The first discovery of an alligatorid (Crocodylia, Alligatoroidea, Alligatoridae) in the Eocene of China
Authors:
Skutschas et al
Abstract:
The Chinese alligator (Alligator sinensis) of the Yangtze area of China is the only living alligatorid in Asia (Wermuth and Mertens, 1961; Pough et al., 2004; Jiang, 2010; Thorbjarnarson and Wang, 2010; Fig. 1). The second modern species of Alligator, the American alligator (A. mississippiensis), is restricted to the southeastern U.S.A. (Wermuth and Mertens, 1961; Pough et al., 2004). The Chinese and American alligators are the only living representatives of Alligatorinae, a group including these two species and their closest relatives (Brochu, 1999). Other living alligatorids—five to seven species of caimans that are distributed in Mesoamerica and South America—are members of Caimaninae (a group including Caiman crocodilus and all crocodylians closer to it than to A. mississippiensis; Brochu, 1999, 2011). The timing of divergence between caimanines and alligatorines was between 71 and 60 million years ago based on molecular data and the fossil record (Brochu, 2011; Oaks, 2011) and that between the extant Alligator species is 58–31 million years based on molecular data (Oaks, 2011) and 20–25 million years based on fossils (Brochu, 2003). If the divergence estimate based on molecular data is correct, it implies that a stock of basal members of the A. sinensis lineage would be predicted to be present in Asiatic Paleogene-aged deposits. Otherwise, if the molecular estimates have overestimated the divergence time and fossils better capture the record, then any alligatorids found in Asia during the early Paleogene would not lie within the crown Alligator clade.
In a study published in Cell Research, Chinese scientists from Zhejiang University and BGI have completed the genome sequencing and analysis of the endangered Chinese alligator (Alligator sinensis). This is the first published crocodilian genome, providing a good explanation of how terrestrial-style reptiles adapt to aquatic environments and temperature-dependent sex determination (TSD).
The Chinese alligator is a member of the alligator family that lives in China. It is critically endangered with a population of ~100 wild and ~10,000 captive individuals in Zhejiang and Anhui Provinces. Great efforts have been put into uncovering the mysteries of this species because of its unique features that allow them being adapted for living in both water and land habitats.
In this study, researchers collected a Chinese alligator sample from Changxing Yinjiabian Chinese Alligator Nature Reserve (Zhejiang Province, China) and sequenced its genome using a whole-genome shotgun strategy. The genomic data yielded a draft sequence of Chinese alligator with the size of 2.3 Gb, and a total of 22,200 genes were predicted.
The genomic data provides a strong evidence from DNA level to illustrate why Chinese alligator can hold its breath under water for long periods of time, such as the duplication of the bicarbonate-binding hemoglobin gene, positively selected energy metabolism, and others. Researchers further identified the genetic signatures of the powerful sensory system and immune system of Chinese alligator. All the results presented evidence for co-evolution of multiple systems specific to the back-to-the water transition.
Chinese alligator exhibits TSD, and does not possess sex chromosomes. The absence of sex chromosomes is another interesting feature. In this study, researchers analyzed the evolutionary mechanism of sex chromosomes, and reported that the alligator was the first TSD species whose genome has been sequenced, which will have great implication in resolving sex chromosome evolution.
