Spread of herbicide‐resistant weedy rice (red rice,Oryza sativa L.) after 5 years of Clearfield rice cultivation in Italy

The weedy relative of cultivated rice, red rice, can invade and severely infest rice fields, as reported by rice farmers throughout the world. Because of its close genetic relationship to commercial rice, red rice has proven difficult to control. Clearfield (Cl) varieties, which are resistant to the inhibiting herbicides in the chemical group AHAS (acetohydroxyacid synthase), provide a highly efficient opportunity to control red rice infestations. In order to reduce the risk of herbicide resistance spreading from cultivated rice to red rice, stewardship guidelines are regularly released. In Italy, the cultivation of Cl cultivars started in 2006. In 2010, surveillance of the possible escape of herbicide resistance was carried out; 168 red rice plants were sampled in 16 fields from six locations containing Cl and traditional cultivars. A first subsample of 119 plants was analysed after herbicide treatment and the resistance was found in 62 plants. Of these 119 plants, 78 plants were randomly selected and analysed at the level of the AHAS gene to search for the Cl mutation determining the resistant genotype: the Cl mutation was present in all the resistant plants. Nuclear and chloroplast microsatellite markers revealed a high correlation between genetic similarity and herbicide resistance. The results clearly show that Cl herbicide‐resistant red rice plants are present in the field, having genetic relationships with the Cl variety. Finding plants homozygous for the mutation suggests that the crossing event occurred relatively recently and that these plants are in the F₂ or later generations. These observations raise the possibility that Cl red rice is already within the cultivated rice seed supply.     

The long and the short of it: SD1 polymorphism and the evolution of growth trait divergence in U.S. weedy rice

Growth-related traits, such as greater height, greater biomass, faster growth rate and early flowering, are thought to enhance competitiveness of agricultural weeds. However, weedy rice, a conspecific weed of cultivated rice (Oryza sativa L.), displays variation for growth traits. In the United States, separately evolved weedy rice groups have been shown to share genomic identity with exotic domesticated cultivars. Through a common garden experiment, we investigated whether growth trait divergence has occurred among U.S. weeds and their putative cultivated progenitors. We also determined polymorphism patterns in the growth candidate gene, SD1, to assess its possible role in the evolution of divergent phenotypes. We found considerable growth trait variation among weed groups, suggesting that growth trait convergence is not evident among weedy populations. Phenotypic divergence of weedy rice from cultivated ancestors is most apparent for flowering time. Introgression of a chromosomal block containing the SD1 allele from tropical japonica, the predominant U.S. rice cultivar, was detected in one weedy rice population and is associated with a change in growth patterns in this group. This study demonstrates the role of introgressive hybridization in evolutionary divergence of an important weed.     

Crop/weed gene flow:Chenopodium quinoa Willd. andC. berlandieri Moq.

Introduction of the Andean grain chenopod (Chenopodium quinoa) into North America placed this crop within the distributional range of a related wild species,C. berlandieri. This wild species, native to the North American flora, is cross-compatible withC. Quinoa. Isozyme analysis of progeny fromC. berlandieri plants growing within and at the periphery of theC. Quinoa fields, combined with fertility assessment and phenetic comparison among putative hybrids and parental types, indicates that over 30% of progeny from wild plants growing as weeds withC. quinoa in 1987 were crop/weed hybrids. This high incidence of interspecific gene flow from crop to weed appears to be the result of asymmetric pollen flow to free-living plants from high-density cultivated populations. The observed level of crop/weed hybridization, combined with heterosis and partial fertility of F₁ crop/weed hybrids, suggests that repeated annual cycles ofC. quinoa cultivation within the North American range ofC. berlandieri could produce introgressive change among sympatric wild populations. In terms of risk assessment for biotechnology, these results suggest that the breeding system may not provide an accurate indication of the potential for genetic interaction among predominately self-pollinating grain crops and their free-living relatives.     

