Fitness of crop-wild hybrid sunflower under competitive conditions: implications for crop-to-wild introgression

The role of intraspecific crop-weed hybridization in the evolution of weediness and invasiveness: Cultivated and weedy radish (Raphanus sativus) as a case study

PREMISE

The phenotype of hybrids between a crop and its wild or weed counterpart is usually intermediate and maladapted compared to that of their parents; however, hybridization has sometimes been associated with increased fitness, potentially leading to enhanced weediness and invasiveness. Since the ecological context and maternal genetic effects may affect hybrid fitness, they could influence the evolutionary outcomes of hybridization. Here, we evaluated the performance of first-generation crop-weed hybrids of Raphanus sativus and their parents in two contrasting ecological conditions.

METHODS

Using experimental hybridization and outdoor common garden experiments, we assessed differences in time to flowering, survival to maturity, plant biomass, and reproductive components between bidirectional crop-weed hybrids and their parents in agrestal (wheat cultivation, fertilization, weeding) and ruderal (human-disturbed, uncultivated area) conditions over 2 years.

RESULTS

Crop, weeds, and bidirectional hybrids overlapped at least partially during the flowering period, indicating a high probability of gene flow. Hybrids survived to maturity at rates at least as successful as their parents and had higher plant biomass and fecundity, which resulted in higher fitness compared to their parents in both environments, without any differences associated with the direction of the hybridization.

CONCLUSIONS

Intraspecific crop-weed hybridization, regardless of the cross direction, has the potential to promote weediness in weedy R. sativus in agrestal and ruderal environments, increasing the chances for introgression of crop alleles into weed populations. This is the first report of intraspecific crop-weed hybridization in R. sativus.

Pollen limitation of native plant reproduction in an urban landscape

PREMISE

Evidence suggests that bees may benefit from moderate levels of human development. However, the effects of human development on pollination and reproduction of bee-pollinated plants are less-well understood. Studies have measured natural variation in pollination and plant reproduction as a function of urbanization, but few have experimentally measured the magnitude of pollen limitation in urban vs. non-urban sites. Doing so is important to unambiguously link changes in pollination to plant reproduction. Previous work in the Southeastern United States found that urban sites supported twice the abundance of bees compared to non-urban sites. We tested the hypothesis that greater bee abundance in some of the same urban sites translates into reduced pollen limitation compared to non-urban sites.

METHODS

We manipulated pollination to three native, wild-growing, bee-pollinated plants: Gelsemium sempervirens, Oenothera fruticosa, and Campsis radicans. Using supplemental pollinations, we tested for pollen limitation of three components of female reproduction in paired urban and non-urban sites. We also measured pollen receipt as a proxy for pollinator visitation.

RESULTS

We found that all three plant species were pollen-limited for some measures of female reproduction. However, opposite to our original hypothesis, two of the three species were more pollen-limited in urban relative to non-urban sites. We found that open-pollinated flowers in urban sites received less conspecific and more heterospecific pollen on average than those in non-urban sites.

CONCLUSIONS

These results suggest that even when urban sites have more abundant pollinators, this may not alleviate pollen limitation of native plant reproduction in urban landscapes.

Consequences of introgression and gene flow on the genetic structure and diversity of Lima bean (Phaseolus lunatus L.) in its Mesoamerican diversity area

We evaluated the role of gene flow and wild-crop introgression on the structure and genetic diversity of Lima bean (Phaseolus lunatus) in the Yucatan Peninsula, an important Mesoamerican diversity area for this crop, using a genotyping-by-sequencing approach (15,168 SNP markers) and two scales. At the local scale, STRUCTURE and NGSEP analyses showed predominantly crop-to-wild introgression, but also evidence of a bidirectional gene flow in the two wild-weedy-crop complexes studied (Itzinté and Dzitnup). The ABBA-BABA tests showed a higher introgression in Itzinté (the older complex) than in Dzitnup (the younger one); at the allelic level, the wild-crop introgression in Itzinté was similar in both directions, in Dzitnup it was higher from crop-to-wild; and at the chromosomal level, introgression in Itzinté was from wild-to-crop, whereas in Dzitnup it occured in the opposite direction. Also, we found H _E values slightly higher in the domesticated accessions than in the wild ones, in both complexes (Itzinté: wild = 0.31, domesticated = 0.34; Dzinup: wild = 0.27, domesticated = 0.36), but %P and π estimators were higher in the wild accessions than in the domesticated ones. At a regional scale, STRUCTURE and MIGRATE showed a low gene flow, predominantly from crop-to-wild; and STRUCTURE, Neighbor-Joining and PCoA analyses indicated the existence of two wild groups and one domesticated group, with a marked genetic structure based in the existence of domesticated MI and wild MII gene pools. Also, at the regional scale, we found a higher genetic diversity in the wild accessions than in the domesticated ones, in all estimators used (e.g., H _E = 0.27 and H _E = 0.17, respectively). Our results indicate that gene flow and introgression are playing an important role at the local scale, but its consequences on the structure and genetic diversity of the Lima bean are not clearly reflected at the regional scale, where diversity patterns between wild and domesticated populations could be reflecting historical events.

