Werren J H, Nur U, Eickbush D
Nature. 1987;327(6117):75-6. doi: 10.1038/327075a0.
Extrachromosomal inheritance is ubiquitous among plants and animals; however, most extrachromosomal factors are uniparentally inherited through females, but not through males. Examples include chloroplasts, mitochondria and a variety of intracellular symbionts. The only known exception to maternal extrachromosomal inheritance in an animal is a paternally transmitted sex ratio factor (psr) which causes all-male families in the parasitic wasp, Nasonia vitripennis. Normally in this wasp, male offspring are haploid and develop from unfertilized eggs whereas females are diploid and develop from fertilized eggs. The psr factor is either a venereally transmitted infection which prevents egg fertilization (and therefore causes all-male families), or a factor transmitted to eggs by the sperm of males carrying psr, which somehow prevents incorporation of the paternal chromosomes. Here we report that sperm from psr males fertilizes eggs, but that the paternal chromosomes are subsequently condensed into a chromatin mass before the first mitotic division of the egg and do not participate in further divisions. Resulting haploid offspring are male, but have inherited the paternal factor. This extrachromosomal factor promotes its own transmission at the expense of the paternal chromosomes, and therefore can be considered a 'selfish' genetic element.
染色体外遗传在动植物中普遍存在；然而，大多数染色体外因子是通过雌性单亲遗传的，而非通过雄性。例子包括叶绿体、线粒体以及多种细胞内共生体。动物中已知的唯一例外是父系传递的性别比例因子（psr），它会导致寄生蜂丽蝇蛹集金小蜂（Nasonia vitripennis）出现全雄家族。通常在这种黄蜂中，雄性后代是单倍体，由未受精的卵发育而来，而雌性是二倍体，由受精卵发育而来。psr因子要么是一种通过交配传播的感染，它会阻止卵子受精（从而导致全雄家族），要么是携带psr的雄性精子传递给卵子的一种因子，这种因子以某种方式阻止父本染色体的整合。在此我们报告，psr雄性的精子使卵子受精，但父本染色体随后在卵子的第一次有丝分裂之前浓缩成染色质团块，并且不参与后续分裂。产生的单倍体后代是雄性，但继承了父本因子。这种染色体外因子以牺牲父本染色体为代价促进自身传播，因此可被视为一种“自私”的遗传元件。
