Ramsey Mike, Vaughton Glenda, Peakall Rod
Botany, University of New England, Armidale, New South Wales, Australia.
Evolution. 2006 Mar;60(3):529-37.
How females establish in populations of cosexuals is central to understanding the evolution of gender dimorphism in angiosperms. Inbreeding avoidance hypotheses propose that females can establish and be maintained if cosexual fitness is reduced because they self-fertilize, and their progeny express inbreeding depression. Here we assess the role of inbreeding avoidance in maintaining sexual system variation in Wurmbea biglandulosa. We estimated costs of self-pollination, mating patterns, and inbreeding depression in gender monomorphic (cosexuals only) and dimorphic (males and females) populations. Costs of selfing, estimated from seed set of experimentally self- and cross-pollinated flowers, were severe in both males and cosexuals (inbreeding depression, sigma = 0.86). In a field experiment, intact males that could self produced fewer seeds than both emasculated males and females, whereas seed set of intact and emasculated cosexuals did not differ. Thus, pollinator-mediated selfing reduces fitness of males but not cosexuals under natural conditions. Outcrossing rates of males revealed substantial selfing (t = 0.68), whereas females and cosexuals were outcrossed (0.92 and 0.97). For males, progeny inbreeding coefficients exceeded parental coefficients (0.220 vs. 0.009), whereas for females and cosexuals these coefficients did not differ and approached zero. Differences in coefficients between males and their progeny indicate that selfed progeny express severe inbreeding depression (sigma = 0.93). Combined with inbreeding depression for seed set, cumulative sigma = 0.99, indicating that most or all selfed zygotes fail to reach reproductive maturity. We propose that present sexual system variation in W. biglandulosa is maintained by high inbreeding depression coupled with differences in selfing rates among monomorphic and dimorphic populations.
雌性个体如何在两性同体的种群中得以确立，是理解被子植物性别二态性进化的核心问题。避免近亲繁殖假说认为，如果两性同体的适合度因自体受精而降低，且其后代表现出近亲繁殖衰退，那么雌性个体就能够得以确立并维持下去。在此，我们评估了避免近亲繁殖在维持大腺毛水麦冬性系统变异中的作用。我们估计了性别单态（仅两性同体）和双态（雄性和雌性）种群中自花授粉的代价、交配模式以及近亲繁殖衰退情况。通过对实验性自花授粉和异花授粉花朵的结实率估算，自交的代价在雄性和两性同体中都很严重（近亲繁殖衰退，σ = 0.86）。在一项田间实验中，能够自交的完整雄性植株产生的种子比去雄的雄性植株和雌性植株都少，而完整的和去雄的两性同体植株的结实率没有差异。因此，在自然条件下，传粉者介导的自交降低了雄性的适合度，但对两性同体没有影响。雄性的异交率显示出大量的自交现象（t = 0.68），而雌性和两性同体则是异交（分别为0.92和0.97）。对于雄性来说，子代的近亲繁殖系数超过了亲本系数（0.220对0.009），而对于雌性和两性同体，这些系数没有差异且接近零。雄性与其子代之间系数的差异表明，自交子代表现出严重的近亲繁殖衰退（σ = 0.93）。结合种子结实的近亲繁殖衰退，累积的σ = 0.99，这表明大多数或所有自交合子未能达到生殖成熟。我们提出，大腺毛水麦冬目前的性系统变异是由高近亲繁殖衰退以及单态和双态种群之间自交率的差异所维持的。
