Natural selection and adaptive evolution of leptin in the ochotona family driven by the cold environmental stress.
BACKGROUND
Environmental stress can accelerate the evolutionary rate of specific stress-response proteins and create new functions specialized for different environments, enhancing an organism's fitness to stressful environments. Pikas (order Lagomorpha), endemic, non-hibernating mammals in the modern Holarctic Region, live in cold regions at either high altitudes or high latitudes and have a maximum distribution of species diversification confined to the Qinghai-Tibet Plateau. Variations in energy metabolism are remarkable for them living in cold environments. Leptin, an adipocyte-derived hormone, plays important roles in energy homeostasis.


RESULTS
To examine the extent of leptin variations within the Ochotona family, we cloned the entire coding sequence of pika leptin from 6 species in two regions (Qinghai-Tibet Plateau and Inner Mongolia steppe in China) and the leptin sequences of plateau pikas (O. curzonia) from different altitudes on Qinghai-Tibet Plateau. We carried out both DNA and amino acid sequence analyses in molecular evolution and compared modeled spatial structures. Our results show that positive selection (PS) acts on pika leptin, while nine PS sites located within the functionally significant segment 85-119 of leptin and one unique motif appeared only in pika lineages-the ATP synthase alpha and beta subunit signature site. To reveal the environmental factors affecting sequence evolution of pika leptin, relative rate test was performed in pikas from different altitudes. Stepwise multiple regression shows that temperature is significantly and negatively correlated with the rates of non-synonymous substitution (Ka) and amino acid substitution (Aa), whereas altitude does not significantly affect synonymous substitution (Ks), Ka and Aa.


CONCLUSIONS
Our findings support the viewpoint that adaptive evolution may occur in pika leptin, which may play important roles in pikas' ecological adaptation to extreme environmental stress. We speculate that cold, and probably not hypoxia, may be the primary environmental factor for driving adaptive evolution of pika leptin.
Yang
Jie
J
Key Laboratory of Qinghai-Tibetan Plateau Biological Evolution and Adaptation, Northwest Institute of Plateau Biology, The Chinese Academy of Sciences, Xining, Qinghai, China.
Wang
Zhen Long
ZL
Zhao
Xin Quan
XQ
Wang
De Peng
de P
Qi
De Lin
de L
Xu
Bao Hong
BH
Ren
Yong Hong
YH
Tian
Hui Fang
HF
D000222:Adaptation, Physiological; D000595:Amino Acid Sequence; D000818:Animals; D003001:Cloning, Molecular; D003080:Cold Temperature; D018076:DNA, Complementary; D007793:Lagomorpha; D020738:Leptin / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D016013:Likelihood Functions; D008969:Molecular Sequence Data; D010802:Phylogeny; D012641:Selection, Genetic; D017386:Sequence Homology, Amino Acid
D018076:DNA, Complementary; D020738:Leptin