Natural hybridisation between two coastal endemic species of Armeria (Plumbaginaceae) from Portugal. 2. Ecological investigations on a hybrid zone

A clinal range of morphological variation between two coastal endemic species of the Genus Armeria in Portugal (A. pseudoarmeria and A. welwitschii) was investigated to test the hypothesis of hybrid advantage in contact zones between species. The putative F1 hybrids morphotypes could be submitted to endogenous selection as most of them show deficiencies in pollen fertility and seed setting in comparison with the parental species. Reciprocal transplant experiments in three sectors along the cline revealed that the intermediate F1 morphotypes are less productive in biomass than the parents even in their sector of origin. These data do not show evidence for the Hybrid Bound Superiority Model. Rather they are in favour of the Tension Zone Model where a dynamic balance is obtained between the continuous production of hybrids and the selection against them. Nevertheless the Tension Zone Model postulates similar fitness for the parents along the cline, which is not the case as the parents are clearly differentiated for nutrient uptake. Soil and plant leaf elemental analysis revealed that, in transplantation, some of the hybrids correspond to a geochemical specialisation diverging from that of the parents suggesting a local adaptation. Owing the high frequency and the healthy developmental aspect of the intermediate morphs in central and northern sector selection must be weak against hybrids. It is concluded that in natural situations the two models could not be strictly exclusive.