NATURAL FORMATION OF IRIS HYBRIDS: EXPERIMENTAL EVIDENCE ON THE ESTABLISHMENT OF HYBRID ZONES

Genomics of Evolutionary Novelty in Hybrids and Polyploids

It has long been recognized that hybridization and polyploidy are prominent processes in plant evolution. Although classically recognized as significant in speciation and adaptation, recognition of the importance of interspecific gene flow has dramatically increased during the genomics era, concomitant with an unending flood of empirical examples, with or without genome doubling. Interspecific gene flow is thus increasingly thought to lead to evolutionary innovation and diversification, via adaptive introgression, homoploid hybrid speciation and allopolyploid speciation. Less well understood, however, are the suite of genetic and genomic mechanisms set in motion by the merger of differentiated genomes, and the temporal scale over which recombinational complexity mediated by gene flow might be expressed and exposed to natural selection. We focus on these issues here, considering the types of molecular genetic and genomic processes that might be set in motion by the saltational event of genome merger between two diverged species, either with or without genome doubling, and how these various processes can contribute to novel phenotypes. Genetic mechanisms include the infusion of new alleles and the genesis of novel structural variation including translocations and inversions, homoeologous exchanges, transposable element mobilization and novel insertional effects, presence-absence variation and copy number variation. Polyploidy generates massive transcriptomic and regulatory alteration, presumably set in motion by disrupted stoichiometries of regulatory factors, small RNAs and other genome interactions that cascade from single-gene expression change up through entire networks of transformed regulatory modules. We highlight both these novel combinatorial possibilities and the range of temporal scales over which such complexity might be generated, and thus exposed to natural selection and drift.

POLLEN-TUBE COMPETITION, SIRING SUCCESS, AND CONSISTENT ASYMMETRIC HYBRIDIZATION IN LOUISIANA IRISES

Postpollination mechanisms can play an important role in limiting natural hybridization in plants. Reciprocal hand pollination experiments were performed to study these mechanisms in two species of Louisiana iris: Iris brevicaulis and I. fulva. Relative pollen-tube growth rates changed significantly through time, with I. fulva tubes increasingly outperforming I. brevicaulis tubes in both conspecific and heterospecific styles. However, this pattern of change in relative performance was a poor predictor of siring success: the majority of seeds sired by both maternal species was conspecific rather than hybrid. Experimental crosses and field studies show consistent asymmetric hybridization in Louisiana irises, with I. fulva being a more successful father and a more selective mother than both I. brevicaulis and a third species, I. hexagona. The cause of this pattern is not yet clear, but the pattern itself is unusual. Typically, short-styled species tend to be less successful in reciprocal crosses than long-styled relatives, but I. fulva has shorter styles than either I. brevicaulis or I. hexagona. The effects of pollen-tube competition, differential fertilization, and selective abortion in causing this pattern of asymmetric hybridization is discussed.

NEUTRAL GENE FLOW ACROSS SINGLE LOCUS CLINES

We study the strength of the genetic barrier to neutral gene flow in a general one-locus, two-allele model that includes viability selection as well as fertility selection and premating isolation. We have separately considered adult migration and pollen migration. Our theoretical results suggest that selection against hybrid formation in the form of fertility selection or assortative mating is more effective in preventing introgression of neutral alleles than is reduction in hybrid viability. We argue that in experimental studies of introgression testing of F₁ hybrids is as important as testing of parental forms. To illustrate the utility of this approach, we estimate the strength of the barrier to neutral gene flow between Piriqueta caroliniana and P. viridis, between Iris hexagona and I. fulva, and between Chorthippus brunneus and C. biguttulus.

A case study of modified interactions with symbionts in a hybrid mediterranean orchid

PREMISE OF THE STUDY

Most studies on orchid hybrids examine separately the effects of hybridization on interactions with pollinators or with mycorrhizal fungi. Here, we simultaneously investigated both interactions in the mediterranean food-deceptive Orchis simia, O. anthropophora, and their hybrid (O. ×bergonii) and tested a possible breakdown of coevolution using a multidisciplinary approach. •

METHODS

We compared leaf growth, seed viability, emitted scent, and mycorrhizal fungi (species and rate of infection) among these three taxa. •

KEY RESULTS

We show that leaf surface is greater in adult hybrids than in the parental species, suggesting a heterosis effect for vegetative growth. We demonstrate that flowers of the two parental species emit well-differentiated bouquets of volatile organic compounds, while hybrids emit larger quantities, accumulating most compounds of the two parental species. However, hybrids fail to attract pollinators and have a 10 times lower fruit set. We determined that closely related Tulasnellales are mycorrhizal in the three taxa, suggesting that the mycorrhizal partner does not impair hybrid survival. We propose an interpretative model for O. ×bergonii compared with its parents. •

CONCLUSIONS

In hybrids, carbon resources normally devoted to reproduction may be reallocated to the mycorrhizal symbiosis as a result of the disruption of the pollination interaction in hybrids. Higher mycorrhizal infection may in turn enhance vegetative growth and scent emission. Such interplay between the two obligate biotic interactions yields new insights into hybridization among orchids.

Asymmetric introgressive hybridization among louisiana iris species

In this review, we discuss findings from studies carried out over the past 20+ years that document the occurrence of asymmetric introgressive hybridization in a plant clade. In particular, analyses of natural and experimental hybridization have demonstrated the consistent introgression of genes from Iris fulva into both Iris brevicaulis and Iris hexagona. Furthermore, our analyses have detected certain prezygotic and postzygotic barriers to reproduction that appear to contribute to the asymmetric introgression. Finally, our studies have determined that a portion of the genes transferred apparently affects adaptive traits.

