Speciation: Splitting when together.
Explore the source record for details and available documents.
Animals↗
Biomedical subjects
L H Rieseberg
Publications and source records attributed to L H Rieseberg.
Identification and mapping of SNPs from ESTs in sunflower.
More than 67,000 expressed sequence tags (ESTs) have recently been generated for sunflower (Helianthus), including 44,000 from cultivated confectionery (RHA280) and oilseed (RHA801) lines of Helianthus annuus and 23,000 from drought- and salt-tolerant wild sunflowers, H. argophyllus and H. paradoxus, respectively. To create a transcript map for sunflower, we identified 605 ESTs that displayed small insertion-deletion polymorphism (SNP) variation in silico, had apparent tissue-specific expression patterns, and/or were ESTs with candidate functions in traits such as development, cell transport, metabolism, plant defense, and tolerance to abiotic stress. Primer pairs for 535 of the loci were designed from the ESTs and screened for polymorphism in recombinant inbred lines derived from a cross between the same cultivars (RHA280 x RHA801) employed for sequencing. In total, 273 of the loci amplified polymorphic products, of which 243 mapped to the 17 linkage groups previously identified for sunflower. Comparisons with previously mapped QTL revealed some cases where ESTs with putatively related functions mapped near QTLs identified in other crosses for salt tolerance and for domestication traits such as stem diameter, shattering, flowering time, and achene size.
Chromosome Mapping↗
The ecological genetics of homoploid hybrid speciation.
Our understanding of homoploid hybrid speciation has advanced substantially since this mechanism of species formation was codified 50 years ago. Early theory and research focused almost exclusively on the importance of chromosomal rearrangements, but it later became evident that natural selection, specifically ecological selection, might play a major role as well. In light of this recent shift, we present an evaluation of ecology's role in homoploid hybrid speciation, with an emphasis on the genetics underlying ecological components of the speciation process. We briefly review new theoretical developments related to the ecology of homoploid hybrid speciation; propose a set of explicit, testable questions that must be answered to verify the role of ecological selection in homoploid hybrid speciation; discuss published work with reference to these questions; and also report new data supporting the importance of ecological selection in the origin of the homoploid hybrid sunflower species Helianthus deserticola. Overall, theory and empirical evidence gathered to date suggest that ecological selection is a major factor promoting homoploid hybrid speciation, with the strongest evidence coming from genetic studies.
Ecology↗
Natural selection for salt tolerance quantitative trait loci (QTLs) in wild sunflower hybrids: implications for the origin of Helianthus paradoxus, a diploid hybrid species.
For a new diploid or homoploid hybrid species to become established, it must diverge ecologically from parental genotypes. Otherwise the hybrid neospecies will be overcome by gene flow or competition. We initiated a series of experiments designed to understand how the homoploid hybrid species, Helianthus paradoxus, was able to colonize salt marsh habitats, when both of its parental species (H. annuusxH. petiolaris) are salt sensitive. Here, we report on the results of a quantitative trait locus (QTL) analysis of mineral ion uptake traits and survivorship in 172 BC2 hybrids between H. annuus and H. petiolaris that were planted in H. paradoxus salt marsh habitat in New Mexico. A total of 14 QTLs were detected for mineral ion uptake traits and three for survivorship. Several mineral ion QTLs mapped to the same position as the survivorship QTLs, confirming previous studies, which indicated that salt tolerance in Helianthus is achieved through increased Ca uptake, coupled with greater exclusion of Na and related mineral ions. Of greater general significance was the observation that QTLs with effects in opposing directions were found for survivorship and for all mineral ion uptake traits with more than one detected QTL. This genetic architecture provides an ideal substrate for rapid ecological divergence in hybrid neospecies and offers a simple explanation for the colonization of salt marsh habitats by H. paradoxus. Finally, selection coefficients of +0.126, -0.084 and -0.094 for the three survivorship QTLs, respectively, are sufficiently large to account for establishment of new, homoploid hybrid species.
Adaptation, Biological↗
Likely multiple origins of a diploid hybrid sunflower species.
