Data are from Taylor (1997) for the approximately 125 genera in the Cactaceae and for species on the CITES Cactaceae checklist (Hunt 1992).
centers of diversity (Table 8.1). Collectively, these four centers of diversity contain approximately half of all known species of cacti and substantial numbers of endemics. Two other regions, i.e., the Caribbean basin (including northern Venezuela, Panama, and northern and western Colombia) and Chile (excluding the northeastern corner [see Table 8.1]), also contain numerous endemics.
A primary objective of conservation is to maintain species diversity within a defined locale or region. In most cases, species diversity has been interpreted as "species richness," i.e., the quantity of species within a country, ecosystem, or region. For cacti, as for other species, species diversity is greatest in the centers of diversity; e.g., one locale in northeast Mexico has 27 species of cacti (Taylor 1997). An emphasis on preserving plant communities with the greatest species richness has meant that the major focus of conservation measures in the Americas has been on tropical moist broadleaf forests ("rainforests") rather than the drier, nonforested or semiforested regions (Redford et al. 1990). However, this latter group of ecosystems contains the vast majority of cacti. Hence, many of the conservation measures designed to preserve tropical rainforests in the Americas have had little impact on conservation of cacti.
Wild species of plants experience numerous changes in their biological and physical environment over time. Their survival, evolutionary potential, and ability to adapt to environmental changes will depend on the existence of genetic diversity. As a consequence, considerable effort has been made to estimate the genetic diversity within wild species of plants. Information on genetic diversity in wild species has come primarily from allozyme surveys. Allo-zymes exhibit simple inheritance, codominance, complete penetrance, and consistency of expression under a wide range of environmental conditions and can be determined for a wide range of plant species irrespective of natural habitat, size, or longevity (Hamrick 1989). These properties make allozymes particularly useful as genetic markers.
Allozyme surveys estimate the level of genetic diversity within individual species and indicate how the genetic variation is distributed within and among populations. A typical allozyme survey consists of three parts: (1) collection of allozyme data from a minimum of one population (usually more) of a species, (2) computation of diversity statistics, and (3) comparison of the species' diversity statistics with those collected from other species with similar life history traits (Hamrick and Godt 1989, 1996). The following statistics are calculated at the species and within-popula-tion levels: (1) percentage of polymorphic loci (P), (2) mean number of alleles per locus (A) for polymorphic as well as monomorphic loci, and (3) genetic diversity (H) of each locus (H = 1 - Ypl, where pi is the mean frequency of the ith allele at a locus). Mean genetic diversity at the species and within-population levels is determined by averaging the H values over all loci. For the average plant species, PA, and Hvalues are 50%, 1.96, and 15%, respectively (Hamrick and Godt 1989). The P A, and H values for the average plant population are 34%, 1.53, and 11%, respectively. For studies that examine allozyme variation in multiple populations, the genetic diversity within populations (Hs) and the total genetic diversity (Ht) are computed for polymorphic loci, and the Gst statistic is calculated to estimate the proportion of total diversity among populations (Gst = [Ht—Hs]/Ht [Nei 1973]). For the average plant species, about 78% of the total genetic diversity (Ht) at polymorphic loci resides within populations, whereas 22% of the Ht is distributed among populations (Hamrick and Godt 1989).
Allozyme surveys have been conducted on natural populations of Opuntia humifusa (Wallace and Fairbrothers 1986), Pachycereusschottii (Parker and Hamrick 1992), and Weberbauerocereus weberbaueri (Sahley 1996). The most thorough genetic diversity study on wild cacti was on the diploid (2n = 22) species P schottii (formerly Lophocereus schottii; Parker and Hamrick 1992). Eight populations of P schottii were examined at the northern extremity of its range (southern Arizona), where reproduction is primarily asexual. At the species level, the percent polymorphic loci was 44%, the mean number of alleles per locus was 1.55, and the mean genetic diversity (He) was 0.145. The P
value for P schottii (44%) was comparable to the mean P value reported by Hamrick and Godt (1989) for dicots (45%), species with a narrow distribution range (45%), and species that reproduce sexually and asexually (44%). The mean proportion of polymorphic loci and mean genetic diversity (He) within populations were 34% and 0.126. The mean proportion of total diversity among populations (Gst) was 0.13. Thus, about 87% of the total genetic variation occurred within populations. The mean Gst value for P schottii was lower than for groups of species with similar ecological traits—dicots, species with a narrow distribution range, species that disperse seed by animal ingestion, and species that reproduce sexually and asexually (Hamrick and Godt 1989). Sporadic long-distance dispersal of stems and occasional sexual reproduction apparently helped sustain a level of genetic diversity in P schottii that is similar to sexually reproducing species. Previous studies (Ellstrand and Roose 1987; Hamrick and Godt 1989) have shown that species that reproduce primarily or exclusively by asexual means (vegetative reproduction and/or agamospermy) maintain as much genetic diversity as species that reproduce sexually.
Information about the extent and distribution of genetic variation in plant species has both scientific merit and practical applications. For example, allozyme data can be useful for determining how to collect and maintain genetically representative samples for conserving genetic diversity in ex situ collections. Information on the distribution of genetic variation within and among populations has aided in the development of sampling strategies for collecting plant materials of endangered species (Brown and Briggs i99i).
