Ågren Jon, Schemske Douglas W
Department of Botany, KB-15, University of Washington, Seattle, WA, 98195, USA.
Evolution. 1993 Feb;47(1):125-135. doi: 10.1111/j.1558-5646.1993.tb01204.x.
Most models of mating-system evolution predict inbreeding depression to be low in inbred populations due to the purging of deleterious recessive alleles. This paper presents estimates of outcrossing rates and inbreeding depression for two highly selfing, monoecious annuals Begonia hirsuta and B. semiovata. Outcrossing rates were estimated using isozyme polymorphisms, and the magnitude of inbreeding depression was quantified by growing progeny in the greenhouse produced through controlled selfing and outcrossing. The estimated single-locus outcrossing rate was 0.03 ± 0.01 (SE) for B. hirsuta and 0.05 ± 0.02 for B. semiovata. In both species, the seed production of selfed flowers was on average 12% lower than that of outcrossed flowers (B. hirsuta P = 0.07, B. semiovata P < 0.05, mixed model ANOVAs). There was no significant effect of crosstype on germination rate or survival, but selfed offspring had a lower dry mass than outcrossed offspring 18 weeks after planting in both species (on average 18% lower in B. hirsuta and 31% lower in B. semiovata). Plants that were the products of selfing began flowering later than plants produced through outcrossing in B. semiovata, but not in B. hirsuta. The effects of crosstype on seed production (B. semiovata), days to first flower and offspring dry mass (both species) varied among maternal parents, as indicated by significant crosstype x maternal parent interactions for these characters. Both species showed significant inbreeding depression for total fitness (estimated as the product of seed production, germination rate, survival and dry mass at 18 weeks). In B. hirsuta, the average total inbreeding depression was 22% (range -57%-98%; N = 23 maternal parents), and in B. semiovata, it was 42% (-11%-84%; N = 21). This study demonstrates that highly selfing populations can harbor substantial inbreeding depression. Our findings are consistent with the hypothesis that a high mutation rate to mildly deleterious alleles contributes to the maintenance of inbreeding depression in selfing populations.
大多数交配系统进化模型预测，由于有害隐性等位基因的清除，近交种群中的近交衰退程度较低。本文给出了两种高度自交的雌雄同株一年生植物——糙叶秋海棠和半卵形秋海棠的异交率和近交衰退估计值。利用同工酶多态性估计异交率，并通过在温室中种植通过控制自交和异交产生的后代来量化近交衰退的程度。糙叶秋海棠估计的单基因座异交率为0.03±0.01（标准误），半卵形秋海棠为0.05±0.02。在这两个物种中，自交花的种子产量平均比异交花低12%（糙叶秋海棠P = 0.07，半卵形秋海棠P < 0.05，混合模型方差分析）。杂交类型对发芽率或存活率没有显著影响，但在两个物种中，自交后代在种植18周后的干重均低于异交后代（糙叶秋海棠平均低18%，半卵形秋海棠低31%）。在半卵形秋海棠中，自交产生的植株比异交产生的植株开花晚，但在糙叶秋海棠中并非如此。杂交类型对种子产量（半卵形秋海棠）、初花天数和后代干重（两个物种）的影响在母本之间存在差异，这些性状的显著杂交类型×母本相互作用表明了这一点。两个物种在总适合度方面均表现出显著的近交衰退（总适合度估计为种子产量、发芽率、存活率和18周时干重的乘积）。在糙叶秋海棠中，平均总近交衰退为22%（范围-57%-98%；N = 23个母本），在半卵形秋海棠中为42%（-11%-84%；N = 21）。本研究表明，高度自交的种群可能存在显著的近交衰退。我们的发现与以下假设一致，即对轻度有害等位基因的高突变率有助于维持自交种群中的近交衰退。
