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Episodic Hybrid Advantage Keeps Species Boundaries Low in Oaks

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Warren B. Chatwin, Mark V. Coggeshall and Jeanne Romero-Severson

Published May 2019 in International Oaks No. 30: 139–144


Permeable species boundaries are common within the order Fagales, suggesting that hybridization has adaptive value. We hypothesize that some hybrid individuals have novel allelic combinations that may enable survival during severe abiotic stress. We also propose that this effect is strongest during the long juvenile period of the life cycle. Our studied species pairs are the sympatric Quercus macrocarpaQ. bicolor and the allopatric Q. roburQ. bicolor. Hybrids were the naturally pollinated progeny of Q. ×warei (Q. robur × bicolor) and Q. ×schuettei (Q. bicolor × macrocarpa). At age two, 500 parentage-known progeny of each hybrid type and 80-100 seedlings of each parent species were moved into a greenhouse. Half of the seedlings were subjected to four consecutive weeks of intense drought, while the other half were well watered. Two leaves per individual were saved for sequencing. Our study showed that the Q. ×warei hybrid seedlings showed higher survival rates than either of its parent species, and the Q. ×schuettei hybrid seedlings showed survival rates closer to the drought-tolerant parent species Q. macrocarpa. This result supports our hypothesis that cyclical abiotic stress events, such as drought, can favor survival of hybrid seedlings. This helps to explain why there is a consistent, but low-level, presence of hybrids in all mixed-species stands of oaks.


Quercus, hybridization, drought, seedling, episodic advantage


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