COMPLETE MITOCHONDRIAL GENOME SUGGESTS DIAPSID AFFINITIES OF TURTLES

Despite more than a century of debate, the evolutionary position of tu rtles (Testudines) relative to other amniotes (reptiles, birds, and ma mmals) remains uncertain. One of the major impediments to resolving th is important evolutionary problem is the highly distinctive and enigma tic morphology of turtles that led to their traditional placement apar t from diapsid reptiles as sole descendants of presumably primitive an apsid reptiles. To address this question, the complete (16,787-bp) mit ochondrial genome sequence of the African side-necked turtle (Pelomedu sa subrufa) was determined. This molecule contains several unusual fea tures: a (TA), microsatellite in the control region, the absence of an origin of replication for the light strand in the WANCY region of fiv e tRNA genes, an unusually long noncoding region separating the ND5 an d ND6 genes, an overlap between ATPase 6 and COIII genes, and the exis tence of extra nucleotides in ND3 and ND4L putative ORFs. Phylogenetic analyses of the complete mitochondrial genome sequences supported the placement of turtles as the sister group of an alligator and chicken (Archosauria) clade. This result clearly rejects the Haematothermia hy pothesis (a sister-group relationship between mammals and birds), as w ell as rejecting the placement of turtles as the most basal living amn iotes. Moreover, evidence from both complete mitochondrial rRNA genes supports a sister-group relationship of turtles to Archosauria to the exclusion of Lepidosauria (tuatara, snakes, and lizards). These result s challenge the classic view of turtles as the only survivors of prima ry anapsid reptiles and imply that turtles might have secondarily lost their skull fenestration.