Graduate School of Agriculture, Kyoto University, Kyoto 606-8502, Japan; National Agricultural and Food Research Organization, Ibaraki 305-8634, Japan; Instituto Gulbenkian de Ciência, Oeiras 2780-156, Portugal; Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-0032, Japan.
Instituto Gulbenkian de Ciência, Oeiras 2780-156, Portugal; Department of Biology, University of Copenhagen, Copenhagen 2200, Denmark.
Curr Biol. 2021 Feb 22;31(4):884-891.e3. doi: 10.1016/j.cub.2020.11.017. Epub 2020 Dec 11.
Animals with exoskeletons molt for further growth. In insects, the number of larval (or nymphal) molts varies inter- and intra-specifically, and it is widely accepted that the variation in the number of larval molts is an adaptive response to diverse environmental conditions. However, the molecular mechanism that underlies the variety and plasticity in the number of larval molts is largely unknown. In the silkworm, Bombyx mori, there are strains that molt three, four, or five times, and these numbers are determined by allelic variation at a single autosomal locus, Moltinism (M). Here, we demonstrate that the Hox gene Sex combs reduced (Scr) is responsible for the phenotypes of the M locus. Scr is selectively expressed in the larval prothoracic gland (PG), an endocrine organ that produces molting hormones.Scr represses the biosynthesis of molting hormones in the PG, thereby regulating the incremental increase in body size during each larval instar. Our experiments consistently suggest that the differential expression levels of Scr among the three M alleles result in different growth ratios that ultimately lead to the different number of larval molts. Although the role of Hox genes in conferring segmental identity along the body axis and in molding segment-specific structure later in development has been well established, the present study identifies an unexpected role of Hox gene in hormone biosynthesis. This new role means that, in addition to shaping segment-specific morphology, Hox genes also drive the evolution of life history traits by regulating animal physiology.
具有外骨骼的动物会蜕皮以促进进一步生长。在昆虫中，幼虫（或若虫）蜕皮的次数在种间和种内都有所不同，人们普遍认为幼虫蜕皮次数的变化是对不同环境条件的适应性反应。然而，导致幼虫蜕皮次数多样性和可塑性的分子机制在很大程度上尚不清楚。在桑蚕（Bombyx mori）中，存在着蜕皮三次、四次或五次的品系，这些数字是由单个常染色体座位 Moltinism（M）上的等位基因变异决定的。在这里，我们证明 Hox 基因 Sex combs reduced（Scr）是 M 基因座表型的决定因素。Scr 在幼虫前胸腺（PG）中选择性表达，PG 是一种内分泌器官，可产生蜕皮激素。Scr 抑制 PG 中蜕皮激素的生物合成，从而调节每个幼虫龄期的体型逐渐增加。我们的实验一致表明，三个 M 等位基因之间 Scr 的差异表达水平导致了不同的生长比率，最终导致了不同的幼虫蜕皮次数。尽管 Hox 基因在沿身体轴赋予节段性身份和在发育后期塑造节段特异性结构方面的作用已得到充分证实，但本研究确定了 Hox 基因在激素生物合成中的一个意想不到的作用。这个新的作用意味着，Hox 基因除了塑造节段特异性形态外，还通过调节动物生理学来推动生命史特征的进化。