The recurrent origin of diploid hybrid species is theoretically improbable because of the enormous diversity of hybrid genotypes generated by recombination. Recent greenhouse experiments, however, indicate that the genomic composition of hybrid lineages is shaped in part by deterministic forces, and that recurrent diploid hybrid speciation may be more feasible than previously believed. Here we use patterns of variation from chloroplast DNA (cpDNA), nuclear microsatellite loci, cross-viability and chromosome structure to assess whether a well-characterized diploid hybrid sunflower species, Helianthus anomalus, was derived on multiple occasions from its parental species, H. annuus and H. petiolaris. Chloroplast DNA and crossability data were most consistent with a scenario in which H. anomalus arose three times: three different H. anomalus fertility groups were discovered, each with a unique cpDNA haplotype. In contrast, there was no clear signature of multiple, independent origins from the microsatellite loci. Given the age of H. anomalus (> 100 000 years bp), it may be that microsatellite evidence for recurrent speciation has been eroded by mutation and gene flow through pollen.
Arizona↗
Polyploid evolution: keeping the peace at genomic reunions.
Incompatible gene or chromosomal combinations brought together in allopolyploid genomes cannot be purged through Mendelian segregation. But recent studies suggest that the elimination of DNA sequences and alteration of DNA methylation patterns may permit the restoration of fertility in some allopolyploids.
Animals↗
The contribution of epistasis to species differences in annual sunflowers.
The contribution of epistasis to 15 morphological traits differentiating two annual sunflowers (Helianthus annuus and H. debilis ssp. cucumerifolius) and to hybrid pollen sterility was estimated in a first generation backcross (BC1) mapping population. Analysis of digenic interactions among quantitative trait loci (QTLs) with significant main effects revealed significant interaction effects for six of the 15 morphological traits and for pollen sterility. Likewise, a genome-wide scan of all possible two-locus combinations detected additional significant interactions for three of the traits with significant epistasis in the original analysis: stem pigmentation, phyllary pubescence, and pollen viability. However, these were the only traits of the 16 examined in which detected interactions explained more than 5% of phenotypic variance. The implications of these findings for adaptive evolution and for the introgression of advantageous morphological QTLs across a natural hybrid zone between these taxa are discussed.
Adaptation, Biological↗
Sex determination in the androdioecious plant Datisca glomerata and its dioecious sister species D. cannabina.
Datisca glomerata is an androdioecious plant species containing male and hermaphroditic individuals. Molecular markers and crossing data suggest that, in both D. glomerata and its dioecious sister species D. cannabina, sex is determined by a single nuclear locus, at which maleness is dominant. Supporting this conclusion, an amplified fragment length polymorphism (AFLP) is heterozygous in males and homozygous recessive in hermaphrodites in three populations of the androdioecious species. Additionally, hermaphrodite x male crosses produced 1:1 sex ratios, while hermaphrodite x hermaphrodite crosses produced almost entirely hermaphroditic offspring. No perfectly sex-linked marker was found in the dioecious species, but all markers associated with sex mapped to a single linkage group and were heterozygous in the male parent. There was no sex-ratio heterogeneity among crosses within D. cannabina collections, but males from one collection produced highly biased sex ratios (94% females), suggesting that there may be sex-linked meiotic drive or a cytoplasmic sex-ratio factor. Interspecific crosses produced only male and female offspring, but no hermaphrodites, suggesting that hermaphroditism is recessive to femaleness. This comparative approach suggests that the hermaphrodite form arose in a dioecious population from a recessive mutation that allowed females to produce pollen.
Chimera↗
Low intraspecific variation for genomic isolation between hybridizing sunflower species.
Barriers to gene flow between species result from selection against foreign linkage blocks in hybrids. When the geographic ranges of taxa meet at multiple locations, the opportunity exists for variation in the genetic architecture of isolating barriers. Hybrid zones between two sunflower species (Helianthus annuus and H. petiolaris) in Nebraska and California exhibited remarkably similar patterns of introgression of mapped molecular markers. Congruence among hybrid zones may result from limited intraspecific variation at loci contributing to isolation and from similar selective effects of alleles in the heterospecific genetic background. The observed consistency of introgression patterns across distantly separated hybrid zones suggests that intrinsic forces predominate in determining hybrid zone dynamics and boundaries between these sunflower species.
Alleles↗
Transgressive character expression in a hybrid sunflower species.
Diploid hybrid lineages often are ecologically distinct from their parental species. However, it is unclear whether this niche divergence is typically achieved via hybrid intermediacy, a mixture of parental traits, and/or the evolution of extreme (transgressive) morphological and ecophysiological features. Here we compare an extensively studied hybrid sunflower species, Helianthus anomalus, with its putative parents, H. annuus and H. petiolaris, for 41 morphological and 12 ecophysiological traits. Helianthus anomalus was morphologically intermediate for one trait (2.4%), parental-like for 23 traits (56.1%), and transgressive for 17 traits (41.5%). For ecophysiological traits, H. anomalus was not significantly different from one or both parents for nine traits (75%), and was transgressive for the remaining three (25%). Thus, H. anomalus appears to be a mosaic of parental-like and transgressive phenotypes. Although the fitness effects of the transgressive characters are not yet known, many of these characters are consistent with adaptations reported for other sand dune plants. Genetic studies are currently underway to ascertain whether these extreme characters arose as a direct byproduct of hybridization or whether they evolved via mutational divergence.
