The Hindu epic Ramayana describes how the protagonist Rama, with help from an army of apes, builds a bridge from the peninsula of India to Sri Lanka, to free his wife Sita from the clutches of Ravana. While the existence of such a bridge is still subject to much debate, geological evidence suggests that a thirty-kilometre long 'bridge of the sea' did form a land connection between India and Sri Lanka not so long ago. In a recent study, researchers Aparna Lajmi and Rohini Bansal in Dr. Praveen Karanth's lab at Indian Institute of Science in collaboration with Dr. Varad Giri, have traced the trail of Indian geckos that were once believed to have crossed this bridge.
Peninsular India and Sri Lanka, which were land fragments that separated from Madagascar about 90 million years ago and merged with Asia, have shared a large part of their geological history. The first separation of India and Sri Lanka happened in the early Miocene era, roughly 15 million years ago. Since then, the two landmasses have reconnected and severed many times, most recently about 10,000 years ago, due to the rising sea levels. This separation has allowed animals in Sri Lanka to become unique and evolve independently, in a process called endemism.
In this study, published in the Zoological Journal of the Linnean Society, the researchers studied the dispersal patterns of the Indian endemic Hemidactylus geckos, or commonly called house geckos, to Sri Lanka, when the land bridge was above sea level. The study was supported by grants from the Department of Science and Technology, Government of India, and The Rufford Small Grants Foundation.
What makes studying the dispersions of geckos interesting? “Island systems provide an exciting setting to study lineage diversification of a species because they are an isolated landmass. However, even though it is an island, Sri Lanka’s intermittent connections with India provides an ideal system to study the role of dispersal in the formation of an island's biodiversity”, says Prof. Praveen Karanth from IISc and an author of the study, in an interview with Research Matters.
Existing research on dispersal patterns of geckos points to two different hypotheses—one, the repeated exchange of fauna between India and Sri Lanka, and two, the in situ formation of endemic new species of geckos in Sri Lanka. To study these hypotheses, the researchers chose the Hemidactylus gecko as they are endemic to the Indian subregion (peninsular India and Sri Lanka) and have more than 27 different species.
The researchers started off by analysing the genetic makeup of the geckos found in the Indian subregion and used their findings to construct an 'evolutionary tree'. The evolutionary tree provides information about the time when the first geckos crossed over from India to Sri Lanka via the sea bridge. Based on this analysis, the researchers then studied how speciation—the formation of a new species due to evolution over a long period—occurred, and reconstructed the historical area of the gecko’s distribution.
“The dispersals of these geckos occurred millions of years ago, and hence their impact on the ecosystem is difficult to access. However, this study and others indicate that the intermittent connection between India and Sri Lanka has had a profound impact on the island's biota”, says Prof. Karanth on whether the geckos crossing over into Sri Lanka affected the ecosystem of the island.
Out of the 27 species that are descendants of the Indian Hemidactylus gecko, the evolutionary tree showed three distinct groups. The first group consists of large geckos with smooth bodies, the second of small-bodied terrestrial geckos endemic to India and Pakistan, and the third, medium to large-sized geckos with rough, bumpy skin. They have dispersed to Sri Lanka several times, and some have now evolved to be endemic to the Sri Lankan ecosystem. The study suggests that the evolution of these species in Sri Lanka is a result of their separation from their Indian counterparts when the sea levels rose in the Miocene era.
The study also found that some gecko species like H. scabriceps (scaly gecko) and H. lankae (Sri Lankan leaf-toed gecko) that inhabit the semiarid open habitats survived in both India and Sri Lanka. The Indian subcontinent underwent a drastic change in vegetation during the Late Miocene era where some forests gave way to open grassland habitats. Although these changes caused the extinction of wetland adapted animals like caecilians and shield-tail snakes, those that were adapted to open habitats, like the Sitana lizards, survived and formed new species. The land bridge between India and Sri Lanka was re-established around this period, allowing for a faunal exchange.
Wet-zone gecko species like H. hunae (spotted giant gecko) and H. depressus (Sri Lankan leaf-nosed gecko) that are now endemic to Sri Lanka are thought to have colonised the country earlier, while the dry zone species appear to have colonized the island in more recently with the establishment of a drier haitat and the reconnection between India and Sri Lanka roughly 5 to 10 million years ago in the late Miocene era. The results of the study show that multiple dispersals from peninsular India to Sri Lanka explain the pattern of distribution of the Hemidactylus geckos. These results support the hypothesis that Sri Lankan fauna is a subset of Indian fauna. However, unlike the other fauna of Sri Lanka like freshwater crabs, shrimp, tree frogs and caecilians, the geckos have not undergone in situ diversification. Also, there is no evidence of the Sri Lankan species dispersing back into India.
On the other hand, some gecko species like the H. frenatus (common house gecko), H. parvimaculatus (spotted house gecko) and H. leschenaultii (Leschenault's leaf-toed gecko), found in both India and Sri Lanka, live in habitats influenced by human settlements and are commensals with humans. They derive some benefits from living near humans but don’t affect the human settlement or cause any harm. The researchers postulate that they might have dispersed to Sri Lanka very recently with the help of human transport—a hypothesis they plan to test in the future.
“A more detailed population genetic study of the dry zone and human commensal species needs to be undertaken. We plan to use population genetics approaches to study the origin and evolution of Sri Lankan commensals”, says Prof. Karanth, on the future directions of his research.