Belgian and US researchers have gained a new understanding of climate change by studying a living fossil tree, Symphonia globulifera. The scientists reconstructed the environmental histories of rainforests of Central America and South America by measuring the genetic diversity of S. globulifera tree populations. The findings are published in the November issue of the journal Evolution. Dr Christopher Dick of the University of Michigan in the US and Myriam Heuertz of the Free University of Brussels in Belgium studied S. globulifera to understand how tropical forests responded in the past to climate changes. They used ribosomal and chloroplast DNA sequences as well as nuclear microsatellite loci (nSSRs, powerful markers for genetic diversity studies) to study S. globulifera's biogeographic history. The goal was to understand exactly where the trees lived during which time period, and how abundant they were. S. globulifera, a rainforest tree commonly called hog-gum, ojeme or okolólo, is a tree native to Africa that has populated the landmasses of Central and South America separately, despite seemingly insurmountable obstacles. S. globulifera's salt-intolerant seeds make it hard to believe that the species dispersed across the seas. However, it has been established that the tree, which populated African rainforests as long as 45 million years ago, travelled to Mesoamerica, the Amazon basin and the West Indies around 15 million years ago, making its way to the New World (the Americas) via oceanic currents at least three times. This dispersal via such a random-seeming route, referred to as 'sweepstakes dispersal', resulted in the establishment of three major clades, or branches, of S. globulifera in the rainforests of the New World. The clades have been genetically isolated ever since, giving S. globulifera the status of 'living fossil'. A living fossil is an organism without any close living relatives that has remained seemingly unchanged for millions of years; it can be any organism that has survived major extinction events and which appears to be the same as a species known otherwise only from fossils. The fossil record of S. globulifera pollen is long, detailed and well studied (likely because of its usefulness as a geological tool for the oil industry). Interestingly, this record reveals that although the New World and African trees now look the same, they have evolved separately. Using the fossil pollen record and measuring genetic diversity amongst existing populations, the researchers were able to reconstruct environmental histories of the areas colonised by S. globulifera. They observed that in Central America, the tree's populations were highly genetically differentiated across the landscape. 'We think the pattern is the result of the distinctive forest history of Mesoamerica,' explained Dr Dick, 'which was relatively dry during the glacial period 10,000 years ago. In many places, the forests were confined to hilltops or the wettest lowland regions. What we're seeing in the patterns of genetic diversity is a signature of that forest history.' They also observed less diversity among populations in the Amazon Basin, which was wet throughout the glacial period and so hosted a continuous forest. 'Genetic drift of S. globulifera was strong in Mesoamerica,' the study reports, while 'Amazon populations seem to have expanded recently'. The study, which is the first to make such comparisons of genetic diversity patterns in Central and South America, answers some questions about how S. globulifera responded to past climate conditions, which may be helpful for predicting how forests will react to future environmental changes. 'We think similar patterns will be found in other widespread species,' Dr Dick said. 'Under scenarios of increased warmth and drying, we can see that populations are likely to be constricted, particularly in Central America, but also that they're likely to persist,' he explained. 'Some things can endure in spite of a lot of forest change,' the study concluded. 'However, past climate changes were not combined with deforestation, as is the case today. That combination of factors could be detrimental to many species - especially those with narrow ranges - in the next century.'
Belgium, United States