Allopatric speciation in cattails: Genomic landscapes of divergence across Typha spp. suggest balancing selection, introgressions, and the absence of adaptive divergence

Speciation can be broadly understood within two non-mutually exclusive frameworks: genetic drift under isolation and natural selection under ecological divergence. Here, we examined the genomic diversity and differentiation of five Typha species, a group of plants foundational to freshwaters with widespread, partially sympatric distributions and at least one widespread hybrid zone. Using genome-wide data from 207 individuals, we examined the contributions of demographic fluctuations, selection, and hybridisation in driving their speciation history. Demographic reconstructions revealed sequential bottlenecks and expansions coincident with lineage splits, and indicated a drift-driven scenario with no migration events for all five species. The genomic landscapes showed balancing selection, sparse divergent selection, and low net divergence. Introgressions from T. latifolia to T. angustifolia and T. domingensis were found. Our findings suggest histories of allopatric speciation followed by range expansions and secondary contacts, leading to contemporary hybridisation between some species. Our results also emphasise the roles of balancing selection and introgression as sources of standing genetic variation. Allopatric speciation in T. latifolia and T. angustifolia could explain their ability to hybridise, highlighting the need to stop the human-mediated dispersal of Typha (e.g., the intercontinental sourcing via garden centres).