Venomous snakes, all of which belong to the superfamily Colubroidea, evolved glands for the storage and dispersal of their saliva approximately 60-80 million years ago. Since that time, various prey-immobilizing toxins have evolved from innocuous proteins that were normally produced in other body tissues.
Scientists believe that snakes, rather than simply tweaking proteins already expressed in their saliva, recruited and altered proteins for their chemical arsenal from other body tissues. This enabled snakes to develop more specific, highly potent toxins, ones that would cause their victims' bodies to turn against themselves upon injection. Over time, these newly derived toxins became a normal part of the saliva protein repertoire. To date, 24 different snake venom toxins have been characterized by scientists, but the evolutionary history ?or tissue origin ?of these proteins has not been documented.
In his March 2005 Genome Research article, Fry, the Deputy Director of the Australian Venom Research Unit, identified the origin of the 24 known snake toxin types. Surprisingly, rather than being saliva-modified proteins, 21 of the toxins were shown to have been originally derived from proteins normally expressed in other body tissues, including brain, eye, lung, heart, liver, muscle, mammary gland, ovary, and testis.
Only two of the toxins were derived from proteins presumably expressed in ancient reptile saliva. Both of these toxin types, CRISP and kallikrein, are closely related to toxins called helothermine and gilatoxin, which are produced by the Beaded Lizard and the Gila Monster, respectively.
One of the toxins in this study (called the
Source:Cold Spring Harbor Laboratory