Gymnocalycium ×applanatum (Cactaceae, Cactoideae) – the first reported nothospecies between the subgenera Gymnocalycium and Trichomosemineum

This study examines an assumed hybrid between Gymnocalycium capillense and G. intertextum discovered in a mixed population of the putative parents at Villa Viso in Córdoba Province, Argentina. Eleven quantitative morphological characters (including two ratios) were measured in the natural populations of the presumed parental species and the assumed hybrid. Five of these characters differed between the parental species whereas the assumed hybrid usually showed intermediate values. Eleven qualitative morphological characters were compared for all three taxa, whereby two characters of the assumed hybrid were closer to G. intertextum, one character was closer to G. capillense, six characters were intermediate (but two of them closer to one presumed parent), and three characters were unique. The seed morphology is the most important character separating the subgenera of Gymnocalycium: the assumed hybrid is most similar in this regard to the nominate subgenus, but the seeds are larger than in both assumed parents and have a very broad hilum-micropylar region and conspicuous strophiole at the margin, like in the subgenus Trichomosemineum. Genome sizes estimated by means of flow cytometry shows that G. capillense is tetraploid and G. intertextum diploid, while the assumed hybrid plants are hexaploid (i.e. allopolyploid hybrids). Sequencing and phylogenetic analysis of a low-copy nuclear PHYC gene also revealed that this genomic region of the hybrid combines sequences present in G. capillense and G. intertextum. In combination with the morphological data, these results support the hybrid nature and parentage of the studied plants. The hybrid is formally described here as G. ×applanatum and is the first documented hybrid between the subgenera Gymnocalycium and Trichomosemineum. The results show the potential of allopolyploidy in which different subgenera participate for the evolution of the genus.