When one phenotype is not enough – divergent evolutionary trajectories govern venom variation in a widespread rattlesnake species

Abstract

32 33 2 SUMMARY 34Understanding the relationship between genome, phenotypic variation, and the ecological 35 pressures that act to maintain that variation, represents a fundamental challenge in evolutionary 36 biology. Functional polymorphisms typically segregate in spatially isolated populations [1, 2] 37 and/or discrete ecological conditions [3][4][5], whereas dissecting the evolutionary processes 38 involved in adaptive geographic variation across a continuous spatial distribution is much more 39 challenging [6]. Additionally, pleiotropic interactions between genes and phenotype often 40 complicate the identification of specific genotype-phenotype links [7][8], and thus of the selective 41 pressures acting on them. Animal venoms are ideal systems to overcome these constraints: they 42 are complex and variable, yet easily quantifiable molecular phenotypes with a clear function and 43 a direct link to both genome and fitness [9]. Here, we use dense and widespread population-level 44 sampling of the Mohave rattlesnake, Crotalus scutulatus, and show that genomic structural 45 variation at multiple loci underlies extreme geographic variation in venom composition, which is 46 maintained despite extensive gene flow. Unexpectedly, selection for diet does not explain venom 47 variation, contrary to the dominant paradigm of venom evolution, and neither does neutral 48 population structure caused by past vicariance. Instead, different toxin genes correlate with 49 distinct environmental factors, suggesting that divergent selective pressures can act on individual 50 loci independently of their genomic proximity or co-expression patterns. Local-scale spatial 51 heterogeneity thus appears to maintain a remarkably ancient complex of molecular phenotypes, 52 which have been retained in populations that diverged more than 1.5-2 MYA, representing an 53 exceptional case of long-term structural polymorphism. These results emphasize how the 54 interplay between genomic architecture and spatial heterogeneity in selective pressures may 55 facilitate the retention of functional polymorphisms of an adaptive phenotype. 57 RESULTS AND DISCUSSION58 Rattlesnake (Crotalus) venoms are among the most complex exocrine secretions in nature, with 59 tens to hundreds of individual components. They display a puzzling phenotypic dichotomy, with 60 two largely mutually exclusive venom strategies: highly lethal type A venoms, characterized by 61 neurotoxic dimeric phospholipase A2, e.g. Mojave toxin (MTX), and less toxic type B venoms, 62 which lack MTX, but are rich in snake venom metalloproteinases (SVMPs) with haemorrhagic 63 activity [10]. Both types seem randomly distributed across the phylogeny of rattlesnakes, and 64 sometimes co-occur within populations of a single species. The Mohave rattlesnake, Crotalus 65 scutulatus, is well-known for displaying both venom strategies across a continuous distributional 66 range in the North American deserts ( Figure 1A) [11][12][13]. This species thus represents an ideal 67 system to investigate the...

When one phenotype is not enough: divergent evolutionary trajectories govern venom variation in a widespread rattlesnake species

“…In fact, the generally positive association between type B venoms and higher winter temperatures runs contrary to the hypothesis that SVMPs are needed to assist digestion at lower temperatures [10,55]. However, climatic stability and seasonality may affect other factors, for instance prey community composition and dynamics [52]. These, in turn, could influence snake foraging strategies, and potentially also the exposure of snakes to predation, an understudied source of selection on venom [56].…”

“…(d) Spatial environmental heterogeneity predicts venom variation Spatial heterogeneity in environmental variables is a key driver of genotypic and phenotypic polymorphism [51]. In the absence of a strong venom-diet association, we performed EAA to understand whether differences in other biotic and/or abiotic factors contribute to geographical variation of venom composition [47,52,53]. Overall venom variation was strongly associated with temperature (table 1), and the longitudinal climatic gradient characterizing the Sonoran desert (electronic supplementary material, figures S9 and S10) was reflected in the differentiation across venom A profiles along the first NMDS axis ( figure 3a).…”

Venomics and Peptidomics of Palearctic vipers: Clade-wide analysis of seven taxa of the generaVipera,Montivipera,MacroviperaandDaboiaacross Türkiye

“…It is important not to generalize the venom composition of a single population to be per se representative for a whole species or subspecies. They should rather be considered as potential source for differing compositions, as it has been already reported for several vipers 16,29,32,33 .…”