Population Genetics Reveals Insights Into Cryophytum Biogeography in South Africa

The Mesembryanthemoideae (Aizoaceae) are a diverse group of succulents in southern Africa, many of which are endemic to the Succulent Karoo and Cape Floristic Region of South Africa. This study investigated the population genetic structure and biogeography of two closely related species, Cryophytum crystallinum and C. guerichianum, and assessed whether abiotic factors, including temperature, precipitation and soil variables, contribute to their genetic differentiation. We also examined a putative hybrid from the Northern Cape Province, showing intermediate morphological traits. In C. crystallinum, populations at either end of the coastal range in the Western Cape Province were genetically similar, with distinct clusters occurring in between. We propose that repeated range shifts and the formation of refugia may explain this pattern and found little evidence that the abiotic variables considered here strongly shaped genetic structure. In C. guerichianum, we identified inland, intermediate and coastal genetic clusters, likely shaped by vicariance associated with the Cape Fold Belt. Across both species, genetic divergence showed some support for the Isolation by Environment hypothesis, with a correlation between genetic and environmental distances independent of geographic distance, although topography was likely the dominant driver. The suspected hybrids were genetically closer to C. guerichianum, suggesting asymmetric introgression and a possible mosaic pattern of hybridisation in areas of sympatry. As C. crystallinum is invasive in parts of the world, accounting for historical population structure and evolutionary history may help identify appropriate source populations for biological control, as genetically distinct populations can respond differently, and geographic distance alone may not reliably predict suitability.