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Pereskia s.l. was found to be separated into the same clades in a 2011 study that used fewer genes but more species, although it was unable to resolve the members of the “core cacti” clade. The relationships depicted above were agreed to be “the most substantial to date.” [33]
Leuenbergeria species (Pereskia s.l. Clade A) consistently lack two essential stem characteristics that are present in the majority of the remaining “caulocacti”: first, like most non-cacti, their stems start to develop bark early in the plants’ lives; second, they lack stomatastructures that regulate air intake into a plant and, consequently, regulate photosynthesis. Contrarily, caulocacti, such as the surviving species of Pereskia s.s. and several Rhodocactus species, generally delay the formation of bark and have stomata on their stems, allowing the stem the ability to play a significant role in photosynthesis. (The two extremely specialized Maihuenia species make an exception.) [34] [36]
The original cacti are believed to have been tiny trees or shrubs with photosynthesis occurring in their leaves. They were residents of tropical regions that occasionally experienced drought. If Leuenbergeria is a suitable representation of these early cacti, then despite having superficial similarities to adjacent trees, they had already developed water-saving techniques (some of which are present in members of other families in the order Caryophyllales). These included the ability to react quickly to periods of rain and minimizing transpiration by making excellent use of water during photosynthesis. The latter was accomplished by closely regulating the stomata’s opening. Early relatives of Pereskia species may have had the ability to change from the typical C3 mechanism, in which carbon dioxide is continuously used in photosynthesis, to CAM cycling, in which carbon dioxide produced during respiration is stored for use during photosynthesis when the stomata are closed. [10]
The evolutionarily significant shift toward stems serving as photosynthetic organs can be seen in the group that includes Rhodocactus and Pereskia s.s. Stomata are present in stems, and bark develops later than it does in typical trees. The “core cacti” exhibit multiple leaf losses that are more-or-less total in the Cactoideae and a continuous rise in stem succulence and photosynthesis. Whether the transition to full CAM photosynthesis in stems occurred separately in Opuntioideae and Cactoideae, in which case it never developed in Maihuenia, or whether it occurred once in the core cacti, in which case it has been lost in Maihuenia. [10]
This article’s “Phylogeny”-related sections need to be updated. Please contribute to updating this article with new information or recent events. (May 2021)
Evolutionary history
Cacti have no fossilized remains that could shed light on how they evolved. [40] Cacti’s global range, however, provides some proof. Cacti are plants native to South America and primarily the southern areas of North America, with the exception of Rhipsalis baccifera, which has only recently expanded to parts of the Old World. This indicates that the family must have originated following the division of the once-common continent Gondwana into South America and Africa during the Early Cretaceous, some 145 to 101 million years ago. [41] It’s less known exactly when cacti evolved after this divide. Older data point to an early origin that occurred in the Late Cretaceous period 9066 million years ago. The origin may have occurred considerably more recently, maybe in the very Late Eocene to Early Oligocene periods, about 3530 million years ago, according to more recent molecular analyses. [40] [42] According to the cacti’s phylogeny, the Leuenbergeria cactus may have originated in Central America and northern South America, but the caulocacti, which have stems that are more or less succulent, evolved later in the southern region of South America before migrating north. [34] It is thought that core cacti, which have firmly succulent stems, first appeared about 25 million years ago. [40] Uplifting in the central Andes, around 2520 million years ago, which was connected to rising and fluctuating aridity, may have served as a catalyst for their evolution. [34] Other succulent plants, such as the Aizoaceae in South Africa, the Didiereaceae in Madagascar, and the genus Agave in the Americas, appear to have diversified at the same time, which coincided with a global expansion of arid environments. However, the current species diversity of cacti is thought to have arisen only in the last 105 million years (from the late Miocene into the Pliocene). [40]