The peduncular section of the hybrid-derived lineage did appear to be where the most successful of the fathers sires its seeds. This is also the section of the fruit were there is a positive effect of seed position and seed weight, in the same lineage in open pollinated fruits. As predicted, pollen donors differ in their ability to sire seeds produced by mixed crosses and the pollen donor identity did influence the offspring fecundity. However, when paternity was not taken into account the relationship between offspring seed weight and fecundity is not so clear. Perhaps, in a competitive environment , these relationships become more apparent. A potential explanation for the reduction in fecundity and low viability in the cultivated Rs might be that during the time the plants spent in the greenhouse, there was an effect of competition. As seedlings in the greenhouse, the plants were very close together in the seed starting trays. The hybrid-derived and the wild lineages germinated and grew faster than the cultivar. Above ground competition for light and space during seedling stage might have decelerated the cultivars’ growth. Competition has been showed to reduce fecundity in Raphanus plants . Similar results were found by Ridley and Ellstrand . Relative to the hybrid-derived CAwr, the cultivar produced fewer fruits and seeds over two years and three replicates period. During their experiments the CAwr, Rs and Rr plants were exposed to open pollination. It is unlikely that the pollination of their cultivar plants was restricted among conspecific plants.
It is likely that all three lineages were pollen sources, procona flower transport containers especially CAwr given that it has ephemeral populations at the experimental sites as well as permanent ones in the surroundings. In the present study, CAwr pollen was used for all hand pollinations on all three lineages potentially reducing compatibility and affecting the cultivar’s viability and fecundity. Individual plants with different types of seeds, e.g. different morphology, have seed heteromorphism . A known trait in heteromorphic seeds is intraspecific seed size variation within the same mother plant or even within the same fruit. To be adaptive, heteromorphic seeds differ in their ecology and fitness offering an advantage in unpredictable environments . Some ecological consequences of seed size heteromorphism include variation in dispersal ability, dormancy and germination requirements with consequences at sibling competition level, final reproductive output, and final seed density-dependent distribution in time and space . Under competitive conditions, larger seeds of Raphanus raphanistrum growth faster, giving them a competitive advantage over smallerones . In our study, peduncular seeds in the hybrid-derived plants produced by open pollination were the only situation where seed position and seed size were significantly influenced. Across environmental hetererogeneity, larger seeds of the annual Cakile edentula have shorter life cycles, an advantageous trait in unpredictable sand dunes environments . There is evidence of sexual selection in Raphanus fruits, manifested in the effect of the pollen-donor identity on fruit set, seeds per fruit, seed abortions and seed weight . Among the fruits and seeds that I was able to genotype and assign paternity, I did find that the fathers that I used as pollen sources did have an effect on seed weight and on final reproductive output.
When paternity was determined, seeds at peduncular ends do produce more seeds that will have higher reproductive output. This is not a universal trait, however. Seeds from peduncular portions of the cucumber fruit are the slowest to attain the optimum conditions for germination and desiccation tolerance . However, effects of within-fruit seed position and within-plant fruit position in cucumber fruits eventually disappear, leveling up final germination rates among all seed positions. In our experiments, I did not find direct effect of seed weight on seed germination and final reproductive output . Our data suggest that seed paternity influences seed weight, final plant height and reproductive output. We also found that seed maternity moderately influences reproductive output, final plant weight and life cycle stages. These results support a hypothesis of complementary maternal and paternal effects in all three lineages. The mechanisms involved in ovule fertilization in Raphanus fruits are determined by maternalchoice and paternal identity, which is especially clear in mixed pollinations . We detected competition among fathers, where single pollination results showed higher reproductive output of some fathers that scarcely were represented in some of the mixed pollinations. When paternity is considered, seeds at peduncular positions in mixed and single crosses produced offspring with higher reproductive output. Combining this result with fruits harder structure at peduncular ends for all three lineages, in particular in the case of CAwr, suggests the intriguing possibility that there is higher protection for higher performing seeds. From an ecological perspective, these results might counteract any potential negative effects of granivores.The fruits of the hybrid invasive lineage, California wild radish , offer better protection to the seeds that will be better competitors, relative to the fruits of both Raphanus progenitors. The most favored and prolific fathers in mixed pollen crosses typically sired the seeds at peduncular and middle portions of the hybrid fruits. Those same portions of the hybrid fruits have the hardest fruit walls. Thus, the fruit structure and the effects of mating selection couple in a favorable manner in the hybrid derived lineage. How much of the fruit structure is the result of inter-specific hybridization alone or a combination of it with natural selection is a question that still needs to be answered. The fruit wall hardness appears to be a defensive trait that provides fitness advantages to the hybrid lineage, in particular in the context of pre-dispersal seed predation by the house finch . This ecological advantage could have been a significant feature in Southern California leading to successful establishment of hybrids over both progenitors. These results now lead to several questions about how broad these patterns may be: How homogeneous is this mechanism along the current distribution on both coastal and inland sites in California, Baja, Mexico and Oregon? How common is the house finch-hybrid lineage interaction along this geographic range? How homogeneous is this trait within the same mother plant and are fruits within the same plant with weaker fruit structure more heavily predated? Of the two progenitors, fruits of the wild Raphanus raphanistrum are much harder than those of the domesticated R. sativus . Patterns in fruit wall hardness ofthe Raphanus system suggest that it is either a neutral or an advantageous trait. In addition, the results described here implicate another mechanism of defense in the wild progenitor. At maturity the fruits in most Rr plants have a weak connection with their pedicel or the fruit-pedicel abscission zone, a trait already described in this species by Panetsos . This characteristic becomes apparent when mature fruits easily fall off shaky branches of Rr plants. Although some CAwr plants present this trait, it is rare and not as widespread as in Rr plants. The combination of shed or “fall off” in fruits allows mature fruits to escape granivory, procona valencia with subsequent fruit fragmentation and seed dispersal in single seeded capsules on the ground. The study of invasiveness, or the evolution of a species’ ability to succeed demographically and geographically in a novel environment, goes back over a decade . A recent multivariate analysis of traits associated with invasiveness concluded that response to damage by herbivores is a common trait among invasive species . As in native populations, animal-plant interactions are of fundamental importance to the invasion success of plants . Furthermore, the complexity of natural communities intensifies when invasive species are added. For example, invasive Medicago polymorpha affects the annual local Lotus wrangelianus by varying its selection pressures from generation to generation .
