Ayoub Nadia A, Hayashi Cheryl Y
Department of Biology, University of California, Riversid, USA.
Mol Biol Evol. 2008 Feb;25(2):277-86. doi: 10.1093/molbev/msm246. Epub 2007 Nov 28.
Spiders spin a functionally diverse array of silk fibers, each composed of one or more unique proteins. Most of these proteins, in turn, are encoded by members of a single gene family thought to have arisen through duplication and divergence of an ancestral silk gene. Because of its remarkable mechanical properties, orb weaver dragline silk, a composite of 2 proteins (MaSp1 and MaSp2), is the best studied. Here, we demonstrate that multiple loci encode MaSp1 in widow spiders (Latrodectus). Because these copies may be the result of more recent duplication events than those leading to the currently recognized silk gene paralogs, they offer insight into the early evolutionary fate of silk gene duplicates. In addition to 3 presumed functional MaSp1 loci in Latrodectus hesperus (Western black widow) and Latrodectus geometricus (brown widow) genomes, we find a MaSp1 pseudogene in L. hesperus, demonstrating the potential for unrecognized extinction of silk gene paralogs. We also document recombination events among L. hesperus MaSp1 loci and between Latrodectus MaSp1 loci and MaSp2. This result supports the hypothesis that concerted evolution occurs not only within an individual silk gene but also among silk gene paralogs. One of the L. geometricus MaSp1 copies encodes a protein that has diverged in amino acid composition and potentially converged on the secondary structure of MaSp2. Based on the presence of multiple MaSp1 loci and the phylogenetic distribution of MaSp1 versus MaSp2, we propose that MaSp2 is derived from an ancestral MaSp1 duplicate. Finally, divergence has occurred in the upstream flanking sequences of the L. hesperus MaSp1 loci, the region most likely to contain regulatory motifs, providing ample opportunity for differential expression. However, the benefits associated with increased protein production may be the primary mechanism maintaining multiple functional MaSp1 copies in widow genomes.
蜘蛛能纺出功能多样的丝纤维，每一种丝纤维都由一种或多种独特的蛋白质组成。反过来，这些蛋白质中的大多数由一个单一基因家族的成员编码，该基因家族被认为是通过一个祖先丝基因的复制和分化而产生的。由于其卓越的机械性能，圆蛛的拖牵丝是研究得最为透彻的，它是由两种蛋白质（MaSp1和MaSp2）组成的复合体。在这里，我们证明在寡妇蜘蛛（寇蛛属）中多个基因座编码MaSp1。因为这些拷贝可能是比那些导致目前已被认可的丝基因旁系同源物的复制事件更新的复制事件的结果，它们为丝基因复制体的早期进化命运提供了见解。除了在西方黑寡妇（间斑寇蛛）和褐寡妇（嗜卷寇蛛）基因组中有3个假定的功能性MaSp1基因座外，我们在西方黑寡妇中还发现了一个MaSp1假基因，这证明了丝基因旁系同源物存在未被识别的灭绝可能性。我们还记录了西方黑寡妇MaSp1基因座之间以及寇蛛属MaSp1基因座与MaSp2之间的重组事件。这一结果支持了这样一种假说，即协同进化不仅发生在单个丝基因内部，也发生在丝基因旁系同源物之间。褐寡妇的一个MaSp1拷贝编码一种在氨基酸组成上已经分化且可能在二级结构上与MaSp2趋同的蛋白质。基于多个MaSp1基因座的存在以及MaSp1与MaSp2的系统发育分布，我们提出MaSp2源自一个祖先MaSp1复制体。最后，西方黑寡妇MaSp1基因座的上游侧翼序列发生了分化，该区域最有可能包含调控基序，这为差异表达提供了充足的机会。然而，与增加蛋白质产量相关的益处可能是维持寡妇基因组中多个功能性MaSp1拷贝的主要机制。
