Kohn Joshua R, Barrett Spencer C H
University of California at San Diego, Department of Biology 0116, La Jolla, California, 92093.
Department of Botany, University of Toronto, Toronto, Ontario, M5S 3B2, Canada.
Evolution. 1994 Oct;48(5):1576-1594. doi: 10.1111/j.1558-5646.1994.tb02197.x.
Floral traits that increase self-fertilization are expected to spread unless countered by the effects of inbreeding depression, pollen discounting (reduced outcross pollen success by individuals with increased rates of self-fertilization), or both. Few studies have attempted to measure pollen discounting because to do so requires estimating the male outcrossing success of plants that differ in selfing rate. In natural populations of tristylous Eichhornia paniculata, selfing variants of the mid-styled morph are usually absent from populations containing all three style morphs but often predominate in nontrimorphic populations. We used experimental garden populations of genetically marked plants to investigate whether the effects of population morph structure on relative gamete transmission by unmodified (M) and selfing variants (M') of the mid-styled morph could explain their observed distribution. Transmission through ovules and self and outcross pollen by plants of the M and M' morphs were compared under trimorphic, dimorphic (S morph absent), and monomorphic (L and S morphs absent) population structures. Neither population structure nor floral morphology affected female reproductive success, but both had strong effects on the relative transmission of male gametes. The frequency of self-fertilization in the M' morph was consistently higher than that of the M morph under all morph structures, and the frequency of self-fertilization by both morphs increased as morph diversity of experimental populations declined. In trimorphic populations, total transmission by the M and M' morphs did not differ. The small, nonsignificant increase in selfing by the M' relative to the M morph was balanced by decreased outcross siring success, particularly on the S morph. In populations lacking the S morph, male gamete transmission by the M' morph was approximately 1.5 times greater than that by the M morph because of both increased selfing and increased success through outcross pollen donation. Therefore, gamete transmission strongly favored the M' morph only in the absence of the S morph, a result consistent with the distribution of the M' morph in nature. This study indicates that floral traits that alter the selfing rate can have large and context-dependent influences on outcross pollen donation.
除非受到近亲繁殖衰退、花粉折扣（自交率增加的个体异交花粉成功率降低）或两者的影响，否则增加自花受精的花部性状预计会传播开来。很少有研究尝试测量花粉折扣，因为这样做需要估计自交率不同的植物的雄性异交成功率。在三型花柱的凤眼蓝自然种群中，中型花柱形态的自交变体在包含所有三种花柱形态的种群中通常不存在，但在非三型种群中往往占主导地位。我们利用遗传标记植物的实验园种群来研究种群形态结构对中型花柱形态的未修饰（M）和自交变体（M'）相对配子传递的影响是否可以解释它们观察到的分布。在三型、二型（无S形态）和单型（无L和S形态）种群结构下，比较了M和M'形态植物通过胚珠以及自花和异花花粉的传递情况。种群结构和花部形态都没有影响雌性繁殖成功率，但两者对雄配子的相对传递都有很大影响。在所有形态结构下，M'形态的自花受精频率始终高于M形态，并且随着实验种群形态多样性的下降，两种形态的自花受精频率都增加。在三型种群中，M和M'形态的总传递没有差异。M'形态相对于M形态自交的小幅、不显著增加被异交授精成功率的降低所平衡，特别是对S形态而言。在缺乏S形态的种群中，由于自交增加和通过异花花粉捐赠获得的成功率提高，M'形态的雄配子传递大约是M形态的1.5倍。因此，配子传递仅在没有S形态时强烈有利于M'形态，这一结果与M'形态在自然界中的分布一致。这项研究表明，改变自交率的花部性状可以对异花花粉捐赠产生巨大且依赖于环境的影响。
