Chromosomes and hybrids of Echeveria (Crassulaceae). VII. Series Gibbiflorae (Baker) Berger

As treated here, series Gibbiflorae of Echeveria appears to be a closely related natural group. About 20 species are recognized, but many intermediate plants make some distinctions almost impossible. All species have n = 27 or 28 or multiples to n = 189. In cultivation they hybridize readily with each other and with most other species of Echeveria and related Mexican genera. Hybrids between species of series Gibbiflorae with n = 27 and 28 are generally fertile, and this may allow successful breeding of new cultivars. Nearly all hybrids of the Gibbiflorae with species of other series and genera having anywhere from n = 12 to n = 34 appear intermediate between their parents, but they are sterile. Each of them never forms more paired elements (bivalents plus multivalents) at meiosis than the number of chromosomes that it received from its parent with fewer chromosomes. This means that no chromosomes from the same parent can pair with each other and that the 27 or 28 chromosomes from the series Gibbiflorae parent comprise only a single genome. Thus they are effectively diploid. Hybrids between polyploids and diploids look more like their polyploid parents, which is evidence that the multiple genomes from the polyploid parent are genetically active in the hybrids.