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Population Biology |
2Centro de Ecología, Instituto Venezolano de Investigaciones Científicas, Apdo. Postal 21827, Caracas 1020-A, Venezuela; 3Biology Department, University of Miami, P.O. Box 249118, Coral Gables, Florida 33124 USA; 4Departments of Botany and Genetics, University of Georgia, 2502 Plant Sciences, Athens, Georgia 30602 USA
Received for publication February 11, 2003. Accepted for publication May 9, 2003.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: allozymes • bats • Cactaceae • Cereus • columnar cacti • Pilosocereus • population structure • Stenocereus • Venezuela
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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, 2001
; Sahley, 1996
; Valiente-Banuet et al., 1996
; Nassar et al., 1997
; Casas et al., 1999
). Today we know that many columnar cacti are partially or completely dependent on bats for pollination. But what are the population genetic consequences of being a chiropterophilous cactus? Empirical evidence suggests that pollen-mediated gene flow is often the predominant form of gene flow in outcrossing plants (Ellstrand, 1992
; Ennos, 1994
; but see Oddou-Muratorio et al., 2001
). Gene flow models predict that the level of population subdivision in a species should be constrained by the extent of genetic exchange among populations, an effect that could be accentuated by the existence of long-distance migrants (Wright, 1943
; Slatkin and Maruyama, 1975
; Slatkin, 1985
). Assuming that plant-feeding bats can transport pollen, and probably seeds, far away from the donor plants, we would expect this process to promote low levels of plant population subdivision and substantial within-population genetic variation. But despite the almost intuitive connection between a plant's dispersal capabilities and levels of population differentiation, the importance of gene flow in shaping the genetic structure of plant populations remains controversial (Hamrick, 1982
; Loveless and Hamrick, 1984
). This is in part due to the lack of control for a broad spectrum of historical, taxonomic, and ecological backgrounds that differentiate sets of plant species that have been the subject of population structure comparisons. In the case of chiropterophilous cacti, we need to examine patterns of genetic variation in species pollinated by bats and to compare observed patterns with those described for other cactus species that share similar life history and ecological traits with bat cacti, but that rely on different gene-dispersal vectors (e.g., bees, butterflies, hawk moths, and birds).
Attributes of genetic diversity of long-lived woody plants adapted to arid conditions have been poorly examined (Keys and Smith, 1994
; Cortés and Hunziker, 1997
; Lia et al., 1999
; Martínez-Palacios et al., 1999
; Nassar et al., 2002
). Levels and patterns of allozyme variation have been investigated for only five species of columnar cacti, four North American taxa, Pachycereus schottii (Engelmann) Hunt, Carnegiea gigantea (Engelmann) Britton & Rose, Stenocereus thurberi (Engelmann) Buxbaum, and Pachycereus pringlei (Watson) Britton & Rose (Hamrick et al., 2002
; Nason et al., 2002
), and one Peruvian cactus, Weberbauerocereus weberbaueri (Schumann ex Vaupel) Backeberg (Sahley, 1996
). Pachycereus pringlei and W. weberbaueri are autotetraploid; the rest are diploid. With the exception of P. schottii, a moth-pollinated cactus (Fleming and Holland, 1998
), all other species include nectar-feeding bats among their pollination and seed-dispersal agents (Fleming et al., 1996
, 2001
; Sahley, 1996
; Fleming and Nassar, 2002
; Sosa and Fleming, 2002
). Overall, North American bat cacti maintain high percentages of polymorphic loci (PS = 83.8–93.3), intermediate to high number of alleles per polymorphic locus (APs = 2.79–3.42), and average to high levels of heterozygosity (HES = 0.129–0.212; Hamrick et al., 2002
). Besides this, most of the genetic diversity in these species resides within populations (GST = 0.075–0.128). These observations contrast with the relatively high levels of genetic structure reported for P. schotti (GST = 0.431; Nason et al., 2002
), the moth-pollinated cactus. Altogether, the genetic structure attributes observed for North American bat cacti are consistent with the foraging behavior characteristic of cactophilous nectar-feeding bats and their potential for long-distance movements (Horner et al., 1998
; Fleming and Nassar, 2002
). However, because other floral visitors (e.g., doves, hummingbirds, honey bees, solitary bees, and hawk moths) also pollinate the species examined, we cannot attribute the patterns of genetic diversity observed to bats exclusively. Ideally, to make more accurate inferences on the influence of flower-visiting bats on the population genetics of columnar cacti, we would need to focus on species exclusively dependent on these vectors for their sexual reproduction.
