Behavioural Ecology, Institute of Ecology and Evolution, University of Bern, Wohlenstrasse 50a, 3032 Hinterkappelen, Switzerland.
Proc Biol Sci. 2012 Feb 7;279(1728):592-600. doi: 10.1098/rspb.2011.1104. Epub 2011 Jul 13.
Large egg size usually boosts offspring survival, but mothers have to trade off egg size against egg number. Therefore, females often produce smaller eggs when environmental conditions for offspring are favourable, which is subsequently compensated for by accelerated juvenile growth. How this rapid growth is modulated on a molecular level is still unclear. As the somatotropic axis is a key regulator of early growth in vertebrates, we investigated the effect of egg size on three key genes belonging to this axis, at different ontogenetic stages in a mouthbrooding cichlid (Simochromis pleurospilus). The expression levels of one of them, the growth hormone receptor (GHR), were significantly higher in large than in small eggs, but remarkably, this pattern was reversed after hatching: young originating from small eggs had significantly higher GHR expression levels as yolk sac larvae and as juveniles. GHR expression in yolk sac larvae was positively correlated with juvenile growth rate and correspondingly fish originating from small eggs grew faster. This enabled them to catch up fully in size within eight weeks with conspecifics from larger eggs. This is the first evidence for a potential link between egg size, an important maternal effect, and offspring gene expression, which mediates an adaptive adjustment in a relevant hormonal axis.
卵的大小通常会提高后代的存活率，但母亲必须在卵的大小和数量之间做出权衡。因此，当后代的环境条件有利时，雌性通常会产下较小的卵，随后通过加速幼体生长来弥补。这种快速生长在分子水平上是如何调节的仍然不清楚。由于躯体发生轴是脊椎动物早期生长的关键调节剂，我们研究了卵的大小对属于这一轴的三个关键基因的影响，这些基因在一种口孵鱼（Simochromis pleurospilus）的不同个体发育阶段。其中一种基因，生长激素受体（GHR）的表达水平在大卵中明显高于小卵，但值得注意的是，这种模式在孵化后发生了逆转：来自小卵的幼体在卵黄囊幼虫和幼体阶段具有显著更高的 GHR 表达水平。卵黄囊幼虫中的 GHR 表达与幼体生长速度呈正相关，相应地，来自小卵的鱼生长速度更快。这使它们能够在八周内完全赶上来自大卵的同种鱼的大小。这是卵大小、重要的母体效应和后代基因表达之间潜在联系的第一个证据，这种联系调节了相关激素轴的适应性调整。