Assessment of gene flow from a herbicide-resistant indica rice (Oryza sativa L.) to the Costa Rican weedy rice (Oryza sativa) in Tropical America: factors affecting hybridization rates and characterization of F1 hybrids

Herbicide-resistant rice cultivars allow selective weed control. A glufosinate indica rice has been developed locally. However, there is concern about weedy rice becoming herbicide resistant through gene flow. Therefore, assessment of gene flow from indica rice cultivars to weedy rice is crucial in Tropical America. A field trial mimicking crop–weed growing patterns was established to assess the rate of hybridization between a Costa Rican glufosinate-resistant rice line (PPT-R) and 58 weedy rice accessions belonging to six weedy rice morphotypes. The effects of overlapping anthesis, morphotype, weedy accession/PPT-R percentage, and the particular weedy accession on hybridization rates were evaluated. Weedy rice accessions with short overlapping anthesis (4–9 days) had lower average hybridization rates (0.1%) than long anthesis overlapping (10–14 days) accessions (0.3%). Hybridization also varied according to weedy rice morphotype and accession. Sativa-like morphotypes (WM-020, WM-120) hybridized more readily than intermediate (WM-023, WM-073, WM-121) and rufipogon-like (WM-329) morphotypes. No hybrids were identified in 11 of the 58 accessions analyzed, 21 accessions had hybridization rates from 0.01% to 0.09%, 21 had rates from 0.1% to 0.9%, and 5 had frequencies from 1% to 2.3%. Another field trial was established to compare the weedy rice-PPT-R F₁ hybrids with their parental lines under noncompetitive conditions. F₁ hybrids had a greater phenotypic variation. They had positive heterosis for vegetative trait and reproductive potential (number of spikelets and panicle length) traits, but negative heterosis for seed set. This study demonstrated the complexity of factors affecting hybridization rates in Tropical America and suggested that the phenotype of F₁ hybrids facilitate their identification in the rice fields.     

Hybridization in the genus Lens by means of embryo culture

Summary The cultivated lentil L. culinaris and the wild lentil L. ervoides are reproductively isolated from one another due to their hybrid embryo breakdowns. Using embryo culture, vegetatively normal hybrids were obtained. One specific hybrid, heterozygous for a reciprocal translocation, had about 50% gamete viability and produced aborted and viable embryos in a 11 ratio. In the F₂, vegetatively normal and highly fertile plants were selected. With the aid of embryo culture techniques, L. ervoides can be included in the wild gene pool of the cultivated lentil.     

The interplay of hybridization and clonal reproduction in the evolution of willows – Experiments with hybrids of S. eriocephala[R] & S. exigua[X] and S. eriocephala & S. petiolaris[P].

Clonal reproduction may contribute to plant evolution either by affecting population biology or by allowing partially sterile individuals (e.g., hybrids) repeated opportunities to reproduce sexually. The interplay of hybridization and clonal reproduction was first proposed by Stebbins (1950), but previously has not been tested experimentally. Alternative models for the effects of hybridization include speciation, introgression, and swamping. Salix spp. were chosen to test comparatively these hypotheses because they are easily hybridized and cloned. Over 500 separate crosses and backcrosses were made and over 6000 separate plants were measured in field experiments and statistically compared for significance to both evolutionary theory and plant breeding for biomass production. The F₁ hybrids in this study always equalled, and in the case of the hybrid PR, outperformed their parents in vegetative parameters. It seems likely that even without reproducing sexually, these F₁ hybrids could exist as successful individuals (sensu Stebbins 1950). However, it also seems likely that they would sexually reproduce: three of four F₁ hybrids studied (RX, RP, and PR) equalled or surpassed their parents in sexual parameters when crossing with at least one other accession. Of the alternative models, experimental data suggest that introgression would be the most likely outcome of a hybridization event. The hybrid XR, however, was partially sterile and performed poorly when crossing with all other accessions in its group except S. exigua (pistillate parent). Thus, this hybrid may fit Stebbins' model of a partially sterile yet vegetatively vigorous plant that can exist as a successful individual and make some contribution to interspecific gene flow over time. This is the first experimental study to confirm the evolutionary importance of clonal reproduction coupled with hybridization. However, distinguishing any of these evolutionary pathways would be difficult in nature using morphological techniques, as interspecific hybrids tend to resemble their pistillate parents in terms of leaf shape.     