The highest levels of bat specialization among cacti have been observed in species restricted to Neotropical regions, including Mexico (Valiente-Banuet et al., 1996
, 1997a
, b
), Central America (Tschapka et al., 1999
), the Caribbean (Petit, 1995
), and Venezuela (Nassar et al., 1997
). In Venezuela, strict bat pollination has been identified in at least five cactus species with similar reproductive characteristics and genetic self-incompatibility. This number represents about 63% of all the Venezuelan columnar cactus species. Three of them, Stenocereus griseus (Haworth) Buxbaum, Cereus repandus (Linnaeus) Miller, and Pilosocereus lanuginosus (Linnaeus) Byles & Rowley, also have similar geographic distributions and habitat affinities across Venezuela and are relatively common species (Ponce, 1989
). The southern long-nosed bat, Leptonycteris curasoae (Glossophaginae, Phyllostomidae), is an active pollinator and seed disperser of the three species (Nassar et al., 1997
). This bat can forage among locations separated by up to 14 km and fly as much as 100 km in a single night (Horner et al., 1998
). On the other hand, seed dispersal of the three cacti is shared between bats and birds (Sosa and Soriano, 1993
; Soriano et al., 1999
); however, bird-mediated gene dispersal of these species is assumed to be spatially restricted based on the home range (<12 ha) assigned to them in Venezuelan arid zones (Bosque, 1984
).
The main goal of this study was to examine and compare levels and patterns of allozyme variation in S. griseus, C. repandus, and P. lanuginosus across their geographic ranges in Venezuela. We hypothesized that patterns of genetic variation in Venezuelan chiropterophilous cacti should reflect the long-distance gene-dispersal properties of their obligate floral visitors, namely high levels of genetic variation within populations and relatively low levels of population differentiation. We compared our estimates of genetic diversity and structure, based on seedling samples, with values reported for other cacti surveyed on the same spatial scales but with contrasting gene-dispersal systems.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Zappi, 1994
). Stenocereus griseus and C. repandus behave genetically as diploid species, while P. lanuginosus showed the allozyme banding segregation pattern of an autotetraploid (this study). Although their geographic distributions do not overlap completely, the three species cover similar spatial ranges, mostly matching the distribution of arid and semiarid lands in Venezuela (Ponce, 1989
) (Fig. 1a). Stenocereus griseus and C. repandus are found below 1000 m, while P. lanuginosus can be found as high as 1800 m in the Andes. The three species are multi-branched, 3–6 m tall, and differ in spatial arrangement of branches and some morphological attributes (Nassar et al., 1997
). Flowers are hermaphrodite, tubular-campanulate, with white internal tepals, night-blooming, with few flowers in anthesis per individual per night. Stenocereus griseus produces flowers during most of the year (9–12 mo). In contrast, C. repandus and P. lanuginosus produce flowers and fruits seasonally (J. M. Nassar, unpublished data). The three species are self-incompatible based on hand-pollination experiments and nectar-feeding bats are nearly the only effective pollinators of these cacti (Petit, 1995
; Nassar et al., 1997
). Fruits are multi-seeded (>1000) berries, and dispersal is mainly performed by bats and also several birds (Bosque, 1984
; Sosa and Soriano, 1993
; Soriano et al., 1999
). Although sexual reproduction is the main reproductive mechanism, these cacti can occasionally spread vegetatively when branches fall from adult individuals and develop root systems. In their natural habitats, population densities vary markedly, from 18 to 1315 individuals/ha. The highest densities occur mostly for S. griseus and C. repandus, which form vegetation units locally known as "cardonales."
