Hybridization and competition between the endangered sea marigold (Calendula maritima, Asteraceae) and a more common congener

Occurrences of hybridization between the rare, endangered Sicilian endemic, Calendula maritima, and other congeners in the wild have been suspected by several authors, but never demonstrated. In San Cusumano (Trapani), one of the only three Sicilian mainland populations, C. maritima occurs in close proximity to the more common congener C. suffruticosa subsp. fulgida, and individuals morphologically intermediate between the two taxa have been observed. In order to determine whether hybridization is occurring at this site, and, if so, begin to assess the impact it could have on C. maritima, two independent tests of hybridity were conducted by studying (1) the pollen morphology and viability and (2) sequence variation in one chloroplast marker (atpI-atpH) and in the nuclear ribosomal internal transcribed spacer region (ITS). Both palynological and molecular data provided evidence of hybridization between C. maritima and C. suffruticosa subsp. fulgida, and suggest that although fertility of hybrids appeared to be reduced, they are likely capable of back-crossing with parents. Thus, hybridization may be putting C. maritima at risk of extinction via introgression. Anthropogenic habitat disturbance and expansion of C. suffruticosa subsp. fulgida into remaining populations of C. maritima could pose a serious threat to its successful conservation.