• Cactaceae;
  • genetic diversity;
  • Melocactus glaucescens;
  • morphological variability;
  • morphometrics


Melocactus glaucescens (Cactaceae) is a critically endangered species endemic to north-eastern Brazil. It grows in sympatry with other congeneric species and there is evidence for hybridization among them. We evaluated the levels of genetic and morphological variability and their substructuring between populations of M. glaucescens and sympatric species, and we tested for the occurrence of natural hybridization. Genetic variability was investigated using 12 allozyme loci, and morphological variability was investigated using multivariate morphometric analyses of 18 vegetative characters in nine natural populations of M. glaucescens, Melocactus ernestii, Melocactus concinnus and two distinct morphs of putative hybrids (Melocactus ×albicephalus). Genetic variability was low in all populations (P = 7.7–41.7, A = 0.3–1.7, He = 0.009–0.096), and all taxa displayed a deficit in heterozygotes. Low genetic structuring and moderate morphological structuring were found for M. glaucescens (FST = 0.045, AMRPP = 0.16) and M. concinnus (FST = 0.022, AMRPP = 0.11). The results obtained are inconclusive with respect to confirming the hypothesis of occurrence of hybridization because of an absence of diagnostic loci in the presumed parental species. The presence of several private alleles and the absence of linkage disequilibrium in the putative hybrids indicate that M. albicephalus should be treated as a distinct species, and not given hybrid status as recently stated by a number of authors. The large number of exclusive alleles and the levels of morphological structuring in the populations of M. glaucescens are important factors that need to be considered in the definition of strategies for the conservation of this species.