Foraminifera from the genus Elphidium are heterotrophic protists that graze on diatoms and sequester chloroplasts from their algal preys, while digesting the rest of the diatom cell. During that process, known as kleptoplastidy, the acquired plastids remain active inside the foraminiferan cell for several months. As most of the genes required to sustain the activity of the chloroplasts are encoded in the diatom nucleus, it is unknown how the host cell can maintain the photosynthetic activity without this information. It has been proposed that maintenance of kleptoplastids could be explained by horizontal gene transfer (HGT). To test this hypothesis we obtained 17,125 EST sequences of Elphidium margaritaceum, and we screened this data set for diatom nuclear-encoded proteins having a function in photosynthetic activity or plastid maintenance. Our analyses show no evidence for the presence of such transcriptionally active genes and suggest that HGT hypothesis alone cannot explain the chloroplast's longevity in Elphidium.
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