Department of Zoology, University of Cambridge, Cambridge CB2 3EJ, UK.
Proc Biol Sci. 2013 Feb 13;280(1756):20122543. doi: 10.1098/rspb.2012.2543. Print 2013 Apr 7.
Biosilicification is widespread across the eukaryotes and requires concentration of silicon in intracellular vesicles. Knowledge of the molecular mechanisms underlying this process remains limited, with unrelated silicon-transporting proteins found in the eukaryotic clades previously studied. Here, we report the identification of silicon transporter (SIT)-type genes from the siliceous loricate choanoflagellates Stephanoeca diplocostata and Diaphanoeca grandis. Until now, the SIT gene family has been identified only in diatoms and other siliceous stramenopiles, which are distantly related to choanoflagellates among the eukaryotes. This is the first evidence of similarity between SITs from different eukaryotic supergroups. Phylogenetic analysis indicates that choanoflagellate and stramenopile SITs form distinct monophyletic groups. The absence of putative SIT genes in any other eukaryotic groups, including non-siliceous choanoflagellates, leads us to propose that SIT genes underwent a lateral gene transfer event between stramenopiles and loricate choanoflagellates. We suggest that the incorporation of a foreign SIT gene into the stramenopile or choanoflagellate genome resulted in a major metabolic change: the acquisition of biomineralized silica structures. This hypothesis implies that biosilicification has evolved multiple times independently in the eukaryotes, and paves the way for a better understanding of the biochemical basis of silicon transport through identification of conserved sequence motifs.
生物硅化广泛存在于真核生物中，需要将硅浓缩在细胞内囊泡中。尽管人们对这一过程的分子机制知之甚少，但在之前研究过的真核生物进化枝中发现了与硅转运蛋白无关的蛋白。在这里，我们从硅质披盔衣有孔虫 Stephanoeca diplocostata 和 Diaphanoeca grandis 中鉴定出了硅转运体（SIT）基因。到目前为止，SIT 基因家族仅在硅藻和其他硅质鞭毛藻类中被发现，而硅藻和披盔衣有孔虫在真核生物中与鞭毛藻类的亲缘关系较远。这是首次发现来自不同真核超群的 SIT 之间具有相似性。系统发育分析表明，披盔衣有孔虫和鞭毛藻类的 SIT 形成了独特的单系群。在其他任何真核生物群中，包括非硅质披盔衣有孔虫，都没有推测的 SIT 基因，这使我们提出 SIT 基因在鞭毛藻类和披盔衣有孔虫之间发生了横向基因转移事件。我们推测，外源 SIT 基因的掺入使鞭毛藻类或披盔衣有孔虫的基因组发生了重大代谢变化：获得了生物矿化的硅质结构。这一假说意味着生物硅化在真核生物中已经独立进化了多次，并为通过鉴定保守序列基序来更好地理解硅转运的生化基础铺平了道路。
