Evolution of a special organ, nuchal gland, based on a molecular phylogeny of the Eurasian natricine snakes (Serpentes: Colubridae)

Abstract

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An Asian natricine snake, Rhabdophis tigrinus, has a unique series o f organs, called nuchal glands, which contain cardiac steroidal toxins known as bufadienolides. Rhabdophis tigrinus sequesters bufadienolides from its toad prey and stores them in the nuchal glands as a defensive substance. Among m ore than 3400 species o f snakes, only 18 Asian natricine species are known to have the nuchal glands. These 18 species belong to three genera, Balanophis, Macropisthodon, and Rhabdophis. In Macropisthodon and Rhabdophis, however, species without the nuchal glands also exist. This evidence suggests multiple independent origin and/or secondarily loss o f the nuchal glands. To infer the evolutionary history o f the nuchal glands, w e investigated the molecular phylogenetic relationships among Eurasian natricine species with and w ithout the nuchal glands, based on variations in partial sequences of the oocyte maturation factor Mos (c-mos) gene, the recom bination-activating gene 1 (Rag 1), and the mitochondrial cytochrome b (cyt.b) gene (total 2.6 kbp). The results supported that species w ith the nuchal glands were all contained in a single clade. Therefore, based on principle o f parsimony, it is inferred that the common ancestor o f the species within this clade has obtained the nuchal glands and that multiple independent evolution has not occurred. Rhabdophis swinhonis, some individuals o f which are known to lack nuchal glands, is considered to have em erged as newly detected position in the nuchal gland clade. Therefore, it is strongly suggested that secondarily loss o f the nuchal glands has occurred at least in some individuals o f R. swinhonis.