Recurrent hybridization between highly divergent paramecium species.

Paramecium sonneborni has hundreds of large DNA segments acquired via recurrent horizontal transfers from multiple very distant lineages, with synonymous divergence around ~1 substitution/site.

• Why it matters: Speciation is usually reversible only while gene flow persists, but strong species barriers are expected to rapidly prevent introgression beyond modest divergence.
• What they did: The authors analyzed the Paramecium sonneborni genome to identify hundreds of horizontally transferred segments, whose near-chromosome sizes indicate introgression through interspecific mating.
• The result: The findings imply that, in P. sonneborni ancestry, repeated hybridization with highly divergent species produced viable and fertile offspring, consistent with germline–somatic separation enabling tolerance of foreign DNA.

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