The Secret Weapon of the Killer Fungus: Sex

This is the frog terminator: Batrachochytrium dendrobatidis, a fungus that is wiping out frogs and other amphibian species all over the world. Now researchers have found a secret weapon that enables the fungus to travel further and withstand harsh conditions longer, turning it more dangerous: sex.

The fungus seems to kill off the frogs by clogging their skin and essentially asphyxiating them, spreading from Australia to the US. In US, the spread of the fungus took place very rapidly towards west across the Sierra Nevada at the speed of about one mile (1.6 km) annually.

The populations of mountain yellow-legged frogs, once the most common amphibian in the area, dropped by 95 % in the last 30 years, so that now the species is on the list of endangered ones.

A new genetic analysis checked if the fungus was new to the area or had been lurking for awhile. In case of a new arrival, the fungus would have presented only one genotype (genetic makeup) as the organism spread by cloning itself. If the fungus had spent more time in the region, it would have had time to mutate and recombine its DNA into various genotypes. Both variants were found.

"We found sites dominated by a single fungal genotype, which suggests recent spread of the pathogen through clonal reproduction," said co-author Jess Morgan, a biologist at the University of California, Berkeley.

But some samples revealed proof of multiple, related genotypes, this pointing to the fact that the fungus is reproducing sexually. Sexual reproduction leads to the production of very resistant spores, as they are more genetically diverse.

"This group of fungi, when it reproduces sexually, can create spores that can last for a decade. That could make this pathogen a harder problem to defeat. As a resistant spore, the fungus could be transported by animals, including humans or birds, or lay dormant in an infected area until a new host comes along." said lead author John Taylor of UC Berkeley.

"This is huge. This is really bad news for the frogs." said frog biologist Karen Lips of Southern Illinois University, not involved in this research.

Finding areas where the fungus is endemic could explain how the fungus could be stopped, as the frogs in the area are likely to be less affected by the fungus, due to co-evolution.