This problem is considered from several perspectives that are described over three chapters. Chapter one concerns seed packaging and addresses three topics: What are the differences in fruit packaging traits such as within-fruit seed distribution, fruit shape, and fruit wall strength? Are there differences in the type and quantity of damage inflicted by the granivore on the fruits? Is the fruit material homogenous across the length of the fruits? Chapter two concerns granivore effects on fecundity by addressing the following topics: What species of bird granivore is the main consumer of radish seeds? How much does the bird granivore affect relative fecundity and relative potential fecundity? Are the variables likely to affect the birds’ selection of individual plants such as days to germination, plant final weight, total fruit production and potential reproduction, correlated with fruit damage? Are there viable seeds in the debris due to granivory resulting from the bird foraging behavior under damaged plants? Chapter three considers within-fruit seed characteristics, and with five major objectives: To compare seed weight among all three Raphanus lineages and their populations; To determine if within-fruit seed positioning influences seed weight, fecundity, and other morphological as well as fitness related characteristics; To compare if single and to mixed hand pollination crosses influence fitness values; To document the occurrence of multiple paternity in all three lineages; To assess maternal and paternal effect on seed weight and other fitness related variables.

Collectively, these studies examine the inter-relationships among fruit mechanical defense, granivory and fitness. The results provide insights into how interspecific hybridization, wholesale plant containers pre-dispersal seed predation defenses and propagule quality determine if a hybrid lineage will become a successful invader that is able to replace its progenitors.Inter and intra-specific hybridization is an important evolutionary force in plant speciation and phenotypic diversification in flowering plants . Successful hybrids display novel combinations of phenotypes that can be selectively advantageous in novel environments compared to progenitor lineages . This same evolutionary force may have significant effects during the invasion process, particularly on the establishment and propagation of invasive species . Relative to both progenitor species, resistance to damage by biotic agents in native hybrids ranges from more susceptible to more resistant . Material properties such as stiffness, extensibility and strength, and structural properties, such as size and shape, determine some of defensive traits that help plants to resist damage . Despite their importance in plant survival and fitness very little is known about the bio-mechanical features underlying invasiveness . The role of the fruit wall as a material and structural defense against pre-dispersal granivory has seldom been explored in the context of plant invasiveness. Seeds are the most vulnerable yet most critical stage of angiosperm plants’ life, and fruits have evolved not only to facilitate their fertilization, maturation and dispersal but also to protect them. For instance, it has been proposed that the initial function of the pulp in fleshy fruits was defensive against seed predators; the pulp has subsequently evolved as a reward to seed dispersers .

In the case of serotinous plants, seeds are stored in the canopy inside protective cones or fruits for a year or more , and the thickness and density of the fruits are correlated to the level of serotiny and protection against granivory and abiotic conditions . As a result, fruit characteristics can be viewed as a set of packaging traits with one function being to provide protection against pre-dispersal biotic conditions. The three Raphanus system has previously been used to study how structural properties, i.e. fruit morphology, has evolved through hybridization. This system consists of the two parental lineages: Raphanus sativus and R. raphanistrum together with their hybrid derived lineage, the highly successful invasive plant named here as California wild radish. The striking variation in fruit structural characteristics among different Raphanus species has being of interest since early 1900’s and it has also been suggested that the material properties, specifically fruit wall strength, may be an additional barrier against seed removal by predators . If true, then the material and/or structural properties of the fruit should also vary among lineages and possibly at different parts of the fruit. Raphanus species have fully indehiscent non-fleshy loments or pod-like fruits that dry out attached to the plant. The fruit develops from the pistil in the flower attached to the plant by a pedicel . In most cases the structure of the fruit narrows into a seedless beak on the stylar end that can account for up to half the total length of the fruit. Recently, Cousens et al. 2010 found that thesegmented fruit wall around single dispersing seeds of Raphanus raphanistrum slows the rate of desiccation for short periods of time, primes germination, and induces dormancy if conditions of moisture prevail for long period of time.

Because fruit structure is one of the genetically based traits that differentiate all three Raphanus lineages , it is possible to perform comparative experiments. Here I address the following questions: what are the differences in the fruit packaging traits of within-fruit seed distribution, fruit shape, and fruit wall strength? are there differences in the type and quantity of damage inflicted by the granivore on the fruits? and are the fruits materially homogenous across the length of the fruits?This study provides evidence for a mechanism that allows a hybrid-derived plant to displace its progenitors and expand its range as an invasive lineage . The innovative combination of structural and material properties of the hybrid California wild radish fruits provides superior protection to its seeds relative to its progenitors. The hybrid has some intermediate characters such as fruit length, beak length, and within fruit seed distribution. Other characters are similar to one or both progenitors, including fruit diameters at specific sections and the positive allometry of fruit length compared to fruit weight. The fruit features that are most different relative to the progenitors are larger volume, greater weight, the strength at the peduncular section, and the tendency to increase the diameter in this area. It is not surprising that the wild lineages have harder fruit walls compared to the domesticated lineages , but the extreme value for fruit wall strength in the penduncular section of the hybrid suggests that hybridization events combined with mechanisms of selection are acting on this section. Uncoupling the effects of these two processes on the structure of the fruit remains to be demonstrated.The parental lineages express divergent values in fruit structure and seed packaging traits. The cultivated Raphanus sativus has easily breakable fruits that contain a large number of seeds, and thus appear to be especially vulnerable to granivores. The wild R. raphanistrum fruits are considerable harder and have uniform fruit wall protection along with internal seed compartments that provide additional shielding for broken fruits. The results presented here indicate that Rr has two additional mechanisms to protect the fruits. The first is the “fell off” mechanism , plastic pot manufacturers which may compensate for lower values in fruit wall strength. The second is the ability to break into individual one-seeded segments offering additional benefits after dispersal from the mother plant . In the case of the hybrid-derived , fruit wall strength is greater particularly at the peduncular section of the fruits, which is also where most double row seeds as well as where the largest seeds dwell in this lineage . The internal structure of the CAwr fruit maintains the seed compartments in most cases providing a similar protection as Rr fruits. Overall, the strength required to break Raphanus fruits is influenced by differences in material properties at the fruit wall among lineages. Our data indicates that the fruit shape alone does not explain the differences among fruit hardness.

