A Model System of Structural Duplication: Homologies of Adductor Mandibulae Muscles in Tetraodontiform Fishes

Abstract

We critically reviewed the homologies of the jaw muscles in tetraodontiform fishes (Triacanthoidea, Balistoidea, Tetraodontoidea), as first described in Winterbottom's phylogenetic monograph (1974, Smithson. Contrib. Zool. 155:1–201), as a case study in structural duplication. Within this order of teleost fishes, the two main adductor mandibulae muscles, A1 and A2, are duplicated one or more times in some subclades. The number of descendant A1 and A2 muscles ranges from as few as the original two muscles in triplespines to as many as eight muscles in some file-fishes. As first pointed out by Winterbottom, the homologies of some muscles are unclear, particularly in comparisons between the superfamilies Balistoidea (boxfishes, triggerfishes, filefishes) and Tetraodontoidea (pursefishes, molas, puffers, porcupinefishes). We reassessed the homologies (orthologs and paralogs) of these A1 and A2 muscles based on their origins, insertions, and relative masses in representative taxa and their congruence with a phylogeny for these taxa. New names that reflect the homologies of these muscles are presented. Ten muscle duplications by subdivision and three phylogenetic losses of muscles have occurred in this system. No relationship was found between the number of separate muscles and the relative masses of the A1 or A2 muscles, suggesting that muscle duplication events essentially repackage existing muscle tissue. However, both A1 and A2 muscle masses are correlated with each other and with the feeding ecology of these fishes. Durophagous taxa have relatively larger A1 and A2 muscles, whereas planktivores and benthic grazers have relatively smaller A2 muscles.