Breeding Systems and Sexual Expression in the Cactaceae
The breeding system markedly affects the magnitude and distribution of genetic diversity in plant species. The Cactaceae can be roughly divided into two groups, depending on whether they are predominantly outcrossing or primarily selfing. Most cacti probably outcross to some degree, and many species are likely to be predominantly out-crossing. Cacti exhibit several mechanisms that encourage outcrossing, including self-incompatibility, herkogamy, and dicliny. Self-incompatibility is a genetically controlled mechanism that promotes outcrossing in fertile hermaphrodites by preventing fertilization when a plant is selfed or outcrossed to another plant with the same incompatibility phenotype (de Nettancourt 1977). It is widespread in the Cactaceae and has been documented in at least 30 of its approximately 120 genera (Boyle 1997). These genera comprise all three of the traditional Cactaceae subfamilies
(Pereskioideae, Opuntioideae, and Cactoideae) and eight of the nine Cactoideae tribes. Self-incompatibility can minimize inbreeding, but, as pointed out by Olmstead (1986), the extent of inbreeding within a population depends primarily on population size rather than on the type of breeding system. Genetic diversity is rapidly lost from small populations due to inbreeding and genetic drift (random changes in allelic frequencies; Wilcox 1984).
Dicliny occurs when some members of a population normally produce flowers that are unisexual instead of hermaphroditic and has been reported in several Cactaceae taxa. Dioecy (co-occurrence of androecious and gynoe-cious plants) occurs in Echinocereus coccineus (Hoffmann 1992) and Opuntia stenopetala (Parfitt 1985). Gynodioecy (consisting of hermaphroditic and gynoecious plants) has been reported in Mammillaria dioica (Ganders and Kennedy 1978; Parfitt 1985) and M. neopalmeri (Parfitt 1985). Trioecy (co-occurrence of androecious, gynoecious, and hermaphroditic plants) occurs in P pringlei (Fleming et al. 1994) and Selenicereus innesii (Innes and Glass 1991). Hermaphroditic, dioecious, and trioecious populations have been documented for Opuntia robusta (Parfitt 1985; del Castillo 1986; Hoffmann 1992). Each type of dicliny promotes outcrossing but to different degrees. Dioecy ensures 100% outcrossing. With gynodioecy and trioecy, the level of outcrossing depends on the frequency of females in the population, degree of selfing in hermaphrodites, and pollinator activity. Murawski et al. (1994) used allozyme analysis to examine the mating system in Ppringlei (a self-compatible trioecious species); the estimated proportion of outcrossing in females was nearly 1.0 (0.949) but the proportion was markedly lower (0.30) in hermaphroditic individuals, indicating that the majority of seeds produced by hermaphrodites result from selfing.
Pollen must be transferred between plants with dissimilar incompatibility phenotypes or dissimilar floral morphologies (diclinous taxa) for seed to set. As far as is known, cacti are pollinated exclusively by animals (Porsch 1938, 1939; Rowley 1980; Grant and Grant 1979; Grant and Hurd 1979; Schlindwein and Wittmann 1997). Animals also serve as dispersal agents for fruit and seed of cacti (Gates 1932; Wallace and Fairbrothers 1986). Hence, cacti and their animal pollinators/dispersers have developed mu-tualistic relationships, with animals receiving nectar and/or pollen as a reward for pollination or, in the case of seed/fruit dispersal, receiving nutrients from the fruit pulp or digested seeds. The incapacity to set seed due to absence of pollinators has profound effects on the genetic structure, mating system, and selection forces of obligate-ly outcrossed species (Olesen and Jain 1994). Any distur bance to mutualistic plant-animal relationships may affect the survival and continuing evolution of either partner. Two night-blooming columnar cacti (Ppringlei and Steno-cereus thurberi) in Sonora, Mexico, may be falling below their reproductive potential possibly due to the scarcity of Leptonycteris bats (Fleming et al. 1996). The population sizes of three columnar cacti (Cereus repandus, Pilosocereus lanuginosus, and Stenocereus griseus) growing on Curaçao, Netherlands Antilles, are decreasing due to unregulated land development threatening the survival of two species of nectar-feeding bats (Leptonycteris curasoae curasoae and Glossophaga longirostris elongata; Petit and Pors 1996). Conservation efforts should focus not only on endangered or threatened cacti but also on those animal species that are essential for their pollination and seed dispersal.
Most of the information available on biodiversity of cultivated cacti comes from allozyme studies of two economically important ornamental cacti: Christmas cactus (Schlumbergera) and Easter cactus (Hatiora; formerly Rhipsalidopsis). The levels of genetic diversity in these two cacti (Table 8.2) are similar to those found in the "average" domesticated crop.
Chessa et al. (1997) used allozymes to analyze an Italian collection of 33 prickly pear (Opuntiaficus-indica) clones. Seven enzyme systems were examined using cladode tissue and 10 enzyme systems using pollen. Allozyme polymorphism was detected for 2 enzyme systems with cladodes and 5 enzyme systems with pollen (29% and 50% polymorphism, respectively). Malate dehydrogenase was the most effective enzyme for distinguishing clones within the collection. Uzun (1997) examined allozymes in 15 Turkish ecotypes and 3 Italian cultivars of prickly pear using 7 enzyme systems. All cultivars displayed the same banding patterns, suggesting that genetic diversity among O. ficus-indica ecotypes and cultivars is low. Neither of these allozyme studies provided genetic interpretations of isozyme banding patterns for O. ficus-indica, which is required for calculating genetic diversity statistics.
Genetic diversity in cultivated cacti is limited by the restricted number of progenitors and the loss of genetic variation in cultivation. Most of the domesticated cacti grown for fruit or ornamental flowers apparently originated from a relatively narrow germplasm base. In the case of Easter cactus, over 100 distinct clones have been described (Meier 1995), but all probably are descendants of three plants (two Hatiora gaertneri and one H. rosea) collected in the field in the late 19th and early 20th centuries. Loss of genetic variation commonly occurs during crop domestication due to
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