Published on December 18, 2008
Plant Speciation & Evolution (PBIO 475/575) : Plant Speciation & Evolution (PBIO 475/575) Hybridization General Principles : General Principles Hybridization sensu lato = cross-fertilization that brings two different genotypes into conjunction May involve different individuals within a population or individuals among populations ("trivial"); or individuals of different taxa (e.g., distinct species--usually what people mean by hybridization) May be restricted to the initial hybridization event, or it may continue through time and across generations where weak isolation mechanisms permit gene flow between original parents and hybrid progeny General Principles : General Principles Previously believed to be unimportant—an evolutionary “dead end” Much new evidence demonstrates that hybridization provides new genetic combinations available for modification by other processes; e.g., polyploidy, chromosomal rearrangements, selection on introgressants Often disrupts isolation mechanisms preventing interspecific gene flow if it occurs over wide geographic areas and impacts many populations of hybridizing species ? “genetic swamping” General Principles : General Principles In groups where hybridization is common, it is involved in formation of new geographic and ecological races e.g., Redroot (Ceanothus) Raven et al. (1992) General Principles : General Principles Hybrids not commonly "perfectly" intermediate in phenotype Hybrid vigor commonly expressed, especially in F1s Different traits often vary toward one of the parental taxa Sometimes a novel expression arises e.g., unique non-glandular hair type in hybrid false foxglove, Aureolaria flava x pedicularis e.g., dense glands of hybrid woodsia fern, Woodsia ilvensis x oregana, that are not found in the parent species General Principles : General Principles Hybrids may be exceedingly localized or rare, or geographically very widespread, depending on frequency of hybridization event e.g., exceedingly rare hybrid cinnamon fern, Osmunda x ruggii known historically from one plant in each of two widely separate sites in the eastern U.S. parents grow sympatrically over thousands of square miles only 1 site now exists extant single plant is probably >1100 years old! General Principles : General Principles Hybrid frequency (cont.) Triploid hybrid woodfern, Dryopteris x triploidea, is a common cross of two widely distributed North American ferns, D. intermedia (4x) and D. carthusiana (2x) Hybrid is often more abundant in a given lowland forest site than either parent; but must be constantly recreated de novo—no ability to reproduce! Triploid genome confers denser glandular hairiness than either parent—”dosage effect” of duplicated genome from D. intermedia General Principles : General Principles Hybrids not always restricted to immediate vicinity of parents, where vegetative or other propagation and dispersal allow e.g., Hybrid horsetail, E. x ferrissii, disperses by stem fragmentation along waterways far outside range of E. laevigatum parent Raven et al. (1992) General Principles : General Principles Where isolation mechanisms are weak and "intermediate" microhabitat for hybrid establishment is extensive, “hybrid swarms” may be produced with all forms of phenotypic (and potentially physiologically adaptive) combinations Not rare in plants Raw material, with new genotypes, upon which selection and other forces may act Hybridization sensu stricto : Hybridization sensu stricto Interbreeding halts at sterile F1 or hybrid breakdown or other post-mating isolation mechanism ? prevents complete interspecific gene flow Very common in land plants—operates (or has operated) in vast majority of plant groups Chromosome doubling or asexual reproduction could still produce evolutionarily important products Classic Introgression : Classic Introgression Active and iterative process, occurs after initial hybridization event Depends on weak interspecific (especially, weak post-mating) isolation Multigenerational process Initial interspecific F1 hybrid produced F1 crosses with parent to produce first-generation backcross progeny Backcross progeny of successive generations continue to hybridize with individuals of same parent Classic Introgression : Classic Introgression May be bidirectional or unidirectional Extent of introgression (and development of “hybrid swarms”) depends on extrinsic factors, e.g., site and environmental conditions, as well as intrinsic factors, e.g., blooming time of parents, nature and strength of isolation limiting gene flow between backcrosses and parents Theoretical consequence is individuals with mostly the genome and (exo)phenotype of one parent but with some genes of the other parent Classic Introgression : Classic Introgression Most profound impact considered to be introgressant plants indistinguishable from one parent but with adaptability to broader range of environments than either Frequency still not fully understood but suspected in many groups where hybridization is common Classic Introgression : Classic Introgression Evidence for introgression frequently demonstrated by comparing nuclear and plastid genomes; e.g., incongruence between nuclear and chloroplast phylogenies of individuals Incongruence in genomic phylogenies termed “plastid capture” and is strong evidence of hybridization with subsequent introgression Sometimes suggests an ancient, cryptic introgressive event e.g., black cottonwood has nuclear genome of BC group but chloroplast of white cottonwood group, but has no clear phenotypic traits of the white cottonwoods! Classic Introgression : Classic Introgression Smith & Sytsma (1990) Red box = white poplar