Wednesday, December 11, 2013

New molecular study provides insights into boa and python evolution

Henophidian snakes (boas, pythons, and their relatives) are one of the most spectacular groups of reptiles and constitute a vast diversity of morphologies, behaviors, body sizes and ecologies. This group include both the shortest (the black-bellied dwarf boa, Tropidophis nigriventris, about 250 mm in total length) and longest (reticulated python, about 10 m in total length henophidian snakes. These are constricting snakes, and include the 13 m Titanoboa cerrejonensis possibly the longest snake to ever exist. Henophidians are also represented by enigmatic and hyper-diverse families such as the Tropidophiidae (the dwarf boas) and Uropeltidae (the shield-tailed snakes); as well as by the nearly extinct insular Mascarene family Bolyeriidae. Boas (Boidae) and pythons (Pythonidae) represent highly diverse families with almost global tropical and subtropical distributions. Molecular phylogenetic and biogeographic studies of the boas and pythons have yielded important insights into evolutionary processes as well as an understanding of taxonomy, divergence, and diversification is becoming especially relevant given that many boa and python species are of significant conservation concern.

In an early online view of a new study Graham Reynolds and colleagues use both new and previously published sequence data, to produced a species-level phylogeny for 84.5% of boid species and 82.5% of pythonid species, set within a larger phylogeny of henophidian snakes. They obtained new sequence data for three boid, one pythonid, and two tropidophiid taxa which have never previously been included in a molecular study, in addition to generating novel sequences for seven genes across an additional 12 taxa. The authors compiled an 11-gene data set for 127 taxa, consisting of the mitochondrial genes CYTB, 12S, and 16S, and the nuclear genes bdnf, bmp2, c-mos, gpr35, rag1, ntf3, odc, and slc30a1, totaling up to 7561 base pairs per taxon.They analyzed this dataset using both maximum likelihood and Bayesian inference and recovered a well-supported phylogeny.

Significant evidence was found for the  discordance between taxonomy and evolutionary relationships in the genera Tropidophis, Morelia, Liasis, and Leiopython, and they found support for elevating two previously suggested boid species. they suggest a revised taxonomy for the boas (13 genera, 58 species) and pythons (8 genera, 40 species).

Some of the highlights of the study:

The authors continue to recognize Aniliidae for the Neotropical pipe snake. Tropidophiidae contains both Tropidophis (which is polyphyletic and Trachyboa). The Southeast Asian dwarf pipe snakes (Anomochilidae) are the sister to the Old World pipe snakes (Cylindrophiidae). Collectively the Eastern Hemisphere pipe snakes are the sister to the shield-tailed snakes (Uropeltidae). The sunbeam snakes form the family Xenopeltidae and the Mexican python is placed in the family Loxocemidae.Within the Pythonidae, Python is an Afro-Asian genus (regius, curtus, brogersmai, molurus, bivittaus, anchietae, and sebae). The new genus Malayopython  contains timorensis and reticulatus. Morelia  (carinata, bredi, spilota, viridis) is polyphyletic.

The new study recovered the monotypic Central American Loxocemidae (Loxocemus bicolor) as sister to the in-group pythons, while the East Asian Xenopeltidae (X. hainanensis and X. unicolor) form the sister to the clade (Pythonidae, Loxocemidae) in both analyses. Among the pythons, they found the African and southern Asian genus Python to be a monophyletic clade basal to the rest of the pythons. Within this genus there was support for a basal placement of the small-bodied West African P. regius and a derived clade of large bodied species (P. bivittatus and P. molurus) from southern Asia and a derived clade of the small-bodied Southeast Asian blood pythons (P. brongersmai and P. curtus). This suggests an evolution of gigantism separate from other giant members of the Australasian Pythonidae (Malayopython and Morelia). They also generated novel sequence data for the small-bodied Kalaharian P. anchietae. The subgeneric name Simalia Gray 1849 is used for the scrub python clade.

As for the Boidae, the authors found weak support for a sister relationship of the non-boid Calabaria to the Madagascan boids. As this is not well supported and inconsistent with previous studies, which have all found Calabaria to be basal to the rest of the boid radiation they consider Calabaria as the closest extant relative to the Boidae. Among Malagasy species they found strong support for the distinction of Sanzinia and Acrantophis.

They recovered the Central American Ungaliophis and Exiliboa as sister taxa to the North American boids Charina and Lichanura, with support for the distinction of C. bottae and C. umbratica.

With the first published sequence data for the African species Eryx muelleri, they found strong support for the placement of this species as sister to the south Asian E. jayakari. And found evidence for interdigitation of Eryx species between Africa and south Asia suggesting repeated dispersal events. No evidence was found for the continued use of the generic name Gongylophis.

Among western hemisphere boids, support was found for the recognition of B. imperator and recommend the epithet B. imperator.

As in previous studies the South American Eunectes and Epicrates were recovered as sisters with support for the distinction of the five mainland species of Epicrates. And, they found support for the continued use of the genus Chilabothrus for the island boas formerly in the genus Epicrates.

Graham Reynolds, R., M. L. Niemiller, L. J. Revell. (2013) Toward a Tree-of-Life for the boas and pythons: Multilocus species-level phylogeny with unprecedented taxon sampling. Molecular Phylogenetics and  Evolution.

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