Utilization of hybrid oak hosts by a monophagous gall wasp: How little host character is sufficient?

Effect of hybridization of the Quercus crassifolia x Quercus crassipes complex on the community structure of endophagous insects

In a previous study, we showed that the geographic proximity of hybrid plants to the allopatric areas of parental species increases their morphological and genetic similarity with them. In the present work, we explored whether the endophagous fauna of hybrid plants show the same pattern. We studied the canopy species richness, diversity and composition of leaf-mining moths (Lepidoptera: Tischeridae, Citheraniidae) and gall-forming wasps (Hymenoptera: Cynipidae) associated with two species of red oaks (Quercus crassifolia and Quercus crassipes) and their interspecific hybrid (Quercusxdysophylla Benth pro sp.) in seven hybrid zones in central Mexico, during four seasons in 2 years. The study was conducted on 194 oak trees with known genetic status [identified by leaf morphology and molecular markers (random amplified polymorphic DNAs)], and the results indicate a bidirectional pattern of gene flow. Hybrid plants supported intermediate levels of infestation of gall-forming and leaf-mining insects compared to their putative parental species. The infestation level of leaf-mining insects varied significantly following the pattern: Q. crassifolia>hybrids>Q. crassipes, whereas the gall-forming insects showed an inverse pattern. A negative and significant relationship was found between these two types of insect guilds in each host taxa, when the infestation percentage was evaluated. It was found that 31.5% (n=11) of the endophagous insects were specific to Q. crassipes, 22.9% (n=8) to Q. crassifolia, and 8.6% (n=3) to hybrid individuals. The hybrid bridge hypothesis was supported in the case of 25.7% (n=9) of insects, which suggests that the presence of a hybrid intermediary plant may favor a host herbivore shift from one plant species to another. Greater genetic diversity in a hybrid zone is associated with greater diversity in the endophagous community. The geographic proximity of hybrid plants to the allopatric site of a parental species increases their similarity in terms of endophagous insects and the Eje Neovolcánico acts as a corridor favoring this pattern.

Genetic architecture of susceptibility to herbivores in hybrid willows

We performed a common garden experiment using parental, F1, F2, and backcross willow hybrids to test the hypothesis that hybrid willows experience breakdown of resistance to herbivores. After exposing plants to herbivores in the field, we measured the densities/damage caused by 13 insect herbivores and one herbivorous mite. Using joint-scaling tests, we determined the contribution of additive, dominance, and epistasis to variation in susceptibility to herbivores (measured either as density or damage level) among the six genetic classes. We found the genetic architecture of susceptibility/resistance in the parental species to be complex, involving additive, dominance, and epistasis for each herbivore species. Although genic interactions altered plant susceptibility for each of the 14 herbivores, three distinct patterns of response of herbivores to hybrids were expressed. One pattern, observed in four herbivore species, supported the hypothesis of breakdown of resistance genes in recombinant hybrids. A second pattern, shown by six other herbivore species, supported the hypothesis of hybrid breakdown of host recognition genes. In other words, epistatic interactions for host recognition traits (probably oviposition/feeding stimulants or attractants) appeared to be important in determining herbivore abundance for those six species. The final patterns supported a structure of dominance, either for host recognition traits (in the case of three herbivore species) or for host resistance traits (for one herbivore species). The combination of differing responses of herbivore species, including members of the same genus and tribe, and the ubiquitous importance of epistasis suggests that many genes affect herbivore resistance in this hybrid willow system.

The population biology of oak gall wasps (Hymenoptera: Cynipidae)

Oak gall wasps (Hymenoptera: Cynipidae, Cynipini) are characterized by possession of complex cyclically parthenogenetic life cycles and the ability to induce a wide diversity of highly complex species- and generation-specific galls on oaks and other Fagaceae. The galls support species-rich, closed communities of inquilines and parasitoids that have become a model system in community ecology. We review recent advances in the ecology of oak cynipids, with particular emphasis on life cycle characteristics and the dynamics of the interactions between host plants, gall wasps, and natural enemies. We assess the importance of gall traits in structuring oak cynipid communities and summarize the evidence for bottom-up and top-down effects across trophic levels. We identify major unanswered questions and suggest approaches for the future.

Resistance of Hybrid Plants and Animals to Herbivores, Pathogens, and Parasites

▪ Abstract  Interspecific hybridization can disrupt normal resistance of plant and animal species to their parasites. Resistance to parasites is affected by hybridization in the following ways: no difference between hybrids and parentals, additivity, hybrid susceptibility, and dominance to susceptibility. Similar patterns were seen across host taxa. Responses of different parasite species vary widely to the same hybrid host, which indicates diverse genetic effects of interspecific hybridization on resistance. Differences between field and common garden or laboratory studies suggest that environmental factors in hybrid zones influence the patterns seen in the field. Based on recent studies of hybrid-parasite interactions, three avenues of future research will provide a more complete understanding of the roles of hybrids and the roles of parasites in host evolution. First, the relationship between inheritance of putative resistance mechanisms of hosts and responses of parasites needs study using analyses of recombinant progenies. Second, the interaction among environmental variation in hybrid zones, resistance mechanisms, responses of parasites, and the impact of parasites on host fitness needs experimental analysis using reciprocal transplant experiments in hybrid zones. Finally, the role of hybrids in the community structure and interactions of parasites needs study.