and the hydrozoan Clytia hemisphaerica (l-r). Scanning electron microscopy image (b) and schematic representation (c) of a sectionned choanocyte chambers of T. wilhelma. Tw-myhc-st-expressing multi-porous cells (b, white arrows, inlet; c, red) are likely involved in water flow (blue dotted arrows) regulation through choanocyte chambers (within dotted white lines). (o) Velum of a young medusa was lifted. Developmental stages: (d-e, h-i) 4 days old planula; (f,k) 9 days old primary polyps; (l-m, p-q) medusal buds; (n-o, r) young medusae; (a-c, g) adults. (a,g) Cross-sections of stained animals; (d-g, l-o) whole-mount micrographs. Views: (d,f,h,k-l,p,r) lateral; (e,i, m-o,q) oral. Aboral towards top (d,h,r) or lower-right (f, k-l, p). Asterisk: mouth. ap: apopyle, cc: choanocyte chamber; exc: excurrent channel; inc: incurrent channel; mh: mesohyl; pp: prosopyle; rc: ring canal; rm: retractor muscle; su: subumbrella; tb: tentacle bulb; tm: tentacle muscle; v: velum. Scale bar: 10μm.

Figure 4. Absence of Clytia hemisphaerica muscleLim and Ch-ldb3/zasp expression in striated muscles

In situ hybridisation (a-d) and schematic representation (e-f) of Ch-muscleLim (a, b), Ch-ldb3/zasp (c, d) expression mainly restricte to the developing radial canal endoderm (a-f). Ch-myhc-st-positive subumbrella striated muscle precursor cells (arrows, compare with Fig. 3m) do not show muscleLim- or ldb3/zasp-expression. Stages: medusal bud (a,c,e), young medusa (b,d,f). All oral views.