Comparative population genetic structure of Venezuelan cacti and estimates of their mating systems

Date of Award




Degree Name

Doctor of Philosophy (Ph.D.)

First Committee Member

Theodore H. Fleming, Committee Chair


This study aimed to answer two questions: (1) what are the levels of genetic diversity and population differentiation in Venezuelan cacti with broad geographic distributions? and (2) how do different pollination and seed dispersal systems influence the distribution of genetic variation within and among populations in these plants? In addition, this study assessed levels of outcrossing, biparental inbreeding and correlated paternity for a subset of the species surveyed. Levels and patterns of allozyme diversity were examined in the cacti Stenocereus griseus, Cereus repandus, Pilosocereus lanuginosus, Pereskia guamacho and Melocactus curvispinus. Compared to other plant taxa and long-lived woody species, Venezuelan cacti had higher percentages of polymorphic loci (species level: Ps = 89.5--100.0%; population level: P p = 45.3--76.1%), more alleles per polymorphic locus (species level: APs = 3.5--3.8; population level: AP p = 2.2--2.7), and average or higher values of heterozygozity (species level: Hes = 0.15--0.26; population level: Hep = 0.10--0.23). Most polymorphic loci of the five cacti had a common allele and several alleles in low frequency at each locus. The highest levels of genetic variation were found in predominantly outcrossing taxa, (all except M. curvispinus) and, in second place, in chiropterophilous species. More than 80% of the genetic diversity in the five species was found within populations. Overall, patterns of gene flow observed in these cacti are best explained by an isolation by distance model. The lowest levels of genetic differentiation among populations and the highest levels of gene flow were associated with obligate outcrossing taxa and chiropterophilous species. S. griseus and C. repandus, both self-incompatible, were predominantly outcrossing (t m = 0.93--1.00) with low levels of biparental inbreeding (<2%). The self-compatible M. curvispinus behaved as a facultatively xenogamous species (tm = 0.76). The three cacti had multiple paternity between and within fruits. The correlation of outcrossed paternity (rp) was relatively low among fruits (rp = 0.05--0.16) and varied widely within fruits (rp = 0.20--0.85), indicating that the pollinators of all the cacti move actively among flowering individuals, but that in some circumstances only one or two fathers can sire the complete progeny array in a maternal plant. Predominantly outcrossing cacti with gene dispersal mediated by bats have the highest levels of genetic diversity and the lowest levels of population differentiation detected in the family. At the other extreme, cacti capable of selfing and with restricted pollen and seed dispersal mechanisms have the lowest values of genetic diversity and the highest levels of population differentiation.


Biology, Botany; Biology, Genetics

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