Chironius phylogeny

Chironius carinatus. JCM

Fourteen colubrid genera are known from South America: including  Chironius Fitzinger, 1826. This is one of the largest genera, with 16 species known to date. Its distribution ranges from the northern coast of Honduras to Uruguay and northeastern Argentina; and they are present in the Lesser Antilles. Chironius are long, slender, oviparous snakes that are mostly diurnal racers of terrestrial and arboreal habits. Most species inhabit lowland tropical rainforests, but also include humid montane forests and open formations. Dixon et al. (1993) made an extensive revision of Chironius and proposed the first morphological phylogeny of the genus, based mostly on hemipenial and external morphology. Their tree topology was not obtained using computer-based techniques, and no outgroup taxa were included in the study. Nevertheless, they discussed the affinities of Chironius with the other South American colubrid snakes, suggesting that Dendrophidion represents the sister-group.

Klaczko and colleagues present a phylogenetic hypothesis for 14species of the genus Chironius based on a combined molecular and morphological data matrix that includes one nuclear (c-mos) and two mitochondrial (12S and 16S) genes, and 37 morphological characters retrieved from scale, skull, and hemipenial features. They test the monophyly of the genus and provide additional evidence for its sister-group relationship within the South American radiation of colubrid snakes.
Phylogenetic relationships were inferred using maximum parsimony (MP) and maximum likelihood (ML). Their combined morphological and molecular analyses strongly support the monophyly of the genus Chironius and its sister-group relationship with a clade formed by the genera Dendrophidion and Drymobius. Phylogenetic relationships within the genus Chironius is still controversial, although five clades are retrieved with medium to strong support.
Their results corroborate three of the clades suggested by Dixon et al. (1993) (i.e. C. scurrulus and C. laevicollis; C. bicarinatus and C. flavolineatus; C. foveatus, C. multiventris, and C. laurenti). But, Dixon et al. (1993) retrieved the clade formed by C. scurrulus and C. laevicollis in the least inclusive position in their tree, whereas the combined analyses of Klaczko et al. reached the opposite result, with this clade in a basal position within the genus. Chironius laevicollisand C. scurrulus form a well-supported clade that corresponds to the most basal clade within Chironius in the MP combined analysis, and to the second most basal clade (including C. fuscus as their sister species) in the ML combined analysis. This clade is supported by seven unambiguous molecular synapomorphies, by the presence of lobular calyces in the hemipenis that gradually decrease towards the base of the organ, and by the absence of a bandeddorsal pattern in juveniles. The clade formed by C. vincenti, C. bicarinatus, and C. flavolineatus received only very low support values, but is corroborated by two unambiguous morphological synapomorphies: a Meckelian fossa delimited by the dentary and the splenial bones, and an anterior extension of the supratemporal that overlaps the parietal–prootic suture. Chironius bicarinatus and C. flavolineatus also form a poorly supported clade) defined exclusively by two unambiguous morphological synapomorphies: presence of apical pits restricted to the neck; and absence of a medial process in the prefrontal. Chironius laurenti, C. multiventris, and C. foveatus form a moderately supported clade defined exclusively by eight molecular unambiguous synapomorphies, whereas C. multiventris + C. foveatu form a clade supported by three molecular and one morphological synapomorphies.
Relationships with the genus Chironius remain controversial and more work needs to done on these interesting and widespread snakes.
Dixon JR, Wiest JA, Cei JM. 1993. Revision of Neotropical snake genus Chironius Fitzinger (Serpentes, Colubridae). Monografie XIII. Torino: Museo Regionale di Scienze Naturali.
Klaczko, J., Montingelli, G. G., & Zaher, H. (2014). A combined morphological and molecular phylogeny of the genus ChironiusFitzinger, 1826 (Serpentes: Colubridae). Zoological Journal of the Linnean Society. doi: 10.1111/zoj.12147.

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