EXPERIMENTAL HYBRIDIZATION OF XANTHIUM STRUMARIUM (COMPOSITAE) FROM ASIA AND AMERICA. I. RESPONSES OF F1 HYBRIDS TO PHOTOPERIOD AND TEMPERATURE

Responses to photoperiod and temperature were compared for hybrids between Asiatic plants in the indigenous strumarium morphological complex and plants in five American complexes: oviforme, italicum, chinense, cavanillesii, and pennsylvancium. The F₁ hybrids between Hong Kong plants and various American plants showed intermediacy in photoperiodic response; however, the hybrid night requirement was more similar to that of the American parent. The Hong Kong plants are difficult to evaluate photoperiodically but showed a night length requirement of 9.25–9.50 hr. Day-neutral plants from India in the strumarium complex produced day-neutral F₁ hybrids in crosses with most American plants having night requirements less than 10 hr. The F₁ hybrids involving the day-neutral Indian plants and either Indian or Australian chinense plants showed a night requirement of 8.25–8.75 hr. The chinense parental plants had apparent critical nights of 10.25–10.50 hr. Crosses between the day-neutral Indian plants and Mexican plants with apparent critical nights of 10.75–11 hr produced F₁ hybrids requiring nights of 8.75–9.00 hr. The various hybrids tended to show the broader temperature tolerances of the American parents. The ripeness-to-flower (maturity) responses of seedlings tended to show the genetic controls of the parent with the faster developmental rate. The hybridization evidence suggests that photoperiodic responses are quantitatively controlled and inherited independently of morphology and ripeness-to-flower responses. The populations of India are highly diverse and probably reflect recombinations of photoperiodic and temperature responses between indigenous day-neutral plants and photoperiodic chinense plants introduced from North America.     

The wild-weed-crop complex in Setaria: a hybridization study

Evidence is given of spontaneous hybridizations between cultivated foxtail millet (Setaria italica) and its wild relative the green foxtail (S. viridis). Such a cross produces F₂ plants similar to the giant wild variant S. viridis var. major found as a weed in cultivated areas. S. viridis major was also crossed to the same cultivar and the two F₂s were compared on the basis of 19 morphological characters. This study indicates that S. viridis major is more closely related to S. italica than S. viridis sensu stricto and indeed could have resulted from a wild x crop hybridization.     

EXPERIMENTAL HYBRIDIZATION OF XANTHIUM STRUMARIUM (COMPOSITAE) FROM ASIA AND AMERICA. II. SESQUITERPENE LACTONES OF F1 HYBRIDS

Effects of hybridization on sesquiterpene lactones in Xanthium strumarium were studied for clues to the relationship of Old and New World populations. In crosses between indigenous Asiatic plants in the “strumarium” morphological complex and various American complexes that produce xanthinin as a major sesquiterpene lactone, the F₁ hybrids contained xanthinin and the related compounds, xanthanol, xanthatin, and xanthinosin. In other crosses with various American complexes that produce xanthumin, the stereoisomer of xanthinin, the F₁ hybrids contained xanthinin and xanthumin as well as xanthinin-related compounds and their stereoisomers, xanthumanol, deacetoxylxanthumin, and tomentosin. The Asiatic plants of “strumarium” from Hong Kong involved in the crosses produce approximately equal percentages of xanthinin, xanthatin and xanthinosin, but those from India contain only xanthinosin. The putative introduction of the American morphological complex, “chinense,” contains xanthumin as the major component but shows chemical diversity that indicates genes derived from the indigenous “strumarium” complex.     

Clarification of the genetic component of hybrids between Phyllodoce caerulea and Phyllodoce aleutica (Ericaceae) in Hokkaido,northern Japan

Natural hybridization provides great opportunities to understand the interaction of genetics and ecology in determining species boundaries. We examined the genetic relationships of Phyllodoce taxa and revealed that most hybrids were fertile F₁s and an extremely small number of backcross and no F₂ plants were established in natural conditions. Because this trend was irrespective of regions, we conclude that negative endogenous selection may act after the germination of F₁ seeds and prevent the establishment of later‐generation hybrids. Based on these results, we discuss the ecological and evolutionary significance of natural hybridization in Phyllodoce taxa.     

Impact of interspecific hybridization between crops and weedy relatives on the evolution of flowering time in weedy phenotypes

Background

Like conventional crops, some GM cultivars may readily hybridize with their wild or weedy relatives. The progressive introgression of transgenes into wild or weedy populations thus appears inevitable, and we are now faced with the challenge of determining the possible evolutionary effects of these transgenes. The aim of this study was to gain insight into the impact of interspecific hybridization between transgenic plants and weedy relatives on the evolution of the weedy phenotype.

