Eberhard W G
Smithsonian Tropical Research Institute, Ciudad Universitaria, Costa Rica.
Q Rev Biol. 1990 Mar;65(1):3-22. doi: 10.1086/416582.
Gene flow between different reproductive units such as bacterial plasmids and chromosomes presents unusual problems for evolutionary analysis. Far more than in eukaryotes, reproductive advantages at several levels of selection--genes, transposons, plasmids, cells, and clones--must be considered simultaneously to understand plasmid evolution. No level consistently prevails in conflict situations, and some reproductive units carry genes that restrain their own reproduction or survival, apparently to enhance the reproduction or survival of the higher-level reproductive units that carry them. Despite gene flow between plasmids and chromosomes, genes for certain functions show strong tendencies to occur on plasmids while others consistently occur on chromosomes. Functions generally associated with plasmids are diverse, but all are useful only in locally restricted contexts; it is argued that the selective consequences of the greater horizontal (within generation) transmission of plasmids are responsible for this pattern. The tendency for prokaryote transposons, which are also horizontally mobile, to carry genes similar to those commonly on plasmids supports this argument. The apparent trends in eukaryote plasmids and transposons to lack these same characters also accords with predictions of the local adaptation hypothesis, because genes on these genetic units are generally no more horizontally mobile than chromosomal genes. There are theoretical reasons to expect that plasmid genes tend to evolve more rapidly than chromosomal genes. "The selfish interests of genes have manifestly produced 'vehicles' in the forms of organelles, cells, individuals and yet higher units. If evolution is to predict as well as describe, then selfish interests must be understood in the framework of the constraints and opportunities generated by these 'vehicles'" (Buss, 1987, p. 182).
不同生殖单位之间的基因流动，如细菌质粒和染色体之间的基因流动，给进化分析带来了不同寻常的问题。与真核生物相比，在多个选择层次——基因、转座子、质粒、细胞和克隆——上的生殖优势必须同时加以考虑，才能理解质粒的进化。在冲突情况下，没有一个层次能始终占据主导地位，而且一些生殖单位携带的基因会抑制它们自身的繁殖或生存，显然是为了增强携带它们的更高层次生殖单位的繁殖或生存能力。尽管质粒和染色体之间存在基因流动，但某些功能的基因在质粒上出现的倾向很强，而其他一些基因则始终出现在染色体上。通常与质粒相关的功能多种多样，但所有这些功能仅在局部受限的环境中才有用；有人认为，质粒更大的水平（代内）传播所产生的选择后果导致了这种模式。原核生物转座子也具有水平移动性，它们携带与质粒上常见基因相似的基因，这一趋势支持了这一观点。真核生物质粒和转座子明显缺乏这些相同特征的趋势也符合局部适应假说的预测，因为这些遗传单位上的基因通常并不比染色体基因更具水平移动性。从理论上讲，质粒基因往往比染色体基因进化得更快。“基因的自私利益显然产生了以细胞器、细胞、个体以及更高层次单位形式存在的‘载体’。如果进化既要进行预测又要进行描述，那么自私利益必须在这些‘载体’所产生的限制和机会的框架内得到理解”（巴斯，1987年，第182页）。
