Proc. Nat. Acad. Sci. USA Vol. 68, No. 9, pp. 1993-1995, September 1971 Natural Hybridization between the Cholla Cactus Species Opuntia spinosior and Opuntia versicolor (vegetative and sexual reproduction/clonal colonies) VERNE GRANT AND KAREN A. GRANT Department of Botany, University of Texas, Austin, Texas 78712 Contributed by Verne Grant, June 7, 1971 ABSTRACT The sexually reproducing cholla cactus shrubby, with short branches in whorls; the stem joints are species, Opuntia spinosior and 0. versicolor, hybridize relatively thick, with high narrow tubercles; there are many naturally in Arizona to produce hybrid swarms and segre- gating introgressive populations with a high degree of in- long spines in each cluster (or areole); and the fruits are yellow dividual variation. The pattern of variation in these hybrid and strongly tuberculate with a deep apical cavity (Figs. 1A, populations can be compared with that in populations de- 2A). Opuntia versicolor is tree-like in form with a central rived from crossing of 0. spinosior and 0. fulgida, where trunk and long divergent branches; the stem joints are thin the hybrids reproduce vegetatively rather than by seeds. The latter hybrid combination, in marked contrast to the with broad low tubercles; the spines are shorter and fewer former, results in clonal colonies with little or no ob- than in 0. spinosior; and the fruits are green or reddish, servable individual variation. smooth-surfaced or slightly tuberculate, and have a shallow apical cup(Figs. 1B, 2B). These character differences provide The shrubby chollas are a group of about 13 interrelated good morphological markers for identifying hybrids. species belonging to the genus and subgenus Opuntia Cylin- Both species reproduce mainly by seeds. The flowers are dropuntia (Cactaceae) and ranging through the desert and visited and pollinated by anthophorid and other bees. semidesert areas of the southwestern United States and adja- Vegetative propagation of detached stem joints is possible cent parts of Mexico. The species overlap in distribution and and doubtless occurs, but is not a common mode of reproduc- hybridize naturally in various combinations. We have pre- tion in either species. viously reported the results of natural hybridization between The flowers are much alike in the two species and are in Opuntia spinosior and 0. fulgida (1). Here we will describe the bloom simultaneously during the month of May. There are very different results of natural hybridization between the no indications of any floral or seasonal isolation. Whether same 0. spinosior and another species, 0. versicolor. The differ- the species are isolated to any significant degree by incompati- ences between the two cases can be correlated with the mode bility barriers is unknown. There is a weak hybrid sterility of reproduction of the hybrids. barrier as will be shown later. The main effective forms of isolation between 0. spinosior and 0. verscolor appear to be The Parental Species spatial and ecological isolation, and these modes of separation Opuntia spinosior occupies a fairly wide area in southern break down in many localities. Arizona, southwestern New Mexico, and northern Sonora and Chihuahua, and ranges altitudinally from 6500 ft (1950 m) Natural Hybrids elevation in the mountains down to 1500 ft (457.2 m) on the We have found natural hybrids in three localities within the desert plains. It reaches its best development in the desert broad zone of sympatric overlap of the parental species. The grassland zone (Fig. 1A), but occurs in various other zones localities are: Park Link road between Red Rock and Florence from pinyon-juniper woodland to creosote-bush scrub (1,2). highway (northwest of Tucson); near Oracle Junction (north Opuntia versicolor has a smaller area in southern Arizona and of Tucson); and Houghton Road (east of Tucson). northern Sonora and a narrower altitudinal range in desert In a mixed colony near Oracle Junction consisting of both foothills and outwash plains from 2000 to 4000 ft (609.6- 0. vereicolor and 0. spinosior, we found a single individual 1219.2 m) elevation (Fig. 1B) (2). The area of 0. versicolor is (plant No. 71-5) which was intermediate in the middle range thus included as an enclave within the area of 0. spinosior. between the two species in habit of branching, stem joint Since 0. spinosior is rare or absent in the desert foothill zone, size, tubercle height, spine length, spine number, and fruit many populations of 0. versicolor in this zone are spatially surface. Plant 71-5 at Oracle Junction is very similar to plant isolated from 0. spinosior. Even more populations of 0. spino- 71-9 at Houghton Road, for which measurements are given sior in the desert grassland and woodland communities are later. allopatric in relation to 0. versicolor. Nevertheless, biotically Four very different individuals in the Houghton Road sympatric contacts between the two species are common on population were selected to show the range of individual desert plains and at the upper edge of desert foothills. variation (see Fig. 3). These four individuals were measured The two species are well differentiated in a series of vegeta- for nine characters, with the results shown in Table 1. The tive and fruit characters. These morphological character characteristics of neighboring allopatric populations of the differences are particularly clearcut in comparisons between parental species are also given in the table as standards for allopatric populations of the two species. Opuntia spinofor is comparison. 