Growth and fecundity of colonizing hybrid Raphanus populations are environmentally dependent

PREMISE

Hybrid gene pools harbor more genetic variation than progenitor populations. Thus, we expect hybrid populations to exhibit more dynamic evolutionary responses to environmental variation. We ask how environmental variation experienced by adapted and transplanted populations influence the success of late-generation hybrid populations during invasion.

METHODS

For four generations, 20 wild (Raphanus raphanistrum) and 20 hybrid radish (R. sativus × R. raphanistrum) plant populations evolved under experimentally manipulated moisture conditions (dry, wet, control-sheltered, or control-unsheltered plots; i.e., evolutionary environment) in old fields near Toronto, Canada. We planted advanced-generation wild and hybrid radishes in sheltered plots and exposed them to either an evolutionary or a novel watering environment. To determine how soil moisture would influence invasion success, we compared the phenotype and fecundity of plants grown in these various environments.

RESULTS

Hybridization produced larger plants. In wet environments, hybrid seedlings emerged more frequently and expressed higher photosynthetic activity. Low-moisture, novel conditions delayed and reduced seedling emergence frequency. Hybrid plants and those that evolved under relatively wet environments exhibited higher aboveground biomass. Hybrid plants from control-sheltered plots colonizing novel moisture environments were more fecund than comparable wild plants.

CONCLUSIONS

Dry environments are less likely than other evolutionary environments to contribute colonists. However, relatively wet locations support the evolution of relatively fecund plants, especially crop-wild hybrid populations. Thus, our results provide a strong mechanistic explanation for variation in the relative success of crop-wild hybrids among study locations and a new standard for studies that assess the risk of crop-wild hybridization events.

Maternal control of early life history traits affects overwinter survival and seedling phenotypes in sunflower (Helianthus annuus L.)

When cultivated and wild plants hybridize, hybrids often show intermediate phenotypic traits relative to their parents, which makes them unfit in natural environments. However, maternal genetic effects may affect the outcome of hybridization by controlling expression of the earliest life history traits. Here, using wild, cultivated and reciprocal crop-wild sunflower (Helianthus annuus L.) hybrids, we evaluated the maternal effects on emergence timing and seedling establishment in the field and on seedling traits under controlled conditions. In the field, we evaluated reciprocal crop-wild hybrids between two wild populations with contrasting dormancy (the high dormant BAR and the low dormant DIA) and one cultivar (CROP) with low dormancy. Under controlled conditions, we evaluated reciprocal crop-wild hybrids between two wild populations (BAR and RCU) and one CROP under three contrasting temperature treatments. In the field, BAR overwintered as dormant seeds whereas DIA and CROP showed high autumn emergence (~50% of planted seeds), resulting in differential overwinter survival and seedling establishment in the spring. Reciprocal crop-wild hybrids resembled their female parents in emergence timing and success of seedling establishment. Under controlled conditions, we observed large maternal effects on most seedling traits across temperatures. Cotyledon size explained most of the variation in seedling traits, suggesting that the maternal effects on seed size have cascading effects on seedling traits. Maternal effects on early life history traits affect early plant survival and phenotypic variation of crop-wild hybrids, thus, they should be addressed in hybridization studies, especially those involving highly divergent parents, such as cultivated species and their wild ancestors.

Germination response of diverse wild and landrace chile peppers (Capsicum spp.) under drought stress simulated with polyethylene glycol

Responses to drought within a single species may vary based on plant developmental stage, drought severity, and the avoidance or tolerance mechanisms employed. Early drought stress can restrict emergence and seedling growth. Thus, in areas where water availability is limited, rapid germination leading to early plant establishment may be beneficial. Alternatively, germination without sufficient water to support the seedling may lead to early senescence, so reduced germination under low moisture conditions may be adaptive at the level of the population. We studied the germination response to osmotic stress of diverse chile pepper germplasm collected in southern Mexico from varied ecozones, cultivation systems, and of named landraces. Drought stress was simulated using polyethylene glycol solutions. Overall, survival time analysis revealed delayed germination at the 20% concentration of PEG across all ecozones. The effect was most pronounced in the genotypes from hotter, drier ecozones. Additionally, accessions from wetter and cooler ecozones had the fastest rate of germination. Moreover, accessions of the landraces Costeño Rojo and Tusta germinated more slowly and incompletely if sourced from a drier ecozone than a wetter one, indicating that slower, reduced germination under drought stress may be an adaptive avoidance mechanism. Significant differences were also observed between named landraces, with more domesticated types from intensive cultivation systems nearly always germinating faster than small-fruited backyard- or wild-types, perhaps due to the fact that the smaller-fruited accessions may have undergone less selection. Thus, we conclude that there is evidence of local adaptation to both ecozone of origin and source cultivation system in germination characteristics of diverse chile peppers.

