EM increased as BRIX decreased and fruit size and firmness increased. Hence, we found that mycelium developed faster on softer, sweeter, and smaller fruit than firmer, less sweet, and larger fruit, as is typical of Royal Royce and other modern LSL cultivars . Finally, LD was weakly negatively genetically correlated with AC, whereas EM was uncorrelated with AC .Although the additive genetic correlation we observed between LD and AC was in the direction predicted by previous studies in strawberry and tomato , LD only slightly decreased as AC increased .Breeding for resistance to necrotrophic pathogens has been challenging in plants . The mechanisms of resistance to necrotrophic pathogens are more subtle, quantitative, and complex than those commonly observed for biotrophic pathogens that trigger pathogen-associated molecular pattern-triggered-immunity and effector-triggered immunity . Our findings were well aligned with previous findings in other B. cinerea hosts and shed light on the genetic complexity of resistance to gray mold in strawberry. Where do we go from here? We are skeptical that significant genetic gains can be achieved for gray mold resistance across the complete shelf life spectrum in strawberry but are confident that post harvest gray mold incidence can be minimized but obviously not eliminated in LSL populations. This conclusion seems well aligned with previous findings in strawberry and other hosts of this pathogen . Because several fruit quality traits pleiotropically affect gray mold resistance in strawberry, 25 litre plant pot the challenge is exponentially greater when breeding for markets where softer fruits with elevated sugars are preferred and LSL phenotypes are neither necessary nor preferred .
However, for markets where LSL cultivars are essential, direct selection for the requisite fruit quality traits seems to confer sufficient resistance to gray mold to ensure marketability under normal post harvest storage conditions and timelines, especially for fruit produced in coastal California and other arid and semi-arid environments with low humidity and rainfall . Although we only sampled 12 coastal California environments to estimate the natural incidence of B. cinerea among eight LSL cultivars, we suspect that deeper sampling will confirm our findings. There are open questions to be addressed and were limitations to our study. First, we did not screen diverse germplasm to identify sources of resistance to gray mold. As our study and others have shown, strong sources of resistance to this pathogen may not exist . The narrow-sense heritability estimates and genomic prediction results for LD and EM in the present study suggest that a deeper exploration of genetic diversity, while challenging, seems worthwhile . The association between resistance and titratable acidity seems to be particularly promising and worthy of further study , particularly if increased acidity is offset by increased sugars to achieve a palatable sugar: acid balance. Second, the parents for this study were selected to assess the effects of fruit quality and shelf life-associated traits on gray mold disease development , not for known intrinsic differences in gray mold resistance that are genetically uncorrelated with fruit quality and shelf life phenotypes. The pleiotropic effects of shelf life-associated fruit quality traits on gray mold susceptibility appear to be inescapable . Our results suggest that resistance can be increased by selecting for increased titratable acidity and firmness and decreased sugars but these phenotypes profoundly affect flavor and cannot be manipulated in a vacuum .
Without more extensive germplasm screening, the data needed to guide the selection of parents for future genetic studies are lacking. Third, the natural incidence of gray mold was only explored in the present study for LSL cultivars commercially grown in California . Highly perishable short- and medium-shelf life cultivars, as typified by the heirloom cultivars we screened , are challenging to grow and phenotype in such studies because they typically have low yields, are easily bruised and wounded, and cannot be harvested and handled with the same robustness as commercially important LSL cultivars. Nevertheless, a study of the natural incidence of gray mold among individuals spanning the shelf life spectrum could shed further light on genetic correlations between fruit quality and gray mold resistance phenotypes and possibly identify sources of favorable alleles underlying intrinsic resistance that are uncorrelated with fruit quality traits, e.g., biochemical phenotypes triggered by defense mechanisms . Because gray mold disease symptoms from artificial inoculation protocols are typically harsher than those observed from natural infections of non-wounded fruit , a deeper exploration of the natural incidence of gray mold seems warranted in strawberry, perhaps by simulating rainfall in field experiments through overhead irrigation or other practices to increase the uniformity and incidence of natural infection. Our results suggest that phenotypic or GS could be effective for gray mold resistance but only in certain populations and only when selection for genetically correlated traits does not antagonistically reduce genetic gains for gray mold resistance phenotypes . Genetic gains for gray mold resistance are affected by shelf life-related traits through additive genetic correlations and could be reversed by simultaneous selection for fruit quality and shelf life traits that antagonistically pleiotropically affect gray mold resistance phenotypes .