1993 Downloaded by guest on October 1, 2021 1994 Botany: Grant and Grant Proc. Nat. Acad. Sci. USA 68 (1971) TABLE 1. Vegetative and fruit characters in a hybrid population of Opuntia versicolor X spinosior and in allopatric populations of the parental species 0. versicolor, Houghton Road population 0. spinosior, Character Rincon Mts. plant 71-7 plant 71-10 plant 71-9 plant 71-8 Santa Rita Mlts. Stem joint, diameter 19 mm 20-21 mm 20-23 mm 27-29 mm 24-26 mm 31-34 mm Tubercle, height 6-7 mm 6-7 mm 6-7 mm 8 mm 7 mm 10-12 mm Tubercle, width at base 7 mm 8 mm 6-7 mm 8-9 mm 7 mm 6 mm Color of sterns reddish reddish reddish reddish green green Spines, length of longest one in cluster 10-11 mm 16-18 mm 11 mm 12 mm 11-14 mm 14-16 mm Spines, number in cluster 6-11 8-14 6-8 8-9 9-11 15-20 Fruit, surface smooth to smooth intermediate intermediate strongly strongly slightly tuberculate tuberculate tuberculate Fruit, color reddish reddish reddish green yellow yellow Fruit, apical cavity shallow shallow intermediate deep deep deep Plants 71-9 and 71-10 in the Houghton Road population spinosior-like traits. Plant 71-8, on the other hand, approaches are generally intermediate between the parental species. 0. spinosior while retaining some intermediate or versicolor- Plant 71-9 is intermediate in the middle range in four of the like traits (Table 1). nine characters scored; it is like or nearly like 0. versicolor The Houghton Road population as a whole exhibits a high in four other characters; and is like 0. spinosior in one char- degree of individual variation, not only in the stem and fruit acter. Plant 71-10 is intermediate in about half the characters characters listed in Table 1, but also in other characters not scored and like 0. versicolor in the others. Plant 71-7 ap- tabulated, such as habit of branching and flower color. The proaches 0. versicolor but possesses some intermediate or wide segregation and recombination in the Houghton Road population indicate that this population is a hybrid swarm. A The Park Link hybrid population is quite variable but has a A .0, B Ar B " -*. e V A-4 P-771A FIG. 1. Opuntia spinosior and 0. versicolar in their typical habitats in an area northwest of Tucson, Arizona. (A) 0. spino- sior in desert grassland. (B) 0. versicolor in saguaro-paloverde FIG. 2. Terminal stem joints and fruits of 0. spinosior (A) scrub. and 0. versicolor (B). Downloaded by guest on October 1, 2021 Proc. Nat. Acad. Sci. USA 68 (1971) Hybridization of Cholla Cactus Species 1995 A the Houghton Road population had the following proportions of morphologically good pollen grains: plant 71-9, 97% good grains; plant 71-7, 91%; plant 71-10, 57%; plant 71-8, 42%. Seed fertility varied in a parallel manner. The first two plants mentioned above were highly fertile as to seeds, and the second two plants were semi-fertile. We examined biotically sympatric stands of 0. versicolor and 0. spinosior in four well-spaced localities. Hybrids were present in three of the four localities. This gives some indica- tion of the frequency of natural hybridization between these two species throughout their zone of contact. The occurrence of individuals in the Houghton Road and B P-ark Link populations which approach one parental species or the other in several different quantitative characters suggests backerossing and introgression. Indeed the Park Link colony appears to be an introgressive population of 0. versicolor segregating in the direction of 0. spinosior. How far the introgression goes in the two species is un- known. There are other variable populations of 0. versicolor, such as one on the north slopes of the Baboquivari Mountains (west of Tucson), which are segregating for characters prob- ably introduced from 0. spinosior. A distinct possibility C ml. W exists that the southern lowland race of 0. spinosior is af- fected on a wide scale by introgression from 0. versicolor. This, however, goes beyond our present story and will require further study. DISCUSSION Opuntia spinosior and 0. versicolor are biotically sympatric in numerous localities and have broadly overlapping seasons of flowering.