Hybrid zones provide interesting systems to study genetic and ecological interaction between different species. The correct identification of hybrids is necessary to understand the evolutionary process involved in hybridization. An oak species complex occurring in Mexico formed by two parental species, Quercus crassifolia H. & B. and Q. crassipes H. & B., and their putative hybrid species, Q. dysophylla, was analyzed with molecular markers (random amplified polymorphic DNA [RAPDs]) and morphological tools in seven hybrid zones (10 trees per taxa in each hybrid zone) and two pure sites for each parental species (20 trees per site). We tested whether geographic proximity of hybrid plants to the allopatric site of a parental species increases its morphological and genetic similarity with its parent. Seventeen morphological traits were measured in 8700 leaves from 290 trees. Total DNA of 250 individuals was analyzed with six diagnostic RAPD primers. Quercus crassifolia differed significantly from Q. crassipes in all the examined characters. Molecular markers and morphological characters were highly coincident and support the hypothesis of hybridization in this complex, although both species remain distinct in mixed stands. Clusters and a hybrid index (for molecular and morphological data) showed that individuals from the same parental species were more similar among themselves than to putative hybrids, indicating occasional hybridization with segregation in hybrid types or backcrossing to parents. Evidence does not indicate a unidirectional pattern of gene flow.

Natural hybridization is a frequent phenomenon among vascular plants. Hybridization is considered an important evolutionary force since it may lead to (1) an increase of the intraspecific genetic diversity of the participating populations, (2) the creation of new species, (3) species extinction through genetic assimilation, and (4) the generation of highly invasive genotypes. Because of the importance of plant hybridization in evolution, it is of great importance to accurately identify hybrid individuals. In this review, we give a general historical background of the study of plant hybridization. Also, we review some of the tools employed for hybrid recognition and their pattern of expression in hybrid individuals (morphological, chemical, chromosome number, and DNA fingerprinting techniques). We emphasize that even when chromosome number, morphological characters, and chemical characters are of limited use for hybrid recognition in the absence of DNA fingerprinting techniques, their exploration may give insights of the ecological performance of hybrids. This is of special importance when hybridization leads to evolutionary novelty in the form of polyploidy, transgressive character expression, or the expression of new secondary metabolites not present in the parental species.

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.

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