Abstract:
A large body of evidence indicates that evolutionary innovations of novel organs
have facilitated the subsequent diversification of species. Investigation of the evolutionary
history of such organs should provide important clues for understanding the
basis for species diversification. An Asian natricine snake, Rhabdophis tigrinus, possesses
a series of unusual organs, called nuchal glands, which contain cardiotonic
steroid toxins known as bufadienolides. Rhabdophis tigrinus sequesters bufadienolides
from its toad prey and stores them in the nuchal glands as a defensive mechanism.
Among more than 3,500 species of snakes, only 17 Asian natricine species are
known to possess nuchal glands or their homologues. These 17 species belong to
three nominal genera, Balanophis, Macropisthodon, and Rhabdophis. In Macropisthodon
and Rhabdophis, however, species without nuchal glands also exist. To infer the evolutionary
history of the nuchal glands, we investigated the molecular phylogenetic
relationships among Asian natricine species with and without nuchal glands, based
on variations in partial sequences of Mt-CYB,
Cmos, and RAG1 (total 2,767 bp).
Results show that all species with nuchal glands belong to a single clade (NGC).
Therefore, we infer that the common ancestor of this clade possessed nuchal glands
with no independent origins of the glands within the members. Our results also
imply that some species have secondarily lost the glands. Given the estimated divergence
time of related species, the ancestor of the nuchal gland clade emerged
19.18 mya. Our study shows that nuchal glands are fruitful subjects for exploring the
evolution of novel organs. In addition, our analysis indicates that reevaluation of the
taxonomic status of the genera Balanophis and Macropisthodon is required. We propose
to assign all species belonging to the NGC to the genus Rhabdophis, pending
further study.
