Holocene climate changes explain the spatial pattern in genetic diversity in populations of Cyperus papyrus from Southeast Africa wetlands

Wetlands are one of the most threatened ecosystems in the world because
more than 70% of the area worldwide has been lost since 1900. Wetland
plant species rely greatly on water for seeds and propagules, which may
lead to a downstream unidirectional dispersal and accumulation of genetic
diversity downstream. However, several species show no support for
unidirectional genetic diversity, revealing the complexity of population
dynamics and gene flow in wetlands. Here, we used microsatellite loci to
address how the past demographic dynamics shaped the contemporary spatial
pattern in genetic diversity and population structure of Cyperus papyrus
in wetlands of Southeast Africa. Using spatially explicit analysis and
coalescent modelling we found no support for unidirectional dispersal.
Instead, we found higher genetic diversity in populations upstream than
downstream in the river basin. We also found high admixture among
populations, most likely due to connections between adjacent river basins
during sporadic floods, and ongoing gene flow due to bird-mediated seed
dispersal. Our results suggest stepping-stone migration due to strong
isolation-by-distance, but not necessarily unidirectional. Moreover, the
past demographic dynamics in the Holocene shaped the current pattern of
genetic diversity and structure, leading to higher genetic diversity in
populations upstream of the Zambezi river basin. Our results also point to
the very low genetic diversity of C. papyrus populations in Southeast
Africa and the need for management and conservation strategies to
guarantee the long-term persistence of the species in the region.