Population genetics of two rare perennials in isolated wetlands: Sagittaria isoetiformis and S. teres (Alismataceae)

We investigated genetic structure in two closely related perennial plants that occur in isolated wetlands: Sagittaria isoetiformis , restricted to the southeastern Coastal Plain of North America, and S. teres , endemic to the northeastern Coastal Plain. Using horizontal starch‐gel electrophoresis, we screened 527 individuals from 11 populations of S. isoetiformis and 367 individuals from seven populations of S. teres. A high proportion of the 16 loci were polymorphic ( %P S = 93.8% in S. isoetiformis and %P S = 75.0% in S. teres ), with higher mean numbers of alleles per polymorphic locus and effective alleles per locus in S. isoetiformis ( AP = 3.27, A E = 1.90) than in S. teres ( AP = 2.58, A E = 1.30). Species‐ and population‐level expected heterozygosities were higher in S. isoetiformis ( H ES = 0.399, H EP = 0.218) than in S. teres ( H ES = 0.177, H EP = 0.101). Jackknife estimates of F statistics indicated moderate levels of inbreeding in S. teres ( F̂ IS = 23.1%). Strong differentiation characterized these geographically isolated populations ( G ST = 39.9% in S. isoetiformis , and G ST = 26.1% in S. teres ). Genetic identities varied substantially within ( Ī = 75%, range = 0.558–0.963 in S. isoetiformis ; Ī = 89%, range = 0.776–0.963 in S. teres ) and among species ( Ī = 81%, range = 0.506–0.882), leading to the discrimination of four regional population clusters using nonmetric multidimensional scaling (NMDS). It appears that S. isoetiformis and S. teres are a progenitor‐derivative species pair.