Origin of Cacti

The fossil record is not of help in determining the origin of cacti because no fossils of cacti have been found. In addition to morphological and molecular data, however, biogeography provides a remarkable amount of information about the place and approximate time cacti originated, as well as their possible ancestors.

Plate tectonics or continental drift is a key phenomenon in understanding the place and time of the origin of cacti. About 200 m.y. b.p. (million years before present), Earth had a single supercontinent, Pangaea. The southern half of Pangaea began to separate about 165 m.y. b.p. from the northern portion, Laurasia, forming Gondwana. That huge landmass, in turn, began to fragment about 130 m.y. b.p., opening the South Atlantic Ocean as the future continents of Africa and South America began to form. Slowly, the future continents separated from each other as their plates drifted apart, though for many millions of years they were close enough that their floras and faunas were the same. Eventually, however, the distances became too great for interchange between the continents, and organisms evolved independently on each.

Cacti are essentially New World plants. Therefore, the family must have arisen after the ancient landmass of Gondwana separated sufficiently into Africa and South America. Otherwise, cacti would be well represented on both continents. Separation of the two continents began during the early Cretaceous, when flowering plants first appeared, about 125-130 m.y. b.p. (Raven and Axelrod 1974, 545). By the close of Cretaceous, 65 m.y. b.p., the two continents were separated by about 800 km (500 miles), which would have prevented most dispersal of organisms between them. Thus the Cactaceae probably did not arise until the very end of or just after the Cretaceous.

Mauseth (1990) suggests that the earliest possible time for the family's origin would be 100-90 m.y. b.p. Chloroplast DNA studies by Hershkovitz and Zimmer (1997), however, suggest that cacti may have arisen as late as 30 m.y. b.p. in the mid-Tertiary, followed by rapid diversification in the newly developing American deserts.

The cactus family is placed in the order Car-yophyllales, also called the Centrospermae or Chenopodiales, on the basis of numerous characteristics (Mabry 1977; Cronquist 1981,235-276; Downie and Palmer 1994). The order, with 12 families, comprises plants that possess several distinct embryological features, a unique type of sieve-tube plastid, and a characteristic spherical, pan-toporate type of pollen grain. Many are succulent, have anomalous secondary growth, and carry out crassulacean acid metabolism. One of the most distinctive features of the order is that 10 of the families, including the Cactaceae, produce nitrogen-containing betalain pigments instead of non-nitrogen-containing anthocyanins. These are the only flowering plant families with these unique pigments. Chloroplast DNA studies have also shown that the rpl2 intron is absent in members of the order (Downie and Palmer 1994,241).

Present biogeography indicates that some members of the Caryophyllales probably arose prior to the separation of Africa and South America (Raven and Axelrod 1974,568). The pokeweed family Phytolaccaceae is most likely the oldest family of the order and may be ancestral to the other families (Cronquist 1981, 238). Subsequently, some families of the order arose and evolved in Africa, while others, including cacti, arose and evolved in South America. Studies based on DNA data suggest that Cactaceae and the purslane family Portulacaceae are probably most closely related to each other. Among the Portulacaceae, sharing of certain DNA restriction sites indicates that cacti are more closely related to Portulaca than to Claytonia (Downie and Palmer 1994; Hershkovitz and Zimmer 1997).

Raven and Axelrod (1974, 605) suggest that there were extensive arid regions within the interior of Gondwana. It is therefore likely that several families arose within this former area—the Madagascan family Didiereaceae (like cacti, spiny stem succulents), some members of the Portula caceae, and the Mesembryanthemum family Aizo-aceae in Africa—and the Cactaceae and other members of the Portulacaceae in South America. Raven and Axelrod (1974,628) further comment that the cacti "appear to have come principally from South America, but they are so well or distinctively represented by endemic elements in North America that they may well have arrived in North America by early Tertiary time " North and South America were not connected by land until about 5.7 m.y. b.p. but it is likely that dispersal by rafting and by means of island bridges could have taken place as long ago as 36 m.y. b.p. through both the Caribbean and along the volcanic chain that makes up present-day Central America.

The South American continent about 100 m.y. b.p. was very different from that today. The continent has been slowly drifting northward, so at the approximate time of origin of cacti, Colombia and Venezuela were at or near the equator. More importantly, the Andes had not yet uplifted, so many regions of northwestern South America were hot, humid, and only seasonally dry. The region had not yet become a desert. Leuenberger (1986,43-45) states that the genus Pereskia probably arose within this area of South America, with three subsequent speciation zones occurring in the Andes, Caribbean, and the chaco and caatinga regions of eastern South America. Pereskia is generally accepted as the genus that most likely resembles what may have been the first cactus (Mauseth and Landrum 1997). Robert S. Wallace (pers. comm.) believes, however, that cacti originated farther south in Peru-Bolivia, for many of the more primitive members of subfamilies Opuntioideae, Cactoideae, and species of Pereskia (P. diaz-romeroana, P. horrida, and P. weberiana) are still found in this region of South America.

About 65 m.y. b.p. the Andes began to uplift, changing both the topography and climate of much of South America. By about 17 m.y. b.p. the Andes were high enough to cause rain-shadow deserts (Mauseth 1990). The latitude of 30° is characterized throughout the world as the region of deserts. During the early evolution and dispersal of cacti, Bolivia and southern Brazil were at this latitude, though the Atacama Desert of Chile toward the west and the Salta-Jujuy region of Ar gentina toward the east now occupy that latitude. It is no surprise, therefore, that both Bolivia and Brazil are important centers of cactus speciation. From these and other areas of South America, cacti slowly migrated both northward and southward into the regions where they are found today.

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