Goodwillie Carol, Knight Mary Catherine
Department of Biology, East Carolina University, Howell Science Complex, Greenville, NC 27858, USA.
Ann Bot. 2006 Aug;98(2):351-60. doi: 10.1093/aob/mcl105. Epub 2006 May 23.
Inbreeding depression is thought to play a central role in the evolution and maintenance of cross-fertilization. Theory indicates that inbreeding depression can be purged with self-fertilization, resulting in positive feedback for the selection of selfing. Variation among populations of Leptosiphon jepsonii in the timing and rate of self-fertilization provides an opportunity to study the evolution of inbreeding depression and mating systems. In addition, the hypothesis that differences in inbreeding depression for male and female fitness can stabilize mixed mating in L. jepsonii is tested.
In a growth room experiment, inbreeding depression was measured in three populations with mean outcrossing rates ranging from 0.06 to 0.69. The performance of selfed and outcrossed progeny is compared at five life history stages. To distinguish between self-incompatibility and early inbreeding depression, aborted seeds and unfertilized ovules were counted in selfed and outcrossed fruits. In one population, pollen and ovule production was quantified to estimate inbreeding depression for male and female fitness.
Both prezygotic barriers and inbreeding depression limited self seed set in the most outcrossing population. Cumulative inbreeding depression ranged from 0.297 to 0.501, with the lowest value found in the most selfing population. Significant inbreeding depression for early life stages was found only in the more outcrossing populations. Inbreeding depression was not significant for pollen or ovule production.
The results provide modest support for the hypothesized relationship between inbreeding depression and mating systems. The absence of early inbreeding depression in the more selfing populations is consistent with theory on purging. Differences in male and female expression of inbreeding depression do not appear to stabilize mixed mating in L. jepsonii. The current estimates of inbreeding depression for L. jepsonii differ from those of previous studies, underscoring the effects of environmental variation on its expression.
近亲繁殖衰退被认为在异花受精的进化和维持中起核心作用。理论表明，近亲繁殖衰退可通过自花受精得以消除，从而为自交选择产生正反馈。 Jepson 薄果草种群在自花受精的时间和速率上存在差异，这为研究近亲繁殖衰退和交配系统的进化提供了契机。此外，还对 Jepson 薄果草中雄性和雌性适合度的近亲繁殖衰退差异能否稳定混合交配这一假说进行了检验。
在生长室实验中，对三个平均异交率在 0.06 至 0.69 之间的种群的近亲繁殖衰退进行了测量。在五个生活史阶段比较了自交和异交后代的表现。为区分自交不亲和性和早期近亲繁殖衰退，对自交和异交果实中的败育种子和未受精胚珠进行了计数。在一个种群中，对花粉和胚珠产量进行了量化，以估计雄性和雌性适合度的近亲繁殖衰退。
在异交程度最高的种群中，合子前障碍和近亲繁殖衰退均限制了自交种子的结实率。累积近亲繁殖衰退范围为 0.297 至 0.501，在自交程度最高的种群中发现了最低值。仅在异交程度较高的种群中发现了早期生活阶段显著的近亲繁殖衰退。花粉或胚珠产量的近亲繁殖衰退不显著。
这些结果为近亲繁殖衰退与交配系统之间的假设关系提供了一定支持。自交程度较高的种群中不存在早期近亲繁殖衰退，这与清除理论一致。近亲繁殖衰退在雄性和雌性中的表达差异似乎并未稳定 Jepson 薄果草中的混合交配。目前对 Jepson 薄果草近亲繁殖衰退的估计与先前研究不同，突出了环境变异对其表达的影响。
