Spines and ribs of Pilosocereus arrabidae (Lem.) Byles & G.D. Rowley and allies (Cactaceae): Ecologic or genetic traits?

• Spine length is strongly influenced by rainfall, but not by solar radiation.
• The number of spines per areole and the number of ribs depend on solar radiation.
• Although related to genetic distance, the number of ribs responds better to latitude.
• Spine length and number of spines per areole exhibited no relationship with genetics.
• Morphometric traits should be used cautiously in cactus taxonomy.

Amongst the more outstanding features of columnar cacti, their ribs and spines may vary greatly between and within species. With the implicit assumption that morphometric traits of spines and ribs are inherited, these features have been historically used for taxonomic purposes in Cactaceae. However, some studies show that environmental variables may influence morphology of cacti and thus, some traits are not solely genetically determined. We aim to test the influence of both genetic and environmental factors upon spine and rib patterns. We tested the correlation between environmental, genetic, and morphological data on three taxa of Pilosocereus from eastern Brazil (from the Pilosocereus arrabidae group). We evaluated the influence of those factors in three traits: the length of the longest spine in the areole, the number of spines per areole and number of ribs. The number of ribs exhibited a significant positive relationship with genetic distance. The length of the longest spine is positively correlated with rainfall, but the number of spines per areole is negatively correlated. Negative relationships were also found between latitude and number of ribs, as well as between latitude and the number of spines per areole. Our results advise towards cautious use of these traits in cactus taxonomy.