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. This region includes most of the coastline, insular areas, and an extensive zone along a north-south axis of continuous xeric formations and small arid patches in western Venezuela, from Paraguaná Peninsula to Ureña, on the frontier with Colombia. The vegetation associated with these regions consists mainly of thorny scrubs, spiny shrubs, and dry forest (Huber and Alarcón, 1988
). Sampled localities and geographic information related to study sites are indicated in Fig. 1 and Table 1.
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Sampling procedures
Tissues used for allozyme analyses were obtained from seedlings of the three species. At the macrogeographic scale, 15 populations of S. griseus, 14 of C. repandus, and 10 of P. lanuginosus were selected. For the mainland region, four, five, and two populations were chosen, respectively. For the peninsula region, five, four, and two populations were sampled, respectively. At each site and when population densities allowed, 48 adult individuals of each species were selected. Conspecific plants were at least 10 m apart to be eligible for sampling. Viable seeds from one fruit per individual were collected and stored under dry conditions. Seeds were germinated directly in trays with potting soil and placed in the greenhouse facilities at the University of Georgia, Athens, Georgia, USA. Seedlings were ready for enzyme extraction when they were about 1 cm tall. One seedling was used from each individual sampled from each population, for a total of 717 seedlings of S. griseus, 648 of C. repandus, and 430 of P. lanuginosus.
Electrophoretic procedures
Seedlings were ground using sand, cold mortar, and pestle. Two polyvinylpyrrolidone-phosphate extraction buffers, one for S. griseus (Wendel and Parks, 1982
) and the other for C. repandus and P. lanuginosus (Mitton et al., 1979
), were added to the tissue to solubilize and stabilize the enzymes. Chromatography paper wicks (Whatman 3 MM, Maidstone, UK) were then soaked with the protein extract, placed into microtest plates, and stored at –70°C until analysis. Horizontal electrophoresis was conducted on 10% potato starch gels (Sigma, St. Louis, Missouri, USA). Combinations of four buffer systems and 15 enzyme systems were used to resolve 18, 17, and 23 putative loci for S. griseus, C. repandus, and P. lanuginosus, respectively. Buffers and enzyme systems included the following: buffer 4, isocitrate dehydrogenase (Idh-1), 6-phosphogluconate dehydrogenase (6-Pgdh-1, 6-Pgdh-2, and 6-Pgdh-3); buffer 8, aspartate aminotransferase (Aat-1 and Aat-2), alcohol dehydrogenase (Adh-1), fluorescent esterase (Fe-1 and Fe-2), glutamate dehydrogenase (Gdh-1 and Gdh-2), menadione reductase (Mnr-1, and Mnr-2, and Mnr-3), phosphoglucoisomerase (Pgi-1 and Pgi-2), triosephosphate isomerase (Tpi-1, Tpi-2, and Tpi-3); buffer 11, adenylate kinase (Ak-1), malate dehydrogenase (Mdh-1, Mdh-2, Mdh-3, and Mdh-4), uridine diphosphoglucose pyrophosphorylase (Ugpp-1 and Ugpp-2); and buffer 34, diaphorase (Dia-1 and Dia-2), leucine aminopeptidase (Lap-1), and phosphoglucomutase (Pgm-1 and Pgm-2). Buffer recipes and stains are modified from Soltis et al. (1983)
and Mitton et al. (1979)
, with the exception of recipes for Aat and Dia (Cheliak and Pitel, 1984
). Loci and alleles were designated by relative protein mobility, with lower numbers assigned to those farther from the origin. For the tetraploid species, banding patterns were examined for relative band intensities interpreted as corresponding to genotypes of different allelic dosage.