Heterotic Effect of Different Cytoplasmic Combinations in Sunflower Hybrids Cultivated Under Diverse Irrigation Regimes

The sunflower hybrids hold a narrow cytoplasmic diversity. Besides, the heterotic effect of wild cytoplasmic combinations of sunflower on important traits under water stress has not been explored in detail. Here, we evaluated the different sunflower cytoplasmic combinations in sunflower hybrids using cytoplasmic male sterile (CMS) sources as female parents. We used a total of sixteen sunflower genotypes representing twelve CMS lines from wild and conventional sources along with four restorer lines. Twelve CMS lines were crossed with four restorer lines to develop a total of 48 F₁ hybrid combinations. The hybrids were evaluated under two different environments (i.e., regular irrigation and water stress) for morphophysiological, yield, and biochemical traits over two years. Heterotic effect for various CMS sources was evaluated on all of the three possible scales, namely, better-parent heterosis (BPH), mid-parent heterosis (MPH), and heterosis as percent of check (PSH-996). For better-parent and mid-parent heterosis, the CMS sources Helianthus annuus, Helianthus argophyllus, and Helianthus debilis demonstrated positive better-parent heterosis for seed yield, oil content, and oleic acid irrespective of the environment. However, the hybrid combinations of different sources when using the genotype RCR8297 as the restorer parent recorded maximum average returns. Furthermore, chlorophyll meter (SPAD) reading positively correlated with days to 50% flowering, days to maturity, plant height, and number of leaves per plant in both the environments. Overall, this study identified and compared the heterotic effect of the different cytoplasmic combinations in sunflower under water stress as well as under normal irrigation environments.

Phenotypic selection under two contrasting environments in wild sunflower and its crop-wild hybrid

Hybridization is a common phenomenon in plants and can lead to the introgression of alleles from one population into another, generate new hybrid lineages, or cause species extinction. The environmental conditions and the genetic background of the participating populations may influence these outcomes since they can affect the fitness of hybrids, thereby increasing or decreasing the chances of introgression. Thus, it is important to understand the context-dependent prospects for introgression of alleles into diverse populations and under multiple ecological environments. Crop-wild hybridization presents an opportunity to explore these dynamics in agroecosystems. To this end, we used diverse wild and hybrid sunflowers from across the northern United States as a basis for evaluating variation in morphological traits and assessing context-dependent selection. These crop-wild hybrids and their wild counterparts were grown under agricultural conditions in the field with and without wheat competition. Interactions between origin and cross type affected expression of early functional traits, while interactions between competition and cross type acted on reproductive traits. A smattering of early and reproductive traits was affected by interactions between cross type and competition that varied by origin (i.e., 3-way interactions). Seven functional traits, especially number of branches and tertiary head diameter, underwent net and direct directional selection, while six out of these seven traits appear to also be experiencing nonlinear selection dynamics. In general, wild-like traits were favored under both sets of conditions, while, under wheat competition, some crop-like traits related to fast growth and primary head diameter became important. These data reaffirm the hypothesis that stressful conditions establish a scenario more suitable for crop introgression and clarify that nonlinear selection dynamics may play a role in this process.

Influence of intergenotypic competition on multigenerational persistence of abiotic stress resistance transgenes in populations of Arabidopsis thaliana

Reducing crop losses due to abiotic stresses is a major target of agricultural biotechnology that will increase with climate change and global population growth. Concerns, however, have been raised about potential ecological impacts if transgenes become established in wild populations and cause increased competitiveness of weedy or invasive species. Potential risks will be a function of transgene movement, population sizes, and fitness effects on the recipient population. While key components influencing gene flow have been extensively investigated, there have been few studies on factors subsequent to transgene movement that can influence persistence and competitiveness. Here, we performed multiyear, multigenerational, assessment to examine fitness effects and persistence of three mechanistically different abiotic stress tolerance genes: C-repeat binding factor 3/drought responsive element binding factor 1a (CBF3/DREB1a); Salt overly sensitive 1 (SOS1); and Mannose-6-phosphate reductase (M6PR). Transgenic Arabidopsis thaliana overexpressing these genes were grown in pure populations and in competition with wild-type (WT) parents for six generations spanning a range of field environment conditions. Growth, development, biomass, seed production, and transgene frequency were measured at each generation. Seed planted for each generation was obtained from the previous generation as would occur during establishment of a new genotype in the environment. The three transgenes exhibited different fitness effects and followed different establishment trajectories. In comparison with pure populations, CBF3 lines exhibited reduced dry weight, seed yield, and viable seed yield, relative to WT background. In contrast, overexpression of SOS1 and M6PR did not significantly impact productivity measures in pure populations. In competition with WT, negative fitness effects were magnified. Transgene frequencies were significantly reduced for CBF3 and SOS1 while frequencies of M6PR appeared to be subject to genetic drift. These studies demonstrate the importance of fitness effects and intergenotype competition in influencing persistence of transgenes conferring complex traits.

