Negre Bárbara, Ranz José María, Casals Ferran, Cáceres Mario, Ruiz Alfredo
Departament de Genètica i de Microbiologia, Universitat Autònoma de Barcelona, Bellaterra (Barcelona) Spain.
Mol Biol Evol. 2003 Dec;20(12):2042-54. doi: 10.1093/molbev/msg238. Epub 2003 Aug 29.
Hox genes encode transcription factors involved in the specification of segment identity in the early metazoan embryo. These genes are usually clustered and arranged in the same order as they are expressed along the anteroposterior body axis. This conserved genomic organization has suggested the existence of functional constraints acting on the genome organization. Partial disassembly of the Hox gene complex (HOM-C) in Caenorhabditis elegans and in two different Drosophila lineages, however, calls into question whether this cluster organization is absolutely required for proper function. Here we report a new split of the HOM-C discovered in the species of the Drosophila repleta group, which relocated the most anterior gene of the complex, lab, to a distant chromosomal site near the two most posterior Hox genes, abd-A and Abd-B. To investigate the evolutionary consequences of natural rearrangements of the Hox gene complex, the gene lab has been cloned and sequenced in D. buzzatii, a member of the D. repleta group with the split, and in D. virilis, a member of a different species group without the split. The results show that the structure of lab in D. buzzatii is intact and place the breakpoint at least 8 kb from its transcription start site. The nucleotide sequence evolution of lab in the genus Drosophila has been investigated by means of maximum likelihood methods. No significant variation has been observed among lineages in the rate of nucleotide substitution or in the nonsynonymous/synonymous substitution ratio. Seemingly, the relocation of lab has not induced a change in evolution rate or degree of functional constraint. Nevertheless, further work is needed to ascertain whether the lab-pb split has had any effects on gene expression.
Hox基因编码参与后生动物早期胚胎节段特性决定的转录因子。这些基因通常成簇排列，并且其排列顺序与它们沿前后体轴的表达顺序相同。这种保守的基因组组织表明存在作用于基因组组织的功能限制。然而，秀丽隐杆线虫以及两个不同果蝇谱系中Hox基因复合体（HOM-C）的部分拆解，让人质疑这种簇状组织对于正常功能是否绝对必要。在此，我们报告在拟暗果蝇组物种中发现的HOM-C的一种新的分裂情况，该分裂将复合体中最前端的基因lab，重新定位到靠近两个最后端的Hox基因abd-A和Abd-B的一个遥远染色体位点。为了研究Hox基因复合体自然重排的进化后果，已在具有这种分裂的拟暗果蝇组成员巴氏果蝇以及没有这种分裂的不同物种组成员粗壮果蝇中克隆并测序了基因lab。结果表明，巴氏果蝇中lab的结构完整，并且断点距离其转录起始位点至少8 kb。已通过最大似然法研究了果蝇属中lab的核苷酸序列进化。在谱系之间，未观察到核苷酸替换率或非同义/同义替换率有显著差异。看起来，lab的重新定位并未导致进化速率或功能限制程度的改变。然而，仍需要进一步的工作来确定lab-pb分裂是否对基因表达有任何影响。
