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Transcript
Hybridization and Conservation Allendorf, Chapter 17 1. Case Study: The New Zealand Black Stilt 2. What is hybridization, and what causes it? 3. What are the potential consequences of hybridization? 4. How do we recognize hybrids and introgressed individuals? 1. Case Study: The New Zealand Black Stilt The New Zealand black stilt - formerly bred throughout New Zealand - now occurs in only one river basin due to predation and loss of breeding habitat. The pied stilt - self-introduced from Australia to the South Island in the early 1800s - spread to the North Island in the 1900s Hybrids - were first reported in the late 1800s - are now widespread. L Will hybridization lead to genetic extinction of the black stilt? 2. What is hybridization, and what causes it? It may occur naturally, e.g.: Hybridization may be defined generally as “mating between individuals from two populations, or groups of populations, which are distinguishable on the basis of one or more heritable characters” (Harrison 1990). - many (10%+) species hybridize occasionally (e.g Darwin's finches) - many sister-taxa form hybrid zones, where hybrids are frequent (e.g. firebellied toads) - many species appear to have arisen from polyploidization following hybridization (e.g. sunflowers) It can also result from anthropogenic changes, especially 3. What are the potential consequences of hybridization? - species introductions (e.g., bull vs. rainbow trout) Hybridization from anthropogenic causes has contributed directly or indirectly to many extinctions.... - habitat fragmentation (e.g., red vs. white mulberry) - e.g., cutthroat trout vs. rainbow trout - habitat degradation (e.g., cichlids in Lake Victoria) - population decline leading to "desperation" (e.g. speckled teal) - New Zealand grey duck vs. mallard duck ....and appears to be increasing However, hybridization can either increase or decrease fitness: Individuals meet Do not breed Fertilization occurs No offspring produced F1 progeny produced F1 have low survival F1 have high survival F1 have low fertility F1 have high fertility F2 or BC have low survival F2 or BC have high survival F2 or BC have low fertility F2 or BC have high fertility hybrid swarm with genetic swamping Fitness effects are largely unpredictable, and depend on - intrinsic and extrinsic factors Intrinsic Causes Demographic swamping or extinction vortex genomic extinction If BCs are fertile, hybridization can lead to a "genomic ratchet", because the introgressed DNA cannot be removed. Extrinsic Causes Fitness - masking of deleterious Increase recessive alleles (hybrid vigor - increase in heterozygote or heterosis) advantage - introduction of new adaptations Fitness - chromosomal Decrease incompatabilities (outbreeding - loss of coadapted depression) gene complexes - loss of local adaptations - time (generations) Generally, the more karyotypically, genetically or ecologically different the organisms are, the more negative the outcome (Fig. 17.3): Potentially, hybridization could lead a species into an extinction vortex. Hybrid Vigor - The shape of the fitness relationship varies both among and within species. outbreeding depression population decline outbreeding limited mate choice 4. How do we recognize hybrids and introgressed individuals? Molecular methods now enable identification of hybrids and BCs: allozymes ¸ PCR ¸ NGS Theoretically, - computer programs (e.g. NewHybrids) enable assignments of individuals to different hybrid classes, Generation [P1] F1 50 BC1 ~25 BC2 ~12.5 BC3 ~ 6.2 ... [P2] 50 ~75 ~87.5 ~93.8 - e.g. Peruvian and blue-footed boobies (Taylor et al. 2012) Until the 1960s, hybrids were identified from morphology, but - not all variation is additive Hybridization is much more extensive that previously recognized! 1. The Black Stilt, Revisited Molecular analyses revealed limited genetic introgression et al. 2010, Mol. Ecol.19: 5090) . 1 0.8 0.6 0.4 © R. Bush 0.2 0 Morphological hybrids are now culled. (Steeves