BIOLOGICAL SCIENCES / EVOLUTION
Phylogeny, evolution, and biogeography of Asiatic Salamanders (Hynobiidae)
Key Laboratory of Gene Engineering of the Ministry of Education, State Key Laboratory of Biocontrol, Zhongshan University, Guangzhou 510275, People’s Republic of China; Department of Biology, Xinjiang Normal University, Xinjiang 830054, People’s Republic of China; and Museum of Vertebrate Zoology, 3101 Valley Life Sciences Building, University of California, Berkeley, CA 94720-3160
Contributed by David B. Wake, March 22, 2006
We sequenced 15 complete mitochondrial genomes and performed comprehensive molecular phylogenetic analyses to study the origin and phylogeny of the Hynobiidae, an ancient lineage of living salamanders. Our phylogenetic analyses show that the Hynobiidae is a clade with well resolved relationships, and our results contrast with a morphology-based phylogenetic hypothesis. These salamanders have low vagility and are limited in their distribution primarily by deserts, mountains, and oceans. Our analysis suggests that the relationships among living hynobiids have been shaped primarily by geography. We show that four-toed species assigned to Batrachuperus do not form a monophyletic group, and those that occur in Afghanistan and Iran are transferred to the resurrected Paradactylodon. Convergent morphological characters in different hynobiid lineages are likely produced by similar environmental selective pressures. Clock-independent molecular dating suggests that hynobiids originated in the Middle Cretaceous [110 million years ago (Mya)]. We propose an "out of North China" hypothesis for hynobiid origins and hypothesize an ancestral stream-adapted form. Given the particular distributional patterns and our molecular dating estimates, we hypothesize that: (i) the interior desertification from Mongolia to Western Asia began 50 Mya; (ii) the Tibetan plateau (at least on the eastern fringe) experienced rapid uplift 40 Mya and reached an altitude of at least 2,500 m; and (iii) the Ailao–Red River shear zone underwent the most intense orogenic movement 24 Mya.
mitochondrial DNA | phylogenetics | homoplasy | Tibetan Plateau
PNAS | May 9, 2006 | vol. 103 | no. 19 | 7360-7365
The Asiatic Salamanders, Hynobiidae, represent an early branch of the caudate lineage (1). All living species (50, in seven to nine genera; http://amphibiaweb.org) occur in Asia. Hynobiids are closely related to the family Cryptobranchidae, with which they form the suborder Cryptobranchoidea. In comparison with other living salamanders, hynobiids are thought to resemble the most recent common ancestor of all salamanders because of three traits that are regarded as ancestral: external fertilization, an angular bone in the lower jaw, and large numbers of microchromosomes (1–3). Although usually thought to be monophyletic, the Hynobiidae also has been considered a paraphyletic stem group (4). Fossil hynobiids are known from the Late Miocene [5 million years ago (Mya)] (5), whereas their relatives, cryptobranchids, can be traced back to the Jurassic (160 Mya) (6). Remarkably, recent fossil findings of salamanders from the Early Cretaceous of North China show strong similarities with the Hynobiidae with respect to many skeletal features (7–9). This finding raises the questions of where and when hynobiids originated and what relationship there is between them and their fossil relatives. To answer these questions, a robust phylogenetic hypothesis of living hynobiids is required. Apart from a tentative phylogeny based on 23 morphological characters (Fig. 1; ref. 10), and another based on an unpublished data set (11), no well supported phylogenetic hypothesis exists.
Although hynobiids are found throughout the Asian continent, their distribution is discontinuous. Given their inability to readily cross deserts and mountains, orogenic movements, as well as inland desertification, may well play important roles in shaping the distribution of lineages. After the collision of India with Asia in the early Cenozoic, the Tibetan plateau began to uplift (12–15). This uplift greatly modified environments all over the Asian continent and has been invoked as the main driving force behind long-term Cenozoic climate change (16, 17). Hynobiids likely experienced effects from this great geologic event because their current distributions are related to geological features such as the Tibetan plateau and its concomitants: loess and deserts. In turn, an accurate estimate of phylogeny and divergence times of living hynobiids will provide clues concerning the effects of geologic events.
In this study, we sequenced 15 previously unreported complete mitochondrial genomes of hynobiids. By combining these sequences with published amphibian mitochondrial genomes, we present a comprehensive molecular phylogeny for living hynobiids. Moreover, by using molecular clock-independent approaches for inferring dating information from molecular phylogenies (18), a timescale for events in hynobiid evolution is given.