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Cottonwood hybrids gain fitness traits of both parents: a mechanism for theirlong-term persistence?¹

Department of Biological Sciences and the Merriam-Powell Center for Environmental Research, Northern Arizona University, Flagstaff, Arizona 86011 USA

Using surveys of natural populations, experimental crosses, and common garden trials, we tested the hypothesis that hybrid cottonwoods (Populus fremontii x P. angustifolia) from the Weber River in northern Utah would produce as many viable offspring as their parental species. We found that both F₁ generations and backcross generations can be just as fit as the parent taxa. First, F₁ hybrids produced as many viable seed as P. angustifolia (but less than P. fremontii), and backcross genotypes produced as many viable seeds as both parent taxa. Second, hybrids produced nearly two times as many ramets from root sprouts as P. angustifolia and four times as many ramets as P. fremontii. Third, the high mortality of germinated seedlings of all tree types (i.e., >90%) and very low mortality of asexually derived ramets provide hybrids with equal sexual reproduction and enhanced asexual reproduction, especially since backcross hybrids exhibit transgressive segregation in ramet production. Our findings suggest that the introgression of P. fremontii seed traits into the hybrid genome is responsible for their equivalent performance (at least to one parent) in sexual reproduction, while the contributions of asexual traits from P. angustifola results in hybrids having equal or greater fitness.

Key Words: hybrid fitness • introgression • Populus • ramets • seed viability • sexual and asexual reproduction • transgressive segregation

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