Reproductive adaptations help tree frogs thrive in different climates

  • A new study shows how the family Rhacophoridae of Old World tree frogs came to occupy a wide variety of environments across Asia and Africa, thanks to adaptations in their breeding methods.
  • Frogs usually spend their larval stage in water as tadpoles, but different species of Rhacophoridae have also adapted other methods: gel nesting, foam nesting, and direct development.
  • Moss and gel nests help frogs lay large numbers of eggs in more open, drier habitats to prevent eggs from drying out, while direct development gives them an advantage in spreading to more open areas. hot and drier where there are no bodies of water.
  • The study authors say that revelations about the evolutionary past of frogs will be useful in predicting their responses to current and future climate change, and therefore their conservation.

COLOMBO — Amphibians evolved from a fish ancestor that had functioning lungs and bony lobed fins, which helped them become the first vertebrates to conquer land. Keeping these earlier paths, today’s frogs and toads also have larvae with gills that require an aquatic environment until they mature into air-breathing adults. But this unique feature did not limit members of the frog family Rhacophoridae to branch out in a wide range of environments, thanks to the adaptation of different modes of reproduction, a new study shows.

With 22 genera and 432 species, the Old World tree frog family Rhacophoridae accounts for about 6% of the world’s frog and toad species. They are thought to have evolved from a common ancestor who lived in northeast Asia around 68 to 53 million years ago.

In their native ranges today in Asia and Africa, Rhacophorid frogs are found in a rich variety of biogeographical regions, from continental mainland bands to archipelagos, and from mainland islands of East Asia and from the South, to mountainous regions such as the Himalayas.

The small Sri Lankan shrub frog (Pseudophilautus tanu) is a tiny endemic Rhacophorid frog, also a user of ‘direct development’ egg laying, in which it lays its eggs on leaf litter. Image courtesy of Madhava Meegaskumbura.

Various breeding methods

“We tried to answer key evolutionary questions, including whether the evolution of reproductive mode in these frogs is an adaptation related to specific climatic events,” said the study’s lead author. Gajaba Ellepolaan evolutionary ecologist University of Peradeniya and doctorate. candidate in College of Forestry, Guangxi University, China.

The earliest ancestors of rhacophorid frogs may have used the primary method of aquatic reproduction of laying eggs directly on bodies of water, with their larvae spending a completely aquatic life until transformed. But Ellepola and her team now show that these frogs use three additional methods of reproduction: laying eggs in gel nests, in foam nests, and a method known as direct development.

Developing embryo of one of Sri Lanka’s smallest frog species, Schneider’s shrub frog (Pseudophilautus schneideri), a direct developer that lays its eggs on moist soil. Image courtesy of Kanishka Ukuwela.

Gel nests and foam nests are built on the edges of water bodies, allowing hatched tadpoles to easily slip into the water. In the case of gel nests, the female frog secretes a gelatinous substance after depositing the eggs. The same is true for foam nests, except that the male frog, while clinging to the female’s body during copulation, flaps its legs so that the gelatinous secretion becomes foamy.

Direct development occurs when the froglets emerge directly from the eggs. Rhacophorid frogs that use this method lay their eggs on a leaf or on moist soil; tadpoles that emerge from the embryo are held captive inside the egg and emerge as froglets when ready.

“Foam nests help the frog lay large numbers of eggs in more open, drier habitats to prevent the eggs from drying out, but ‘direct development’ is an adaptability that gives the frog an advantage to spread to warmer, drier areas where there are no bodies of water,” Ellepola told Mongabay.

The different methods of reproduction of members of Rhacophorid frogs and their distribution in different regions. Species that lay their eggs in an aquatic environment are restricted to East and Southeast Asia, where the ancestors of the Rhacophorid frogs lived, indicating that others may have spread more widely due to their adaptability . Image from Ellepola et al. (2022).

Key adaptations

The researchers identified these methods as key evolutionary innovations, or KEIs, that allowed Rhacophorid frogs to diversify and establish themselves in niches that were not previously accessible to them. When the researchers mapped the areas where these tree frogs live, they found that the typical method of laying eggs directly in water was limited to species found in East and Southeast Asia.

“This indicates that the other species adapted to ‘direct development’ and ‘moss nesting’ which could spread to other areas, while those that needed water to lay eggs were restricted to the the family’s area of ​​origin,” Ellepola said.

Tadpoles are the aquatic larval stage of frogs and toads that breathe through internal gills before undergoing metamorphosis. Image by Marjan No via Pixabay.

The researchers also constructed a phylogenetic tree to study the evolutionary history of Rhacophorid frogs. They used a new method to map this family tree of 415 species of Rhacophoridae.

“This phylogeny or evolutionary history map is the most comprehensive species tree for rhacophorid frogs to date,” said the study’s co-author. Madhava Meegaskumburaevolutionary biologist at the College of Forestry, Guangxi University.

The study confirms that ancestral members of the Rhacophoridae family lived in northeast Asia. They then began to spread to cool, humid areas from their center of origin by adapting to new climatic conditions or dispersing during periods of favorable weather, the study shows.

“We found nearly constant rates of diversification across phylogeny, but a highly patchy, phylogenetically clustered distribution of species richness across the range of Rhacophorids,” Meegaskumbura told Mongabay.

The study also confirmed that mountain regions on islands and some continental regions with cool and humid climatic conditions have higher genetic diversity and endemism. These include the highland islands of Sri Lanka and Indonesia, as well as the high regions around the Himalayas – regions where Rhacophoridae have more recently emerged as endemic.

Multiple males may be involved in copulation and moss nest formation, as with these Indian tree frogs (Polypedates maculatus), a species commonly found in home gardens. Image courtesy of Gajaba Ellepola.

The results of the genetic study indicate that after the first ancestors of Rhacophoridae colonized the region of East and Southeast Asia, their descendants adapted to the rising temperatures during an event of global warming that occurred about 40 million years ago, during the middle Eocene epoch. The researchers’ mapping finds that these adaptations coincided with the evolution of these frogs’ terrestrial modes of reproduction, i.e. the use of foam nests and direct development. A subsequent global cooling event followed, reducing sea levels and allowing these heat-adapted ancestors to jump into adjacent areas between climatically harsh regions.

Meegaskumbura said the significance of this study is that it tested how lines or sections of family trees used optimal climatic conditions and also how they adapted to new conditions. The knowledge generated, he said, will be useful in predicting the responses of these frogs to current and future climate change, and therefore their conservation.

Quote:

Ellepola, G., Pie, MR, Pethiyagoda, R., Hanken, J. and Meegaskumbura, M. (2022). The role of climate and islands in species diversification and breeding patterns of Old World tree frogs. Communications Biology, 5(1), 347. doi:10.1038/s42003-022-03292-1

Banner image of a pair of round-nosed pygmy frogs (Pseudophilautus femoralis), a species found only in Sri Lanka, by laying eggs on a leaf using the “direct development” method. Image courtesy of Madhava Meegaskumbura.