Majure L. C., Judd W. S., Soltis P. S., Soltis D. E., Cytogeography of the Humifusa clade of Opuntia s.s. Mill. 1754 (Cactaceae, Opuntioideae, Opuntieae): Correlations with pleistocene refugia and morphological traits in a polyploid complex, Comparative Cytogenetics, 2012, 6(1):53-77, doi: 10.3897/CompCytogen.v6i1.2523
A compilation of chromosome numbers, morphology and distribution of members of the Humifusa clade. The authors examine the geographical separation of diploids and polyploids, and present a hypothesis of Pleistocene refugia and subsequent range expansion.