The world’s tiniest salamanders are so small that some body parts appear to get short shrift. Those in the genus Thorius, for example, have heads that are “mind bogglingly small, maybe half the size of a pencil eraser,” says herpetologist and evolutionary biologist Sean Rovito of the Center for Research and Advanced Studies of the National Polytechnic Institute in Mexico. Within this tiny skull, the eyes bulge and the brain is, in relative terms, massive. The teeth on the upper jaw are usually missing entirely.
The major challenge for miniaturized organisms is packing enough cells into a tiny form to build these complex structures. In salamanders, cells can only be so small, because they are chock-full of DNA. Their genomes range from about three to 40 times the size of the human genome.
Now, Rovito and collaborators in Slovenia and the United States have discovered that genome size tends to decrease with body size across a group of 60 salamander species. The research, reported in The American Naturalist, suggests that salamander genomes may evolve to make room for more cells in a miniature frame.
Genome size balloons in salamanders owing, at least in part, to an abundance of repetitive DNA sequences known as retrotransposons that copy and paste themselves across the genome. Rovito hypothesized that miniature salamanders might evolve to ditch some of this excess DNA.
To explore this possibility, he collected salamanders in the forests of Mexico and Guatemala that belong to a group known as the bolitoglossines. The group includes salamanders spanning a wide range of sizes, from the smallest Thorius species to the roughly eight times longer Isthmura gigantea.
The smallest salamanders were the hardest to find. “First of all, they are tiny. Secondly, they are brown,” says Rovito. He and his students combed the forests for their tiny figures, lifting bark from fallen trees and peeling back bromeliad leaves one by one. Ultimately, they assembled a collection that spanned 60 species and eight genera. They then used an image analysis technique to measure genome size in each sample’s red blood cells.
The team also estimated the physical size of each species based on head volume and measures of length from nose to waist. They then looked at the relationship between genome size and physical size, while accounting for relatedness between species. Genome size tended to decrease as physical size decreased, suggesting that evolution may favor smaller genomes for smaller salamanders.
The team also constructed a phylogenetic tree using previously published genetic data to reveal how genome size changed over time as salamander species diverged. The tree shows several instances of genome size reduction, including in the ancestor of all living Thorius species.
The researchers calculated a measure called “biological size” for each species, which is essentially the number of cells an organism can fit within a fixed amount of space. One species has a greater biological size than another of equal physical stature when its cells are smaller, thanks to a smaller genome. Salamanders with these relatively smaller cells essentially have more cellular building blocks available, meaning they’re capable of having more complex bodies. The team found that species within Thorius are actually larger in terms of biological size than some of the physically larger salamanders, perhaps helping them compensate for their miniature form.
But the current study can’t prove that a smaller body in salamanders selects for a smaller genome just yet. “We don’t actually know if the smaller salamanders had selection to make their genomes smaller,” says Rovito, “or if they were able to become smaller because they already had smaller genomes to permit it.”
Both possibilities may be true, says evolutionary biologist Ryan Gregory of the University of Guelph in Canada, who was not involved in the study. Gregory points to an earlier study suggesting that small genome size evolved in the dinosaur ancestors of birds before they learned to fly, as well as his own research suggesting that the metabolic demands of flight cause modern-day birds to continue to evolve smaller genomes. “You can have the prerequisite and the enhancement,” he explains.
Other factors could also be at play, says evolutionary biologist Ivan Gomez-Mestre of the Doñana Biological Station in Seville, Spain, who was not involved in the study. Gomez-Mestre’s own research suggests that climate can have an indirect effect on genome size in frogs. The recent salamander study provides “a nice foundation to go even further,” he says, perhaps taking into account variables such as microhabitat or climate data.
“I think this [study] should inspire some more interesting cases,” says Gregory. He’s keen to try this team’s comparison of genome size and body size on insects, which also include cases of extreme miniaturization. “It would be really interesting,” he says, “if [the pattern] holds.”