Stein Adam C, Uy J Albert C
Department of Biology, Syracuse University, 108 College Place, Syracuse, New York 13244, USA.
Evolution. 2006 Jul;60(7):1476-85.
Hybridization can be an evolutionary creative force by forming new polyploid species, creating novel genetic variation or acting as conduits of potentially advantageous traits between hybridizing forms. Evidence for the latter is often difficult to find because alleles under positive selection can spread rapidly across a hybrid zone and sweep to fixation. In Western Panama, an avian hybrid zone between two species of manakins in the genus Manacus exists where the unidirectional introgression of bright, yellow plumage into a white population provides evidence for the importance of hybrid zones as conduits of advantageous traits. Several lines of indirect evidence suggest that sexual selection favoring yellow plumage drives this asymmetrical spread, but more direct evidence is lacking. Along the edge of the hybrid zone, both yellow- and white-collared manakins are found in the same mating arenas or leks and compete for the same females ("mixed leks"), providing us with a unique opportunity to understand the dynamics of yellow plumage introgression. We studied these mixed leks to determine whether yellow males have a mating advantage over white males and, if so, whether the mating advantage is driven by male-male interactions, female choice, or both. We found that yellow males mated more than white males, suggesting that sexual selection favoring yellow males can, indeed, explain the spread of yellow plumage. However, we found that this advantage occurred only in mixed leks where the frequency of yellow males is greater than white males. This suggests that the advantage of yellow males may depend on the presence of other yellow males, which may slow the rate of introgression in leks where yellow frequency is low such as in areas where yellow males are beginning to colonize the white population. This, along with the geographic barrier posed by major rivers in the hybrid zone, may initially limit or slow the spread of yellow plumage. Finally, we found that yellow and white males were similar in aggression and body size, and held comparable positions within leks. Because these traits or factors are often important in or dictated by aggressive male-male interactions, these comparisons indicate that male-male interaction is not the primary mechanism for the spread of yellow plumage. However, white and yellow males received similar numbers of courtship visits from females but differed in the number of matings, suggesting that females actively rejected white in favor of yellow males. Our results indicate that sexual selection by female choice has driven the unidirectional introgression of yellow plumage into the white population, providing a mechanism for how hybrid zones act as conduits of novel and advantageous traits.
杂交可以通过形成新的多倍体物种、创造新的遗传变异或作为杂交形式之间潜在有利性状的传导途径，成为一种进化的创造性力量。后一种情况的证据往往很难找到，因为受到正选择的等位基因可以迅速在杂交区域扩散并固定下来。在巴拿马西部，存在一个白翅曼燕属两种侏儒鸟之间的鸟类杂交区域，明亮黄色羽毛向白色种群的单向基因渗入为杂交区域作为有利性状传导途径的重要性提供了证据。几条间接证据表明，对黄色羽毛的性选择推动了这种不对称扩散，但缺乏更直接的证据。在杂交区域边缘，黄领和白领侏儒鸟出现在相同的交配场所或求偶场中，争夺相同的雌性（“混合求偶场”），这为我们提供了一个独特的机会来了解黄色羽毛基因渗入的动态过程。我们研究了这些混合求偶场，以确定黄色雄鸟是否比白色雄鸟具有交配优势，如果是，这种交配优势是由雄雄互动、雌鸟选择还是两者共同驱动的。我们发现黄色雄鸟的交配次数多于白色雄鸟，这表明对黄色雄鸟的性选择确实可以解释黄色羽毛的扩散。然而，我们发现这种优势仅出现在黄色雄鸟频率高于白色雄鸟的混合求偶场中。这表明黄色雄鸟的优势可能取决于其他黄色雄鸟的存在，这可能会减缓黄色频率较低的求偶场中的基因渗入速度，比如在黄色雄鸟开始进入白色种群的区域。这一点，再加上杂交区域主要河流构成的地理屏障，可能最初会限制或减缓黄色羽毛的扩散。最后，我们发现黄色和白色雄鸟在攻击性和体型上相似，在求偶场中的位置相当。因为这些特征或因素在雄雄攻击性互动中通常很重要或由其决定，这些比较表明雄雄互动不是黄色羽毛扩散的主要机制。然而，白色和黄色雄鸟从雌鸟那里得到的求偶次数相似，但交配次数不同，这表明雌鸟会主动拒绝白色雄鸟而选择黄色雄鸟。我们的结果表明，雌鸟选择的性选择推动了黄色羽毛向白色种群的单向基因渗入，为杂交区域如何作为新的和有利性状的传导途径提供了一种机制。
