LEAF FLUCTUATING ASYMMETRY INCREASES WITH HYBRIDIZATION AND INTROGRESSION BETWEEN QUERCUS MAGNOLIIFOLIA AND QUERCUS RESINOSA (FAGACEAE) THROUGH AN ALTITUDINAL GRADIENT IN MEXICO Ana Luisa Albarra ´n-Lara, 1, * Luis Mendoza-Cuenca, y Susana Valencia-Avalos, z Antonio Gonza ´lez-Rodrı ´guez,* and Ken Oyama* *Centro de Investigaciones en Ecosistemas, Universidad Nacional Auto ´noma de Me ´xico, Antigua Carretera a Pa ´tzcuaro Numero 8701, Col. Ex-Hacienda de San Jose ´ de la Huerta, Morelia, 58190 Michoaca ´n, Mexico; y Facultad de Biologı ´a, Universidad Michoacana de San Nicola ´s de Hidalgo, Francisco J. Mu ´gica, Colonia Felı ´citas del Rı ´o, Morelia, 58030 Michoaca ´n, Mexico; and zDepartamento de Biologı ´a, Facultad de Ciencias, Universidad Nacional Auto ´noma de Me ´xico, Me ´xico, D.F. 04510, Mexico We tested the effects of hybridization and introgression on the levels of leaf fluctuating asymmetry (FA) in a hybrid zone between Quercus magnoliifolia and Quercus resinosa at the Tequila volcano, Jalisco state, Mexico, in which the species are distributed along an altitudinal gradient ranging from 1400 to 2100 m. Bayesian clustering analysis was performed with STRUCTURE on data for eight nuclear microsatellite loci to assign individuals from reference populations and from the hybrid zone to pure or hybrid genotypic classes. To test the performance of the assignment procedure and to determine optimal thresholds for genetic assignment pure, hybrid and backcrossed genotypes were simulated (from the allelic frequencies found in real pure populations of the two species) and reanalyzed with STRUCTURE. Leaf FA and morphological identification of parental and hybrid individuals were obtained using geometric morphometric techniques. We found genetic and morphological evidence of a hybrid zone by secondary contact at the Tequila volcano. The genotypes and phenotypes were significantly correlated with altitude along the mountain, but no correlation between them was found. FA was higher in F 1 hybrids and backcrossed genotypes than in pure species. Levels of FA were more correlated with the proportion of genetic ancestry of each individual than with leaf morphology or altitude, supporting the hypothesis that hybridization is associated with development instability potentially caused by disruption of coadapted gene complexes characteristic of each species. Keywords: genetic assignment, hybridization, introgression, leaf fluctuating asymmetry, Quercus. Introduction Developmental instability arises from genetic or environmen- tal stressors that disturb the normal developmental pathways of different continuous characters, producing developmental noise, which is commonly measured as fluctuating asymmetry (FA) in phenotypic traits (Van Valen 1962; Palmer and Stro- beck 1986; Leamy and Klingenberg 2005). FA is the variance in subtle differences between the left and the right sides in bilaterally symmetrical organisms or parts of them, and it pro- vides a measure of how well an individual can buffer its devel- opment against internal genetic and external environmental stress during ontogeny (Van Valen 1962; Palmer 1996). Hybridization is one of the biological factors underlying the changes in levels of FA in plants (Siikama ¨ki and Lammi 1998; Wilsey et al. 1998). Two closely related hypotheses have been proposed to explain the levels of FA in hybrids. The first one is that FA is reduced in hybrids because of the increase in genetic heterozygosity (Soule ´ 1979). The second hypothesis is that FA increases by disruption of coadapted gene complexes in hybrids (Soule ´ 1967; Levin 1970; Graham and Felley 1985; Wilsey et al. 1998; Hochwender and Fritz 1999; Siikama ¨ki 1999). It seems that increments of FA in hy- brids depend on how closely related the parents of the hybrids are; intraspecific hybrids (closely related parents) present lower levels of FA than do hybrids of interspecific species (dis- tantly related parents; Markow 1995). Hybridization is a frequent and important natural process involved in plant evolution and speciation, with at least 25% of plant species engaged in hybridization and potential intro- gression events with other species (Stebbins 1959; Grant 1981). Hybrids and hybrid zones entail the breeding between genetically distinct groups or taxa, resulting in offspring of mixed ancestry (Barton and Hewitt 1985; Arnold 1997). Hy- brid zone structure is influenced by dispersal ability of parental and hybrid individuals and by both environment-independent (‘‘endogenous’’) and environment-dependent (‘‘exogenous’’) se- lection (Barton and Hewitt 1985; Arnold 1997; Howard et al. 2003). Endogenous selection against hybrids results from geno- mic incompatibilities due to the disruption of parental co- adapted gene complexes and/or the interaction between genes that have diverged functionally between the species, giving rise 1 Author for correspondence; e-mail: aalbarran@oikos.unam.mx. Manuscript received September 2009; revised manuscript received November 2009. 310 Int. J. Plant Sci. 171(3):310–322. 2010. Ó 2010 by The University of Chicago. All rights reserved. 1058-5893/2010/17103-0009$15.00 DOI: 10.1086/650317