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The population genetics of gene flow and hybridization Implications for in situ managed populations of crop wild relatives and landraces Norman Ellstrand Professor of Genetics University of California - Riverside CWRs and landraces are evolutionarily l i il dynamic d i Gene flow’s role? The Th population l ti genetics ti off gene flflow** Population genetics primer – Relationship of gene flow to the other evolutionary forces When do immigrant alleles p persist and spread? g p What does it all mean for in situ conservation of plant genetic resources? Hybridization = Intertaxon gene flow *Hybridization Population genetics (microevolution) Focuses on 4 evolutionary “forces” and their interactions 1. Mutation: spontaneous allelic or cytogenetic change source of all genetic variation rate (μ): generally, 10-4-10-6 / generation, but varies with organism, gene, etc. 2 Selection: 2. S l ti genetically--based differences in reproductive success genetically many kinds, directional (Darwinian), disruptive, stabilizing, balancing, etc. 1-5% selective disadvantage (s) of immigrant allele not unusual 3. Drift: chance events that alter allele frequencies generally, drift becomes more important as population size (N) decreases founder effect, skewed sex ratio, bottleneck, chronically small populations, etc. 4 Gene Flow (migration) 4. allele frequency changes due to immigration of individuals or gametes – e.g., spores, sperm, seeds, pollen, eggs, etc. generally, tends to homogenize populations gene flow rate (m) 11-5% / generation not unusual for outcrossing plant populations separated by >100 m Relationship of gene flow to the other evolutionary forces Gene flow vs. opposing mutation: – Gene flow is more important p than mutation when m > μ Gene flow mutation Relationship of gene flow to the other evolutionary forces Gene flow vs. opposing selection: – Gene flow is more important p than selection when m > s Gene flow selection Relationship of gene flow to the other evolutionary forces Gene flow and selection in concert: – augment g each other,, speeding p g adaptive p evolution Selection Selection & Gene Flow Gene flow Relationship of gene flow to the other evolutionary forces Gene flow vs. drift: – One or more successful immigrants g p per everyy other generation (Nm > 0.5) are sufficient to counteract drift Gene flow drift Relationship of gene flow to the other evolutionary forces Summary Summary Gene flow vs. Mutation: Gene flow prevails when m > μ Mutation: - Selection Selection:: Gene flow prevails when m > s + Selection Selection:: Gene flow augments selection Drift:: Gene flow prevails when Nm > 0.5 Drift Gene flow prevails frequently For detailed discussion of assumptions, see C. 4, Ellstrand. 2003. Dangerous Liaisons? Johns Hopkins University press Implications for in situ management Generally, plant conservation managers will like to see some immigrant alleles persistence and spread p p – and others g go extinct When do immigrant alleles persistence and spread – and others go extinct? Persistence and spread of immigrant allele depends on … Whether gene flow is oneone-time or recurrent How that allele affects fitness Whether that allele is tightly linked to other alleles with strong fitness effects One locus expectations: p Immigrant allele fate X generations after a single unliateral gene flow episode single, Neutral allele – persists, more or less, in its original frequency as immigrant. E.g., if the initial frequency of an immigrant allele is 5%, it should persist at ca. 5%. Detrimental allele – decreases to extinction Beneficial allele – increases to fixation IImportant t t assumption: ti The sink population is large (N N >> 100); that is, drift is negligible. One locus expectations: p Immigrant allele fate after X generations of recurrent unilateral gene flow recurrent, Neutral allele – persists and increases to match its frequency in the source population. e.g., if an immigrant allele has a 5% frequency in its source population, will evolve to 5% in the sink population. Detrimental allele – persists in “migration-selection” “migration selection” equilibrium Beneficial allele – increases to fixation IImportant t t assumption: ti The sink population is large (N N >> 100); that is, drift is negligible. Multi-locus expectations: Multip Immigrant allele fate after X generations following unilateral gene flow If an immigrant allele is tightly linked to an allele at another locus with a stronger fitness effect, its fate will be largely determined by “hitch-hiking” with that allele. ¾ e.g., selective sweep Genomic G i linkage li k varies i with ith ““recombination bi ti system” t ” Ö Apomixis – linkage more or less absolute Ö Selfing – linkage decays relatively slowly Ö Outcrossing – linkage decays relatively quickly Some possible consequences of unintended gene flow for the recipient population. #1 Some interesting documented consequences Changes g in diversity y ((increase OR decrease)) Evolution of new species Evolution of increased weediness/invasiveness Extirpation/Extinction via … – genetic swamping – outbreeding depression Some possible consequences of unintended gene flow for the recipient population. #2 One hypothetical??? yp consequence q CWRs as a repository p y for “heirloom” crop p alleles? Gene flow as a potential tool: Some examples “Genetic rescue” (inc. “assisted migration”?) – Adding diversity to a genetically depauperate population – Should population genetics Use scalpel, p , not a chainsaw Genetic pest management (hypothetical, at the moment) – Gould, G ld G Gressel, l and d many more When is unintended gene flow important in plant conservation? Straightforward rule of thumb: Gene flow becomes an important p concern in plant conservation ONLY when it changes SUBSTANTIALLY compared to recent g gene p flow levels (Ellstrand & Elam. 1993. Annual Review of Ecology & Systematics) Gene flow becomes an important concern in plant conservation ONLY when it changes SUBSTANTIALLY compared to recent gene fl flow llevels l What is a SUBSTANTIAL S S change? ? “No” gene flow to “some” gene flow (or vice versa) “Some” gene flow to “lots” of gene flow (or vice versa) Et cetera How can you estimate “recent gene flow levels”? Population p g genetic structure statistics ((e.g., g , FST) reflect historic, not current, gene flow levels Alternatively, information from ethnobotany, history, breeding system, dispersal ecology, etc. Qualitative “Levels” Levels of Gene Flow No gene flow or essentially “no” gene flow –m<μ “Some” gene flow (stochastically significant) – ca. 5% > m > μ “Lots of” gene flow (adaptively significant) – m > ca ca. 5% Conclusions Gene flow G fl is i a common and d potentially t ti ll important evolutionary force Generally, an unmanaged immigrant allele persist in a p population p will p Plant conservation managers should be mindful of gene flow’s flow s potential as a problem, as a tool, or something to be left alone In conservation, only SUBSTANTIAL gene fl flow changes h are worthy th off concern Thank you! Funding from John Simon Guggenheim Memorial Foundation Fellowship and prior support from USDA NSF USDA, NSF, EPA EPA, SJFR, SJFR Fulbright Fellowship Fellowship, etc etc. “Team Ellstrand” Crop Gene Flow Scientists Janet Clegg Terrie Klinger Lesley y Blancas Sylvia Heredia Caroline Ridley S b Subray H Hegde d Roberto Guadagnuolo Melinda Zaragoza Detlef Bartsch Diane Marshall Janet Leak Leak--Garcia P Pesach hL Lubinsky bi k Diane Elam Paul Arriola Jutta Burger g Bernie Devlin Karen Goodell J Joanne H Heraty t