The Phylogeny of Xantusiidae, and its Placement with Scleroglossa (Reptilia: Squamata)

Brigham Young University, Provo UT

Investigators

Abstract

Relationships among the "night lizards" (Xantusiidae) have been problematic for over a century. Because of the group's antiquity, knowledge of its most primitive members is critical for understanding the deep evolutionary history of the larger group of lizards called scleroglossans. Scleroglossa, which includes snakes and their ancestors, is unique among amniotes (reptiles, birds, and mammals) in the repeated evolution of body elongation and limb reduction, Xantusiidae is a "tree-changing" group in that its alternative placements can alter hypothesized relationships among most other scleroglossan groups, but too few data are available to infer the exact placement of Xantusiidae. Thus, this project will: (1) reconstruct the phylogeny of the Xantusiidae based on the most extensive taxonomic sampling attempted to date; (2) use this phylogeny as a framework for a detailed study of the mutational dynamics and evolution of the mitochondrial cytochrome-b (cyt-b) gene; and (3) use the phylogeny to infer the exact position of Xantusiidae within Scleroglossa. This study will use both molecular and morphological data sets. The molecular sampling of relevant xantusiid species/populations is virtually complete; approximately 4500 base pairs (bp) of DNA sequence data will be collected for about 80 individuals, representing regions of several nuclear and mitochondrial genes (including the entire cyt-b gene). Extensive morphological data will be collected, including internal cranial characters revealed via high-resolution X-ray CT (HRCT) scans of all named fossil and extant xantusiids. HRCT will permit nondestructive collection of novel characters from fossils still embedded in matrix and critically important type specimens too precious to dissect. All characters will be combined and subjected to extensive computer analyses run on parallel clusters to ensure exhaustive searches of tree space. Mitochondrial cyt-b sequences will be mapped onto the resulting phylogenetic hypothesis for statistical studies of mutational dynamics across all levels of divergence in the Xantusiidae. This project will also support a large sequencing effort to address placement of Xantusiidae within Scleroglossa, including at least 8500 bp from 6 - 8 nuclear genes and some of the same mtDNA gene regions used for Xantusiidae. Preliminary data (HRCT scans) will be collected for selected scleroglossan taxa for preparation of a proposal to support a complimentary morphological study of the placement of Xantusiidae within Scleroglossa. This study will provide extensive morphological and molecular data sets, and a complete survey of cyt-b mutational dynamics, for an entire group of lizards (Xantusiidae). The group's small size will permit thorough explorations of tree search strategies for both data sets, sensitivity analyses to different character coding schemes and assumptions about character evolution, and determination of the group's basal members. This will in turn provide an understanding of character state distributions within Xantusiidae, which is key to its placement within Scleroglossa and therefore to understanding the evolution of this larger group. Extensive data sets will be generated including novel nuclear genes (optimized and made available for studies of other lizard groups) and novel cranial characters revealed via HRCT (and made available as an online resource). The study will provide extensive opportunities for undergraduate student training.