![]() ![]() 1989, 1992 Muller 1992 Isbister and Gray 2003b).Ĭomplementary DNA (cDNAs) for two families of venom protein components-latrodectins and latrotoxins-have been cloned from the venom glands of the Eurasian black widow L. geometricus), also inflict painful bites with similar physiological effects to those from black widows but with varying degrees of potency ( Muller et al. hasselti) and highly invasive brown widow ( L. Other Latrodectus species, including the sexually cannibalistic Australian red-back spider ( L. variolus), which are widely recognized and feared because of the extreme neurotoxicity of their venom and their abundance in human-inhabited areas ( Clark et al. “Black widow” is a common name referring to several Latrodectus species (e.g., L. 1990, Kiyatkin, Dulubova, and Grishin 1993 Dulubova et al. A striking example of this dearth of studies is the molecular composition of black widow spider venom, which has been extensively characterized from only 1 of the 31 Latrodectus species in the family Theridiidae ( Latrodectus tredecimguttatus Knipper et al. 2004), and evolutionary analyses of spider venoms have been limited ( Sollod et al. The order Araneae (spiders) is the largest clade of venomous organisms, but biochemical characterization of their venoms has been restricted to a small sampling of species ( Fletcher et al. Notably, venom molecular evolution is highly dynamic and appears to be shaped by frequent gene duplications and strong diversifying selection, as well as co-evolution and convergence ( Duda and Palumbi 1999 Li et al. Genes encoding venom toxins are also of significant evolutionary interest because of their direct role in organismal fitness and ecological adaptation. Venom toxins have drawn enormous scientific attention because of their applications as pharmaceuticals and as probes for isolating cellular receptors ( Adams and Olivera 1994 Lewis and Garcia 2003 Veiseh et al. Several organismal lineages have independently become venomous and exhibit morphological convergence in venom-injecting organs, as well as similarities at the molecular level in their toxin repertoire due to independent recruitment of related genes for venom production ( Fry et al. Venoms are chemically complex secretions produced by some animals for defense or prey acquisition. The greater differences between Latrodectus and Steatoda α-latrotoxin, and their relationships to invertebrate-specific latrotoxins, suggest a shift in α-latrotoxin toward increased vertebrate toxicity coincident with the evolution of widow spiders. Long-term purifying selection on α-latrotoxin indicates its functional importance in black widow venom, even though vertebrates are a small fraction of their diet. ![]() hasselti, antivenom in treating bites from other Latrodectus species, and the weaker neurotoxic symptoms associated with Steatoda and Parasteatoda bites. Such variation likely explains the efficacy of red-back spider, L. ![]() However, α-latrotoxin is highly divergent in amino acid sequence between these genera, with 68.7% of protein differences involving non-conservative substitutions, evidence for positive selection on its physiochemical properties and particular codons, and an elevated rate of nonsynonymous substitutions along α-latrotoxin’s Latrodectus branch. Multiple codon and branch-specific estimates of the nonsynonymous/synonymous substitution rate ratio also suggest a long history of purifying selection has acted on α-latrotoxin across Latrodectus and Steatoda. Across black widow species, α-latrotoxin shows ≥94% nucleotide identity and variability consistent with purifying selection. In this study, ∼4 kb α-latrotoxin sequences and their homologs were characterized from a diversity of Latrodectus species, and representatives of Steatoda and Parasteatoda, establishing the wide distribution of latrotoxins across the mega-diverse spider family Theridiidae. The evolution of this toxin is enigmatic because only two α-latrotoxin sequences are known. α-Latrotoxin is the vertebrate-specific toxin responsible for the dramatic effects of black widow envenomation. Black widow spiders (members of the genus Latrodectus) are widely feared because of their potent neurotoxic venom.
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