Origin of Tetrapods Inferred from Their Mitochondrial DNA Affiliation to Iungfish Axel Meyer" and Allan Ci Wilson Diviaies of Miochemiatry and Meleculor Biolney, Univenity of Californin, Derkelicy, Californis 94720, USA Summary. This paper shows that questions of an tree - Statistical testing - Morphological traits unexpected phylogenetic depth can be addressed by Conquest of land the stady of mitochondrial DNA (mtDNA) sequences. For decades, it has been unclear whether coelacanth fishes or lunglishes are the closest living relatives of land vertebrates (Tetrapoda). Segments Introduction of mtDNA from a lungfish, the coelacanth, and a ray-finned fish were seeuenced and compared to the The recognision that land vertebrates (tetrapods). published sequence of a froe miDNA. A tree based evolved from lobe-finned fishes was one of the great on inferred amino ocid replacements, silent trans- achievements of comparative morphology (Romer versions, and ribosomal RNA (rRNA) substitutions 1966). For more that a century, however, there has sbowed with statistical confidence that the lungfish been uncertainty (Forey 1988) about which of the mADNA is more closely related to that of the frog two lobe- finned groups, the Crossopterygii or lung. than in the mtDNA of the coelafanth. This result fishes, are more akin to tetrapods (Fig. 1, 1ee legend). appears to rule out the possibility fatt the coclacanth So long as this matter was unsettled, it was imposlineage gave rise to land vertebrates; hence, mor- sibie to develop a detailed model of hoa fishes conphological characters that link the latter two groups quered the land, are possibly due to convergent evolution or reversals Fifty years ago, the discovery of a living species. and not to common descent. Besides supporting the representiag an otherwise extinct group of coelaTheory that land vertebrates arose from an offhoot canths (Smith 1939,1953 ) , which comprise a part of the lineage leading to lungfishes, the molecular of the Crossopterygi,, created hope that this living tree facilitabes an evolutionary interpeetation of the fossil was the missing link and close to the ancestry. morphological differences among the living forms. of tetrapods. However, even with extant material at It would appear that the commos ancestor of lung- hand, cormparative morphologists and paleontolofises and tetrapods already possessed multiple gists favored different relationships (Fig. IA) (Wahmorphological traits preadspting their locomotion, lert 1968; Lavtrup 1977; Wiley 1979; Resen et al. circulation, and respiration for life on land. 1981; Lagios 19s2; Forcy 1987; Fritasch 1987 ; Northeut 1987 ; Schultze 1987). Although the above Key werds: Direct sequencing - Polymerase chain question is unsolved, it is generally accepted, as reaction - Cytochrome b 12 S rRNA - Cocla- shown in Fit. 1A, that ray-finned frshes (Actinopcanth - Ray- finned fishes - Frog - Evolutionary terygii) are more distantly related to the tetrapods and a.

1. Origin of Tetrapods Inferred from Their Mitochondrial DNA Affiliation to Iungfish Axel Meyer"
and Allan Ci Wilson Diviaies of Miochemiatry and Meleculor Biolney, Univenity of Californin,
Derkelicy, Californis 94720, USA Summary. This paper shows that questions of an tree -
Statistical testing - Morphological traits unexpected phylogenetic depth can be addressed by
Conquest of land the stady of mitochondrial DNA (mtDNA) sequences. For decades, it has been
unclear whether coelacanth fishes or lunglishes are the closest living relatives of land vertebrates
(Tetrapoda). Segments Introduction of mtDNA from a lungfish, the coelacanth, and a ray-finned
fish were seeuenced and compared to the The recognision that land vertebrates (tetrapods).
published sequence of a froe miDNA. A tree based evolved from lobe-finned fishes was one of
the great on inferred amino ocid replacements, silent trans- achievements of comparative
morphology (Romer versions, and ribosomal RNA (rRNA) substitutions 1966). For more that a
century, however, there has sbowed with statistical confidence that the lungfish been uncertainty
(Forey 1988) about which of the mADNA is more closely related to that of the frog two lobe-
finned groups, the Crossopterygii or lung. than in the mtDNA of the coelafanth. This result
fishes, are more akin to tetrapods (Fig. 1, 1ee legend). appears to rule out the possibility fatt the
coclacanth So long as this matter was unsettled, it was imposlineage gave rise to land
vertebrates; hence, mor- sibie to develop a detailed model of hoa fishes conphological characters
that link the latter two groups quered the land, are possibly due to convergent evolution or
reversals Fifty years ago, the discovery of a living species. and not to common descent. Besides
supporting the representiag an otherwise extinct group of coelaTheory that land vertebrates arose
from an offhoot canths (Smith 1939,1953 ) , which comprise a part of the lineage leading to
lungfishes, the molecular of the Crossopterygi,, created hope that this living tree facilitabes an
evolutionary interpeetation of the fossil was the missing link and close to the ancestry.
morphological differences among the living forms. of tetrapods. However, even with extant
material at It would appear that the commos ancestor of lung- hand, cormparative morphologists
and paleontolofises and tetrapods already possessed multiple gists favored different relationships
(Fig. IA) (Wahmorphological traits preadspting their locomotion, lert 1968; Lavtrup 1977; Wiley
1979; Resen et al. circulation, and respiration for life on land. 1981; Lagios 19s2; Forcy 1987;
Fritasch 1987 ; Northeut 1987 ; Schultze 1987). Although the above Key werds: Direct
sequencing - Polymerase chain question is unsolved, it is generally accepted, as reaction -
Cytochrome b 12 S rRNA - Cocla- shown in Fit. 1A, that ray-finned frshes (Actinopcanth - Ray-
finned fishes - Frog - Evolutionary terygii) are more distantly related to the tetrapods and are the
sister-group of all lobe-tinned fishes Univenily of Niew York, Soey Broek, New York l1JH,
USA A molecular approach to defining the eencalogical Konstunser Omline-Putvalaniosi-Syarm
(KOPS)