Environmental impacts of genetically modified plants: A review

Powerful scientific techniques have caused dramatic expansion of genetically modified crops leading to altered agricultural practices posing direct and indirect environmental implications. Despite the enhanced yield potential, risks and biosafety concerns associated with such GM crops are the fundamental issues to be addressed. An increasing interest can be noted among the researchers and policy makers in exploring unintended effects of transgenes associated with gene flow, flow of naked DNA, weediness and chemical toxicity. The current state of knowledge reveals that GM crops impart damaging impacts on the environment such as modification in crop pervasiveness or invasiveness, the emergence of herbicide and insecticide tolerance, transgene stacking and disturbed biodiversity, but these impacts require a more in-depth view and critical research so as to unveil further facts. Most of the reviewed scientific resources provide similar conclusions and currently there is an insufficient amount of data available and up until today, the consumption of GM plant products are safe for consumption to a greater extent with few exceptions. This paper updates the undesirable impacts of GM crops and their products on target and non-target species and attempts to shed light on the emerging challenges and threats associated with it. Underpinning research also realizes the influence of GM crops on a disturbance in biodiversity, development of resistance and evolution slightly resembles with the effects of non-GM cultivation. Future prospects are also discussed.

Applying gene flow science to environmental policy needs: a boundary work perspective

One application of gene flow science is the policy arena. In this article, we describe two examples in which the topic of gene flow has entered into the U.S. national environmental policymaking process: regulation of genetically engineered crops and clarification of the jurisdictional scope of the Clean Water Act. We summarize both current scientific understanding and the legal context within which gene flow science has relevance. We also discuss the process by which scientific knowledge has been synthesized and communicated to decision-makers in these two contexts utilizing the concept of 'boundary work'. Boundary organizations, the work they engage in to bridge the worlds of science, policy, and practice, and the boundary objects they produce to translate scientific knowledge existed in both examples. However, the specific activities and attributes of the objects produced varied based on the needs of the decision-makers. We close with suggestions for how scientists can contribute to or engage in boundary work with policymakers.

Life history traits and phenotypic selection among sunflower crop-wild hybrids and their wild counterpart: implications for crop allele introgression

Hybridization produces strong evolutionary forces. In hybrid zones, selection can differentially occur on traits and selection intensities may differ among hybrid generations. Understanding these dynamics in crop-wild hybrid zones can clarify crop-like traits likely to introgress into wild populations and the particular hybrid generations through which introgression proceeds. In a field experiment with four crop-wild hybrid Helianthus annuus (sunflower) cross types, we measured growth and life history traits and performed phenotypic selection analysis on early season traits to ascertain the likelihood, and routes, of crop allele introgression into wild sunflower populations. All cross types overwintered, emerged in the spring, and survived until flowering, indicating no early life history barriers to crop allele introgression. While selection indirectly favored earlier seedling emergence and taller early season seedlings, direct selection only favored greater early season leaf length. Further, there was cross type variation in the intensity of selection operating on leaf length. Thus, introgression of multiple early season crop-like traits, due to direct selection for greater early season leaf length, should not be impeded by any cross type and may proceed at different rates among generations. In sum, alleles underlying early season sunflower crop-like traits are likely to introgress into wild sunflower populations.

Seed fates in crop-wild hybrid sunflower: crop allele and maternal effects

Domestication has resulted in selection upon seed traits found in wild populations, yet crop-wild hybrids retain some aspects of both parental phenotypes. Seed fates of germination, dormancy, and mortality can influence the success of crop allele introgression in crop-wild hybrid zones, especially if crop alleles or crop-imparted seed coverings result in out-of-season germination. We performed a seed burial experiment using crop, wild, and diverse hybrid sunflower (Helianthus annuus) cross types to test how a cross type's maternal parent and nuclear genetic composition might affect its fate under field conditions. We observed higher maladaptive fall germination in the crop- and F₁- produced seeds than wild-produced seeds and, due to an interaction with percent crop alleles, fall germination was higher for cross types with more crop-like nuclear genetics. By spring, crop-produced cross types had the highest overwintering mortality, primarily due to higher fall germination. Early spring germination was identical across maternal types, but germination continued for F₁-produced seeds. In conclusion, the more wild-like the maternal parent or the less proportion of the cross type's genome contributed by the crop, the greater likelihood a seed will remain ungerminated than die. Wild-like dormancy may facilitate introgression through future recruitment from the soil seed bank.