Data analysis
Allele frequencies and standard genetic diversity parameters following Hedrick (1985)
and Berg and Hamrick (1997)
were estimated at the species (subscript "s"), regional (Mainland and Peninsula, subscript "r"), and population (subscript "p") levels for the three species. For this purpose, we used the program LYNSPROG, written by M. D. Loveless (College of Wooster, Wooster, Ohio, USA) and A. F. Schnabel (University of Indiana, South Bend, Indiana, USA). The following estimates were obtained: proportion of polymorphic loci (P), mean number of alleles per locus (A) and per polymorphic locus (AP), effective number of alleles per locus (Ae = 1/
fi2, where fi is the frequency of the ith allele), observed heterozygosity (Ho), and Nei's (1973)
gene diversity (H = 1 – fi2). Estimates were calculated for each locus and averaged over all loci. Population level estimates were averaged over all populations to obtain means and standard errors. Departures from Hardy-Weinberg expectations were evaluated for each polymorphic locus in each population by calculating Wright's (1931)
fixation index (F = 1 – [Ho/He]). Significant deviations of F from zero were tested using a chi-squared test formulated in terms of the fixation index F (Li and Horvitz, 1953
). In addition, we determined the percentage of alleles found per population relative to the total number of alleles scored for the species (% Al).
Measures of the spatial partitioning of genetic variation were performed at both macrogeographic and regional scale. At macrogeographic scale, we included populations from all over Venezuela. Because populations are close to each other (<50 km) within the peninsula and mainland regions, we randomly chose one population from each of these areas to maintain the macrogeographic range of analysis. We repeated this subsampling method for all possible combinations of populations from the two regions. At the regional level, all the populations within each the mainland and peninsula regions were analyzed. Nei's parameter of population differentiation GST, which is the proportion of total genetic diversity (HT) from differences among populations, was estimated for each polymorphic locus (Nei, 1973
, 1977
). This method to estimate population differentiation is applicable to organisms of any ploidy (Nei, 1973
). Differences in allele frequencies among populations were examined for each polymorphic locus using a heterogeneity chi-square analysis, 
2 = 2NGST(a – 1), df = (a – 1)(n – 1); for a alleles, n populations, N = total individuals, and df = degrees of freedom (Workman and Niswander, 1970
). Chi-square values and degrees of freedom were summed over all loci to conduct overall tests for the multilocus estimates of GST (Berg and Hamrick, 1997
). Wright's (1978)
within-population inbreeding coefficient (FIS) was also estimated for each polymorphic locus and its significance tested by random permutations of genes among individuals within populations using the program SPAGeDi 1.0 (Hardy and Vekemans, 2002
), which accepts genotype data of any ploidy level. Overall means and standard errors for GST and FIS estimates were obtained by jackknifing over loci (Weir, 1996
). We performed jackknifing procedures across populations of each species to calculate variance and standard errors of the GST estimates and compared mean GST estimates among the columnar cacti species using a one-way ANOVA.
Isolation by distance was tested using Rousset's (1997)
method, based on the computation of a linear regression of pairwise FST/(1 – FST) estimates to the natural logarithm of geographic distances between pairs of populations. A positive correlation between the two variables is indicative of isolation by distance. A Mantel test of association (9000 permutations) between pairwise FST/(1 – FST) and log10 of geographic distance matrices was used to test for significance of the isolation by distance pattern (Mantel and Valand, 1970
; Heywood, 1991
). Nei's (1972)
genetic identities (I) and distances (D) were estimated between all pairs of populations of each species to generate average clusterings using the neighbor-joining method (Romesburg, 1984
). Dendrograms plotted using this procedure helped in understanding the genetic relationship among populations. Mantel tests and neighbor-joining clustering were performed using the programs NTSYS-pc version 1.8 (Rohlf, 1993
) and Mega version 2.1 (Kumar et al., 1993
), respectively.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Genetic diversity results at the species, region, and population levels are summarized in Table 2. A total of 60, 63, and 72 alleles were resolved for C. repandus, S. griseus, and P. lanuginosus, respectively. At the species level, 21 (91.3%), 16 (94.1%), and 18 (100.0%) loci were polymorphic for P. lanuginosus, C. repandus, and S. griseus, respectively. The three species had comparable average number of alleles per locus (As = 3.3–3.5) and per polymorphic locus (APs = 3.5–3.7). The average effective number of alleles per locus (Aes) varied slightly among species, from 1.35 in S. griseus to 1.47 in C. repandus. The marked decay in the number of alleles per locus from A to Aes is to the substantial proportion of loci (46–67%) with one common allele (fi > 0.95) and several rare alleles for each of the three species. Overall gene diversity was high for the two diploid (Hs = 0.182–0.242) and the tetraploid (Hs = 0.274) species.