Reproductive isolation on interspecific backcross of F1 pollen to parental species, Hemerocallis fulva and H. citrina (Hemerocallidaceae)

Reproductive failure of backcross could play important roles in determining the evolutionary outcome of hybridization. However, such studies have been somewhat fewer than those of the F1-producing cross and F1 x F1 cross. We conducted hand-pollination backcross experiments using Hemerocallis fulva, H. citrina, and their F1 hybrids. Seed set per flower of the backcrosses reduced to about 50% of the control cross, irrespective of ovule parent species. This symmetrical seed set reduction on the backcross might be caused by the death of backcross embryo as a result of Dobzhansky-Muller incompatibility by recessive alleles.

Microarray analysis reveals differential gene expression in hybrid sunflower species

This paper describes the creation of a cDNA microarray for annual sunflowers and its use to elucidate patterns of gene expression in Helianthus annuus, Helianthus petiolaris, and the homoploid hybrid species Helianthus deserticola. The array comprises 3743 ESTs (expressed sequence tags) representing approximately 2897 unique genes. It has an average clone/EST identity rate of 91%, is applicable across species boundaries within the annual sunflowers, and shows patterns of gene expression that are highly reproducible according to real-time RT-PCR (reverse transcription-polymerase chain reaction) results. Overall, 12.8% of genes on the array showed statistically significant differential expression across the three species. Helianthus deserticola displayed transgressive, or extreme, expression for 58 genes, with roughly equal numbers exhibiting up- or down-regulation relative to both parental species. Transport-related proteins were strongly over-represented among the transgressively expressed genes, which makes functional sense given the extreme desert floor habitat of H. deserticola. The potential adaptive value of differential gene expression was evaluated for five genes in two populations of early generation (BC2) hybrids between the parental species grown in the H. deserticola habitat. One gene (a G protein-coupled receptor) had a significant association with fitness and maps close to a QTL controlling traits that may be adaptive in the desert habitat.

Mapping quantitative trait loci in plants: uses and caveats for evolutionary biology

Gregor Mendel was either clever or lucky enough to study traits of simple inheritance in his pea plants; however, many plant characters of interest to modern geneticists are decidedly complex. Understanding the genetic basis of such complex, or quantitative, traits requires a combination of modern molecular genetic techniques and powerful statistical methods. These approaches have begun to give us insight into understanding the evolution of complex traits both in crops and in wild plants.

Intimately linked or hardly speaking? The relationship between genotype and environmental gradients in a Louisiana Iris hybrid population

Several models of hybrid zone evolution predict the same spatial patterns of genotypic distribution whether or not structuring is due to environment-dependent or -independent selection. In this study, we tested for evidence of environment-dependent selection in an Iris fulva x Iris brevicaulis hybrid population by examining the distribution of genotypes in relation to environmental gradients. We selected 201 Louisiana Iris plants from within a known hybrid population (80 m x 80 m) and placed them in four different genotypic classes (I. fulva, I. fulva-like hybrid, I. brevicaulis-like hybrid and I. brevicaulis) based on seven species-specific random amplified polymorphic DNA (RAPD) markers and two chloroplast DNA haplotypes. Environmental variables were then measured. These variables included percentage cover by tree canopy, elevation from the high water mark, soil pH and percentage soil organic matter. Each variable was sampled for all 201 plants. Canonical discriminant analysis (CDA) was used to infer the environmental factors most strongly associated with the different genotypic groups. Slight differences in elevation (-0.5 m to +0.4 m) were important for distinguishing habitat distributions described by CDA, even though there were no statistical differences between mean elevations alone. I. brevicaulis occurred in a broad range of habitats, while I. fulva had a narrower distribution. Of all the possible combinations, I. fulva-like hybrids and I. brevicaulis-like hybrids occurred in the most distinct habitat types relative to one another. Each hybrid class was not significantly different from its closest parent with regard to habitat occupied, but was statistically unique from its more distant parental species. Within the hybrid genotypes, most, but not all, RAPD loci were individually correlated with environmental variables. This study suggests that, at a very fine spatial scale, environment-dependent selection contributed to the genetic structuring of this hybrid zone.

Morphometric variation in a hybrid zone between the weed, Silene vulgaris, and the endemic, Silene uniflora ssp. petraea (Caryophyllaceae), on the Baltic island of Öland

On the island of Öland the weed, Silene vulgaris (Moench) Garcke, and the endemic, Silene uniflora Roth ssp. petraea, hybridize when brought into contact by anthropogenic disturbance. Variation was studied in transects across a hybrid zone where a linear population of S. vulgaris crossed the native habitat of S. uniflora ssp. petraea. Plants were scored for 20 morphological characters. Although individual characters showed clinal trends between weed and endemic, all plants were assignable to one or other parental species. Only 14% of the 554 scored plants showed intermediacy in one or a few characters, and ordinations showed two separate groups of samples. The low number of intermediates is discussed in terms of character choice, habitat separation, disturbance history, and reproductive ecology. The results of the study are consistent with the earlier observation that the species have remained morphologically distinct on Öland, despite evidence of sparse introgression of allozymes from weed to endemic. Disturbance is necessary not only for the creation of intermediate (hybrid) habitats but also for the establishment of the weedy parent. The transient nature of S. vulgaris populations is likely to be important in limiting introgression into S. uniflora ssp. petraea under the present disturbance regime.Key words: genetic assimilation, hierarchical partitioning of diversity, habitat disturbance, introgression, rare species, clines.

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.