Methodology/Principal Findings

Experimental populations of weedy birdseed rape (Brassica rapa) and transgenic rapeseed (B. napus) were grown under glasshouse conditions. Hybridization opportunities with transgenic plants and phenotypic traits (including phenological, morphological and reproductive traits) were measured for each weedy individual. We show that weedy individuals that flowered later and for longer periods were more likely to receive transgenic pollen from crops and weed×crop hybrids. Because stem diameter is correlated with flowering time, plants with wider stems were also more likely to be pollinated by transgenic plants. We also show that the weedy plants with the highest probability of hybridization had the lowest fecundity.

Conclusion/Significance

Our results suggest that weeds flowering late and for long periods are less fit because they have a higher probability of hybridizing with crops or weed×crop hybrids. This may result in counter-selection against this subset of weed phenotypes, and a shorter earlier flowering period. It is noteworthy that this potential evolution in flowering time does not depend on the presence of the transgene in the crop. Evolution in flowering time may even be counter-balanced by positive selection acting on the transgene if the latter was positively associated with maternal genes promoting late flowering and long flowering periods. Unfortunately, we could not verify this association in the present experiment.     

野生稻与栽培稻及种间杂种F1叶表面亚显微结构的研究

用扫描电镜对原产中国的３种野生稻、２个栽培稻品种及栽培稻与野生稻间的杂种Ｆ１,就气孔频度、气孔器乳突、大瘤状乳突及本栓细胞乳突等叶片表面亚显微结构作了比较观察研究。结果显示这些性状具有种的特性,发现种间杂种的亚星微性状与两个亲本 特性及亲缘关系远近有关。     

Identification and fine mapping of <Emphasis Type="Italic">S-d</Emphasis>, a new locus conferring the partial pollen sterility of intersubspecific F<Subscript>1</Subscript> hybrids in rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.)

The partial pollen abortion of hybrids between the indica and japonica subspecies of Asian cultivated rice is one of the major barriers in utilizing intersubspecific heterosis in hybrid rice breeding. Although a single hybrid pollen sterility locus may have little impact on spikelet fertility, the cumulative effect of several loci usually leads to a serious decrease in spikelet fertility. Isolating of the genes conferring hybrid pollen sterility is necessary to understand this phenomenon and to overcome the resulting genetic barrier. In this study, a new locus for F₁ pollen sterility, S-d, was identified on the short arm of chromosome 1 by analyzing the genetic effect of substituted segments of the near-isogenic line E11-5 derived from the japonica variety Taichung 65 (recurrent parent) and the indica variety Dee-geo-woo-gen (donor parent). The S-d locus was first mapped to a 0.8 cM interval between SSR markers PSM46 and PSM80 using a F₂ population of 125 individuals. The flanking markers were then used to identify recombinants from a population of 2,160 plants derived from heterozygotes of the primary F₂ population. Simultaneously, additional markers were developed from genomic sequence divergence in this region. Analysis of the recombinants in the region resulted in the successful mapping of the S-d locus to a 67-kb fragment, containing 17 predicted genes. Positional cloning of this gene will contribute to our understanding of the molecular basis for partial pollen sterility of intersubspecific F₁ hybrids in rice.     

Detecting hybridization in mixed populations ofRhinanthus minor andRhinanthus angustifolius

Rhinanthus minor andRhinanthus angustifolius are known to hybridize in mixed populations in nature. These hybridization events can have important evolutionary consequences. The development and use of species-specific RAPD and ISSR markers allowed the detection of hybrid individuals not always distinguishable with morphological characters. Two mixed populations of different ages were studied. In a young mixed 2-year-old population, both individuals of the two parental species and F₁ hybrids were found using genetic analysis, showing that hybridization occurred rapidly. Flower morphology of F₁ hybrids was too variable to distinguish all these hybrids from the parental species. This morphological variability of F₁ hybrids was also confirmed in artificial crosses in the greenhouse. In an old and no longer mixed 30-year-old population, onlyR. angustifolius plants and a few genetically introgressed individuals close toR. angustifolius were present. Genetic markers showed traces of past hybridization and introgression. Unidirectional introgression ofR. minor intoR. angustifolius with the complete disappearance ofR. minor from this population was observed.     

Fertile asymmetric somatic hybrids between Lycopersicon esculentum Mill. and Lycopersicon peruvianum var. dentatum Dun.

