FIG. 27.20

How did photosynthesis spread through the Eukarya?

BACKGROUND Many different branches on the eukaryotic tree include photosynthetic species, commonly interspersed with non-photosynthetic lineages. Did eukaryotes gain photosynthesis once, by the endosymbiotic incorporation of a cyanobacterium, and then lose this capacity multiple times? Or, is the history of photosynthetic endosymbioses more complicated, involving multiple events of symbiont capture and transformation?

HYPOTHESIS 1 Photosynthesis was established early in eukaryotic evolution and was subsequently lost in some lineages.

HYPOTHESIS 2 Eukaryotes acquired photosynthesis multiple times by repeated episodes of endosymbiosis.

PREDICTIONS The two hypotheses make different predictions. If photosynthesis were acquired in a single common ancestor of all living eukaryotes and then lost in some lineages, we would expect chloroplast and nuclear gene phylogenies to show the same pattern of branching. If photosynthesis were acquired multiple independent times, we would expect chloroplast and nuclear phylogenies to show different patterns.

EXPERIMENT Chloroplasts have DNA, so scientists developed a phylogenetic tree based on molecular sequence comparison of chloroplast genes (Fig. 27.20a). This phylogeny was then compared with a second phylogeny, for all eukaryotes, based on molecular sequence comparison of nuclear genes (Fig. 27.20b).

RESULTS Molecular sequence comparisons show that evolutionary relationships among chloroplasts do not mirror those based on nuclear genes. The chlorarachniophyte algae and photosynthetic euglenids (shown in green) fall in a monophyletic group with the green algae when chloroplast DNA is analyzed (Fig. 27.20a). However, these groups lie far from green algae in the phylogeny based on nuclear genes (Fig. 27.20b). Similarly, brown algae, diatoms, most photosynthetic dinoflagellates, and cryptophyte algae (shown in red) form a monophyletic group with the red algae in the phylogeny based on chloroplast DNA (Fig. 27.20a), but analysis of nuclear genes places them in different groups (Fig. 27.20b).

CONCLUSION The chloroplast and nuclear phylogenies show different patterns of branching, supporting the hypothesis that photosynthesis spread through the Eukarya by means of multiple eukaryotic endosymbionts.

SOURCE Hackett, J. D., et al. 2007. “Plastid Endosymbiosis: Sources and Timing of the Major Events.” In Evolution of Primary Producers in the Sea, edited by P. G. Falkowski and A. H. Knoll, 109–132. Boston: Elsevier Academic Press.