Gene duplication and horizontal gene transfer (HGT) are two important but different forces for adaptive genome evolution. In eukaryotic organisms, gene duplication is considered to play a more important evolutionary role than HGT. However, certain fungal lineages have developed highly efficient mechanisms that avoid the occurrence of duplicated gene sequences within their genomes. While these mechanisms likely originated as a defense against harmful mobile genetic elements, they come with an evolutionary cost. A prominent example for a genome defense system is the RIP mechanism of the ascomycete fungus Neurospora crassa, which efficiently prevents sequence duplication within the genome and functional redundancy of the subsequent paralogs. Despite this tight control, the fungus possesses two functionally redundant sterol C-5 desaturase enzymes, ERG-10a and ERG-10b, that catalyze the same step during ergosterol biosynthesis. In this study, we addressed this conundrum by phylogenetic analysis of the two proteins and supporting topology tests. We obtained evidence that a primary HGT of a sterol C-5 desaturase gene from Tremellales (an order of Basidiomycota) into a representative of the Pezizomycotina (a subphylum of Ascomycota) is the origin of the ERG-10b sequence. The reconstructed phylogenies suggest that this HGT event was followed by multiple HGT events among other members of the Pezizomycotina, thereby generating a diverse group with members in the four classes Sordariomycetes, Xylonomycetes, Eurotiomycetes and Dothideomycetes, which all harbor the second sterol C-5 desaturase or maintained in some cases only the ERG-10b version of this enzyme. These results furnish an example for a gene present in numerous ascomycetous fungi but primarily acquired by an ancestral HGT event from another fungal phylum. Furthermore, these data indicate that HGT represents one mechanism to generate functional redundancy in organisms with a strict avoidance of gene duplications.

基因复制和水平基因转移（HGT）是适应性基因组进化的两个重要但不同的力量。在真核生物中，基因复制被认为比HGT具有更重要的进化作用。但是，某些真菌谱系已开发出高效的机制，可避免在其基因组内出现重复的基因序列。尽管这些机制可能起源于对有害的移动遗传元素的防御，但它们却伴随着进化的代价。基因组防御系统的一个突出例子是子囊真菌Neurospora crassa的RIP机制。，可有效防止基因组内的序列重复和后续旁系同源物的功能冗余。尽管进行了严格控制，但真菌仍具有两种功能上多余的固醇C-5去饱和酶ERG-10a和ERG-10b，它们在麦角固醇生物合成过程中可催化同一步骤。在这项研究中，我们通过对两种蛋白质的系统发育分析并支持拓扑测试来解决这个难题。我们获得的证据表明，从银耳（Basidiomycota的一个订单）到Pezizomycotina（子囊的一个亚门）的代表，固醇C-5去饱和酶基因的主要HGT是ERG-10b序列的起源。重建的系统发育表明，此HGT事件之后是Pezizomycotina的其他成员之间的多个HGT事件，从而产生了一个由四类伞形霉菌，木霉菌，Eurotiomycetes和Dothideomycetes中的成员组成的多样化群体，它们都带有第二种固醇C-5去饱和酶，或者在某些情况下仅维持该酶的ERG-10b版本。这些结果为存在于众多子囊真菌中但主要由另一真菌门的祖先HGT事件获得的基因提供了实例。此外，这些数据表明，HGT代表了一种在严格避免基因重复的情况下在生物体中产生功能冗余的机制。这些结果提供了存在于众多子囊真菌中但主要由另一真菌门的祖先HGT事件获得的基因的实例。此外，这些数据表明，HGT代表了一种在严格避免基因重复的情况下在生物体中产生功能冗余的机制。这些结果为存在于众多子囊真菌中但主要由另一真菌门的祖先HGT事件获得的基因提供了实例。此外，这些数据表明，HGT代表了一种在严格避免基因重复的情况下在生物体中产生功能冗余的机制。