Aim: The evolution of key innovations promotes adaptive radiations by opening access to novel ecological opportunity. The acquisition of viviparity (live-bearing reproduction) has emerged as one such innovation explaining reptile proliferations into extreme climates. By evolving viviparity, females provide embryos with internally stable environments to complete development. The classical hypothesis suggests that natural selection for viviparity arises from low temperatures in cold-climates, which promote prolonged egg retention in the mother’s body. An alternative hypothesis proposes that declines in atmospheric oxygen at high elevations create natural selection for embryo retention to provide them with optimal oxygen levels during development. However, although experimental studies support the negative effects of low oxygen on egg development, this ‘hypoxia’ hypothesis has never been tested quantitatively. Here, we compete the hypoxia hypothesis against the ‘cold-climate’ hypothesis, using a highly-diverse lizard genus.\ud Location: South America\ud Major taxa: Liolaemus lizards.\ud Methods: We employ a multivariate dataset covering 121-species varying extensively in geographic and climatic distribution (including extreme thermal and oxygen gradients), and parity mode. Based on a new molecular phylogeny for the genus, we use phylogenetic logistic regressions to generate a range of models ranking environmental factors as a function of their effects on parity mode transitions.\ud Results: Elevation and oxygen declines correlate nearly perfectly, and both were identified as the dominant predictors of oviparity-to-viviparity transitions, while the role for temperature (dominated by the coldest winter temperatures and daily fluctuations) is significant but secondary. Overall, we show that oxygen-deprivation and low temperatures both play a role in the evolution of viviparity.\ud Main conclusions: Our findings support the role for selection from declines in oxygen concentrations as the primary driver behind viviparity. However, selection arising from cold temperatures and from reduced fluctuations in daily temperatures contribute to the evolution of these transitions by creating multivariate selection on parity mode

The major venom component of Micrurus mipartitus, a coral snake distributed from Nicaragua to northern South America, was characterized biochemically and functionally. This protein, named mipartoxin-I, is a novel member of the three-finger toxin superfamily, presenting the characteristic cysteine signature and amino acid sequence length of the short-chain, type-I, α-neurotoxins. Nevertheless, it varies considerably from related toxins, with a sequence identity not higher than 70% in a multiple alignment of 67 proteins within this family. Its observed molecular mass (7030.0) matches the value predicted by its amino acid sequence, indicating lack of post-translational modifications. Mipartoxin-I showed a potent lethal effect in mice (intraperitoneal median lethal dose: 0.06 μg/g body weight), and caused a clear neuromuscular blockade on both avian and mouse nerve-muscle preparations, presenting a post-synaptic action through the cholinergic nicotinic receptor. Since mipartoxin-I is the most abundant (28%) protein in M. mipartitus venom, it should play a major role in its toxicity, and therefore represents an important target for developing a therapeutic antivenom, which is very scarce or even unavailable in the regions where this snake inhabits. The structural information here provided might help in the preparation of a synthetic or recombinant immunogen to overcome the limited venom availability.

The uplift and final connection of the Central American land bridge is considered the major event that allowed biotic exchange between vertebrate lineages of northern and southern origin in the New World. However, given the complex tectonics that shaped Middle America, there is still substantial controversy over details of this geographical reconnection, and its role in determining biogeographic patterns in the region. Here, we examine the phylogeography of Bothrops asper, a widely distributed pitviper in Middle America and northwestern South America, in an attempt to evaluate how the final Isthmian uplift and other biogeographical boundaries in the region influenced genealogical lineage divergence in this species. We examined sequence data from two mitochondrial genes (MT-CYB and MT-ND4) from 111 specimens of B. asper, representing 70 localities throughout the species’ distribution. We reconstructed phylogeographic patterns using maximum likelihood and Bayesian methods and estimated divergence time using the Bayesian relaxed clock method. Within the nominal species, an early split led to two divergent lineages of B. asper: one includes five phylogroups distributed in Caribbean Middle America and southwestern Ecuador, and the other comprises five other groups scattered in the Pacific slope of Isthmian Central America and northwestern South America. Our results provide evidence of a complex transition that involves at least two dispersal events into Middle America during the final closure of the Isthmus.

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All genome sequencing data have been deposited in GenBank.