Rising temperatures are threatening many frog species, and pose a particular challenge for burrowing frogs, which survive long dry periods in arid zones by burying themselves and waiting for rain. However, three Australian species appear to be coping surprisingly well and they have one feature in common – four sets of chromosomes instead of the usual two.
A surfeit of chromosomes, known as polyploidy, is common in plants, but rare in animals, particularly vertebrates. Polyploidy sometimes occurs when animals hybridize, Dr Ian Brennan of the Australian National University told IFLScience, with offspring somehow ending up with both sets of chromosomes from each parent. Unusual conditions, such as exposure to certain chemicals or freezing cycles, can induce what is known as autopolyploidy, where a doubling up occurs without hybridization. Even more exotic events can leave an organism with three or six sets of chromosomes.
Extra chromosomes usually interfere with breeding, however, so polyploid animals usually reproduce asexually, making the Australian genus Neobatrachus an exception among exceptions. There are nine known species of Neobatrachus frogs, six diploid (the usual two chromosome sets), while three are tetraploid (four chromosome sets). All are sexual.
Brennan is part of a team that describe the Neobatrachus family tree, particularly its tetraploid species in PLOS Genetics. Astonishingly, Brennan and co-authors report that the tetraploid species do not all descend from a single event. They're sufficiently evolutionarily distant from each other, with diploid species in-between, and they must have acquired the extra chromosomes independently.
"It doesn't happen often – but here it's happened three times in one genus. And we're talking about three independent events,” Brennan said in a statement. As to why this one genus is so prone to something almost unheard of elsewhere, Brennan could only tell IFLScience “we have some ideas, but are not really confident.”
Four chromosomes certainly messes with the dominant/recessive Mendelian genetics we learned at school, and Brennan told IFLScience understanding of what occurs instead is still hazy. “Sometimes only two copies are used to determine genetic influences, while the others just accumulate mutations,” he added.
For all its drawbacks, polyploidy seems to work for these frogs, and indeed the authors found it is the tetraploid species that are adapting best as rainstorms become rarer and the heat between more intense.
"We think Neobatrachus diploids might be suffering the early impacts of climate-induced habitat loss, while the polyploid species seem to be avoiding this fate," Brennan said.
The authors think this is because the tetraploid species have greater genetic diversity. A lot of hybridization occurs between the species, such that their family tree lacks a neat branching structure, more closely resembling a rose bush. Novel genes get absorbed into tetraploid populations more easily than diploid ones, offering more flexibility when conditions change.