Heterosis and hybrid breakdown in the Dimorphandra hybrid zone in a savanna/forest ecotone

The fitness of hybrids in comparison with their parents determines the fate of hybrid zones. Here, we evaluated the fitness of the parental tree species, Dimorphandra exaltata and D. mollis, and their hybrids, identified as D. wilsonii, in a hybrid zone from an ecotonal area between the Cerrado (savanna) and Atlantic Forest. We aimed to answer two principal questions: Do the hybrids express heterosis or hybrid breakdown? Do patterns of seed germination of hybrids suggest local adaptation or intrinsic hybrid incompatibilities? We analysed biometry of fruit and seed traits and germination response to a wide temperature range (12-40 °C) to assess the fitness of F1 and backcross compared to parental species. We identified heterosis in F1 hybrids, since their fruits were almost a third larger, and their seeds heavier than both parental species. The backcross hybrid showed a loss of vigour in fruit traits compared to F1. A much lower number of filled seeds per fruit, with a high ratio of malformed seeds/total seeds per fruit and a lower germination in almost all temperatures was found in the backcross compared to parental species and F1. These results suggest a hybrid breakdown. The heterosis found in the F1 hybrids coupled with the hybrid breakdown in backcross suggests that the maintenance of hybrid zone is dependent on the recurrent crossing between D. exaltata and D. mollis. The rarity of advanced generations of hybrids can be explained at least partially by intrinsic incompatibility, independent of the environment, consistent with intrinsic post-zygotic reproductive isolation.