Department of Biology, College of William and Mary, Williamsburg, VA 23187, USA.
Integr Comp Biol. 2009 Nov;49(5):550-62. doi: 10.1093/icb/icp014. Epub 2009 May 22.
A key question in the evolution of life history and in evolutionary physiology asks how reproductive and other life-history traits evolve. Genetic variation in reproductive control systems may exist in many elements of the complex inputs that can affect the hypothalamic-pituitary-gonadal (HPG) or reproductive axis. Such variation could include numbers and other traits of secretory cells, the amount and pattern of chemical message released, transport and clearance mechanisms, and the number and other traits of receptor cells. Selection lines created from a natural population of white-footed mice (Peromyscus leucopus) that contains substantial genetic variation in reproductive inhibition in response to short winter daylength (SD) have been used to examine neuroendocrine variation in reproductive timing. We hypothesized that natural genetic variation would be most likely to occur in the inputs to GnRH neurons and/or in GnRH neurons themselves, but not in elements of the photoperiodic pathway that would have pleiotropic effects on nonreproductive functions as well as on reproductive functions. Significant genetic variation has been found in the GnRH neuronal system. The number of GnRH neurons immunoreactive to an antibody to mature GnRH peptide under conditions maximizing detection of stained neurons was significantly heritable in an unselected control (C) line. Furthermore, a selection line that suppresses reproduction in SD (photoperiod responsive, R) had fewer IR-GnRH neurons than a selection line that maintains reproduction in SD (photoperiod nonresponsive, NR). This supports the hypothesis that genetic variation in characteristics of GnRH neurons themselves may be responsible for the observed phenotypic variation in reproduction in SD. The R and NR lines differ genetically in food intake and iodo-melatonin receptor binding, as well as in other characteristics. The latter findings are consistent with the hypothesis that genetic variation occurs in the nutritional and hormonal inputs to GnRH neurons. Genetic variation also exists in the phenotypic plasticity of responses to two combinations of treatments, (1) food and photoperiod, and (2) photoperiod and age, indicating genetic variation in individual norms of reaction within this population. Overall, the apparent multiple sources of genetic variation within this population suggest that there may be multiple alternative combinations of alleles for both the R and NR phenotypes. If that interpretation is correct, we suggest that this offers some support for the evolutionary "potential" hypothesis and is inconsistent with the evolutionary "constraint" and "symmorphosis" hypotheses for the evolution of complex neuroendocrine pathways.
一个关键的问题在生命史的进化和进化生理学中问到生殖和其他生命史特征如何进化。生殖控制系统的遗传变异可能存在于影响下丘脑-垂体-性腺（HPG）或生殖轴的复杂输入的许多因素中。这种变异可能包括分泌细胞的数量和其他特征、释放的化学信息的数量和模式、运输和清除机制以及受体细胞的数量和其他特征。从一个自然种群的白足鼠（Peromyscus leucopus）中创建的选择线，该种群在对短冬日照长度（SD）的生殖抑制中存在大量遗传变异，已被用于研究生殖时间的神经内分泌变异。我们假设，自然遗传变异最有可能发生在 GnRH 神经元的输入中，或者在 GnRH 神经元本身中，但不会发生在光周期途径的元素中，这些元素会对非生殖功能以及生殖功能产生多效性影响。已经发现 GnRH 神经元系统中存在显著的遗传变异。在最大限度地检测染色神经元的条件下，对成熟 GnRH 肽的抗体呈免疫反应的 GnRH 神经元的数量在未选择的对照（C）线中具有显著的遗传性。此外，在 SD 中抑制生殖的选择线（光周期反应性，R）比在 SD 中维持生殖的选择线（光周期非反应性，NR）具有更少的 IR-GnRH 神经元。这支持这样的假设，即 GnRH 神经元自身特征的遗传变异可能是导致在 SD 中观察到的生殖表型变异的原因。R 和 NR 线在食物摄入和碘-褪黑素受体结合以及其他特征上存在遗传差异。后一种发现与这样的假设一致，即遗传变异发生在 GnRH 神经元的营养和激素输入中。对两种处理组合（1）食物和光周期以及（2）光周期和年龄的反应的表型可塑性也存在遗传变异，表明该群体中个体反应规范存在遗传变异。总的来说，该群体中明显的多种遗传变异来源表明，对于 R 和 NR 表型，可能存在多种替代等位基因组合。如果这种解释是正确的，我们认为这为进化“潜力”假说提供了一些支持，并且与复杂神经内分泌途径进化的进化“约束”和“对称”假说不一致。