Journal Article↗
Possible consequences of genes of major effect: transient changes in the G-matrix.
Understanding the process of evolutionary divergence requires knowledge of the strength, form, and targets of selection, as well as the genetic architecture of the divergent traits. Quantitative genetic approaches to understanding multivariate selection and genetic response to selection have proven to be powerful tools in this endeavor, particularly with respect to short-term evolution. However, the application of quantitative genetic theory over periods of substantial phenotypic change is controversial because it requires that the requisite genetic parameters remain constant over the period of time in question. We show herein how attempts to determine the stability of key genetic parameters may be misled by the 'many genes of small effect' type of genetic architecture generally assumed in quantitative genetics. The presence of genes of major effect (GOMEs) can alter the genetic variance-covariance matrix dramatically for brief periods of time, significantly alter the rate and trajectory of multivariate evolution, and thereby mislead attempts to reconstruct or predict long term evolution.
Biological Evolution↗
The likelihood of homoploid hybrid speciation.
New species may be formed through hybridization and without an increase in ploidy. The challenge is for hybrid derivatives to escape the homogenizing effects of gene flow from parental species. The mechanisms hypothesized to underlie this process were modelled using a computer simulation. The model is of recombinational speciation, in which chromosomal rearrangements between parental species result in poor fertility of F1 hybrids, but through recombination, novel homozygous types are formed that have restored fertility. In simulations, stable populations bearing the recombinant karyotypes originated frequently and were maintained when the fertility of F1 hybrids was high. However, this high rate of origination was offset by low genetic isolation, and lower F1 hybrid fertility increased the evolutionary independence of derived populations. In addition, simulations showed that ecological and spatial isolation were required to achieve substantial reproductive isolation of incipient species. In the model, the opportunity for ecological isolation arose as a result of adaptation to extreme habitats not occupied by parental species, and any form of spatial isolation (e.g. founder events) contributed to genetic isolation. Our results confirmed the importance of the combination of factors that had been emphasized in verbal models and illustrate the trade-off between the frequency at which hybrid species arise and the genetic integrity of incipient species.
Animals↗
Two independent loci control agamospermy (Apomixis) in the triploid flowering plant Erigeron annuus.
Asexual seed production (agamospermy) via gametophytic apomixis in flowering plants typically involves the formation of an unreduced megagametophyte (via apospory or diplospory) and the parthenogenetic development of the unreduced egg cell into an embryo. Agamospermy is almost exclusively restricted to polyploids. In this study, the genetic basis of agamospermy was investigated in a segregating population of 130 F(1)'s from a cross between triploid (2n = 27) agamospermous Erigeron annuus and sexual diploid (2n = 18) E. strigosus. Correlations between markers and phenotypes and linkage analysis were performed on 387 segregating amplified fragment length polymorphisms (AFLPs). Results show that four closely linked markers with polysomic inheritance are significantly associated with parthenogenesis and that 11 cosegregating markers with univalent inheritance are completely associated with diplospory. This indicates that diplospory and parthenogenesis are unlinked and inherited independently. Further, the absence of agamospermy in diploid F(1)'s appears to be best explained by a combination of recessive-lethal gametophytic selection against the parthenogenetic locus and univalent inheritance of the region bearing diplospory. These results may have major implications for attempts to manipulate agamospermy for agricultural purposes and for interpreting the evolution of the trait.
Chromosome Mapping↗
Evolutionary changes over the fifty-year history of a hybrid population of sunflowers (Helianthus).
Although there are many studies of the evolution and ecology of hybrid zones, few extend over long enough time periods to track evolutionary changes in the zones or assess the ultimate outcome of hybridization. Here we describe the current genetic and morphological composition of a hybrid sunflower population relative to its initial makeup 50 years ago. It appears that few genetically pure parental plants remain in the hybrid population and the average phenotype has shifted from an initial bias toward Helianthus bolanderi to a predominance of H. annuus-like plants. The similarity to H. annuus is more pronounced for morphology than for neutral genetic markers. In contrast to the shift in morphology that occurred primarily in the past 40 years, overall pollen viability increased to its current level during the first 10 to 15 years of hybridization, indicating the presence of strong fertility selection. Dramatic differences are seen in morphology, genetics, and pollen viability between the eastern and western halves of the population, thus confirming observations by previous authors and suggesting that selection pressures in the two halves differ. The trends seen in this hybrid population over the past 50 years suggest that H. bolanderi is undergoing genetic assimilation, and this trend may be representative of its fate throughout its range.