In some years, M. polymorpha directly affects L. wrangelianus reducing its fitness, but in other years, the negative effect comes indirectly from increasing the density of the weevil herbivoreHypera brunneipennis . In another recent example from the island of Mauritius, the endemic lizard frugivore Leiolopisma telfairii, ingests the fruits of introduced plants, thereby improving the competitive conditions of local plants by reducing germination time, increasing local plants seedling survival relative to the introduced plants . In Tahiti, seed dispersal networks in heavily invaded areas are influenced by introduced fruit trees and introduced frugivores . Invasive species impact biodiversity and can also have negative economic consequences. Specifically, U.S. agriculture weeds have been reported to impact 12% of the country’s crop production. When the cost of herbicide application is added, the final cost is approximately $27 billion dollars in losses . However, invasive species also offer large-scale natural experiments to study basic processes in populations , proposing hypotheses for understanding population regulation, niche concepts, competition and species coexistence, and community assembly and succession. Why some organisms can successfully invade new habitats is a question that is difficult to answer for a single taxon, and even more challenging to determine at a general level. As is the case for all ecological interactions, invasive species exist in a complex network of interactions, and like most ecological problems, there exists a wealth of hypotheses for answering this deceptively simple question. Hierro et al. provide a summary of the theories pertaining to ecological invasions, and concluded that no single hypothesis can be unambiguously supported. Furthermore, the authors conclude that the most salient weakness of past research is a failure to consider the bio-geography of both original and novel ranges of the focal invasive taxa. This hypothesis has yet to be evaluated, but it is clear that despite steadily increasing research over the last two decades , two fundamental question have yet to be answered: why do some taxa have the capacity to invade new ranges? and what are the predictors of invasion success? The ability to compare a well studied hybrid derived lineage with its two progenitor lineages, with distinct geographical and domestication history, represents a powerful natural experiment to study evolution of plant traits because it is possible to compare the ancestral condition with a new derived state. Likewise, in invasion biology comparing hybrid derived invaders with their progenitor lineages from their native geographic range provides understanding of the evolution of invasive species and the evolution of invasiveness. Among the most experimentally valuable comparisons are progenitor lineages and the derived invasive when grown side by side . In addition, several transcripts involved in ethylene signaling and response were more abundant in symptomatic fruits, suggesting that HLB may have a stronger effect on ethylene signaling than ethylene biosynthesis. The general up-regulation of ethylene-related genes does not agree with the lower ethylene concentration previously reported in HLB symptomatic fruits [28]. This may be due to differences in fruit developmental stages and health status. Furthermore, gene networks responsible for ethylene biosynthesis and perception are still not fully elucidated. A large number of genes annotated as ethylene-related occur as parologs playing tissue-specific roles, and many play additional roles in biotic stress response. Their expression may be drastically affected by the complex gene regulatory network involved in immune responses without directly affecting ethylene levels. The increased number of ERF- and AP2/ EREBP-related genes modulated by HLB supports this notion. These factors control expression of many PR proteins and defense response effectors. In addition, the induction of ethylene biosynthesis and signal transduction could profoundly modify fruit metabolism by accelerating senescence linked to the typical fruit malformations caused by HLB. Salicylic acid and jasmonates are hormones involved in activating defense responses to pests and pathogens. Pathways for both hormones were activated in CaLas-infected citrus. It is known that different hormone-regulated defense pathways are activated depending on whether the pathogen is a necrotroph or biotroph. The mechanisms of CaLas pathogenesis are poorly understood. The putative bacterial pathogen is closely related to bacterial families with symbiotic properties. However, the gene expression in challenged fruit showed an upregulation of jasmonate-induced defense responses, typical of a host response to localized necrotroph invasion. This pathway is also stimulated by long distance signaling involving volatile compounds. Salicylic acid methyl-transferase was up-regulated in early, asymptomatic disease stages, potentially leading to production of volatile methylsalicylate. Nonexpressor of pathogenesisrelated genes1 was up-regulated in symptomatic fruits . In Arabidopsis, NPR1 is required for SA-mediated suppression of JA-dependent defenses. Also, ethylene modulates the NPR1 dependency of SA-JA antagonism, compensating for enhanced allocation of NPR1 to functions in SAdependent activation of PR genes. In plants, IRE1 gene is considered to be involved in unfolded protein response mediated by SAR with the engagement of BiP.