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.

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The impacts of the historical geologic and climatic events on the diversity and genetic structure of Neotropical taxa have recently become a subject of study. However, annual plants associated with tropical dry forests remain under‐studied. The exploration of additional taxa in contrasting environments will improve the current understanding of responses of the Neotropical biota to these events. Here, we explore the species distribution and geographic structure of the annual herb Tithonia rotundifolia. We sampled 175 individuals from 19 populations of T. rotundifolia. Species distribution modelling and six microsatellite chloroplast loci were used to infer its population history. We identified areas of historical climate suitability and then tested if there is genetic structuring among these areas. Haplotypes showed strong phylogeographic structure. Historical climatic suitability areas were found along the Pacific coast; however, a gap was found at the Isthmus of Tehuantepec (IT). Although Bayesian analysis showed population structuring, amova revealed that the IT is not its main driver. Instead, a subdivision into a higher number of regions had higher FCT values. Also, populations to the east of the IT showed evidence of recent population expansion and migration in a south–north direction. Pleistocene climate fluctuations partially explain the geographic structure of T. rotundifolia. However, life‐history characteristics such as limited seed dispersal and the patchy distribution of suitable habitats explain the high haplotype diversity and population sub‐structuring and diversity. Lastly, the absence of geographic structure of some haplotypes may indicate long‐distance dispersal, or hybridisation with the closely related T. tubaeformis.

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Background: Hybridization leads to individuals with atypical phenotypes, leading to taxonomic confusion when hybridizing populations are studied. Micromorphological characters may enable taxonomists to discriminate between parental and hybrid categories due to their simple genetic inheritance. Species study: Three oak taxa distributed in the montane regions of the Mexico were studied: Quercus crassifolia, Q. crassipes and their hybrid Q. × dysophylla. Hypothesis: We describe the leaf micromorphological and macromorphological variation of these taxa. Specifically, we searched for a unique combination of micromorphological characters in hybrids. We hypothesize that spatial micromorphological variation will match the spatial pattern revealed by a previous genetic study. Study sites: Two allopatric stands for each parental species and seven hybrid zones were studied. Individuals within each stand were identified as belonging to parental or hybrid categories following previous genetic analyses. Methods: Stomata and trichome types for each taxa were determined through Scanning Electron Microscopy. Eight micromorphological characters for trichomes and stomata and four foliar macromorphological characters were measured. Results: We found the presence of both multirradiate and simple stellate non-glandular trichomes as a diagnostic feature of Q. × dysophylla. Overall populations, Q. × dysophylla showed intermediate phenotypes in 70 % of morphological characters. However, hybrid phenotype exhibited geographical variation. Lastly, spatial hybrid's phenotype variation did not correlate with the spatial genetic pattern previously identified. Conclusions:The micromorphological features of Q. × dysophylla may enable taxonomists to accurately discriminate between this taxon and its parental species. Finally, we suggest that micromorphological expression of both parental species and hybrids may be influenced by environmental gradients and microclimates. Keywords: hybrid phenotype, morphometrics, stomata, trichome. ResumenAntecedentes: La hibridación resulta en individuos con morfologías atípica, lo cual lleva a confusiones taxonómica al estudiar poblaciones en las que ocurre hibridación. Los caracteres micromorfológicos pueden ser útiles para diferenciar entre individuos híbridos y parentales. Especies de estudio: Se estudiaron tres taxones de encinos: Quercus crassifolia, Q. crassipes y Q. × dysophylla, el híbrido de estas especies. Hipótesis: Se describe la expresión de caracteres micromorfológicos y macromorfológicos de estos taxones. En particular, se buscó una combinación única de caracteres micromorfológicos en los híbridos. Asimismo, esperamos que la variación espacial fenotípica sea similar a la encontrada en un estudio genético previo. Sitio de estudio: Se estudiaron dos poblaciones alopátridas para cada especie parental y siete zonas híbridas. Los individuos fueron considerados como parentales o híbridos de acuerdo al estudio genético previo. Métodos: Los tipos de estomas y tricomas para cada taxón fueron ident...

Background: Morphological variability can lead to serious taxonomic problems in species with wide distribution ranges. Although morphological variability is partly due to ontogenetic programming, abiotic variables can also exert a significant effect on micro-and macromorphological characters. In this paper, we studied the morphological variability (43 characters) of Cosmos bipinnatus associated to different vegetation types in central Mexico. We searched for significant correlations between the overall morphology of C. bipinnatus and abiotic variables such as altitude and soil parameters (pH, organic matter content, NH 4 , NO 3 , PO 4 , total N and total P content). We also analyzed the Simplified Relative Distance Plasticity Index (RDPI s ). Results: Locality had a significant effect in all but three morphological characters measured. Also, 71.43 % of the characters had a significant correlation with at least one abiotic variable. PO 4 content was significantly correlated with paleae characters, while pH had a significant effect in ligule coloration. Discriminant function analysis revealed that C. bipinnatus individuals collected at grasslands and Pinus forests form separate clusters, while individuals collected at scrubs and Quercus forests showed considerable overlap. The RDPI s across all sites showed very low levels of plasticity in almost all characters. Conclusions: Some abiotic variables (altitude, soil NH 4 and PO 4 content, and soil pH) largely contribute to the differential phenotypic expression of C. bipinnatus in central Mexico. However, we found that the number of external phyllaries, the trichome length, and the petiole area can be considered diagnostic traits of C. bipinnatus as they did not show differences within and between collected sites. We hypothesize that the low levels of plasticity found in C. bipinnatus across sites is due to the high tolerance of the species to different environmental conditions.