Host range expansion of asexual parasite can be explained by loss of adaptions in Muller’s Ratchet

Swapnil Tichkule, Cock van Oosterhout, Simone M. Cacciò, Filip Weisz, Balu Balan, Eva Nohýnková, Myo Naung, Brendan R. E. Ansell, Samantha J. Emery-Corbin, Louise Baker, Marco Lalle, Staffan Svärd, Robin B. Gasser and Aaron R. Jex ()
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Swapnil Tichkule: Walter and Eliza Hall Institute, Infection and Global Health
Cock van Oosterhout: Norwich Research Park, School of Environmental Sciences, University of East Anglia
Simone M. Cacciò: Istituto Superiore di Sanità, Department of Infectious Diseases
Filip Weisz: Charles University, Institute of Medical Microbiology, First Faculty of Medicine
Balu Balan: Walter and Eliza Hall Institute, Infection and Global Health
Eva Nohýnková: Charles University, Institute of Medical Microbiology, First Faculty of Medicine
Myo Naung: Walter and Eliza Hall Institute, Infection and Global Health
Brendan R. E. Ansell: Walter and Eliza Hall Institute, Infection and Global Health
Samantha J. Emery-Corbin: Walter and Eliza Hall Institute, Infection and Global Health
Louise Baker: Walter and Eliza Hall Institute, Infection and Global Health
Marco Lalle: Istituto Superiore di Sanità, Department of Infectious Diseases
Staffan Svärd: Uppsala University, Department of Cell and Molecular Biology
Robin B. Gasser: University of Melbourne, Melbourne Veterinary School
Aaron R. Jex: Walter and Eliza Hall Institute, Infection and Global Health

Nature Communications, 2025, vol. 16, issue 1, 1-14

Abstract: Abstract Sexual recombination is a hallmark of eukaryotic evolution. Without recombination, asexual eukaryotes should succumb to deleterious mutations and more rapidly evolving pathogens. Giardia duodenalis, a parasitic protist, sits within one of the earliest-branching eukaryotic lineages and has no known sexual stage. Whether Giardia are ‘ancient asexuals’ has been long explored but is unresolved. Here, we find clear evidence of sex in Giardia and also discover an asexual sublineage that has a broader host range than its sexual ancestor. This asexual lineage is not ancient, and is accumulating deleterious mutations. Unlike its sexual counterparts, its genetic variation lacks the signatures of selection and Red Queen coevolution. We propose a new hypothesis that explains how a mutational meltdown during Muller’s Ratchet might enable asexual pathogens to expand their host ranges transiently. Fittingly, our results suggest that Giardia is not the last exception to, but rather further evidence of, the essentiality of eukaryotic sex.

Date: 2025
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DOI: 10.1038/s41467-025-65843-4

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