MOLECULAR EVOLUTION OF THE MITOCHONDRIAL 12S RIBOSOMAL-RNA IN UNGULATA (MAMMALIA)

The complete 12S rRNA gene has been sequenced in 4 Ungulata (hoofed eu therians) and 1 marsupial and compared to 38 available mammalian seque nces in order to investigate the molecular evolution of the mitochondr ial small-subunit ribosomal RNA molecule. Ungulata were represented by one artiodactyl (the collared peccary, Tayassu tajacu, suborder Suifo rmes), two perissodactyls (the Grevy's zebra, Equus grevyi, suborder H ippomorpha; the white rhinoceros, Ceratotherium simum, suborder Cerato morpha), and one hyracoid (the tree hyrax, Dendrohyrax dorsalis). The fifth species was a marsupial, the eastern gray kangaroo (Macropus gig anteus). Several transition/transversion biases characterized the patt ern of changes between mammalian 12S rRNA molecules. A bias toward tra nsitions was found among 12S rRNA sequences of Ungulata, illustrating the general bias exhibited by ribosomal and protein-encoding genes of the mitochondrial genome. The derivation of a mammalian 12S rRNA secon dary structure model from the comparison of 43 eutherian and marsupial sequences evidenced a pronounced bias against transversions in stems. Moreover, transversional compensatory changes were rare events within double-stranded regions of the ribosomal RNA. Evolutionary characteri stics of the 12S rRNA were compared with those of the nuclear 18S and 28S rRNAs. From a phylogenetic point of view, transitions, transversio ns and indels in stems as well as transversional and indels events in loops gave congruent results for comparisons within orders. Some compe nsatory changes in double-stranded regions and some indels in single-s tranded regions also constituted diagnostic events, The 12S rRNA molec ule confirmed the monophyly of infraorder Pecora and order Cetacea and demonstrated the monophyly of suborder Suiformes, However, the monoph yly of the suborder Ruminantia was not supported, and the branching pa ttern between Cetacea and the artiodactyl suborders Ruminantia and Sui formes was not established. The monophyly of the order Perissodactyla was evidenced, but the relationships between Artiodactyla, Cetacea, an d Perissodactyla remained un resolved. Nevertheless, we found no suppo rt for a Perissodactyla + Hyracoidea clade, neither with distance appr oach, nor with parsimony reconstruction. The 12S rRNA was useful to so lve intraordinal relationships among Ungulata, but it seemed to harbor too few informative positions to decipher the bushlike radiation of s ome Ungulata orders, an event which has most probably occurred in a sh ort span of time between 55 and 70 MYA.