Regarding the continuing controversy on the hybrid origin of the cork oaks producing nail-type cork, which is an essential issue in the recurring discussion over the cork-holm oak mixed groves in Portugal, and to obtain credible estimates of the hybridization rates in these groves, genetic criteria for the discrimination between both species and the hybrids were established. The present work is an account of the following progress in this research:
I. Discovery and validation of isoenzyme markers for distinguishing cork oak and hybrids or their descendants;
II. Confirmation of the hybrid status of trees known as “cerqueiro” oaks, using genetic markers ;
III. Estimation of the hybridization rate at a very low value (less than 0.1% ) in the mixed groves studied;
IV. Detection in cork as well as holm oak, of introgressed markers from the other species;
V. Preliminary analysis of nail-type cork-producing oaks;
VI. Preliminary analysis of the progenies from hybrids, including the demonstration that pollen grains of either parental species are involved, as well as a relatively high rate of apparent selfings;
VII. Preliminary analysis of the allelic frequencies in two marker loci, one in each species, and computation of a relatively low neutral estimate of Ne (between 5 and 7);
VIII. Illustration of the same methodology with samples from other species of Quercus and related taxa.
Although an authoritative value for the rate of hybridization between cork and holm oak is yet to be obtained, the current estimates indicate that generally it is a rather exceptional event, but a relatively high local occurrence is admissible.
The developed methodology can be applied on a large scale for the detection of hybrids in propagation materials for cork oak, with relatively low cost, and it can contribute for an elucidation of various questions on the reproduction biology of both species and the hybrids between them, in line with the preliminary studies included in the present work.