EVOLUTION OF CHLOROPHYLL AND BACTERIOCHLOROPHYLL - THE PROBLEM OF INVARIANT SITES IN SEQUENCE-ANALYSIS

Competing hypotheses seek to explain the evolution of oxygenic and ano xygenic processes of photosynthesis, Since chlorophyll is less reduced and precedes bacteriochlorophyll on the modern biosynthetic pathway, it has been proposed that chlorophyll preceded bacteriochlorophyll in its evolution, However, recent analyses of nucleotide sequences that e ncode chlorophyll and bacteriochlorophyll biosynthetic enzymes appear to provide support for an alternative hypothesis, This Is that the evo lution of bacteriochlorophyll occurred earlier than the evolution of c hlorophyll, Here we demonstrate that the presence of invariant sites i n sequence datasets leads to inconsistency in tree building (including maximum-likelihood methods). Homologous sequences with different biol ogical functions often share invariant sites at the same nucleotide po sitions, However, different constraints can also result in additional invariant sites unique to the genes, which have specific and different biological functions, Consequently, the distribution of these sites c an be uneven between the different types of homologous genes. The pres ence of invariant sites, shared by related biosynthetic genes as well as those unique to only some of these genes, has misled the recent evo lutionary analysis of oxygenic and anoxygenic photosynthetic pigments, We evaluate an alternative scheme for the evolution of chlorophyll an d bacteriochlorophyll.