Prior studies have reported Raphanus fruit characteristics, mostly based on observations and general measurements. Fruits of Raphanus sativus or cultivated radish have been described as smooth, spongy and corky and at maturity easily crushed whereas the fruits of R. raphanistrum are harder to break and shrink in diameter around the seed compartments when ripeningeventually breaking into individual seed segments . The hybrid fruits are intermediate with hard fruit walls that do not break at maturity, smooth surfaces , and heavier and larger volumes . The present study provides additional detail of the fruit structure and experimentally examines the interaction of the fruit traits and the seed predators. Finch granivory and seed predation has been previously reported in Raphanus plants but never tested. To our knowledge there are no reports on European finches feeding in the wild on R. raphanistrum, but there are numerous granivore species in Europe . In addition, seeds of R. sativus are typically found in commercially manufactured bird food and have been reported as part of the diet in granivore birds in northern European farms . Post-invasion success has received considerable attention with most mechanistic studies being focused on chemical defenses. The evolution of bio-mechanical defense in invasive plants is largely unexplored despite being a genetically based trait but the few available studies indicate its importance. Seeds from roadside populations of introduced rose clover in California were found to retain hairier calyx than populations from old rose clover plantations . This feature is associated with enhanced seed survival. Fruit wall thickness was measured during a common garden experiment at the invaded range between European originated and the North American invasive counterpart of Silene latifolia plants . The European originated fruits appeared to have better protected fruits when seed mass was considered compared to fruits from the invasive range. The influence of invasive plant fleshy fruit characteristics on seed dispersal agents has received more attention although these studies do not focus on fruit defenses . Invasive plants are not exempted of mutualistic and antagonistic interactions, even if the composition and nature of those agents changes at the novel range . Natural or novel enemies drive the evolution of plant characters in the same way native or introduce host plants might drive the enemies characters . Fruit morphology is a fundamental trait in plant evolution . Fruit and/or seeds produced within the same plant that vary in morphology are heteromorphic . The genus Raphanus belongs to the Brassiceae, a tribe in the Brassicaceae family. Fewer than half of the species in the Brassiceae tribe have heteromorphic fruits, in particular heteroarthrocarpic fruits, characterized by having segments . Heteroarthrocarpic fruits, in contrast with non-heteroarthrocarpic ones, do not release their seeds from a particular fruit and/or plant at once into the environment at maturity . Seed dispersal variations depend on different heteroarthrocarpic fruit types, described in detail elsewhere . As a consequence, different proportions of seeds from the same fruits disperse without pericarp and others disperse in units that include entire fruits or sections of the fruits, distal/stylar or proximal/peduncular . Systematic, developmental, and ecological aspects of these characteristics have been studied in different species of Brassiceae . Understanding the genetics of fruit development, morphology and physiology has also been key to improve crop yield and agriculture in general . In natural and agricultural environments fruit traits are essential to protect seeds from both biotic and abiotic factors at pre-dispersal, dispersal and post-dispersal seed stages . The present study supports that fruit structure and morphology is part of plant structural defenses in a hybrid Raphanus lineage. It remains to study to what extend this structural defense influences pre-dispersal seed predation and plant fitness. Also, since the fruit wall hardness varies along the fruit, it would be worthwhile to correlate the fruit wall hardness with within-fruit seed characteristics.These biotic interactions can range from mutualistic to antagonistic and frequently span a broad range of this spectrum . It is therefore plausible that biotic interactions can help to determine the success of an invasive plant with respect to another invasive plant lacking those components. Work in applied bio-control suggests an important role for predispersal seed predation in invasive plants . Differential pre-dispersal seed predation has also been shown to influence the relative production of viable seeds in closely related species . Thus, how plants respond to predation of seeds may be anevolutionary response that increases invasiveness. One key challenge to testing this hypothesis is that successfully invasive plants are only defined as such retroactively. Pre-dispersal seed predators consume seeds from the mother plant and can influence native and invasive plants through: 1) plant community interactions and demography , 2) other animal-plant interactions such as frugivory and 3) the evolution of plant traits .