Holtsford Timothy P, Ellstrand Norman C
Department of Botany and Plant Sciences, University of California, Riverside, CA, 92521, USA.
Evolution. 1990 Dec;44(8):2031-2046. doi: 10.1111/j.1558-5646.1990.tb04309.x.
Inbreeding depression is commonly observed in natural populations. The deleterious effects of forced inbreeding are often thought to be less pronounced in populations with self-pollinating mating systems than in primarily outcrossing populations. We tested this hypothesis by comparing the performance of plants produced by artificial self- and cross-pollination from three populations whose outcrossing rate estimates were 0.03, 0.26, and 0.58. Outcrossing rates and inbreeding coefficients were estimated using isozyme polymorphisms as genetic markers. Analysis of F statistics suggests that biparental inbreeding as well as self-fertilization contribute to the level of homozygosity in the seed crop. Biparental inbreeding will reduce the heterozygosity of progeny produced by outcrossing, relative to random outcrossing expectations, and hence will reduce the effects of outcrossing versus self-fertilization. Heterotic selection may increase the average heterozygosity during the life history. Selfed and outcrossed seeds from all three populations were equally likely to germinate and survive to reproduce. However, inbreeding depression was observed in fecundity traits of plants surviving to reproduction in all three populations. Even in the population whose natural self-fertilization rate was 97%, plants grown from seed produced by self-pollination produced fewer fruits and less total seed weight than plants grown from outcrossed seed. There was no detectable inbreeding depression in estimated lifetime fitness. Inbreeding effects for all reproductive yield characters were most severe in the accession from the most outcrossing population and least severe in the accession from the most self-fertilizing population.
近亲繁殖衰退在自然种群中普遍存在。人们通常认为，与主要进行异交的种群相比，具有自花授粉交配系统的种群中，强制近亲繁殖的有害影响不那么明显。我们通过比较人工自花授粉和异花授粉产生的植株的表现，对这一假设进行了检验，这些植株来自三个异交率估计值分别为0.03、0.26和0.58的种群。利用同工酶多态性作为遗传标记来估计异交率和近亲繁殖系数。F统计量分析表明，双亲近亲繁殖以及自交都对种子作物的纯合度水平有影响。相对于随机异交预期，双亲近亲繁殖会降低异交产生的后代的杂合度，因此会减少异交与自交的影响差异。杂种优势选择可能会在生活史中增加平均杂合度。来自所有三个种群的自交种子和异交种子发芽和存活至繁殖的可能性相同。然而，在所有三个种群中，存活至繁殖阶段的植株的繁殖力性状都出现了近亲繁殖衰退。即使在自然自交率为97%的种群中，自花授粉产生的种子长成的植株比异花授粉种子长成的植株结出的果实更少，种子总重量也更低。在估计的终生适合度中没有检测到近亲繁殖衰退。所有生殖产量性状的近亲繁殖效应在异交程度最高的种群的材料中最为严重，在自交程度最高的种群的材料中最不严重。