group Chloroplast phylogeny Nuclear phylogeny P. nigra P. nigra Second “Introgression” Process? : Second “Introgression” Process? PhD student Aurea Cortes studied extensively hybridizing violet species pairs in Mexico over 3 field seasons Found phenotypic and nuclear genetic marker evidence of extensive gene flow ? apparent introgression (many “parental” individuals had genetic markers of other parent) Found virtually no pollinator movement over 3 seasons, suggesting that interspecific hybridization is rare “Pseudo-introgression”? : “Pseudo-introgression”? Violet species have mixed breeding system--produce outcrossing showy flowers in spring and numerous cleistogamous (strictly selfing) flowers later Species reproduce largely by cleistogamous flowers, hybrid individuals reproduce entirely by cleistogamy ? meiotic segregation in progeny of hybrids? ? is not introgression, which is an active backcrossing process Needs confirmation, and further study in other groups with mixed breeding systems; could be common? Consequences of Introgression : Consequences of Introgression True gene flow accomplished across species boundaries Locally significant if restricted to a particular population, but much more significant if it occurs to extent that introgressants proliferate and disperse widely May ultimately affect the competitive ability, long-term survival of one or both parental species May also be complicated by polyploidy, etc. Consequences of Introgression : Consequences of Introgression May yield a new taxon isolated from either parent that is uniquely adapted to a particular set of conditions Some modern authors use the term "introgression" interchangeably to include both hybridization in the narrow sense, and introgression—this is sloppy, and the two processes have very different evolutionary consequences More on Hybrid Speciation : More on Hybrid Speciation Homoploid ("diploid") hybrid speciation Parents differ by two or more translocations; begins with chromosomally sterile hybrid Self-fertilization and meiotic recombination generates structural homozygote Recombinant progeny are true-breeding but yield sterile hybrids in progeny-parent crosses Could also be accomplished by recombination of genic sterility factors (theoretical possibility) Experimentally documented in Wild rye (Elymus),Gilia (Gilia),Tobacco (Nicotiana), Rice (Oryza), etc. More on Hybrid Speciation : More on Hybrid Speciation Recombinational speciation through external barriers = ecological speciation following initial hybridization Parents isolated temporally or ecologically Begins with fertile/subfertile hybrid Self-fertilization and recombination generates externally isolated progeny Eventually produces species isolated from parents Inferred in amaranth (Amaranthus), larkspur (Delphinium), willow-herb (Epilobium), oaks (Quercus), etc.; proven with molecular data in Beardtongues (Penstemon), Wild irises (Iris), several other groups More on Hybrid Speciation : More on Hybrid Speciation Allopolyploid speciation Believed very common in plants--47% of angiosperms, 97% of ferns and fern allies probably polyploid; majority presumably of allopolyploid origin Begins with a hybrid of two different species or "races" (subspecies or varieties) Sterile hybrid undergoes chromosome doubling (both means below appear to be common events) Somatic cells of the stem may generate a tetraploid branch that produces tetraploid flowers; diploid gametes produce tetraploid progeny Diploid plant produces two sets of unreduced (diploid, not haploid) gametes; union produces tetraploid progeny More on Hybrid Speciation : More on Hybrid Speciation Tetraploid is fertile--2 sets of chromosomes representing the 2 parental genomes pair up, yielding "balanced" gametes Famous examples of whole species groups include spleenwort ferns (Asplenium), henbit (Galeopsis), cotton (Gossypium), wheat (Triticum) Multiple origins of allopolyploid species documented in many groups, e.g., Goat's-beards Maybe more common than we ever thought More on Hybrid Speciation : More on Hybrid Speciation Segmental allopolyploidy (older term for “infraspecific” allopolyploidy) Series of races of one species hybridize with another species in different places Chromosome doubling yields a "continuum" of very closely related allopolyploids Slightly different taxa with same ploidy level all interfertile More on Hybrid Speciation : More on Hybrid Speciation Factors fostering allopolyploidy Long life cycle, with some mode of vegetative reproduction "Primary" speciation completed, resulting in chromosomal rearrangements Frequent natural hybridization Bibliography : Bibliography Anonymous. Flora of North America Editorial Committee (ed.). 1993. Flora of North America north of Mexico, vol. 2-Pteridophytes and gymnosperms. Oxford University Press, New York, New York. 475 pp. Briggs, D. and S. M. Walters. 1997. Plant variation and evolution, 3rd ed. Cambridge University Press, Cambridge, United Kingdom. 512 pp. Futuyma, D. J. 1979. Evolutionary biology. Sinauer Associates, Inc., Sunderland, Massachusetts. 565 pp. Grant, V. 1971. Plant speciation. Columbia University Press, New York, New York. 435 pp. Bibliography : Bibliography Grant, V. 1991. The evolutionary process: A critical study of evolutionary theory, 2nd ed. Columbia University Press, New York, New York. 487 pp. Raven, P. H., R. F. Evert, and S. E. Eichhorn. 1992. Biology of plants, 5th ed. Worth Publishers, New York, New York. 791 pp. Smith, R. L. and K. J. Sytsma. 1990. Evolution of Populus nigra (sect. Aigeiros): Introgressive hybridization and the chloroplast contribution of Populus alba (sect. Populus). American Journal of Botany 77:1176-1187.