Natural selection and adaptive evolution of leptin in the ochotona family driven by the cold environmental stress

Abstract

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.

Methodology/principal findings: 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/significance: 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.

Figure 1. Multiple alignments of leptin amino acids sequences.

Residues identical to Gorilla leptin are presented as dots (.). The predicted motifs are shaded by different colors [protein kinase C phosphorylation sites (PKC) are yellow; casein kinase II phosphorylation sites (CK2) are red; N-glycosylation sites are green). Underlined amino acid sequences indicate the motif of the ATP synthase α and β subunit signature site. The numbers at the right are the total numbers of amino acids. Two cysteine residues at 96 and 146 are indicated by asterisks.

Figure 2. Maximum likelihood tree of the ob gene and mitochondrial cytochrome b (cytb) gene.

(A) shows the phylogenetic tree based on nucleotide sequences of ob gene. Node A indicates reconstructed ancestral sequences. Branch B indicates the pika branch. (B) shows the phylogenetic tree based on nucleotide sequences of mitochondrial cytochrome b (cytb) gene. The scale bar of “0.1” means 0.1 nucleotide substitution per site.

Figure 3. Relationship between mean rates of synonymous substitution (Ks), non-synonymous substitution (Ka) and amino acid substitution (Aa) relative to outgroup and altitude (in m) and mean January actual temperature (Tjanu, °C).

(A) shows the relationship between mean rates of Ks, Ka and Aa relative to outgroup and altitude (in m). (B) shows the relationship between mean rates of Ks, Ka and Aa relative to outgroup and mean January actual temperature ((Tjanu, °C).