Researchers investigate the genetic variability of the Brazilian mangrove and identify "dramatic" differences between the mangroves that grow along the country's coast
Half of the original area of Brazilian mangroves has disappeared. Mangroves are among the most neglected ecosystems in the world. "Between 1983 and 1997, practically half (46%) of the original area occupied by mangroves in the Brazilian coast disappeared, destroyed by human activities, such as real estate speculation," says biologist Gustavo Maruyama Mori of the Institute of Biosciences of the State University of São Paulo (Unesp), "Litoral Paulista" campus, in São Vicente-SP.
Despite the loss of half of its original area, Brazil still has the second largest mangrove area in the world. By 2014, it is estimated that there were 81,500 km2 of mangroves in the tropical and subtropical regions around the planet. Of this total, 9.5% (or 7,600 km2) are in Brazil. It is only behind Indonesia (23,100 km2), and well ahead of the third place, Malaysia, with 4,700 km2.
"Despite the large loss of mangrove area recorded in the last two decades of the 20th century, since 2000 there has been a significant reduction in the rate of deforestation of mangroves," says Mori. "In 2000, there were 7,700 km² of mangrove swamps in Brazil. In 2014, the area fell to 7,600 km², a loss of less than 1%."
Mangroves spread over many hundreds of miles along the Brazilian coast, from the border with French Guiana in the Amazon, to the state of Santa Catarina in the South Atlantic. Mangrove forests, also known as mangals, are ecosystems that function as an interface between the sea and rivers that flow in them. Bathed daily by the nutrients brought by freshwater, this biome is extremely important environments such as the nursery of marine fish of commercial value, such as snooks, and crustaceans such as shrimps and crabs.
"The destruction of the mangroves is an irreparable loss, with serious consequences for the fishing activity and for the local populations that depend on the mangrove for their sustenance," says Mori.
In the Molecular Ecology Laboratory of Unesp in São Vicente, Mori and his students are carrying out an important study to understand the genetic diversity of the Brazilian mangrove. Mori's research began more than 10 years ago, when he was still researching with Anete Pereira de Souza, head of the Laboratory of Molecular Genetic Analysis of the Biology Institute of the State University of Campinas (Unicamp). A geneticist who studies plants, Souza leads several research groups that investigate the genetic diversity of plants of great commercial and ecological value, such as rubber trees, sugarcane, pasture grass, mangrove, among others.
"From my laboratory came a new generation of researchers: 50 doctors in the last 20 years," says Souza. "They were able to train and learn the most modern genetic techniques using sophisticated equipment and material, all of which was only possible thanks to the practically uninterrupted support I received from research funding agencies in Brazil: Fapesp, CNPq, Capes and Finep."
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| Gustavo Mori collecting specimens in the mangrove (copyright: Mariana Vargas Cruz) |
"Gustavo is passionate about the mangrove. His work gradually reveals aspects of the evolutionary history of Brazilian mangroves, something about which one knew practically nothing until he became interested in the subject and began to investigate it," says Souza.
"The mangrove is made up of very particular plants," says Mori. "The fact that they disperse their seeds through the water is not something we usually see. Another thing that drew my attention was to see that most plants do not survive in the mangrove environment."
The group led by Mori and Souza, with the participation of Patrícia Mara Francisco (with her PhD work on the genetic variation of three mangrove species along the coast of Brazil), has already discovered two remarkable things. The first of these has to do with the diversity of Brazilian mangroves. Although composed of the same species, the mangroves of the northern region of Brazil are genetically different from the mangroves of the Southeastern and Southern regions of Brazil. This is because the species that grow in the mangrove scatter their seeds in the water, which transports them to the ocean, where they are carried by the marine currents that circulate there. Thus, the seeds of the northern mangroves fluctuate only towards the northwest, while the seeds of the mangroves of the Southeast and South fluctuate towards the South.
It works like this: the South Equatorial Current is a marine current that crosses the Atlantic Ocean from the African coast to the Brazilian Northeast, where its waters bifurcate forming two new currents. To the North, the South Equatorial Current runs along the coasts of the states of Rio Grande do Norte, Ceará, Maranhão, Pará and Amapá. The Brazil Current flows along the Brazilian coast, from Northeast to Southeast and South.
