Evolution in island birds and the ‘insularity’ syndrome
Islands are renowned natural laboratories for the study of ecology and evolution given their isolation, small area and simple biotas. However, many aspects of island ecology and evolution remain poorly understood. Our studies on island birds have shown that insularity favours reduced fecundity, extended developmental periods, increased sociality, and reduced sexual dimorphism in plumage (and a tendency for duller plumage in island birds). Our research is now focusing on the mechanisms underlying these patterns. Current research topics include host-parasite dynamics and immunity, evolutionary genomics as well as the role of inter-specific competition on song evolution.
The low species richness on islands results in increased population densities and niche enlargement among island birds, but it remains unclear if these characteristics extend to the parasite community. Martim Melo and Rita Covas have been working in the Gulf of Guinea islands and nearby mainland areas for over 15 years and, in collaboration with Claire Loiseau (CIBIO, University of Porto) analysed c. 2,000 blood samples for ‘avian malaria’ Haemosporidia parasites. The results confirmed lower parasite pressure on the islands. In addition, the island parasite assemblage was distinct, comprising a mix of generalist and specialist parasites that we interpret to represent, respectively, recent arrivals and ancient colonisation events (which have evolved into endemic lineages, as proposed by E.O. Wilson’s taxon cycle hypothesis).
But these results may not hold for different types of parasites. Similar analyses conducted for Trypanosoma parasites did not reveal any striking dissimilarities between the island and mainland communities. Finer scale analyses are currently underway, but these results appear to indicate that Trypanosoma are generalist parasites with widespread distributions.
An important and unresolved question is whether reduced parasite pressure on islands is associated with reduced immune function. Our previous results showed that different immune parameters responded differently to the island environment, prompting further study. A new application for funding in collaboration with Benoit Nabholz (University of Montpellier, France) has been made in order to conduct a new approach to investigate the effects of insularity on the evolution of toll-like receptors(TLRs), a class of proteins that play a key role in the innate immune system.
Work on avian diversification at the Gulf of Guinea islands conducted by Melo in collaboration with Martin Stervander and Bengt Hansson (University of Lund, Sweden) has shed light on the evolutionary history of one of the few candidate cases for sympatric speciation in birds: the enigmatic São Tomé Grosbeak Crithagra concolor and its sister species, the Príncipe Seedeater C. rufobrunnea. Using a genome-wide approach, they showed that the two ancestral populations on São Tomé hybridized, leading to extensive introgression. The genomes of both species are now a complex mosaic made of each other’s genes. Of interest is the fact that the genomic segments that remain unique to each species are close to regions containing genes determining bill morphology. This strongly suggests disruptive selection between the two species during divergence in sympatry.
Another project is investigating the role of reduced interspecific competition in the evolution of bird song on islands. PhD student Alois Robert (University of Montpellier, France) conducted field work together with Martim Melo, Claire Doutrelant and Thierry Lengagne (University of Lyon, France) in tropical and temperate island locations (São Tomé, Cameroun, Madeira and southern France) to record the acoustic space of islands and mainland populations using SM3 boxes. Initial results indicate that there is less competition for acoustic space on tropical island populations compared to mainland ones, but no differences for the temperate communities.
Activities in 2017
- A comprehensive study of the evolutionary genomics of one of the few candidate cases for sympatric speciation among island birds was concluded.
- Analyses on the distribution of Trypanosoma indicated that, unlike avian malaria, these parasites do not appear to be influenced by insularity.
- A grant application was submitted to investigate the effects of insularity on the immune system using toll-like receptors.
- Claire Doutrelant, Alois Robert and Thierry Lengagne (CNRS, France) conducted fieldwork in Madeira and South of France to record individual songs and the dawn chorus of the bird communities.
- A paper was published showing that birds in the Gulf of Guinea islands generally enjoy reduced parasitism from avian malaria (Loiseau et al. 2017. J. Biogeography).
- A paper was published uncovering the sister-species relationship between the São Tomé Grosbeak and the Príncipe Seedeater. We demonstrated that the grosbeak is a giant canary, by far the largest in the genus. This result was picked up by many media outlets, including Scientific American and received an Altmetric score of 178 (i.e. top 5% of all research outputs scored by Altmetric).
- The first capture of a Príncipe Scops Owl, a species only discovered in 2016 after almost 20 years of surveys. Preliminary genetic analyses demonstrate that it is a new, highly distinct, species.
- PhD student Alois Robert presented his work at the International Bio-Acoustics Conference in India.
Impact of the project
This project is uncovering novel patterns of adaptation in island birds and investigating the mechanisms underlying these adaptations. The results make a significant contribution to our understanding of the ecology and evolution of island environments. Given the very large number of species endemic to islands worldwide and the severe threats to these species from invasive species and habitat loss, our work will help to understand and conserve island species.
FCT (Portugal); National Geographic; British Ecological Society; DST-NRF CoE; CNRS (PEPS); University of Montpellier (PhD grant to Aloïs Robert).
Dr Rita Covas (FIAO and CIBIO, U. Porto)
Dr Martim Melo (FIAO and CIBIO, U.Porto)
Dr Claire Doutrelant (CNRS and FIAO)
Dr Claire Loiseau (CIBIO, U. Porto)
Dr Martin Stervander (U. Lund, Sweden)
Dr Bengt Hansson (U. Lund, Sweden)
Dr Thierry Lengagne (CNRS, U. Lyon)
Student: Aloïs Robert (PhD, CNRS and U. Montpellier).