title: Snake venoms are integrated systems, but abundant venom proteins evolve more rapidly.
abstract: BACKGROUND
  While many studies have shown that extracellular proteins evolve rapidly, how selection acts on them remains poorly understood. We used snake venoms to understand the interaction between ecology, expression level, and evolutionary rate in secreted protein systems. Venomous snakes employ well-integrated systems of proteins and organic constituents to immobilize prey. Venoms are generally optimized to subdue preferred prey more effectively than non-prey, and many venom protein families manifest positive selection and rapid gene family diversification. Although previous studies have illuminated how individual venom protein families evolve, how selection acts on venoms as integrated systems, is unknown.
  
  
  RESULTS
  Using next-generation transcriptome sequencing and mass spectrometry, we examined microevolution in two pitvipers, allopatrically separated for at least 1.6 million years, and their hybrids. Transcriptomes of parental species had generally similar compositions in regard to protein families, but for a given protein family, the homologs present and concentrations thereof sometimes differed dramatically. For instance, a phospholipase A2 transcript comprising 73.4 % of the Protobothrops elegans transcriptome, was barely present in the P. flavoviridis transcriptome (<0.05 %). Hybrids produced most proteins found in both parental venoms. Protein evolutionary rates were positively correlated with transcriptomic and proteomic abundances, and the most abundant proteins showed positive selection. This pattern holds with the addition of four other published crotaline transcriptomes, from two more genera, and also for the recently published king cobra genome, suggesting that rapid evolution of abundant proteins may be generally true for snake venoms. Looking more broadly at Protobothrops, we show that rapid evolution of the most abundant components is due to positive selection, suggesting an interplay between abundance and adaptation.
  
  
  CONCLUSIONS
  Given log-scale differences in toxin abundance, which are likely correlated with biosynthetic costs, we hypothesize that as a result of natural selection, snakes optimize return on energetic investment by producing more of venom proteins that increase their fitness. Natural selection then acts on the additive genetic variance of these components, in proportion to their contributions to overall fitness. Adaptive evolution of venoms may occur most rapidly through changes in expression levels that alter fitness contributions, and thus the strength of selection acting on specific secretome components.
authors: Okinawa Institute of Science and Technology Graduate University, Tancha 1919-1, Onna-son, Kunigami-gun, Okinawa-ken, 904-0412, Japan. steven.aird@oist.jp.
authors: Steven D
authors: SD
authors: Aird
authors: Okinawa Institute of Science and Technology Graduate University, Tancha 1919-1, Onna-son, Kunigami-gun, Okinawa-ken, 904-0412, Japan. aggarwalshikha91@yahoo.in.|University School of Environment Management, Guru Gobind Singh Indraprastha University, Sector 16C, Dwarka, New Delhi, 110078, India. aggarwalshikha91@yahoo.in.
authors: Shikha
authors: S
authors: Aggarwal
authors: Okinawa Institute of Science and Technology Graduate University, Tancha 1919-1, Onna-son, Kunigami-gun, Okinawa-ken, 904-0412, Japan. avillar@oist.jp.
authors: Alejandro
authors: A
authors: Villar-Briones
authors: Okinawa Institute of Science and Technology Graduate University, Tancha 1919-1, Onna-son, Kunigami-gun, Okinawa-ken, 904-0412, Japan. mandytin@oist.jp.
authors: Mandy Man-Ying
authors: MM
authors: Tin
authors: Okinawa Prefectural Institute of Health and the Environment, Biology and Ecology Group, 2003 Ozato, Ozato, Nanjo-shi, Okinawa, 901-1202, Japan. teradakk@pref.okinawa.lg.jp.
authors: Kouki
authors: K
authors: Terada
authors: Okinawa Institute of Science and Technology Graduate University, Tancha 1919-1, Onna-son, Kunigami-gun, Okinawa-ken, 904-0412, Japan. alexander.mikheyev@oist.jp.|Research School of Biology, Australian National University, Canberra, ACT 0200, Australia. alexander.mikheyev@oist.jp.
authors: Alexander S
authors: AS
authors: Mikheyev
mesh_terms: D000818:Animals; D005075:Biological Evolution; D020869:Gene Expression Profiling / Q000379:methods; D005786:Gene Expression Regulation; D059014:High-Throughput Nucleotide Sequencing; D013058:Mass Spectrometry; D040901:Proteomics / Q000379:methods; D012641:Selection, Genetic; D012910:Snake Venoms / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D059467:Transcriptome
chemical_list: D012910:Snake Venoms