64 Nucleus‐encoded, plastid‐targeted glyceraldehyde‐3‐phosphate dehydrogenase (GAPDH) indicates a single origin for chromalveolate plastids

Abstract

Plastids (the photosynthetic organelles of plants and algae) ultimately originated through an endosymbiosis between a cyanobacterium and a eukaryote. Subsequently, plastids spread to other eukaryotes by secondary endosymbioses that took place between a eukaryotic alga and a second eukaryote. Recently, evidence has mounted in favour of a single origin for plastids of apicomplexans, cryptophytes, dinoflagellates, haptophytes, and heterokonts (together with their non‐photosynthetic relatives, collectively termed chromalveolates). As of yet, however, no single molecular marker has been described which supports a common origin for all of these plastids. One piece of the evidence for a single origin of chromalveolate plastids came from plastid‐targeted glyceraldehyde‐3‐phosphate dehydrogenase (GAPDH), which originated by a gene duplication of the cytosolic form. However, no plastid GAPDH has been characterized from haptophytes, leaving an important piece of the puzzle missing. We have sequenced genes encoding cytosolic, mitochondrial‐targeted, and plastid‐targeted GAPDH proteins from a number of haptophytes and heterokonts, and found the haptophyte homologues to branch within the strongly supported clade of chromalveolate plastid‐targeted GAPDH genes. Interestingly, plastid‐targeted GAPDH genes from the haptophytes were more closely related to apicomplexan genes than was expected. Overall, the evolution of plastid‐targeted GAPDH reinforces other data for a red algal ancestry of apicomplexan plastids, and raises a number of questions about the importance of plastid loss and the possibility of cryptic plastids in non‐photosynthetic lineages such as ciliates.