Boudreau E, Turmel M
Program in Evolutionary Biology, Canadian Institute for Advanced Research, Département de biochimie, Faculté des sciences et de génie, Université Laval.
Mol Biol Evol. 1996 Jan;13(1):233-43. doi: 10.1093/oxfordjournals.molbev.a025560.
We have constructed a physical and gene map for the chloroplast DNA (cpDNA) of the unicellular green alga Chlamydomonas gelatinosa, a close relative of Chlamydomonas reinhardtii. At 285 kb, the C. gelatinosa cpDNA is 89 kb larger than its C. reinhardtii counterpart. The alterations in the order of 77 genes on the cpDNAs of these green algae are attributable to nine inversions and one event of expansion/contraction of the inverted repeat. These rearrangements are much more extensive than those previously reported between the cpDNAs of the closely related Chlamydomonas moewusii and Chlamydomonas pitschmannii. Because the divergence level of the C. gelatinosa and C. reinhardtii chloroplast-encoded large subunit rRNA gene sequences is equivalent to that of the corresponding C. moewusii and C. pitschmannii sequences, our results may suggest that, in the same period of time, there have been more numerous rearrangements in the lineage comprising C. gelatinosa and C. reinhardtii than in the lineage comprising C. moewusii and C. pitschmannii. Alternatively, given that substitution rates in chloroplast genes are not necessarily uniform across lineages, the extensive rearrangements between the C. gelatinosa and C. reinhardtii cpDNAs may reflect a longer divergence period for this pair of Chlamydomonas species compared to that for the C. moewusii/C. pitschmannii pair. We have also found that, like its C. reinhardtii homologue but unlike its C. moewusii and C. pitschmannii counterparts, the C. gelatinosa cpDNA features a large number of dispersed repeated sequences that are readily detectable by Southern blot hybridization with homologous fragment probes. Assuming that the two pairs of closely related Chlamydomonas species diverged at about the same time, these data suggest that the susceptibility of Chlamydomonas cpDNAs to rearrangements is correlated with the abundance of repeated sequences. Preliminary characterization of a 345-bp C. gelatinosa cpDNA region containing a repeated sequence by both DNA sequencing and Southern blot analysis has revealed no sequence homology between this region and the cpDNAs of C. reinhardtii and other Chlamydomonas species.
我们构建了单细胞绿藻胶衣藻叶绿体DNA（cpDNA）的物理图谱和基因图谱，胶衣藻是莱茵衣藻的近亲。胶衣藻的cpDNA大小为285 kb，比莱茵衣藻的cpDNA大89 kb。这些绿藻cpDNA上77个基因顺序的改变归因于9次倒位和一次反向重复序列的扩增/收缩事件。这些重排比先前报道的近缘种莫氏衣藻和皮氏衣藻的cpDNA之间的重排更为广泛。由于胶衣藻和莱茵衣藻叶绿体编码的大亚基rRNA基因序列的分歧水平与相应的莫氏衣藻和皮氏衣藻序列的分歧水平相当，我们的结果可能表明，在同一时期内，包含胶衣藻和莱茵衣藻的谱系中的重排比包含莫氏衣藻和皮氏衣藻的谱系中的重排更为频繁。或者，鉴于叶绿体基因的替代率在不同谱系中不一定均匀，胶衣藻和莱茵衣藻cpDNA之间广泛的重排可能反映了这对衣藻物种的分歧时间比莫氏衣藻/皮氏衣藻对更长。我们还发现，与莱茵衣藻的同源物一样，但与莫氏衣藻和皮氏衣藻的对应物不同，胶衣藻的cpDNA具有大量分散的重复序列，通过与同源片段探针的Southern印迹杂交很容易检测到。假设这两对近缘衣藻物种大约在同一时间分化，这些数据表明衣藻cpDNA对重排的敏感性与重复序列的丰度相关。通过DNA测序和Southern印迹分析对包含重复序列的345 bp胶衣藻cpDNA区域进行的初步表征表明，该区域与莱茵衣藻和其他衣藻物种的cpDNA之间没有序列同源性。
