Population genetics elucidates invasion pathways and informs biocontrol strategies for the Crystalline Ice Plant ( Cryophytum crystallinum ) in North America

Abstract

Crystalline ice plant [ Cryophytum crystallinum (L.) N.E. Br.] is a succulent plant indigenous to South Africa and is a highly invasive weed in North America. There are currently three weevil species from the native range that show promise as potential biological control agents for these invasive populations. The present study used a ddRADseq population genetic technique to match the invasive populations in North America to their source of origin to prioritize survey areas where the best-matched agent genotypes are likely to occur. We found that populations in the Canary Islands and Morocco closely matched those in the invaded North American range, and that this genetic cluster was most similar to the South African native range sites surrounding the Cape Town area and those from Aniston Village further east along the coastline. There was evidence of an invasion route from South Africa to the Mediterranean Basin, and that a secondary bridgehead invasion likely took place from there to North America. Genetic structuring within the native range revealed a total of four distinct populations, where sites north and south of Cape Town were unique and did not match the invasive populations. We found a unique outlier population in the Northern Cape which displayed intermediate morphological traits between C. crystallinum and its close relative, ice plant ( Mesembryanthemum guerichianum Pax; syn. Cryophytum guerichianum Pax). Surveys for biological control agents should focus on the areas between Kalk Bay and Yzerfontein in the Western Cape, and sites surrounding Arniston Village.