According to Dr. Anna Ronikier from the Molecular Biogeography and Systematics Group at the Institute of Botany of the Polish Academy of Sciences, the mould found in the earth’s southern hemisphere “changed its reproductive strategy from sexual to asexual,” so that it “could quickly spread in the northern hemisphere.”

Slime moulds are a little-known group of fungus-like organisms. Quite inconspicuous, they have no economic significance: they are not parasites, they do not damage crops, and they are not a common research subject for biologists.

Researchers from the Institute of Botany of the Polish Academy of Sciences are among the few specialists, who study these organisms. 

Dr. Anna Ronikier from the Molecular Biogeography and Systematics Group at the Institute of Botany of the Polish Academy of Sciences said: “For us, slime moulds are extremely interesting, especially the group of mountain slime moulds, which have chosen specific conditions to live in. Amoeba populations function in the soil under the snow cover, and just after the winter snow melts in spring, immobile, visible to the naked eye sporangia are formed. For years we have been studying these slime moulds that are little known and occur in all mountain massifs of the world.”

She added that slime moulds have a very important role in nature; feeding on bacteria and other microorganisms, they take part in the circulation of matter in nature and the regulation of soil microorganisms.

A few years ago, Dr. Ronikier was invited to a project to study the diversity of slime moulds in South America. She focused on mountain slime moulds. Among the material collected at that time, one species (Didymium nivicola) constituted as much as 1/4 of all collections. She said: “It was interesting because this species is rare in the northern hemisphere. The contrast between the frequent occurrence of this species in the Andes and South America and its rarity in the northern hemisphere intrigued me.”

To investigate this unexpected situation, she and PhD student Paulina Janik worked on samples of the species collected in the world herbariums and representing all areas, in which it was ever recorded. The researchers were interested in the genetic and morphological variability of this species in South America compared to its entire range.

The results of Janik's doctoral dissertation, based on sequencing of DNA fragments and recently published in the journal Protist(https://www.sciencedirect.com/science/article/abs/pii/S1434461020300766), show that all populations of the species originating from the Northern Hemisphere: North America, Asia and Europe, are genetically identical. 

But the South American populations turned out to be very diverse. Dr. Ronikier said: “It was surprising for us, because when looking for data on this type of genetic variation, also for other organisms, we could not find a similar example.”

Based on this research and data on species diversity, the researchers hypothesized that South America is the centre of mountain slime mould diversity. The studied species could have travelled to the Northern Hemisphere from there. Ronikier said: “Slime moulds spread as spores blown by the wind. On the globe, winds blow mainly along the circles of latitude. We can assume that long-range transport within the hemispheres is possible without hindrance, but transport between hemispheres is limited. In the past, there had to be a one-off transport of spores from the southern hemisphere to the northern hemisphere. After the transport event, the organism changed its reproductive strategy from sexual to asexual and could quickly spread in the northern hemisphere.”

The genetic diversity observed in South American populations is beneficial to slime moulds because more diverse organisms can potentially respond better to changes in the environment. On the other hand, in the case of a northern hemisphere line, it is possible that one species that reproduces asexually shortens its life cycle and has a chance of conquering new habitats more quickly.

Until recently, it was thought that protists - organisms that also include slime moulds - did not have any distribution patterns around the globe due to their small size and ease of spread. Recent studies show that this is not the case. 

Roniker said: “Our research shows that these patterns exist and can vary. Some organisms can be very widespread across the globe, but their distribution pattern is not necessarily consistent. These types of studies on a global scale are rare, however, due to the difficulty of collecting appropriate biological material. 

“In this respect, slime moulds are unique among protists because they produce sporangia that can be collected and stored in permanent herbarium collections. They are therefore a very convenient object for further research, important for a better understanding of the biogeography of microorganisms.”

PAP - Science in Poland

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