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Average percentage of alleles (Al%) captured in a given population ranged from 51% for S. griseus to 72% for P. lanuginosus (Table 2). For P. lanuginosus, the tetraploid cactus, the proportion of individuals possessing three different alleles at least at one locus was relatively high across populations, with population MOC01 having the highest proportion (85.4%) and Margarita Island and the Andean populations having the lowest proportions (42.6–58.3%). Individuals with four different alleles at a locus were less common (<10.4%) across populations. The lowest estimates of genetic diversity for all the parameters examined at the population level were obtained for S. griseus, Pp = 56.7%, Ap = 1.78, APp = 2.36, Aep = 1.30, Ho = 0.145 and Hp = 0.161. On the other hand, P. lanuginosus had the highest values for all parameters, Pp = 76.1%, Ap = 2.29, APp = 2.69, Aep = 1.41, Hop = 0.320, and Hp = 0.253. Cereus repandus had intermediate average estimates. Population-level estimates differed significantly among species for Pp (one-way ANOVA; F = 19.0, df = 2, P < 0.001), Ap (F = 15.6, df = 2, P < 0.001), APp (F = 13.7, df = 2, P < 0.001), Aep (F = 12.7, df = 2, P < 0.001), and Hp (F = 28.5, df = 2, P < 0.001). Overall, genetic diversity estimates varied slightly among populations within species, with the widest ranges of variation occurring in Pp and Hp for all cacti.
From a geographic perspective, and with few exceptions, the three species converged in having the highest levels of genetic diversity for all parameters in populations located in northwestern Venezuela (Fig. 1). For diploid cacti, populations PAR04, PAR05, PAR01, PAR07, LAF11, LAF22, LAF34, and LAF35 had the highest Pp (67.0–88.24%) and Hp (0.182–0.265) values, while most Andean and northeastern populations including AND21, AND22, AND32, GUA01, BUC01, and ARA01 had comparatively lower estimates of Pp (44.0–64.7%) and Hp (0.110–0.180). For the tetraploid cactus, populations PAR05, MOC01, LAF41, LAF38, and LAF34 had the highest estimates of Pp (78.3–87.0%) and Hp (0.271–0.295), while Andean and Margarita Island populations including AND11, AND31, NUE03, and NUE05 had the lowest values of Pp (65.2–69.6%) and Hp (0.191–0.253).
Genetic structure
A substantial proportion of the observed heterozygosities calculated per locus and population were significantly smaller than expected heterozygosities under Hardy-Weinberg equilibrium in the three species, suggesting a moderate deficiency of heterozygotes across species (data available upon request from J. M. N.). For S. griseus, 19 of 130 (14.6%) fixation indices were positive and significantly different from zero (P < 0.05). For C. repandus, 30 of 148 (20.2%) fixation indices were positive and significantly different from zero (P < 0.05). For P. lanuginosus, 89 of 173 (51.4%) fixation indices were positive and significantly different from zero (P < 0.05). Mean estimates of Wright's (1978)
within-population inbreeding coefficient (FIS) were positive and low to moderate for the three cacti at both spatial scales (Table 3).
