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Multicellular animals possess two to three different types of muscle tissues. Striated muscles have considerable ultrastructural similarity and contain a core set of proteins including the muscle myosin heavy chain ( Mhc ) protein. The ATPase activity of this myosin motor protein largely dictates muscle performance at the molecular level. Two different solutions to adjusting myosin properties to different muscle subtypes have been identified so far: Vertebrates and nematodes contain many independent differentially expressed  Mhc  genes while arthropods have single  Mhc  genes with clusters of mutually exclusive spliced exons (MXEs). The availability of hundreds of metazoan genomes now allowed us to study whether the ancient bilateria already contained MXEs, how MXE complexity subsequently evolved, and whether additional scenarios to control contractile properties in different muscles could be proposed, By reconstructing the  Mhc  genes from 116 metazoans we showed that all intron positions within the motor domain coding regions are conserved in all bilateria analysed. The last common ancestor of the bilateria already contained a cluster of MXEs coding for part of the loop-2 actin-binding sequence. Subsequently the protostomes and later the arthropods gained many further clusters while MXEs got completely lost independently in several branches (vertebrates and nematodes) and species (for example the annelid  Helobdella robusta  and the salmon louse  Lepeophtheirus salmonis ). Several bilateria have been found to encode multiple  Mhc  genes that might all or in part contain clusters of MXEs. Notable examples are a cluster of six tandemly arrayed  Mhc  genes, of which two contain MXEs, in the owl limpet  Lottia gigantea  and four  Mhc  genes with three encoding MXEs in the predatory mite  Metaseiulus occidentalis . Our analysis showed that similar solutions to provide different myosin isoforms (multiple genes or clusters of MXEs or both) have independently been developed several times within bilaterian evolution.

Shared Gene Structures and Clusters of Mutually Exclusive Spliced Exons within the Metazoan Muscle Myosin Heavy Chain Genes