How have apicomplexan plastids evolved?
A hypothesis
Bodyl A.
Zoological Institute, University of Wroclaw, ul. Sienkiewicza 21, 50-335 Wroclaw, Poland
Köhler et al. [1] provided first evidence that the apicomplexan plastids (apicoplasts) were surrounded by four membranes, and a green alga was their ancestor. They suggested that the apicoplasts had evolved independently from the dinoflagellate plastids in an aplastidal protist. However, the conversion of a eukaryotic alga into a plastid may face an array of very serious difficulties [2] which seem to exclude the polyphyletic origin of multi-membraned plastids. Thus, it should be supposed that the apicoplasts originated in another way. According to my recent hypothesis, four-membrane plastids have evolved via the replacement of three-membrane plastids [3]. This assumption makes it possible to overcome the difficulties encountered by the secondary endosymbioses, and so provides a good theoretical basis for the hypothesis on the polyphyly of four-membrane plastids. Although very diverse kinds of plastids are known in dinoflagellates [4], the three-membrane plastids containing peridinin are their typical form. The remaining plastids most likely have a recent and independent eukaryotic origin. It can be supposed that these "unusual" plastids have evolved via the substitution of the typical dinoflagellate plastids. This model is supported by the case of some dinoflagellates, e.g. Peridinium balticum, which harbour two kinds of plastids of different origin. I suggest that such a dinoflagellate-like protist, which had three-membrane plastids as well as four-membrane plastids, was the ancestor of the phylum Apicomplexa. If so, also the apicoplasts could have evolved via the replacement of three-membrane plastids.
[1] Köhler S. et al. (1997) Science 275: 1485-1489.
[2] Cavalier-Smith T. (1986) In: Progress in Phycological Research, vol. 4 (F.E. Round and D.J. Chapman, eds.). Biopress, Bristol, pp. 309-347.
[3] Bodyl A. (1997) Bot. Acta 110: 395-400.
[4] Schnepf E. (1993) In: Origins of Plastids (R.A. Lewin, ed.). Chapman & Hall, New York, pp. 53-76.
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