After 80 years stumping scientists, strange Philippine frog unmasked as hybrid
University of Kansas
image:
Distribution of the frogs studied and examples of their finger disc widths. Distribution of the Kaloula conjuncta complex A and B K. picta. Diamond = location of samples sequenced for this study; star = type locality of K. c. stickeli; red box = location of the putative hybrid zone where K. c. meridionalis, K. c. stickeli, and K. picta co-occur. C Differences in widths of the finger discs in the focal taxa.
view moreCredit: Chan et al
LAWRENCE — Evolutionary biologists working in the Philippines long have puzzled over a hard-to-find amphibian mystery: the Leyte Chorus Frog. It’s a small, ground-dwelling frog from Leyte Island in the Visayan province of the eastern Philippines. The country is an island archipelago once blanketed with dense forests that today exist in small fragments — an unfortunate consequence of intensive logging.
The Leyte Chorus Frog was collected in 1944 and noted for its odd toe discs when it was described in an academic paper in 1954, then not seen again for decades despite a concerted hunt by researchers.
Now, a report from the University of Kansas appearing in the Genetics Society journal Heredity confirms that Kaloula conjuncta stickeli (the scientific name of the strange chorus frog) is a hybrid of two species with overlapping ranges whose intermingling likely was driven by deforestation.
“It’s this really weird, enigmatic frog in the Philippines,” said Chan Kin Onn, a postdoctoral researcher at KU’s Biodiversity Institute and Natural History Museum, who served as lead author on the paper. Chan credits his mentor Rafe Brown, curator-in-charge of the herpetology division at the iInstitute, for years of fieldwork that undergird the findings.
“The reason the frog’s enigmatic is that it was collected a long time ago,” Chan said. “It looks really distinct — you can just eyeball a specimen and immediately tell it's not this, it's not that; it's something else. When it was originally described, it was classified as a subspecies.”
Chan said evolutionary biologists tend to think of a “subspecies” as a population that looks slightly different, but not obviously unique enough to qualify as a separate species.
The problem was that only a few Leyte Chorus Frogs were ever discovered despite regular sampling by scientists in the region — its rarity was a puzzle that raised questions about its status as a subspecies.
“The person who first discovered it collected only two specimens,” Chan said. “Over the course of several decades, despite many researchers returning to the same locations, only five more specimens have been found. The lack of material makes it really difficult to study — only seven specimens exist in collections worldwide. For the longest time, we didn’t have the tools to figure it out, so people considered it either a subspecies or questioned its validity. Some hypothesized that it might be a full species, one that might already be extinct. No one really knew for sure, so it remained in this gray area.”
Fieldwork in the Philippines by co-author Brown, who also serves as KU professor of ecology & evolutionary biology, turned up several of the Leyte Chorus Frogs specimens — but this time from the southern island of Mindanao. Today, they’re housed in collections of the National Museum of the Philippines, the Texas Memorial Museum and the University of Texas at Austin.
“Ever since catching these curious, oddball specimens in 1996, the summer before I started grad school at UT Austin, the unanswered question of what they really were — or what they could possibly represent — has bugged me on and off, occasionally resurfacing from memory,” Brown said. “It has been like a note-to-self: Don’t forget to get back to those weird frogs from Mindanao because we still don’t know the answer.”
Following up on Brown’s questions, Chan looked into the mystery of the Leyte Chorus Frog using the same specimens from 1944 and 1996, as well as the latest tools of the evolutionary biology trade. He used genetic material from specimens in biodiversity collections and analyzed relevant portions of the genome using labs at KU and offsite, looking to accurately place the frog in its family tree.
“Today we have a bunch of those ‘Star Trek’-style science gizmos,” Brown said. “We’ve got genomic resources, bioinformatic tools, really powerful computers, and most importantly Dr. Chan, whose insight provided the final pieces to come together, for a much anticipated resolution of the mystery.”
“We thought, ‘Let’s see if genomics can answer this question,’” Chan said. “So, we did, and the result was surprising — this frog is actually a hybrid. Not just any hybrid — an ‘F1 hybrid,’ meaning it's the first-generation offspring of two distinct parent species. Because of that, it has 50% of its genetic material from one parent and 50% from the other.”
The analysis found the odd Leyte Chorus Frogs are a hybrid between K. c. meridionalis and K. picta.
“Normally, these two frog species wouldn’t meet because one lives in trees and bushes, but the other on the ground, and they breed in completely different places,” Chan said. “But deforestation changed their environment, creating new open spaces where their habitats overlapped. This led to tree-dwelling females being drawn to the calls of ground-dwelling males, resulting in unexpected hybridization. Scientists suspected this years ago when they observed the frogs interacting in the wild, but they lacked the tools to prove it.”
Chan said the hybrid Leyte Chorus Frogs were particularly interesting because their two parent species look and act so differently.
“One is large, with big expanded toe pads for climbing,” he said. “The other is smaller, with almost no expanded toe pads because it’s a ground dweller. This enigmatic frog is a perfect intermediate between them — it has intermediate-sized toe pads. Just by eyeballing the size of the toe pads, you can tell what’s what.”
In the end, the KU researchers determined the hybrid isn’t a distinct species and doesn’t warrant its own scientific name. Further, it’s rarity comes down to the fact that it cannot reproduce and is an evolutionary dead end.
“When they breed, we think it likely that genetic incompatibilities form,” Chan said. “Apparently, the hybrids are incompatible with each other, and with the two species that produced them, so they cannot progress beyond that first generation F1 hybrid. At least, this is what we suspect, and it opens the door for future research to verify this.”
Brown emphasized the critical role played by specimen-based museum research for deciphering biological problems that rely on clues only to be gleaned from biological specimens properly preserved and cared for in natural history museums.
“Chan’s work is really impressive, and with this new publication he closed the book on the Leyte Chorus Frog ‘stickeli’ mystery,” Brown said. “That’s thanks to his hard work, analytical abilities, writing skills and cutting-edge genomic data. That said, this project would not have been possible if we had been unable to access the exact same museum specimens that stupefied field biologists 30 to 80 years ago. Lucky for us, they made note of their observations, collected the specimens and preserved them for use by future researchers.”
Journal
Heredity
DOI
One of just seven specimens of the mysterious Leyte Chorus Frog collected by W. H. Stickel on Nov. 20, 1944, from Leyte, Philippines. Part of the collection of the Field Museum of Natural History.
Credit
Photo by Sarah Drasner.
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