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) showed comparatively more population subdivision than S. griseus (t = 17.3, P < 0.0001, df = 15), C. repandus (t = 10.3, P < 0.0001, df = 14), and P. lanuginosus (t = 28.7, P < 0.0001, df = 14). An insect-pollinated Venezuelan cactus (Pereskia guamacho, GST = 0.111 ± 0.003; Nassar et al., 2002
) also showed more population subdivision than S. griseus (t = 4.3, P < 0.001, df = 14) and P. lanuginosus (t = 19.2, P < 0.0001, df = 13), but lower population structure than C. repandus (t = 2.6, P = 0.02, df = 13). Isolation by distance was detected at the macrogeographic level for the three columnar cacti (Fig. 2). The association between pairwise log10 geographic distances and pairwise FST/(1 – FST) values was positive and highly significant for the three species of cactus at the macrogeographic scale (Mantel test; r = 0.63, one-tailed P < 0.001 for S. griseus; r = 0.65, one-tailed P < 0.001 for C. repandus; r = 0.78, one-tailed P < 0.001 for P. lanuginosus). Regression coefficients were positive for the three species (ß = 0.155 for S. griseus, ß = 0.151 for C. repandus, and ß = 0.109 for P. lanuginosus), and regression lines explained 39.6%, 42.2%, and 60.1% of the variation in FST/(1 – FST), respectively. Population pairs with the largest geographic separations had considerable variation in FST/(1 – FST) values for S. griseus and C. repandus. When meaningful to calculate, no significant associations between pairwise log10 geographic distances and pairwise FST/(1 – FST) values were detected at the regional level (r = 0.67, one-tailed P < 0.21 for S. griseus in the mainland subset; r = –0.43, one-tailed P < 0.1 for S. griseus in the peninsula subset; r = 0.53, one-tailed P < 0.13 for C. repandus in the mainland subset). This was mainly due to the low number of possible pairwise comparisons that could be conducted within regional subsets, thus reducing the statistical power to reject the null hypotheses of no isolation by distance.
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genetic identities (I) were relatively high for all species (mean ± 1 SE; I = 0.966 ± 0.002 for S. griseus, I = 0.940 ± 0.004 for C. repandus, and I = 0.978 ± 0.003 for P. lanuginosus), as expected for conspecific populations (Gottlieb, 1977
; Crawford, 1989
). The lowest genetic identities were mostly associated with population pairs including Andean populations. Neighbor-joining dendrograms based on Nei's genetic distances between populations partially matched geographic relationships among populations in the three species (Fig. 3). The trees for P. lanuginosus and C. repandus had the best correspondence between genetic and geographic distances. In S. griseus, some groupings were incongruent with geographic relationships, especially between eastern (ARA01, GUA01) and midwestern (LAF31, LAF32, LAF35) populations. Well-defined geographic units of closely related populations included Andean populations (AND-), midwestern (LAF-) and eastern (NUE-, ARA1, BUC1) regions, the Paraguaná Peninsula cluster (PAR-), and populations in western Venezuela, including the Andes, and the mainland and peninsula subsets. Andean (AND-) and Margarita Island (NUE-) populations had the highest genetic differences with respect to other populations.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Among long-lived woody plants adapted to xeric environments, the three species had intermediate to high values of gene diversity (Hp = 0.17–0.29 in the Zygophyllaceae, Cortés and Hunziker, 1997
; Hp = 0.28–0.39 in the Agavaceae, Eguiarte et al., 2000
). Within the Cactaceae, species- and population-level estimates of genetic variation for Venezuelan columnar cacti were similar to average values obtained from four North American species, Carnegiea gigantea, Stenocereus thurberi, Pachycereus pringlei, and Lophocereus schotii, and one Peruvian species, Weberbauerocereus weberbaueri (Ps = 89.8%, Pp = 58.9%, APs = 3.09, APp = 2.45, Aes = 1.33, Aep = 1.27, Hs = 0.189, Hp = 0.177; Hamrick et al., 2002
). These species share with Venezuelan cacti similar life-history and ecological traits, including long-lived woody condition, columnar form, a predominantly animal-outcrossing breeding system, and self-incompatibility, with the exception of P. pringlei and W. weberbaueri (Murawski et al., 1994
; Fleming et al., 1996
; Sahley, 1996
; Nassar et al., 1997
, 2002
). All columnar cacti surveyed up to now form a compact group with respect to their genetic diversity profiles: very high percentage of polymorphic loci, high number of alleles per polymorphic locus, and intermediate to high heterozygosity. Except for the moth-pollinated P. schotii, all other species have bats as facultative or obligate gene-dispersal agents. With the exception of C. gigantea, all other bat cacti had comparatively higher within-population genetic variation than the moth cactus (Pp = 49.5%, APp = 2.33, Hp = 0.144; Hamrick et al., 2002
). Among North American bat cacti, C. gigantea is the species with the northernmost distribution and the one that least relies on bats as pollinator agents (Fleming et al., 2001
). Altogether, the available data on columnar cacti suggest that bat-mediated gene dispersal enhances genetic variation in this group of plants, probably by facilitating genetic exchange among populations.