Biological Evolution↗
Crossing relationships among ancient and experimental sunflower hybrid lineages.
Reproductive barrier formation between newly derived hybrid taxa and their parental species represents a major evolutionary hurdle. Here, I examine the development of a sterility barrier during hybrid speciation by examining the fertility of progeny from all combinations of crosses involving three experimentally synthesized sunflower hybrid lineages, their natural hybrid counterpart, Helianthus anomalus, and their parents, H. annuus and H. petiolaris. Crosses between the parental species and H. anomalus generated almost completely sterile offspring (pollen viability < 5%; seed set < 1%). A fairly strong sterility barrier also has developed between three hybrid lineages and both parental species (pollen viability 11.1-41.6%; seed set 0.84-20.1%). In contrast, the three hybrid lineages are almost fully interfertile (pollen viabilities 83.1-88.6%; seed set 72.1-75.3%), as predicted by molecular mapping studies that indicate they have converged on a similar set of gene combinations and chromosomal rearrangements. A modest decline in compability is observed in crosses between the three hybrid lineages and H. anomalus (pollen viabilities 64.1-70.7%; seed set 37-43%), a result that agrees well with prior data demonstrating significant congruence between the genomes of the natural and experimental hybrid lineages. These observations not only indicate that reproductive isolation can arise as a by-product of fertility selection in hybrid populations, but also testify to the repeatability of this mode of speciation.
Breeding↗
Hybridization, introgression, and linkage evolution.
Genetic mapping methods provide a unique opportunity to study the interactions of differentiated genes and genomes in a hybrid genetic background. After a brief discussion of theoretical and analytical concerns, we review the application of these methods to a wide range of evolutionary issues. Map-based studies of experimental hybrids indicate that most postzygotic reproductive barriers in plants are polygenic and that the expression of extreme or novel traits in segregating hybrids (transgressive segregation) results from the complementary action of divergent parental alleles. However, genetic studies of hybrid vigor do not concur in their interpretations of the relative roles of dominance, overdominance, and epistasis. Map-based studies of natural hybrids are much rarer, but the few existing studies confirm the polygenic basis of postzygotic barriers and demonstrate the utility of genetic linkage for detecting cryptic introgression. In addition, studies of experimental and natural hybrid lineages provide compelling evidence that homoploid hybrid speciation has occurred in nature, and that it represents a rapid and repeatable mode of speciation. Data further indicate that this mode is facilitated by strong fertility selection and high chromosomal mutation rates. We recommend that future studies of hybrid genomes focus on natural hybrids, not only because of the paucity of data in this area, but also because of the availability of highly recombinant hybrid genotypes in hybrid zones. Of particular value will be studies of long-lived or difficult-to-propagate organisms, which previously have not been amenable to genetic study.
Crosses, Genetic↗
How reliable is science information on the web?
Explore the source record for details and available documents.
Information Management↗
Transgressive segregation, adaptation and speciation.
The production of extreme or 'transgressive' phenotypes in segregating hybrid populations has been speculated to contribute to niche divergence of hybrid lineages. Here, we assess the frequency of transgressive segregation in hybrid populations, describe its genetic basis and discuss the factors that best predict its occurrence. From a survey of 171 studies that report phenotypic variation in segregating hybrid populations, we show that transgression is the rule rather than the exception. In fact, 155 of the 171 studies (91%) report at least one transgressive trait, and 44% of 1229 traits examined were transgressive. Transgression occurred most frequently in intraspecific crosses involving inbred, domesticated plant populations, and least frequently in interspecific crosses between outbred, wild animal species. Quantitative genetic studies of plant hybrids consistently point to the action of complementary genes as the primary cause of transgression, although overdominance and epistasis also contribute. Complementary genes appear to be common for most traits, with the possible exception of those with a history of disruptive selection. These results lend credence to the view that hybridization may provide the raw material for rapid adaptation and provide a simple explanation for niche divergence and phenotypic novelty often associated with hybrid lineages.
Adaptation, Biological↗