Department of Biological Sciences and the Merriam-Powell Center for Environmental Research, Northern Arizona University, Flagstaff, Arizona 86011 USA.
Am J Bot. 2002 Jun;89(6):981-90. doi: 10.3732/ajb.89.6.981.
Using surveys of natural populations, experimental crosses, and common garden trials, we tested the hypothesis that hybrid cottonwoods (Populus fremontii × P. angustifolia) from the Weber River in northern Utah would produce as many viable offspring as their parental species. We found that both F(1) generations and backcross generations can be just as fit as the parent taxa. First, F(1) 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.
利用自然种群调查、实验杂交和共同花园试验，我们检验了这样一个假设，即来自犹他州北部韦伯河的杂交棉白杨（Populus fremontii×P. angustifolia）将与亲本种产生同样多的有活力的后代。我们发现，无论是 F1 代还是回交代，其适应性都与亲本类群相当。首先，F1 杂种产生的有活力种子与 P. angustifolia 一样多（但比 P. fremontii 少），回交基因型产生的有活力种子与双亲种一样多。其次，杂种从根蘖中产生的无性繁殖体几乎是 P. angustifolia 的两倍，是 P. fremontii 的四倍。第三，所有树种（即>90%）的萌发幼苗死亡率很高，而无性繁殖体的死亡率非常低，这为杂种提供了平等的有性繁殖和增强的无性繁殖，特别是因为回交杂种在繁殖体产生方面表现出超越性分离。我们的研究结果表明，P. fremontii 种子特征的基因渗入杂种基因组是其有性生殖中表现相当（至少与一个亲本相当）的原因，而 P. angustifola 的无性特征的贡献导致杂种具有同等或更高的适应性。