The highest values of genetic diversity in the three Venezuelan columnar cacti occurred in populations located in northwestern Venezuela. The same observation was reported by Nassar et al. (2002)
for the cactus Pereskia guamacho, surveyed over the same geographic range. In general, large and continuously distributed habitats facilitate the maintenance of high levels of genetic diversity in plants (Barrett and Kohn, 1991
; Hamrick and Godt, 1989
, 1996
; Ellstrand and Elam, 1993
). The most extensive arid zones in Venezuela are located in northwestern Venezuela (Sarmiento, 1976
). This region has remained dry and suitable for xerophilous vegetation at least since the late Tertiary (Ochsenius, 1983
), despite a history of cyclic expansions and contractions (Gentry, 1982
; Schubert, 1988
; Rull, 1996
). It is possible that this paleoclimatic condition might have contributed to the existence of stable and genetically diverse populations of cacti in the region. Other less extensive arid zones in Venezuela, like the isolated arid patches in the Andes and the eastern coastline, contain columnar cacti populations with some of the lowest within-population genetic diversity values observed in this study.
Regional comparisons showed that S. griseus and C. repandus had more genetic diversity in the peninsula subset than in the mainland subset. This result is contrary to expectations, considering that populations on the Paraguaná Peninsula are currently more geographically isolated than mainland populations. However, this was not the situation during the Late Pleistocene (13 000–18 000 yr BP), when the Caribbean Sea was substantially lower and an extensive land bridge connected the Paraguaná Peninsula with continental Venezuela (Ochsenius, 1983
). Contemporary gene flow between the Paraguaná Peninsula and adjacent mainland is possible, because nectar-feeding bats that pollinate and disperse columnar cacti in Venezuela and roost in the peninsula have been shown to fly across the isthmus towards the mainland (Martino et al., 1998
). It is feasible that during these long-distance flights, bats could transport viable cactus pollen and seeds between the two regions. This does not seem to be the case for the hummingbird-pollinated cactus, Melocactus curvispinus (Nassar et al., 2001
), and an insect-pollinated cactus, Pereskia guamacho (Nassar et al., 2002
), surveyed for allozyme diversity in the same Venezuelan locations. Those species had significantly more genetic variation in the mainland region than in the peninsula region, a pattern suggesting comparatively more limited gene-dispersal capabilities than chiropterophilous cacti.
Genetic structure of columnar cacti
Fixation indices and FIS statistics for the three species indicated that there is a slight to moderate deficiency of heterozygous individuals within populations at both macrogeographic and regional scales. It is not clear why. Because these cacti are self-incompatible and are nearly obligate outcrossers, the possibility that inbreeding caused the deviations from H-W equilibrium is low. Biparental inbreeding could contribute to reduced heterozygosity in relatively small and partially isolated populations (Handel, 1983
; Heywood, 1991
), but estimates of biparental inbreeding conducted on populations of S. griseus and C. repandus using allozyme markers and following methods by Ritland and Jain (1981)
indicated a very low percentage of consanguineous matings (tm – ts = 0.02, where tm is the family level multilocus outcrossing rate and ts is the single-locus outcrossing rate averaged across loci) for the two species (Nassar, 1999
).