It is the opposite direction of these two currents that makes the populations of mangrove trees that grow in the North and the South of Brazil do not exchange genes among themselves. It follows that, over thousands of years of evolution, mangrove species were having their characteristics selected so that the two populations adapted to the conditions of the different regions of the Brazilian coast. The researchers' detailed genetic research has found that mangroves in northern Brazil are, for example, adapted to higher equatorial sunshine, while southern mangroves can grow on a less sunny day throughout the year.
"We found that the difference between the same mangrove species that occur in the North and the South is genetically dramatic," says Souza.
Here's a parenthesis to explain exactly what defines a mangrove. Mangrove environment or mangal is the forest composed of mangrove plants, which is a specific type of trees adapted to grow in coastal environments flooded daily by high tide. In other words, it is an environment that almost no terrestrial plant would tolerate... except for the mangrove.
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| Rizophora aerial roots (copyright: Mariana Vargas Cruz) |
There are two genera that occur in mangroves all over the world, Avicennia and Rhizophora. Although they belong to completely different families and orders, that is, they are evolutionarily very distant, having no close common ancestors, Avicennia and Rhizophora have adapted to the specific conditions of the mangrove environment.
The conditions of those coastal areas that are influenced by the tide have required Avicennia and Rhizophora to develop ingenious and similar adaptive solutions. Avicennia and Rhizophora support living in environments flooded by both freshwater rivers and salt water from the tides. Avicennia and Rhizophora germinate and grow by planting their roots in the mangrove sludge, an unstable substratum that, although rich in nutrients brought by rivers and tide, is almost completely anaerobic, that is, devoid of oxygen. As an adaptation to the oxygen poverty of the unstable mud, Avicennia and Rhizophora developed aerial roots, that allow to deal with the lack of oxygen and to support the tree even when the tide rises and the soil soaks. "Avicennia has small aerial roots, up to 15 centimeters long and can breathe. Rhizophora stabilizing roots can reach up to several meters in length," explains Mori.
Because of the tides, the mangrove mud is extremely saline. And the presence of salt in the soil is an inhibitory factor for the germination of almost all terrestrial plants - except mangrove trees. They have adaptations that allow their roots to absorb salt water and extract the sea salt, which is then expelled, for example, through the surface of its leaves. Wind and rainwater perform the rest of the service by blowing or washing the surface of the leaves, sweeping all the salt accumulated.
To disperse their seeds, plants generally make use of various strategies. The seeds can simply fall to the ground and germinate right there, they can be carried by the wind, or they can still be dispersed in the feces of the animals that feed on the fruits that shelter the seeds. With the mangrove, what happens is quite different. The seeds are dispersed in the water of the ebb tide. They are very resistant to the corrosive action of sea water, and once in the ocean they can float for several weeks and even months conserving their germinative power until they reach an area where they can then germinate and grow. It is because of this dispersion strategy that the mangroves of the North of Brazil are genetically different from the mangroves of the Southeast and South.
There are dozens of species of Avicennia and Rhizophora growing in mangroves all over the world. In Brazil, there are only five. They are two species of Avicennia and three of Rhizophora (R. mangle, R. racemosa and a hybrid species among them, R. harrisonii), being this last genus popularly known like red-mangrove. "Rhizophora is the common symbol of the mangrove. Because it is the most resistant to the influence of the tide, it often grows at the water's edge, forming the most visible postcard of the mangrove swamp for bathers and tourists who follow the coast and, in order to reach the beaches, have to cross areas of mangrove," explains Mori.
The genus Avicennia is commonly called the black-mangrove, and has two species that occur in Brazil, Avicennia schaueriana and A. germinans. Both grow on land a little further away from the water's edge, where Rhizophora usually dominates.
"The genetic tools at our disposal allow us to make a very accurate genetic diagnosis of the devastation suffered by the mangrove. The technique of molecular marker analysis called microsatellites allows us to identify how much of a forest region has been degraded, "explains Souza.
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| Specimens of mangroves selected for laboratory study at Unicamp (copyright: Mariana Vargas Cruz) |
The degree of degradation is revealed by comparing the microsatellites of the nuclear genome (DNA) of the trees that make up a given mangrove area. "The work is done by sampling and comparing the genome of plants. In this way, we can know how different or similar are the individuals of the same population," explains Souza.
When the DNA of the trees is very similar, very close, this suggests that they descend from the same small group of ancestral plants, probably those that survived the deforestation, thus being able to repopulate the area. "When we find many plants with the same genetic variations, it means that there is no more expressive genetic diversity there," says Souza.
