Cetaceans (whales, dolphins, and porpoises) are a group of mammals adapted to various aquatic habitats, from oceans to freshwater rivers. We report the sequencing, de novo assembly and analysis of a finless porpoise genome, and the re-sequencing of an additional 48 finless porpoise individuals. We use these data to reconstruct the demographic history of finless porpoises from their origin to the occupation into the Yangtze River. Analyses of selection between marine and freshwater porpoises identify genes associated with renal water homeostasis and urea cycle, such as urea transporter 2 and angiotensin I-converting enzyme 2, which are likely adaptations associated with the difference in osmotic stress between ocean and rivers. Our results strongly suggest that the critically endangered Yangtze finless porpoises are reproductively isolated from other porpoise populations and harbor unique genetic adaptations, supporting that they should be considered a unique incipient species.

Although great progress has been made in resolving the relationships of placental mammals, the position of several clades in Laurasiatheria remain controversial. In this study, we performed a phylogenetic analysis of 97 orthologs (46,152 bp) for 15 taxa, representing all laurasiatherian orders. Additionally, phylogenetic trees of laurasiatherian mammals with draft genome sequences were reconstructed based on 1608 exons (2,175,102 bp). Our reconstructions resolve the interordinal relationships within Laurasiatheria and corroborate the clades Scrotifera, Fereuungulata, and Cetartiodactyla. Furthermore, we tested alternative topologies within Laurasiatheria, and among alternatives for the phylogenetic position of Perissodactyla, a sister-group relationship with Cetartiodactyla receives the highest support. Thus, Pegasoferae (Perissodactyla + Carnivora + Pholidota + Chiroptera) does not appear to be a natural group. Divergence time estimates from these genes were compared with published estimates for splits within Laurasiatheria. Our estimates were similar to those of several studies and suggest that the divergences among these orders occurred within just a few million years.

The baiji, or Yangtze River dolphin (Lipotes vexillifer), is a flagship species for the conservation of aquatic animals and ecosystems in the Yangtze River of China; however, this species has now been recognized as functionally extinct. Here we report a high-quality draft genome and three re-sequenced genomes of L. vexillifer using Illumina short-read sequencing technology. Comparative genomic analyses reveal that cetaceans have a slow molecular clock and molecular adaptations to their aquatic lifestyle. We also find a significantly lower number of heterozygous single nucleotide polymorphisms in the baiji compared to all other mammalian genomes reported thus far. A reconstruction of the demographic history of the baiji indicates that a bottleneck occurred near the end of the last deglaciation, a time coinciding with a rapid decrease in temperature and the rise of eustatic sea level.

Background: The phylogeny of Cetacea (whales) is not fully resolved with substantial support. The ambiguous and conflicting results of multiple phylogenetic studies may be the result of the use of too little data, phylogenetic methods that do not adequately capture the complex nature of DNA evolution, or both. In addition, there is also evidence that the generic taxonomy of Delphinidae (dolphins) underestimates its diversity. To remedy these problems, we sequenced the complete mitochondrial genomes of seven dolphins and analyzed these data with partitioned Bayesian analyses. Moreover, we incorporate a newly-developed "relaxed" molecular clock to model heterogenous rates of evolution among cetacean lineages.

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