<b>Diversification and hybridization within the </b><b><i>Spirochona gemmipara</i></b><b> complex driven by gammarid host range shifts</b>

Using integrated morphological, morphometric, and multilocus phylogenetic data, we show that <i>Spirochona gemmipara</i>, a widespread ciliate epibiont of European freshwater gammarids, constitutes a hybridising network of at least nine mitochondrial lineages forming a sympatric multispecies syngameon. Time-calibrated phylogenies indicate that its five main lineages originated in the Miocene through allopatric divergence on isolated European palaeoislands colonised independently by freshwater gammarids.

Subsequent reconnection of continental freshwater systems following the regression of the Paratethys Sea enabled secondary contact and hybridisation among emerging <i>S. gemmipara</i>-like species, with later Pleistocene range shifts likely maintaining compatibility within the syngameon. Discordant mitochondrial signals within the syngameon are consistent with heteroplasmy and may also reflect phylogenetic noise or NUMTs, and only tentatively suggest the possibility of mitogenome recombination.