title: Early evolution of metazoan transcription factors
1. 发现大多数转录因子(Tf)家族起源于动物王国的黎明，是在当代动物谱系分化之前，在生物发育过程中表达的家族。
A large majority of the signaling pathway components and transcription factors (TFs) that lie at the heart of bilaterian developmental gene regulatory networks (GRNs). Also exist in sponges, placozoans, and cnidarians.
Last common ancestor to all living metazoans (LCAM)
The interaction between a TF and its target cis-regulatory elements is central to GRN outputs, and ultimately to the expression of cell lineage-specific and territory-specific differentiation products. 转录因子和它的靶顺式作用元件的相互作用是基因调节网络的关键。并最终表达细胞系特异性和地域特异性分化产物。
因此，了解后生动物TF‘工具箱’的起源对于破译支持动物多细胞进化的祖先基因组结构至关重要。
通过对海绵，胎盘动物，以及刺胞动物的基因组推断，与多有代表性的转录因子存在于LCAM中。 像很多经典的转录因子的进化发生在海绵和原生动物分化之前，如helix–loop–helix (bHLH), Mef2, Fox, Sox, T-box, ETS, nuclear receptor, Rel/NF-kB, bZIP, Smad, and diversity of homeobox-containing classes, including ANTP, Prd-like, Pax, POU, LIM-HD, Six, and
TALE。 栉水母类动物也有许多典型的转录因子家族。
zootype a (hypothetical) classification of several phyla or groups of animals together based on shared chromosomes, etc。
这一概念最初是指Hox基因和其他发育调节因子的保守表达和结构，这一概念也可延伸到动物特有的基因组方面，包括转录因子，信号配体以及其他被认为在调节发育方面起到关键作用的因子。
Members of many of these gene families and classes are positioned within known bilaterian developmental GRNs, where they often have conserved roles and localized developmental expression patterns. 许多这些基因家族存在于已知的两侧对称动物的GRN中，在这个调节网络中它们有保守的角色和发育表达模式。
后生动物基因组zootype 也包括转录因子涉及到的信号通路。
##后生动物转录因子基因家族的发生 三种类型
The genesis of metazoan-specific TFs can be classed into three groups。 these are type I, II, and III novelties.
Animal TF genes that have no clear relatives outside the Metazoa are considered a type I novelty and currently include ETS and nuclear receptor families。
在后生动物之外没有明确亲缘关系的动物TF基因被认为是一种新的I型基因，目前包括ETS和核受体家族。
POU, Pax, and Six homeobox classes all can be classified as type II novelties, where the animal restricted POU, Six, and Pax domains are combined with the more ancient homeodomain to produce the metazoan novelty。
第二类是指这些转录因子区域结合更古老的同源域去产生后生动物的新颖性。
type III novelties are those where ancient premetazoan domains combine in novel ways to generate a metazoan specific domain architecture;
第三类是指古老的前后生动物结合域以一种新的方式结合去产生后生动物特有的结合域结构。
##早期后生动物转录因子基因家族的扩增
The cnidarian Nematostella has a TF gene content that is remarkably similar to bilaterians, being in general two to three times larger than is observed in Amphimedon or Trichoplax.
Given that cnidarians are classified consistently as the sister group to the bilaterians. 刺胞动物和两侧对称动物。
在后生生物早期的进化过程中，动物基因的复制和缺失的推论是往往是基于系统发育分析最简练的解释。
##本文研究方法：
we can nonetheless use gene phylogenies to identify orthologs in distant taxa and uncover the minimal number of
gene families and classes in a particular group of animals.
转录因子家族进化的一般趋势:
first, the origin of all the major metazoan-specific classes, often from a more ancestral prototype.
second, an initial expansion phase before the divergence of sponge and eumetazoan lineages.
third, a second period of expansion early in eumetazoan evolution, resulting often in a twofold to fourfold increase in gene number.
four, a third and typically less extensive phase of expansion in the bilaterian stem.