Smith Jeff
Department of Biology, Emory University, Atlanta, Georgia 30322, USA.
Am Nat. 2007 Oct;170(4):542-50. doi: 10.1086/521236. Epub 2007 Aug 15.
Symbiotic associations between species are ubiquitous, but we only poorly understand why some symbioses evolve to be mutualistic and others to be parasitic. One prominent hypothesis holds that vertical transmission of symbionts from host parents to their offspring selects for symbionts that are benign or beneficial, while horizontal transmission of symbionts among unrelated hosts selects for symbionts that are less beneficial or outright harmful. A long-standing challenge to this hypothesis, however, is the existence of selfish genetic elements (SGEs). SGEs are passed exclusively from parent to offspring and are able to spread and persist in populations despite reducing the fitness of their hosts. Here I show that SGEs are in fact consistent with the transmission mode hypothesis if one measures transmission from the perspective of host genes instead of host organisms. Both meiotic drive genes and cytoplasmic sex ratio distorters require horizontal transmission, in the form of outbred sex, to spread as parasites. Transmission from parent to offpsring does not constrain SGEs to evolve toward mutualism. The gene-centered perspective I present here is applicable to symbioses at all levels of selection and brings closer together our understandings of cooperation within and between species.
物种间的共生关系无处不在，但我们对为何有些共生关系会进化为互利共生而有些则进化为寄生关系的了解还很有限。一个著名的假说是，共生体从宿主亲本垂直传递给后代会选择良性或有益的共生体，而共生体在不相关宿主之间的水平传递则会选择益处较少或完全有害的共生体。然而，这一假说长期面临的一个挑战是自私遗传元件（SGEs）的存在。SGEs仅从亲本传递给后代，尽管会降低宿主的适应性，但仍能在种群中传播并持续存在。在这里我表明，如果从宿主基因而非宿主生物体的角度衡量传递，那么SGEs实际上与传递模式假说相符。减数分裂驱动基因和细胞质性别比例畸变基因都需要以远交性别的形式进行水平传递，才能作为寄生虫传播。从亲本到后代的传递并不会限制SGEs向互利共生方向进化。我在此提出的以基因为中心的观点适用于所有选择层面的共生关系，并使我们对物种内部和物种之间合作的理解更加统一。
