Evolutionary genomics of sexual and asexual lineages of Giardia duodenalis

Sexual recombination is an essential hallmark of eukaryotic evolution. Without recombination, asexual eukaryotes should succumb to deleterious mutations and more rapidly evolving pathogens. Giardia duodenalis is an important parasitic protist, sits among the earliest-branching eukaryotic lineages and has no known sexual stage. Whether Giardia is an 'ancient asexual' is long explored but unresolved. Here, we find clear evidence for sex in Giardia but also a major asexual sublineage that has a broader host range than its sexual counterpart. This asexual lineage is not ancient and likely in mutational meltdown. The asexual lineage accumulates mutations, but unlike its sexual counterparts, lacks the signatures of selection and Red Queen coevolution. We present a new hypothesis that could explain how a mutational meltdown during Muller's Ratchet might enable asexual pathogens to transiently expand their host ranges. Fittingly, Giardia is not a last exception to but further proof of essentiality of sex for the long-term viability of eukaryotic lineages.