"This discovery is surprising because no previous theories of how the frogs arrived had predicted a single origin for Caribbean terrestrial frogs and because groups of close relatives rarely dominate the fauna of an entire continent or major geographic region," explained Penn State's Blair Hedges, the evolutionary biologist and professor of biology who directed the research. "Because land connections among continents have allowed land-dwelling animals to disperse freely over millions of years, the fauna of any one continent is usually a composite of many types of animals."
The field work for the study required nearly three decades to complete because many of the species are restricted to remote and isolated mountain tops or other inaccessible areas. Some species included in the study now are believed to be extinct because of habitat degradation and possibly other causes such as climate change.
A recent global assessment of frogs found that the Caribbean Islands have the highest proportion of frog species threatened with extinction. Hedges and coauthor William Duellman, a professor emeritus of the University of Kansas, were involved in much of the field work. A third co-author of the study is Penn State graduate student Matthew Heinicke, who performed DNA sequencing and analyses.
One prominent theory had proposed that frog species on the large islands of Cuba, Jamaica, Hispaniola, and Puerto Rico had walked there across land bridges that existed when those islands were connected in a geologic arc about 70-to-80-million years ago. A second major theory proposed that they arrived, instead, by rafting across the Caribbean Sea after the giant asteroid impact near Cuba 65-million years ago, which is widely believed to have exterminated the dinosaurs. "Both theories acknowledged that the frog faunas must have arrived by rafting over water to the smaller and younger islands, the Lesser Antilles, because they never were connected by land to South America, but neither theory proposed that all of the Caribbean island frog species had a single common ancestor," Hedges said.
The anatomy of Caribbean frogs previously had led the advocates of both theories to conclude that species in Cuba and other western-Caribbean islands were related to different mainland species than were the species on Puerto Rico and other eastern-Caribbean islands, regardless of how they got there. "Discovering a single origin for all of these species from throughout the Caribbean islands was completely unexpected," Hedges said.
To make their discovery, the researchers sequenced the DNA of nearly 300 species of Caribbean, Central American, and South American frogs and used three mitochondrial genes and two nuclear genes in their study, building trees of relationships among the species and timing the divergences of the species with molecular-clock methods. "Molecular clocks work by counting the number of DNA-sequence mutations separating two species and then dividing that number by the rate of change, which is established with the help of fossils and geologic information," explained Hedges.
The study's DNA research revealed that, while many ocean dispersals may have occurred over time, only two led to the current faunas: one for the Caribbean islands and another for Central America. The scientists speculate that it may not be coincidental that these ancient and successful dispersals happened after the asteroid collision rather than earlier. "The asteroid impact generated giant waves that devastated the islands, probably eliminating any existing fauna at that time," Hedges said.
The original frogs that successfully colonized the Caribbean islands likely hitched a ride on floating mats of vegetation called flotsam, which is the method typically used by land animals to travel across salt water. "Some rafts of flotsam, if they are washed out of rivers during storms and caught in ocean currents, can be more than a mile across and could include plants that trap fresh water and insect food for frogs," Hedges said. It is not likely that the frog species dispersed simply by swimming because frogs dry easily and are not very tolerant of salt water.
In addition to the study's discoveries about Caribbean and Central American frogs, the research also revealed and defined an unusually large and unpredicted group of species in South America. "The South American group may have more than 400 species and is mostly associated with the large Andes mountains of South America," Hedges said.
"Until now, the entire group of these terrestrial, tropical frog species -- the eleutherodactylines -- have been considered a "black hole" in frog biology because of the poor understanding of their evolutionary history," explained Hedges. Scientists consider the knowledge of evolutionary relationships, also called "phylogeny," to be fundamental to many fields of biology, including medicine, anatomy, physiology, ecology, and conservation.
This research was supported by the National Science Foundation's Biotic Surveys and Inventories Program, Systematic Biology Program, and Assembling the Tree of Life (AToL) Program. The latter program is an effort to understand the "tree of life," or the relationships among all organisms. The research also was supported by the National Aeronautics and Space Administration's Astrobiology Institute (NAI). A major goal of NASA's Astrobiology Roadmap is to understand how past life on Earth interacted with its changing planetary environment, such as asteroid impacts and connections of continents.
Barbara K. Kennedy | EurekAlert!
Climate Impact Research in Hannover: Small Plants against Large Waves
17.08.2018 | Leibniz Universität Hannover
First transcription atlas of all wheat genes expands prospects for research and cultivation
17.08.2018 | Leibniz-Institut für Pflanzengenetik und Kulturpflanzenforschung
New design tool automatically creates nanostructure 3D-print templates for user-given colors
Scientists present work at prestigious SIGGRAPH conference
Most of the objects we see are colored by pigments, but using pigments has disadvantages: such colors can fade, industrial pigments are often toxic, and...
Scientists at the University of California, Los Angeles present new research on a curious cosmic phenomenon known as "whistlers" -- very low frequency packets...
Scientists develop first tool to use machine learning methods to compute flow around interactively designable 3D objects. Tool will be presented at this year’s prestigious SIGGRAPH conference.
When engineers or designers want to test the aerodynamic properties of the newly designed shape of a car, airplane, or other object, they would normally model...
Researchers from TU Graz and their industry partners have unveiled a world first: the prototype of a robot-controlled, high-speed combined charging system (CCS) for electric vehicles that enables series charging of cars in various parking positions.
Global demand for electric vehicles is forecast to rise sharply: by 2025, the number of new vehicle registrations is expected to reach 25 million per year....
Proteins must be folded correctly to fulfill their molecular functions in cells. Molecular assistants called chaperones help proteins exploit their inbuilt folding potential and reach the correct three-dimensional structure. Researchers at the Max Planck Institute of Biochemistry (MPIB) have demonstrated that actin, the most abundant protein in higher developed cells, does not have the inbuilt potential to fold and instead requires special assistance to fold into its active state. The chaperone TRiC uses a previously undescribed mechanism to perform actin folding. The study was recently published in the journal Cell.
Actin is the most abundant protein in highly developed cells and has diverse functions in processes like cell stabilization, cell division and muscle...
17.08.2018 | Event News
08.08.2018 | Event News
27.07.2018 | Event News
17.08.2018 | Physics and Astronomy
17.08.2018 | Information Technology
17.08.2018 | Life Sciences