Comparing Relative Rates of Pollen and Seed Gene Flow in the Island Model Using Nuclear and Organelle Measures of Population Structure

Comparing Relative Rates of Pollen and Seed Gene Flow in the Island Model Using Nuclear and Organelle Measures of Population Structure

Matthew B. Hamilton^a,b and Judith R. Miller^c
^a Department of Biology, Georgetown University, Washington, DC 20057,
^b Biological Dynamics of Forest Fragments Project, National Institute for Research in the Amazon, Manaus, AM 69011-970, Brazil
^c Department of Mathematics, Georgetown University, Washington, DC 20057

Corresponding author: Matthew B. Hamilton, Reiss 406, Georgetown University, 37th and O Streets NW, Washington, DC 20057., hamiltm1{at}georgetown.edu (E-mail)

Communicating editor: D. CHARLESWORTH

We describe a method for comparing nuclear and organelle populationdifferentiation (F_ST) in seed plants to test the hypothesisthat pollen and seed gene flow rates are equal. Wright's infiniteisland model is used, with arbitrary levels of self-fertilizationand biparental organelle inheritance. The comparison can alsobe applied to gene flow in animals. Since effective populationsizes are smaller for organelle genomes than for nuclear genomesand organelles are often uniparentally inherited, organelleF_ST is expected to be higher at equilibrium than nuclear F_STeven if pollen and seed gene flow rates are equal. To rejectthe null hypothesis of equal seed and pollen gene flow rates,nuclear and organelle F_ST's must differ significantly from theirexpected values under this hypothesis. Finite island model simulationsindicate that infinite island model expectations are not greatlybiased by finite numbers of populations ( $>=$ 100 subpopulations).The power to distinguish dissimilar rates of pollen and seedgene flow depends on confidence intervals for fixation indexestimates, which shrink as more subpopulations and loci aresampled. Using data from the tropical tree Corythophora alta,we rejected the null hypothesis that seed and pollen gene flowrates are equal but cannot reject the alternative hypothesisthat pollen gene flow is 200 times greater than seed gene flow.

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