Krackow S, Schmidt T A, Elepfandt A
Verhaltensbiologie, Zoologisches Institut, Universität Zürich-Irchel, Winterthurerstrasse 190, 8057 Zürich, Switzerland.
Proc Biol Sci. 2003 May 7;270(1518):943-7. doi: 10.1098/rspb.2002.2310.
We present the first empirical evidence that mammalian sex-ratio deviations result from variation in adult-weight sexual dimorphism via correlated effects on blastocyst development. Two selection lines of mice exhibiting high and low sexual dimorphism in adult weight showed correlated sexual weight differences at birth and at weaning, caused by relatively decelerated growth of males in the low line from before birth. The sex ratio at birth was significantly female-biased in the low line, and significantly lower than in the highly dimorphic line. Concomitantly, blastomere numbers were at significantly higher variance in the low than in the highly dimorphic line, owing to an increased frequency of slowly growing blastocysts. Since low-dimorphism mice produced more corpora lutea and more female pups than the high-dimorphism mice, but not more males, birth sex-ratio bias most parsimoniously resulted from the loss of slowly growing male blastocysts. This is in agreement with the observation that sex-ratio skews in mammals arise when timing of uterine responsiveness (i.e. its temporally limited capacity for implantation) varies in relation to sex-specific embryonic growth rates. Hence, natural mammalian sex-ratio variation that stems from developmental asynchrony might be a by-product of natural selection for sexual dimorphism in adult weight.
我们提供了首个实证证据，证明哺乳动物的性别比例偏差是由成年体重性二态性的变化通过对囊胚发育的相关影响导致的。两个品系的小鼠在成年体重上表现出高性二态性和低性二态性，它们在出生时和断奶时出现了相关性的体重差异，这是由于低性二态性品系中的雄性在出生前生长相对减速所致。低性二态性品系出生时的性别比例显著偏向雌性，且显著低于高性二态性品系。与此同时，低性二态性品系中卵裂球数量的方差显著高于高性二态性品系，这是因为生长缓慢的囊胚频率增加。由于低性二态性小鼠比高性二态性小鼠产生更多的黄体和更多的雌性幼崽，但雄性幼崽数量没有增加，出生时的性别比例偏差最简约的解释是生长缓慢的雄性囊胚的损失。这与以下观察结果一致：当子宫反应性的时间（即其植入的时间限制能力）相对于性别特异性胚胎生长速度发生变化时，哺乳动物会出现性别比例偏差。因此，源于发育异步性的自然哺乳动物性别比例变化可能是成年体重性二态性自然选择的副产品。
