Research developed at IB-Unicamp seeks to understand the changes undergone by biomes over the last five million years
Analysis of nuclear and mitochondrial DNA from populations of Nephila clavipes, a common species of spider that lives in the warmer regions of the Americas, are helping scientists better understand the evolutionary history of the Amazon Rainforest and Atlantic Forest over the last five million years. The work, carried out for the first time with invertebrates, was developed for the doctoral thesis of biologist Luiz Filipe de Macedo Bartoleti, defended at the Institute of Biology (IB) at Unicamp, under the guidance of professor Vera Solferini.
According to Luiz Filipe, the study is part of an area of knowledge called Historical Biogeography, which deals with investigating the geographic distribution patterns of living beings in a given period of time. By analyzing the DNA of different populations of the same species, researchers look for evidence of possible changes that have occurred in biomes over time. Such indications come from the fact that DNA is a molecule that undergoes several changes. “Mutations can be adaptive, but they can also be neutral; in the latter case, they are cumulative”, informs the author of the thesis.
But, after all, what can the DNA of a simple spider “tell” about the evolution of the Amazon Rainforest and the Atlantic Forest? Professor Vera Solferini explains that her advisor collected samples from 40 populations of Nephila clavipes in different regions of Colombia and Brazil. In the national territory alone, Luiz Filipe visited 20 states. “This broad sampling design allows us to have a very good idea of what the evolutionary history of these biomes was like,” says the professor.
In practice, what researchers do is analyze how the different populations of this species of arachnid are related. Depending on the genetic variability found, they are able to say whether populations found at different points maintain contact or whether they split a long time ago. “Imagine two populations of Nephila clavipes, one from the Amazon and the other from the Atlantic Forest. If they stop, for some reason, maintaining contact with each other, each one will accumulate its own mutations. In other words, they become genetically distant from each other over time. As long as they stay in contact, they will exchange these mutations”, details Luiz Filipe.
It is precisely this genetic variability that bears signs of demographic events that occurred in the past. In his analysis of the DNA of the spiders he collected, the author of the doctoral thesis found that populations west and east of the Colombian Andes accumulated different mutations, which indicates that some event prevented gene flow between them. “In this case, the barrier to contact was the final uplift of the Andes Mountains, which occurred about three million years ago. This hypothesis reinforces the results of studies in other areas about the evolutionary history of that biome”, points out the biologist.
The populations collected in Brazil showed more recent genetic diversification, dating back to the last 350 thousand years. The separation of Brazilian lineages, according to Professor Vera Solferini, is probably associated with climatic changes characteristic of the Pleistocene period, which must have isolated populations in different biomes, causing the emergence of the lineages found. These same climatic events, the professor considers, must have been responsible for recurrently modifying the geographic distribution of biomes, which may have created “bridges” between the Amazon and the Atlantic Forest, through which spiders were able to mix during the last glacial cycles.
In the last two million years, adds Luiz Filipe, Brazilian biomes have undergone many transformations. As one expanded the other retracted. They assumed their current characteristics in a more recent period, around 20 thousand years. “According to our hypothesis, these changes helped to establish these 'bridges'. As forests changed shape and position, the organisms that lived in them also changed. We concluded that a corridor appeared in the middle of the Cerrado that allowed spiders to move from one biome to another, promoting the exchange of mutations that we found in DNA analyses”, explains the researcher.
Studies of this type, according to Professor Vera Solferini, contribute to the understanding of the complex diversification of South American biomes, in addition to demonstrating that spiders are suitable organisms for studying the biogeographic history of widely diverse regions. In this sense, the professor points out, the work offered another important conclusion, this one relating exclusively to the Atlantic Forest.
Many studies divide the biome into two parts – north and south – and consider that each has a distinct climate regime. “In our research, we were unable to find this division. The spiders that occurred in the north are genetically similar to those in the south. This shows that the response to changing climate regimes is specific to each species”, says the professor. Still according to her, the set of information generated by the study is also relevant because facts from the past can help project the future.
Luiz Filipe details this issue further: “It is essential that we understand how diversity was built over geological time. When we understand how species respond in the present to climate changes that have occurred in the past, we are able to create models to predict how they will behave in the future in relation to ongoing climate change. Thus, we can make predictions about species that could be maintained or extinct and climatically stable areas that could serve for conservation purposes.”
The next step in this line of research, says Professor Vera Solferini, will be the incorporation of RNA analysis, that is, the gene expression of organisms. Similar studies are being carried out by other researchers, mainly with plants and birds. “We maintain contact with teams in Brazil and abroad to exchange information. With some of them, we maintain valuable collaborations, which allows studies to gain a multidisciplinary approach and advance in a more solid way”, considers the professor, who also highlights the importance of funding from development agencies, particularly the Fundação de Amparo à Research from the State of São Paulo (Fapesp) and the National Council for Scientific and Technological Development (CNPq).