Summary Thirteen nuclear asymmetric hybrids were regenerated under selective conditions following fusion of chlorophyll-deficient protoplasts from cultivated tomato (Lycopersicon esculentum Mill.) and -(-irradiated protoplasts from the wild species Lycopersicon peruvianum var. dentatum Dun. All hybrid plants were classified as being asymmetric based on morphological traits, chromosome numbers and isozyme patterns. The majority of the hybrids inherited Lycopersicon peruvianum var. dentatum chloroplasts. Mitochondrial DNA analysis revealed mixed mitochondria populations deriving from both parents in some of the hybrids and rearranged mitochondrial DNA in others. The asymmetric hybrids express some morphological traits that are not found in either of the parental species. Fertile F₁ plants were obtained after self-pollination of the asymmetric hybrids in four cases. The results obtained confirm the potential of asymmetric hybridization as a new source of genetic variation, and as a method for transferring of a part of genetic material from donor to recipient, and demonstrate that it is possible to produce fertile somatic hybrids by this technique.     

NATURAL AND ARTIFICIAL HYBRIDS OF HELIANTHUS MAXIMILIANI X H. GROSSESERRATUS

Long , Robert W. (Ohio Wesleyan U., Delaware.) Natural and artificial hybrids of Helianthus Maximiliani × H. grosseserratus. Amer. Jour. Bot. 46(10): 687–692. Illus. 1959.—An investigation of the occurrence of natural hybridization in two perennial sunflowers, Helianthus Maximiliani and H. grosseserratus, was begun in 1950. Subsequently, artificial F₁, F₂, and first and second backcross generations were produced. Fertility and vigor were high in all these plants, but F₁ plants appeared to excel the others in these characteristics. Observations in the experimental garden were supplemented by examination of chromosomes in pollen mother cells, comparisons of herbarium collections, and study of wild populations. Evidence pointed to close genetic relationship of the species and to the occurrence of natural hybridization in areas of distributional overlap. In 1957 and 1958, field work in these areas resulted in the scoring of 18 natural populations, 3 of which consisted of both parental species plus putative F₁ hybrids. Two explanations are offered to account for the seeming absence of introgression. The results support the conclusion that natural hybridization leads to the establishment of F₁ hybrids and that introgression does not occur to any significant extent. Although both species display a high degree of interfertility, they are distinct morphologically. For this reason, it is advisable to maintain them as separate species.     

Heterosis breeding in rice (Oryza sativa L.)

Summary Studies conducted at the International Rice Research Institute (IRRI) during 1980 and 1981 have shown up to 73% heterosis, 59% heterobeltiosis and 34% standard heterosis for yield in rice. The latter was estimated in comparison to commercial varieties: IR36 and IR42 (yield 4–5 t/ha in wet season trials and 7–8 t/ha in dry season trials). Generally speaking, absolute yield was lower and extent of standard heterosis was higher in wet season than in dry season with some exception. Yields up to 5.9 t/ha (22% standard heterosis) in the wet season and 10.4 t/ha (34% standard heterosis) in the dry season were obtained. Most of the hybrids performed better in some season while some performed better in both seasons. Hybrids showed better lodging resistance although they were 5–10 cm taller. F₁ hybrids had significant positive correlations with the parental traits viz., yield (r = 0.446), tillering (r = 0.746), height (r = 0.810) and flowering (r = 0.843). Selection of parents among elite breeding lines on the basis of their per se yield performance, diverse origin and resistance to insects and diseases should give heterotic combination. Yield advantage of hybrids was due primarily to increase in number of spikelets per unit area even though tiller number was reduced. Grain weight was either the same or slightly higher. High yielding hybrids also showed significant heterosis and heterobeltiosis for total dry matter and harvest index. For commercial utilization of heterosis in rice, effective male sterility and fertility restoration systems are available and up to 45% natural outcrossing on male sterile lines has been observed. Consequently, F₁ rice hybrid have been successfully developed and used in China. Prospects of developing hybrid rice varieties elsewhere appear bright especially in countries that have organized seed production, certification and distribution programs and where hybrid seed can be produced at a reasonable cost.     