Conversely, if the DNA of the mangrove trees is diverse, revealing a great genetic variety within the same population, this suggests that there was a long time to accumulate genetic diversity, through mutations, within the same population, so it is a forest formed by old-growth forest or primeval forest.
"When plants of a certain population lose diversity, that's a problem. They may wither, they may present growth or adaptation problems. Plants that do not have the genes that confer, for example, resistance to lack of water, can die when drought comes, "explains Souza. "In this sense, mangrove populations with low genetic diversity may suffer more from the effects of climate change."
Take the example of Avicennia. The trees that grow in the mangroves of the Amazon River Delta are more suited to greater sunshine and warmer weather. On the other hand, Avicennia that grows in the mangroves in the region of Florianópolis survives well in not so warm environments, with less luminosity, and low temperatures during the winter. "This means that mangrove reforestation projects in the North should not be done with seedlings brought in from the South, and vice versa. The seedlings would die," says Souza.
Mori and Francisco developed a marker system to identify similarities and differences between the same mangrove species that grow in different regions. More than 40 microsatellites were used to study the mangrove species that grow in Brazil.
One of the most surprising discoveries was the occurrence of hybridization between different species of Avicennia. "Hybridization is an evolutionary process that allows gene flow, or gene exchange between different species. We not only identified hybrids in Avicennia, but found that individuals from this first generation of hybrids were also viable and fertile. We now want to find out what process is behind the appearance of these hybrids."
The next steps of the research, already in progress, involve the identification and analysis of another type of molecular markers, the so-called SNPs (single nucleotide polymorphism). "In the case of microsatellites, we develop a few dozen markers. In the case of SNPs markers, we can develop thousands of them, "says Mori. "So we can conduct a much more refined genetic analysis of the genetic variability that exists in mangrove species."
Mori wants answers to the following questions: "How will the species respond to the new climatic conditions? Will they adapt? if so, in what way? What are the genes that allow two populations of the same species to occupy such diverse environments?"
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| Gustavo Mori collecting specimens in the mangrove (copyright: Mariana Vargas Cruz) |
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| Collecting specimens in the mangrove (copyright: Mariana Vargas Cruz) |
PRESS CONTACT:
Cel: 5511-96574-5091
CONTACT WITH THE AUTHORS:
Gustavo Mori
Universidade Estadual Paulista (Unesp)
Campus do Litoral Paulista - Unidade São Vicente.
São Vicente-SP
Telefone: (13) 3569-7186
celular: (19) 98149-0381
Anete Pereira de Souza
Prof Titular Genética Vegetal
Laboratório de Análise Genética Molecular
Instituto de Biologia (IB)
Universidade Estadual de Campinas (UNICAMP)
Campinas-SP
Telefone: (19) 3521-1132
Patrícia M. Francisco, Gustavo M. Mori, Fábio M. Alves, Evandro V. Tambarussi, Anete P. de Souza. 2018. Population genetic structure, introgression, and hybridization in the genus Rhizophora along the Brazilian coast. Ecology and Evolution 8(6):3491-3504.
DOI: 10.1002/ece3.3900
Gustavo M Mori, Maria I Zucchi, Iracilda Sampaio and Anete P Souza. 2015. Species distribution and introgressive hybridization of two Avicennia species from the Western Hemisphere unveiled by phylogeographic patterns. BMC Evolutionary Biology 15:61.
Gustavo M Mori, Maria I Zucchi, Iracilda Sampaio and Anete P Souza. 2015. Species distribution and introgressive hybridization of two Avicennia species from the Western Hemisphere unveiled by phylogeographic patterns. BMC Evolutionary Biology 15:61.
DOI: 10.1186/s12862-015-0343-z
Mori GM, Zucchi MI, Souza AP (2015) Multiple-Geographic-Scale Genetic Structure of Two Mangrove Tree Species: The Roles of Mating System, Hybridization, Limited Dispersal and Extrinsic Factors. PLoS ONE 10(2): e0118710. DOI: 10.1371/journal.pone.0118710
This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).
Mori GM, Zucchi MI, Souza AP (2015) Multiple-Geographic-Scale Genetic Structure of Two Mangrove Tree Species: The Roles of Mating System, Hybridization, Limited Dispersal and Extrinsic Factors. PLoS ONE 10(2): e0118710. DOI: 10.1371/journal.pone.0118710
This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).
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