Hybridization and Conservation Allendorf, Chapter 17

Transcription

1 1. Overview of Genetic Concerns Anthropogenic Problem Overexploitation Habitat degradation (inc. pollution) Alien species (inc. disease) Climate change Habitat loss Habitat fragmentation Genetic Effect Inbreeding and inbreeding depression Reduced population viability Disruptions to gene flow Hybridization and introgression Unnatural selection HW Violation Random mating Infinite population size No gene flow No selection Hybridization and Conservation Allendorf, Chapter Case Study: The New Zealand Black Stilt 2. What is hybridization, and what causes it? 3. What are the potential genetic consequences of hybridization? 4.How do we recognize hybrids and introgressed individuals?

2 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? 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).

3 It may occur naturally, e.g.: - many (10%+) species hybridize occasionally (e.g Darwin's finches) - many sister-taxa form hybrid zones, where hybrids are frequent (e.g. fire-bellied toads) - many species appear to have arisen from polyploidization following hybridization (e.g. sunflowers) It can also result from anthropogenic changes, especially - species introductions (e.g., bull vs. rainbow trout) - habitat fragmentation (e.g., red vs. white mulberry) - habitat degradation (e.g., cichlids in Lake Victoria) - population decline leading to "desperation" (e.g. speckled teal)...and appears to be increasing

4 3. What are the potential consequences of hybridization? Hybridization from anthropogenic causes has contributed directly or indirectly to many local and some global extinctions... - e.g., cutthroat trout vs. rainbow trout - New Zealand grey duck vs. mallard duck Hybridization can either increase or decrease fitness: Individuals meet Do not breed Fertilization occurs F1 progeny produced No offspring produced F1 have low survival F1 have high survival F1 have high fertility F2 or BC have high survival F2 or BC have high fertility F1 have low fertility F2 or BC have low survival F2 or BC have low fertility hybrid swarm with genetic swamping 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.

5 Fitness effects are largely unpredictable, and depend on - intrinsic and extrinsic factors Intrinsic Causes Extrinsic Causes Fitness - masking of deleterious - introduction of Increase recessive alleles new adaptations (hybrid vigor - increase in heterozygote or heterosis) advantage Fitness - chromosomal - loss of local Decrease incompatabilities adaptations (outbreeding - loss of coadapted depression) gene complexes - time (generations) Generally, the more karyotypically, genetically or ecologically different the organisms are, the more negative the outcome (Fig. 17.3): - The shape of the fitness relationship varies both among and within species. Hybrid Vigor

6 Potentially, hybridization could lead a species into an extinction vortex. overexploitation, introduced species or habitat decline outbreeding depression population decline outbreeding limited mate choice 4.How do we recognize hybrids and introgressed individuals? Theoretically, Generation [P1] [P2] F BC1 ~25 ~75 BC2 ~12.5 ~87.5 BC3 ~ 6.2 ~ x Until the 1960s, hybrids were identified from morphology, but - not all variation is additive?

7 Molecular methods now enable identification of hybrids and BCs: allozymes PCR NGS - computer programs (e.g. NewHybrids) enable assignments of individuals to different hybrid classes, - e.g. Peruvian and blue-footed boobies (Taylor et al. 2012) Hybridization is much more extensive than previously recognized! 1. The Black Stilt, Revisited Molecular analyses revealed limited genetic introgression (Steeves et al. 2010, Mol. Ecol.19: 5090) R. Bush Morphological hybrids are now culled.