Heiser David A, Shaw Ruth G
Peabody Museum of Natural History, Yale University, New Haven, Connecticut 06520, USA.
Evolution. 2006 Jan;60(1):64-76.
Small and relatively isolated populations that occupy fragmented habitat are at risk of local extinction. However, fitness consequences of fragmentation related to mating distance, such as inbreeding depression following increased self- and near-neighbor mating, may not follow standard expectations in species with specialized genetic systems. We investigated the effect of mating distance on progeny fitness in Calylophus serrulatus, a primarily autogamous, permanent translocation heterozygote that is restricted to prairie fragments in the North American tallgrass prairie region. We pollinated flowers by hand in the field with pollen sampled at various distances from the maternal parent within and between three populations in southeastern Minnesota. We raised the progeny in a greenhouse and measured fitness-related characters. Because their genetic system prevents loss of heterozygosity throughout much of the genome, regardless of inbreeding, permanent translocation heterozygotes are not expected to exhibit inbreeding depression. Consistent with this expectation, in no case did progeny of self matings suffer significantly reduced mean fitness compared to progeny from crosses between plants. Crosses between plants in the two closely situated (2 km) populations yielded progeny with fitness intermediate to their parents, but crosses between each of those populations and the more distant (20 km) population yielded progeny with reduced fitness, suggesting outbreeding depression at this largest spatial scale. Similarly, fitness of self-pollinated progeny and progeny from "near" crosses (<2 m) within populations tended to be higher than "mid" (10-25 m) and "far" (>35 m) cross-progeny fitness. Under the current conditions of fragmentation, it seems likely that the distant matings that produce outbreeding depression are rare. It appears that mean fitness in this species is maintained in the context of severe fragmentation of its populations, largely because of its genetic system.
占据破碎化栖息地的小型且相对孤立的种群面临着局部灭绝的风险。然而，与交配距离相关的破碎化对适合度的影响，比如自交和近邻交配增加后出现的近亲繁殖衰退，在具有特殊遗传系统的物种中可能并不符合标准预期。我们研究了交配距离对锯齿草（Calylophus serrulatus）子代适合度的影响，锯齿草是一种主要进行自花授粉的永久易位杂合子，仅分布于北美高草草原地区的草原碎片中。我们在明尼苏达州东南部的三个种群内部及之间，用从母本植株不同距离采集的花粉，在田间对花朵进行人工授粉。我们在温室中培育子代，并测量与适合度相关的性状。由于它们的遗传系统能防止基因组大部分区域杂合性的丧失，无论近亲繁殖情况如何，永久易位杂合子预计不会表现出近亲繁殖衰退。与这一预期一致，自交子代的平均适合度与植株间杂交产生的子代相比，在任何情况下都没有显著降低。两个距离较近（2公里）的种群之间的植株杂交产生的子代适合度介于其亲本之间，但这些种群与距离较远（20公里）的种群之间的杂交产生的子代适合度降低，这表明在这个最大空间尺度上存在远交衰退。同样，种群内自花授粉子代和“近”距离杂交（<2米）产生的子代适合度往往高于“中”距离（10 - 25米）和“远”距离（>35米）杂交子代的适合度。在当前的破碎化条件下，产生远交衰退的远距离交配似乎很少见。看起来该物种的平均适合度在其种群严重破碎化的情况下得以维持，这很大程度上是由于其遗传系统。