Genetic variation for fruit quality traits strongly affected the phenotypic differences we observed for LD and EM. Most importantly, the fruit quality traits associated with enhanced flavor were antagonistically genetically correlated with gray mold resistance phenotypes . Cross-validation of genomic predictions in the present study shed light on the complexity of genetic mechanisms underlying gray mold resistance phentoypes and highlighted the challenges inherent in breeding for increased resistance to gray mold in strawberry . The three WGR methods we applied to the prediction problem strongly shrunk breeding values to the population mean for LD in the multifamily and EM in the Royal Royce Tangi populations. Such shrinkage is typical for moderately heritable complex diseases in plants . The prospects for identifying superior genotypes through GS were greater for LD in the Royal Royce Tangi population and EM in the multifamily population than vice versa. Whether applying phenotypic or GS, the probability of selecting superior genotypes can be exceedingly low when breeding for resistance to genetically complex diseases in plants . Nevertheless, with cost-effective genome-wide genotyping, GS has the potential to increase genetic gains by increasing the number of selection candidates that can be screened per unit of time and space . Even in the small populations we studied , nearly 20,000 phenotypic observations were collected to quantitatively assess resistance to gray mold, which included image analyses at each post inoculation time point . The phenotyping throughput needed to effectively evaluate post harvest traits in strawberry can be limiting, particularly when multiple harvests are factored into the equation, e.g., day-neutral cultivars are typically harvested twice weekly over a period of six to eight months in coastal California. Because the progression of disease symptoms was continuous and genotype storage time interactions were negligible , phenotyping throughput can be increased by employing a response surface experiment design, e.g., by phenotyping fruit at three equally spaced time points spanning the range needed to estimate response curves and accurately predict LD and EM phenotypes . This is precisely the sort of breeding problem where genomic prediction methods have the greatest potential utility and applicability , 30 litre plant pots bulk despite the complexity of the underlying genetic mechanisms and variable prediction accuracies across populations, which nevertheless affect phenotypic and GS equally .Damage by biotic agents is one the most important challenges to plant survival and reproduction . Animal influences on plants may be antagonistic , mutualistic , or some combination of the two. The responses by plants to herbivory influence their chemistry, morphology, phenology, and ecology , and can also lead to the evolution of defense traits. The research described here was motivated by the expectation that this evolutionary process would be especially apparent by comparing an invasive hybrid-derived plant with its two progenitor lineages. Biological defenses are defined as adaptive responses to selective forces imposed via interactions with other organisms . In plant biology, chemical defenses have received considerably more attention than structural defenses . Many structural defenses also have physiological effects such as aiding in water retention and cold tolerance, and it has been suggested that the physiological role is typically the original target of selection with anti-herbivore defensive features evolving thereafter . Structural defenses can take several forms such as spinescence, pubescence and sclerophylly . Plants have structural defenses that protect both vegetative and reproductive components. For the latter, the investment of the plant in the propagule generally increases with phenology and is at its highest level during the period of seed dispersal. Seeds represent one of the most vulnerable stage in a plant’s life cycle, so their defense is essential to the parent plant’s reproductive success . A common mechanism of defense is to package seeds in fruits, which are evolved structures that mediate maturation and seed dispersal, and can also protect seeds by reducing consumption from granivores . Pre-dispersal seed predation has the potential to affect the ecology and evolution of plant populations .
Some plants lose a significant proportion of their seeds to seed predators and these loses have the potential to change the number of subsisting individuals, their occurrence and dissemination modifying the structure and interactions of the ecosystem. Each species responds differently based on intrinsic as well as the environmental characteristics. At the evolutionary level, seed predators may act as selective agents when feeding differentially among plants with different heritable traits . This can be assessed by considering how plant traits are altered in response to predation pressure. However, it is still difficult to predict the ecological and evolutionary consequences of seed predation with considering plant-animal interactions in spatially- and temporally-explicit frameworks. For example, the amount of seeds lost is not necessarily a good estimate of the impact of the granivores . Factors other than predator pressure, such as variation in seed density, seed predator density, or presence of other interacting species, can contribute to predation rate. Arguably, one of the most useful features of hybrid plant systems is that one can regenerate or work with existent hybrid lineages and compete it with its pure lines of progenitors to examine the interactions between ecological and evolutionary processes with spatial and temporal precision. Many invasive plants are hybrid-derived. There are multiple examples of invasive lineages derived from progenitors in which both members are introduced , as well as from crosses between introduced and native plants . A recent study illustrates how hybridization between native Rosa mollis and invasive R. rugosa is reducing the genetic diversity of R. mollis as well as its abundance . When referring to hybrid plant susceptibility, Fritz et al. have proposed four alternative hypotheses to the null hypothesis that the are no differences in susceptibility to damage when comparing F1 hybrid-derived with progenitor lineages. The four alternative hypotheses consider how resistant traits are inherited and expressed as follows: additive hypothesis: when hybrid susceptibility is intermediate. The average resistance is not different from both progenitors; dominance: when the mean susceptibility of the F1 resembles to one of the progenitors; hybrid susceptibility: the F1 susceptibility exceeds both progenitors. Mixed stands of progenitors and hybrids will have more herbivores residing on hybrid plants ; finally hybrid resistance: F1 hybrids are more resistant than either progenitor. Experimentally testing hybrids produced de novo allows for the determination of inheritance patterns. In contrast, a comparison of inherited traits in advanced hybrids with pure lines of both progenitors is likely to be confounded by evolutionary history.Radish plants in the genus Raphanus are a traditional model system for research in plant ecology and plant evolution . The genus Raphanus belongs to the Brassicaceae family. Raphanus flowers are insect pollinated, self-incompatible based on a single S-allele locus and hermaphroditic. The genus has a well-studied reproductive and sexual selection system and high genetic polymorphism . The fruit develops from the pistil to form an indehiscent silique pod-like fruit attached to the plant by a pedicel . Each fruit holds from 1 to up to 12 seeds in total. Mature seeds have a globular shape weighing 2mg to 12 mg . Several features of reproductive biology of Raphanus suggest that this is a tractable system for assessing how the evolution of plant characters and plant-enemy interactions may have contributed to its invasive success. The central question motivating this dissertation research is: are there enhanced seed packaging and seed characteristics in a hybrid-derived invasive lineage relative to its progenitors?