Conservation Genetics of Endangered Suisun Thistle and its Congeners

Abstract

The native California thistle species Cirsium hydrophilum occurs endemically in serpentine wetlands and tidal salt marsh soils, surviving in edaphic conditions that prove toxic to most plants. The two varieties of C. hydrophilum are separately adapted to life in different stressful environments in Northern California. C. hydrophilum var. hydrophilum, Suisun thistle, is restricted to the brackish tidal salt marsh of Rush Ranch in Solano County while C. hydrophilum var. vaseyi, Mt. Tamalpais thistle, sustains on the metallic serpentinite soils of Mt.Tamalpais fresh watershed in Marin. Both varieties of C. hydrophilum are recognized as endangered at the federal level due to their low total occurrences. Suisun thistle's case of endemism is hypothesized to be a result of both environmental conditions and genetic factors.The prevalent paradigm we investigate is founded on the understanding that small plant populations with limited distributions are prevented from expanding due to their own lack of alleles, or genetic diversity, making them inept to survive in distinctive neighboring habitats. We have developed a reduced representation genome and cross analyzed the geographic positions of individuals from clustered subpopulations with their genetic content to investigate the genomic nature of this restricted endemic. Genetic analyses have substantiated the hypothesized inbreeding in Suisun thistle and our biogeographic analyses show that the individuals sampled from Rush Ranch thus far comprise a single population. This data ultimately guides our drafting of regional recommendations for out-planting of Suisun thistle to maintain native Cirsium in Northern California.