Significant but low genetic differentiation among populations was detected at both macrogeographic and regional scales. Between 4% and 13% of the total genetic diversity (HT) of the species was found among populations. Mean GST estimates for the two regional subsets were somewhat lower (0.009–0.044) than those found at the macrogeographic level, indicating that there is little interpopulation differentiation within arid regions for the three species. Pilosocereus lanuginosus, the autopolyploid cactus, had the lowest level of population structure. This pattern is in agreement with the fact that gene flow events in a tetraploid species involve the movement of twice the number of genes transported in a diploid species. Overall, such levels of population structure are substantially lower than mean GST values reported for 655 plants across taxa (0.228), but similar to estimates obtained from 195 long-lived woody plants (0.084) (Hamrick et al., 1992
). Low population structure in bat cacti is consistent with the flight capabilities of bats in general and the southern long-nosed bat, Leptonycteris curasoae, in particular. This nectar-feeding bat pollinates and disperses columnar cacti in North America (Fleming et al., 1996
), the Netherlands Antilles (Petit, 1995
), and northern South America (Nassar et al., 1997
) and can forage among locations separated by up to 14 km and fly as much as 100 km in a single night (Horner et al., 1998
). Compared to bats, birds that disperse cactus seeds in Venezuela seem to have a relatively restricted radius of activity based on their reported home ranges (<12 ha) and their resident status (Bosque, 1984
). Cactus species with spatially restricted gene-dispersal systems, such as the insect-pollinated P. guamacho (GST = 0.112; Nassar et al., 2002
), the hummingbird-pollinated M. curvispinus (GST = 0.189; Nassar et al., 2001
), and the moth-pollinated P. schottii (FST = 0.431; Nason et al., 2002
), have comparatively more genetic structure than North American bat-pollinated cacti (GST = 0.075–0.128; Hamrick et al., 2002
) and the Venezuelan columnar cacti considered in this study. Overall, bat-mediated gene dispersal within the Cactaceae appears to confer higher levels of gene exchange among populations than other animal-mediated gene-dispersal systems.
For the three Venezuelan columnar cacti, genetic differentiation among populations increased with geographic distance at the macrogeographic scale. This result indicates that isolation by distance determines the mode that genes move across the landscape for the three species. Levels of genetic identity observed for the three cacti (I > 0.94) are within the range of values reported for conspecific populations of plants (Gottlieb, 1977
; Crawford, 1989
). The topology of neighbor-joining trees based on Nei's genetic distances (D) corresponded with important geographic relationships between populations of the three cacti. Populations located in Margarita Island (C. repandus and P. lanuginosus), the Andean arid patches (S. griseus, C. repandus), and the Paraguaná Peninsula (C. repandus) were grouped together and well separated from the others, reflecting how water and topographic isolation can influence genetic relatedness among populations. Also, populations distributed in the mainland region (C. repandus), the central coast (S. griseus), and western Venezuela (C. repandus and P. lanuginosus), tended to be grouped together, reflecting their geographic relationships. Overall, at the level of resolution allowed by allozyme analyses, geographic barriers that separate populations of the three species do not seem important enough to promote speciation in these cacti.
In summary, the results of this research indicate that Venezuelan chiropterophilous cacti, and in general all bat columnar cacti studied to date, form a relatively uniform group in terms of their genetic diversity attributes. Bat-mediated gene dispersal is associated with high levels of genetic diversity and low levels of interpopulation differentiation across cactus species. From a conservation perspective, there are two important elements. First, because most of the total genetic variation detected in Venezuelan columnar cacti resides within populations, the risk of negatively affecting the species' gene pool by localized population extinctions should be relatively low. However, the concentration of high levels of genetic diversity for the three cacti in northwestern Venezuela should be taken into consideration for the delimitation of conservation areas containing plant genetic diversity reservoirs within arid and semiarid zones in northern South America.
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FOOTNOTES |
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5 E-mail: jnassar{at}oikos.ivic.ve
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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