When it comes to surviving in hot, dry environments, no plant does it better than the cactus. A suite of evolutionary adaptations, both anatomical and physiological, has allowed them to live where water is scarce. Cacti have shallow roots that quickly soak up infrequent desert rainfalls, and most species do not have leaves, which are easily desiccated. Instead, photosynthesis has moved indoors, taking place in their thick, succulent stems. How did these special features arise? A new study of cactus evolution suggests that the plant's water-saving strategies might have come first, followed only later by dramatic changes in the plant's anatomy.
Michael Donoghue and Erika Edwards, plant evolution researchers at Yale University in New Haven, Connecticut, analyzed water conservation mechanisms in the cactus genus Pereskia, an assortment of leafy shrubs and trees that are thought to represent the ancestral state of living cacti. In work published last year, the team sequenced selected DNA regions of 38 cactus species and concluded that modern Pereskia actually represents two evolutionary groups, one of which probably split off from the rest of the cacti before they had undergone their dramatic anatomical changes. The second Pereskia group is more closely related to other cacti even though it does not resemble them anatomically.
In the new work, Donoghue and Edwards (who is now at the University of California, Santa Barbara) looked at seven of the 17 known Pereskia species, including members of both groups. As the pair reports in the June issue of The American Naturalist, these seven species--which come from South and Central America--have water conservation features similar to leafless, thick-stemmed cacti, and very different from other leafy plants living in the same localities. For example, the leaves of Pereskia plants can store much more water, and they seem to use an alternative, water-conserving photosynthetic pathway typical of the leafless cacti when conditions are particularly dry. Since these physiological features are shared by both Pereskia groups as well as other cacti, despite their ancient evolutionary split, the team concludes that they must have evolved first, and that the first cacti were "shrubs or small trees with photosynthetic leaves."
It's "a wonderful study" which demonstrates that the evolution of water conservation "set the stage for the loss of leaves and the evolution of succulence," says David Ackerly, a plant evolution researcher at the University of California, Berkeley. Robert Wallace, a plant researcher at Iowa State University in Ames, agrees. "The authors provide the first empirical evidence [of] this scenario," he says, adding that some cactuslike traits may have evolved "long before cacti were cacti."