Abstract
Bats are the main pollinators and seed dispersers of Stenocereus thurberi, a xenogamous columnar cactus of northwestern Mexico and a good model to illustrate spatial dynamics of gene flow in long-lived species. Previous studies in this cactus showed differences among populations in the type and abundance of pollinators, and in the timing of flowering and fruiting. In this study we analyzed genetic variability and population differentiation among populations. We used three primers of ISSR to analyze within and among populations genetic variation from eight widely separated populations of S. thurberi in Sonora, Mexico. Sixty-six out of 99 of the ISSR bands (P = 66.7%) were polymorphic. Total heterozygosity for all populations sampled revealed high genetic diversity (Hsp = 0.207, HBT = 0.224). The AMOVA showed that most of the genetic variation was within populations (80.5%). At the species level, estimates of population differentiation, θ = 0.175 and θB = 0.194, indicated moderate gene flow among populations. The absence of a significant correlation between genetic and geographic distances indicated little isolation by geographic distance. The large genetic variation and diversity found in S. thurberi is consistent with its open reproductive system and the high mobility of bats, a major pollinator. However, small changes in number or kind of pollinators and seed dispersal agents, in the directionality of migratory routes, and/or in the timing of flowering and fruiting among populations, can critically affect gene flow dynamics.
Highlights
Genetic diversity and its spatial distribution is the result of mutation, genetic drift, gene flow and natural selection acting within and among populations of a species [1, 2]
We found that for the total of the populations studied, S. thurberi displayed relatively high percentages of genetic variation, both measured as expected heterozygosity and polymorphic loci (Hsp = 0.207±0.016 S.E., P = 66.66%, HBT = 0.224±0.009 S.D.)
The northern population of Sonoyta showed the maximum value of genetic variation (Hpop = 0.203±0.018 SE, P = 61.62%), while the minimum was found in the southern population of Las Guásimas (Hpop = 0.146±0.018 SE, P = 41.41%; Table 2)
Summary
Genetic diversity and its spatial distribution is the result of mutation, genetic drift, gene flow and natural selection acting within and among populations of a species [1, 2] The relevance of these factors in determining genetic diversity is related to the heterogeneity of the physical environment, the intrinsic biological traits of the species (phylogenetic constraints), and the network of ecological interactions within the populations studied. Features such as the mode of reproduction and dispersal, and longevity have been related to variation and genetic structure in several groups of plants [3, 4]. It has been demonstrated that cross-pollinated species usually have
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