Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138.
Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138;
Proc Natl Acad Sci U S A. 2017 Jun 6;114(23):5784-5791. doi: 10.1073/pnas.1610600114.
In animals, primordial germ cells (PGCs) give rise to the germ lines, the cell lineages that produce sperm and eggs. PGCs form in embryogenesis, typically by one of two modes: a likely ancestral mode wherein germ cells are induced during embryogenesis by cell-cell signaling (induction) or a derived mechanism whereby germ cells are specified by using germ plasm-that is, maternally specified germ-line determinants (inheritance). The causes of the shift to germ plasm for PGC specification in some animal clades remain largely unknown, but its repeated convergent evolution raises the question of whether it may result from or confer an innate selective advantage. It has been hypothesized that the acquisition of germ plasm confers enhanced evolvability, resulting from the release of selective constraint on somatic gene networks in embryogenesis, thus leading to acceleration of an organism's protein-sequence evolution, particularly for genes expressed at early developmental stages, and resulting in high speciation rates in germ plasm-containing lineages (denoted herein as the "PGC-specification hypothesis"). Although that hypothesis, if supported, could have major implications for animal evolution, our recent large-scale coding-sequence analyses from vertebrates and invertebrates provided important examples of genera that do not support the hypothesis of liberated constraint under germ plasm. Here, we consider reasons why germ plasm might be neither a direct target of selection nor causally linked to accelerated animal evolution. We explore alternate scenarios that could explain the repeated evolution of germ plasm and propose potential consequences of the inheritance and induction modes to animal evolutionary biology.
在动物中，原始生殖细胞（PGC）产生生殖系，即产生精子和卵子的细胞谱系。PGC 通常通过两种模式之一在胚胎发生中形成：一种可能是祖先模式，其中生殖细胞通过细胞间信号（诱导）在胚胎发生中被诱导；另一种是衍生机制，其中生殖细胞通过使用生殖质（即母体指定的生殖系决定因素）来指定（遗传）。在某些动物类群中，PGC 特异性指定向生殖质转变的原因在很大程度上仍然未知，但它的重复趋同进化提出了一个问题，即它是否可能是由内在的选择优势导致或赋予的。有人假设，生殖质的获得赋予了更高的进化能力，这是由于胚胎发生中对体细胞基因网络的选择约束的释放，从而导致生物体的蛋白质序列进化加速，特别是对于在早期发育阶段表达的基因，并导致含有生殖质的谱系的高物种形成率（在此表示为“PGC 特异性假说”）。尽管如果该假说得到支持，可能对动物进化具有重大影响，但我们最近从脊椎动物和无脊椎动物进行的大规模编码序列分析提供了重要的例子，这些例子不支持生殖质下的约束释放假说。在这里，我们考虑生殖质为什么既不是选择的直接目标，也与加速动物进化没有因果关系的原因。我们探讨了可以解释生殖质重复进化的替代方案，并提出了遗传和诱导模式对动物进化生物学的潜在后果。