Interspecific hybridization between common and tepary beans: increased hybrid embryo growth,fertility, and efficiency of hybridization through recurrent and congruity backcrossing

Cultivated common bean (Phaseolus vulgaris L.) and tepary bean (Phaseolus acutifolius A. Gray) genotypes possessing desirable agronomic traits were hybridized. The F₁ hybrids were backcrossed twice with the common bean (i.e., recurrent backcrossing). Also, alternate backcrosses with common and tepary beans (i.e., congruity backcrossing) were carried out. Embryo culture was necessary for all initial interspecific crosses, and its requirement was proportionally lower when the common bean was used as the recurrent parent and as the last parent of congruity backcrosses. Modification of the embryo culture technique was necessary to produce congruity hybrids. Effects of both tepary and common bean genotypes on the success rate of hybridization were observed. Tepary accession G 40001 and common bean cultivar ICA Pijao facilitated interspecies hybridization. Growth of hybrid embryos before rescue, recovery of mature hybrid plants, and the vigor and fertility of F₁ hybrids all increased with increased recurrent and congruity backcrosses and intermatings between male-sterile F₁ and selected fertile F₂ plants of the third and fifth congruity backcrosses. Introgression of tepary genes was verified by means of seed protein electrophoretic analysis and morphological markers. The results suggest that congruity backcrossing can help to gradually reduce or overcome P. vulgaris x P. acutifolius hybridization barriers such as genotype incompatibility, early embryo abortion, hybrid sterility, and lower frequencies of hybridization.     

Allele Distributions at Hybrid Incompatibility Loci Facilitate the Potential for Gene Flow between Cultivated and Weedy Rice in the US

The accumulation of independent mutations over time in two populations often leads to reproductive isolation. Reproductive isolation between diverging populations may be reinforced by barriers that occur either pre- or postzygotically. Hybrid sterility is the most common form of postzygotic isolation in plants. Four postzygotic sterility loci, comprising three hybrid sterility systems (Sa, s5, DPL), have been recently identified in Oryza sativa. These loci explain, in part, the limited hybridization that occurs between the domesticated cultivated rice varieties, O. sativa spp. japonica and O. sativa spp. indica. In the United States, cultivated fields of japonica rice are often invaded by conspecific weeds that have been shown to be of indica origin. Crop-weed hybrids have been identified in crop fields, but at low frequencies. Here we examined the possible role of these hybrid incompatibility loci in the interaction between cultivated and weedy rice. We identified a novel allele at Sa that seemingly prevents loss of fertility in hybrids. Additionally, we found wide-compatibility type alleles at strikingly high frequencies at the Sa and s5 loci in weed groups, and a general lack of incompatible alleles between crops and weeds at the DPL loci. Our results suggest that weedy individuals, particularly those of the SH and BRH groups, should be able to freely hybridize with the local japonica crop, and that prezygotic factors, such as differences in flowering time, have been more important in limiting weed-crop gene flow in the past. As the selective landscape for weedy rice changes due to increased use of herbicide resistant strains of cultivated rice, the genetic barriers that hinder indica-japonica hybridization cannot be counted on to limit the flow of favorable crop genes into weeds.     

Fitness of reciprocal F1 hybrids between Rhinanthus minor and Rhinanthus major under controlled conditions and in the field

The performance of first‐generation hybrids determines to a large extent the long‐term outcome of hybridization in natural populations. F₁ hybrids can facilitate further gene flow between the two parental species, especially in animal‐pollinated flowering plants. We studied the performance of reciprocal F₁ hybrids between Rhinanthus minor and R. major, two hemiparasitic, annual, self‐compatible plant species, from seed germination to seed production under controlled conditions and in the field. We sowed seeds with known ancestry outdoors before winter and followed the complete life cycle until plant death in July the following season. Germination under laboratory conditions was much lower for the F₁ hybrid formed on R. major compared with the reciprocal hybrid formed on R. minor, and this confirmed previous results from similar experiments. However, this difference was not found under field conditions, which seems to indicate that the experimental conditions used for germination in the laboratory are not representative for the germination behaviour of the hybrids under more natural conditions. The earlier interpretation that F₁ hybrid seeds formed on R. major face intrinsic genetic incompatibilities therefore appears to be incorrect. Both F₁ hybrids performed at least as well as and sometimes better than R. minor, which had a higher fitness than R. major in one of the two years in the greenhouse and in the field transplant experiment. The high fitness of the F₁ hybrids confirms findings from naturally mixed populations, where F₁ hybrids appear in the first year after the two species meet, which leads to extensive advanced‐hybrid formation and introgression in subsequent generations.