Diversification of the genus GeckoellaSun, 2013-09-01 16:05
By Ishan Agarwal
I’ve been obsessed with reptiles since I was in school. My initial fascination was with snakes – diverse, beautiful and potentially deadly; but more recently, I’ve been bitten by the gecko `bug’! Geckos are the most diverse lizard family in India, with more than 70 described species found across India. Geckos are also the most familiar of Indian lizards, with most of our homes harboring between one and three species of house geckos (Hemidactylus). However, Hemidactylus is only one genus out of about 15 Indian gecko genera, with the vast majority found living away from human habitations. Geckos are nocturnal and can be found on rocks, trees or on the ground; in a range of habitats such as forests, scrub and even deserts. Geckos also have a corresponding diversity in form and pattern, from the tiny Tropiocolotes persicus euphorbiacola in the Thar Desert with adult males no bigger than 5 cm in total length, to Gekko gecko in northeastern India that can grow up to over 34 cm total length. As I read more about geckos and travelled across India in the years leading up to my PhD, I began to ask questions about the diversity of these poorly understood and beautiful creatures. I wanted to know where different species were found and how they got there. How do we go about addressing such questions on geckos?
A brief description of how we go about addressing these questions:
Research in ecology and evolution that aim to understand diversification patterns within a group of species include two broad disciplines: systematics and biogeography. Systematics attempts to classify biological diversity based on the evolutionary relationships between species. Biogeography, on the other hand, is the study of species in space and time; and strives to identify historical events that may have shaped their spatial and temporal distribution (Cox & Moore 2005; Lomolino et al 2006). We should not confuse these with traditional taxonomy, which involves the cataloguing of biological diversity – naming, describing, and providing the means with which to identify species without an emphasis on evolutionary relationships. For a true understanding of the biogeographic patterns we see in nature, such as why and how different geckos are distributed across India, it is essential to know the evolutionary relationships between taxa.
Traditionally, evolutionary relationships were inferred based on morphological similarity. The use of morphological data has a long history, and has wide applicability due to the ease with which it can be used with ancient museum specimens and fossil data. While doing away with morphology entirely is neither feasible nor warranted, there are clear limitations in the use of morphological data. Imagine trying to reconstruct the vertebrate phylogeny using morphological data – what characters could one use to compare fish and mammals! Unlike molecular data, which can be used more generally, many morphological characters have broad applicability only within a taxonomic group.
Molecular data, in the form of DNA and RNA, are shared across all extant life forms and provide a potentially huge number of characters that can be used to determine true evolutionary relationships between species. Each position in a DNA sequence is a character, and the pattern of mutations or changes that occur between two lineages allows us to retrace the history of the group. These changes can also be modelled, and fairly accurate dates can be assigned to the splits between lineages. The result is a tree depicting evolutionary relationships through time, also called a phylogeny. These phylogenies then allow us to address a multitude of questions related to systematics and biogeography of a particular group of species.
Back to the Geckos:
Bent-toed geckos are the most diverse lizard group across South Asia. When I began my PhD, the Indian bent-toed geckos had been placed into Cyrtodactylus (S. Asian), Cyrtopodion (Palaearctic) and Geckoella genera (endemic to Peninsular India and Sri Lanka). Cyrtodactylus live on rocks and trees in moist forests of Southeast Asia (SE Asia) including Sri Lanka, and are found as far west as the Western Himalayas, with about 5 species in India. Cyrtopodion are mainly rock-dwelling, arid zone species found across the Palaearctic, reaching its eastern limit in north and northwest India, with about 7 species in India. Geckoella, on the other hand, includes terrestrial, forest dwelling species in both wet and dry zones and is endemic to Peninsula India and Sri Lanka, with five Indian species and two Sri Lankan species (Fig. 1) Little was known of the relationships between these genera. Based on morphological affinities, Bauer (2002) considered Geckoella a subgenus of Cyrtodactylus, while the only phylogenetic study recovered Cyrtodactylus and Geckoella as sister taxa (Feng et al. 2007), though Cyrtopodion was not included in the study. The evolutionary relationships within these groups remain unresolved, with both higher order as well as intrageneric relationships ambiguous. It is simply not possible to determine the evolutionary origin of the genus Geckoella without molecular data. This became the basis of the first chapter in my thesis,.
Working in Praveen Karanth’s lab at CES, I constructed a molecular phylogeny of the bent-toed geckos which revealed a number of fascinating patterns of west-to-east diversification, with a probable Palearctic origin of this clade. The basal group in this radiation turns out to be the palearctic Cyrtopodion, followed by a clade that has Hemidactylus sister to Cyrtodactylus including Geckoella. Cyrtodactylus has a number of well-defined subclades. The basal radiation is a Himalayan clade, followed by an Indo-Burma clade and then finally the rest of Cyrtodactylus including Geckoella. This implies that Geckoella is simply a divergent lineage of Cyrtodactylus, and is probably not a valid genus.
The question then arises, how might Geckoella have come into India from far eastern Southeast Asia? There are two possible explanations, the first has been invoked for freshwater crabs and considers that Southeast Asia and the Indian plate were in close proximity in the middle to late Eocene (~50 million years ago) with possible temporary land bridges connecting the landmasses. The alternative is that Geckoella came into Sri Lanka via a trans-oceanic dispersal event, as geckos are known to have crossed marine barriers at various points in their evolutionary history. Molecular dating estimates and sampling of Cyrtodactylus from Sri Lanka and the Andaman and Nicobar Islands are vital in determining the evolutionary origin of Geckoella. I hope to explore these questions in the future.
Caption for Fig 1: This picture shows the Indian species of Geckoella – albofasciatus from the central Western Ghats, deccanensis from northern WG, collegalensis from the drier foothills of the Western Ghats, from Southern Gujarat till South India with unconfirmed reports from Sri Lanka; nebulosus across the Eastern Ghats from Ranchi down to Chennai, extending west into the Satpuras; and jeyporensis from the Eastern Ghats.
Bauer, A. M. 2002. Two new species of Cyrtodactylus (Squamata:
Gekkonidae) from Myanmar. Proceedings of California Academy of Science
Feng, J., Han, D., Bauer, A. M. and K. Zhou. 2007. Interrelationships
among Gekkonid geckos inferred from mitochondrial and nuclear gene
sequences. Zoological Science 24: 656–665.