Archambeault Sophie, Taylor Julia Ann, Crow Karen D
Department of Biology, San Francisco State University, 1600 Holloway Ave, San Francisco, CA 94132 USA.
Evodevo. 2014 Nov 19;5:44. doi: 10.1186/2041-9139-5-44. eCollection 2014.
Hox genes are master regulatory genes that specify positional identities during axial development in animals. Discoveries regarding their concerted expression patterns have commanded intense interest due to their complex regulation and specification of body plan features in jawed vertebrates. For example, the posterior HoxD genes switch to an inverted collinear expression pattern in the mouse autopod where HoxD13 switches from a more restricted to a less restricted domain relative to its neighboring gene on the cluster. We refer to this program as the 'distal phase' (DP) expression pattern because it occurs in distal regions of paired fins and limbs, and is regulated independently by elements in the 5' region upstream of the HoxD cluster. However, few taxa have been evaluated with respect to this pattern, and most studies have focused on pectoral fin morphogenesis, which occurs relatively early in development.
Here, we demonstrate for the first time that the DP expression pattern occurs with the posterior HoxA genes, and is therefore not solely associated with the HoxD gene cluster. Further, DP Hox expression is not confined to paired fins and limbs, but occurs in a variety of body plan features, including paddlefish barbels - sensory adornments that develop from the first mandibular arch (the former 'Hox-free zone), and the vent (a medial structure that is analogous to a urethra). We found DP expression of HoxD13 and HoxD12 in the paddlefish barbel; and we present the first evidence for DP expression of the HoxA genes in the hindgut and vent of three ray-finned fishes. The HoxA DP expression pattern is predicted by the recent finding of a shared 5' regulatory architecture in both the HoxA and HoxD clusters, but has not been previously observed in any body plan feature.
The Hox DP expression pattern appears to be an ancient module that has been co-opted in a variety of structures adorning the vertebrate bauplan. This module provides a shared genetic program that implies deep homology of a variety of distally elongated structures that has played a significant role in the evolution of morphological diversity in vertebrates.
Hox基因是动物轴向发育过程中指定位置身份的主要调控基因。由于其在颌类脊椎动物身体结构特征的复杂调控和指定方面的作用，关于它们协同表达模式的发现引起了人们的浓厚兴趣。例如，在小鼠的 autopod 中，后部HoxD基因切换为反向共线性表达模式，其中HoxD13相对于其在基因簇上的相邻基因，从一个更受限的区域切换到一个限制较少的区域。我们将这个程序称为“远端阶段”（DP）表达模式，因为它发生在成对鳍和肢体的远端区域，并由HoxD基因簇上游5'区域的元件独立调控。然而，很少有分类群针对这种模式进行评估，并且大多数研究都集中在胸鳍形态发生上，胸鳍形态发生在发育过程中相对较早出现。
在这里，我们首次证明DP表达模式也存在于后部HoxA基因中，因此并不完全与HoxD基因簇相关。此外，DP Hox表达不仅限于成对鳍和肢体，还出现在各种身体结构特征中，包括匙吻鲟的触须——从第一下颌弓（以前的“无Hox区”）发育而来的感觉附属物，以及泄殖腔（一种类似于尿道的内侧结构）。我们在匙吻鲟触须中发现了HoxD13和HoxD12的DP表达；并且我们首次提供了三种硬骨鱼后肠和泄殖腔中HoxA基因DP表达的证据。HoxA DP表达模式是由最近在HoxA和HoxD基因簇中发现的共享5'调控结构所预测的，但之前在任何身体结构特征中都未观察到。
Hox DP表达模式似乎是一个古老的模块，已被多种装饰脊椎动物基本结构的结构所采用。这个模块提供了一个共享的遗传程序，意味着各种远端拉长结构具有深度同源性，这在脊椎动物形态多样性的进化中发挥了重要作用。
