A phylogenetic analysis of myosin heavy chain type II sequences corroborates that Acoela and Nemertodermatida are basal bilaterians.
Bilateria are currently subdivided into three superclades: Deuterostomia, Ecdysozoa, and Lophotrochozoa. Within this new taxonomic frame, acoelomate Platyhelminthes, for a long time held to be basal bilaterians, are now considered spiralian lophotrochozoans. However, recent 18S rDNA [small subunit (SSU)] analyses have shown Platyhelminthes to be polyphyletic with two of its orders, the Acoela and the Nemertodermatida, as the earliest extant bilaterians. To corroborate such position and avoid the criticisms of saturation and long-branch effects thrown on the SSU molecule, we have searched for independent molecular data bearing good phylogenetic information at deep evolutionary nodes. Here we report a phylogenetic analysis of DNA sequences from the myosin heavy chain type II (myosin II) gene from a large set of metazoans, including acoels and nemertodermatids. Our study demonstrates, both for the myosin II data set alone and for a combined SSU + myosin II data set, that Platyhelminthes are polyphyletic and that acoels and nemertodermatids are the extant earliest bilaterians. Hence, the common bilaterian ancestor was not, as currently held, large and complex but small, simple, and likely with direct development. This scenario has far-reaching implications for understanding the evolution of major body plans and for perceptions of the Cambrian evolutionary explosion.
Departament de Genètica and Departament de Biologia Animal, Universitat Barcelona, Av. Diagonal, 645 08028 Barcelona, Spain.
I
I
Ruiz-Trillo
J
J
Paps
M
M
Loukota
C
C
Ribera
U
U
Jondelius
J
J
Baguna
M
M
Riutort
D000818:Animals; D001483:Base Sequence; D005075:Biological Evolution; D003063:Cnidaria / Q000145:classification* / Q000235:genetics; D017931:DNA Primers; D008969:Molecular Sequence Data; D018995:Myosin Heavy Chains / Q000235:genetics*; D024501:Nonmuscle Myosin Type IIB; D010802:Phylogeny; D016133:Polymerase Chain Reaction; D013045:Species Specificity
D017931:DNA Primers; D024501:Nonmuscle Myosin Type IIB; C103815:nonmuscle myosin type IIB heavy chain; D018995:Myosin Heavy Chains