Evolutionary consequences of the loss of photosynthesis in the nonphotosynthetic chlorophyte alga Polytoma.

摘要

Chlamydomonad algae in the genus Polytoma have lost the ability to do photosynthesis. This study involved an investigation of two plastid (leucoplast) genes from several Polytoma species, looking for evolutionary consequences, if any, of the loss of photosynthetic ability. In other non-photosynthetic chlorophyte or plant lineages, most leucoplast genes involved in the expression (transcription or translation) of other leucoplast genes tend to be selectively retained and appear intact and functional. The first hypothesis tested in this study was that the leucoplast genome in Polytoma is still present, functional, and codes for functional expression genes. Another observation in other non-photosynthetic lineages is accelerated evolution in many functional leucoplast genes, compared to close photosynthetic relatives. The second hypothesis tested was that Polytoma leucoplast expression genes would also show accelerated nucleotide substitution rates. DNA sequences for two leucoplast expression genes (rrn16 and tufA) from two or more Polytoma species were analyzed for evidence of functional features, and substitution rates were compared to rates in close photosynthetic relatives. In Polytoma sp. SAG 62-27, all features for both leucoplast genes were consistent with functionality, but increased substitution rates were not observed in either gene. P.62-27 may be a lineage that lost photosynthesis more recently than Polytoma uvella UTX 964 and Polytoma obtusum DH1, because rrn16 genes from P.u.964 and P. obtusum displayed functional features but showed increased substitution rates compared to close photosynthetic relatives. Increased evolution was also observed in tufA from P. obtusum, which displayed higher substitution rates for both synonymous and non-synonymous types of substitutions. These data imply that selection is relaxed in this expression gene, and that the plastid mutation rate in P. obtusum is higher than in chloroplasts of photosynthetic relatives. Codon bias is also much reduced in this tufA gene, compared to photosynthetic relatives, again implying that selection is relaxed in this gene. A hypervariable region of the tufA gene displays several chlamydomonad-specific insertions.