The genus Quercus is known for the occurrence of frequent hybridization events between species. Although this phenomenon is not common among holm oak (Q. ilex) and cork oak (Q. suber), these species can hybridize when they coexist in mixed stands. The result of hybridization is a viable hybrid progeny with very heterogeneous leaf morphology. Literature concerning the leaf morphology of suber-ilex hybrid seedlings is scarce, and non-existent from a quantitative point of view. In the case of the leaf morphology of hybrids and their progeny, it has been observed a high frequency of leaves with fluctuating asymmetry or developmental abnormalities, which can have a marked effect on fitness. In this work, we have characterized seedlings' leaf morphology corresponding to two- and four-year-old half-sib progenies of holm oak, cork oak and their hybrids. For this purpose, three to ten leaves of each individual were collected, and two methodologies were used for analysis. Firstly, we used a classic morphological analysis of twelve variables that were reduced using multivariate techniques. On the other hand, shape of the leaves was thoroughly analyzed by geometric morphometric analysis methods. The extent of fluctuating asymmetry and the presence of developmental abnormalities of seedlings were analyzed calculating an asymmetry index. The results indicate that thickness is the most discriminating trait between species, and that the hybrid progenies do not show a third different phenotype compared to the parental species. However, half-siblings tend to show similar leaf morphology between them, depending on the genetic adscription of the parents. While fluctuating asymmetry was found in half-sib progenies of the parental species and the hybrids, a significant proportion of hybrid half-sibs showed strong leaf asymmetry, probably due to modifications of the epigenetic systems that control leaf development at the shoot apical meristems and leaf primordia.
