The molecular evolution of spiggin nesting glue in sticklebacks

Gene duplication and subsequent divergence can lead to the evolution of new functions and lineage-specific traits. In sticklebacks, the successive duplication of a mucin gene (MUC19) into a tandemly arrayed, multigene family has enabled the production of copious amounts of ‘spiggin’, a secreted adhesive protein essential for nest construction. Here, we examine divergence between spiggin genes among three-spined sticklebacks (Gasterosteus aculeatus) from ancestral marine and derived freshwater populations, and propose underpinning gene duplication mechanisms. Sanger sequencing revealed substantial diversity among spiggin transcripts, including alternatively spliced variants and interchromosomal spiggin chimeric genes. Comparative analysis of the sequenced transcripts and all other spiggin genes in the public domain support the presence of three main spiggin lineages (spiggin A, spiggin B and spiggin C) with further subdivisions within spiggin B (B1, B2) and spiggin C (C1, C2). Spiggin A had diverged least from the ancestral MUC19, while the spiggin C duplicates had diversified most substantially. In silico translations of the spiggin gene open reading frames predicted that spiggins A and B are secreted as long mucin-like polymers, while spiggins C1 and C2 are secreted as short monomers, with putative antimicrobial properties. We propose that diversification of duplicated spiggin genes has facilitated local adaptation of spiggin to a range of aquatic habitats.

基因重复及其后续分化可推动新功能演化与谱系特异性性状的形成。在棘鱼中，一类粘蛋白基因（MUC19）经连续重复后形成串联排布的多基因家族，由此得以产生大量的“spiggin”——一种对筑巢必不可少的分泌型粘附蛋白。本研究针对源自祖先海洋种群与衍生淡水种群的三棘刺鱼（Gasterosteus aculeatus）的spiggin基因间的分化展开分析，并提出支撑其演化的基因重复机制。桑格测序（Sanger sequencing）结果显示，spiggin转录本存在显著多样性，涵盖可变剪接变体与染色体间spiggin嵌合基因。对已测序转录本与公共数据库中所有其他spiggin基因的比较分析表明，存在三大主要spiggin谱系：spiggin A、spiggin B与spiggin C，其中spiggin B可进一步分为B1、B2两个亚型，spiggin C则分为C1、C2两个亚型。spiggin A与祖先MUC19的分化程度最低，而spiggin C的重复基因则发生了最为显著的多样化。对spiggin基因开放阅读框的计算机模拟（in silico）翻译结果显示，spiggin A与B以长链粘蛋白样聚合物形式被分泌，而spiggin C1与C2则以短单体形式分泌，且二者具备潜在抗菌活性。本研究认为，重复spiggin基因的多样化过程推动了spiggin对多种水生栖息地的局部适应性演化。