Secondary Endosymbiosis
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Secondary endosymbiosis occurs when a eukaryote engulfs and retains another eukaryote that had a chloroplast from primary endosymbiosis.
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Euglenoids acquired their chloroplasts by engulfing a green alga, gaining a plastid surrounded by extra membranes marking its two-step origin.
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A secondary plastid's extra surrounding membranes are the structural evidence of this two-step history— a primary plastid has only two membranes.