On the evolution of chloroplasts from eukaryotic endosymbionts: A study of Glenodinium foliaceum (Dinophyceae)

Abstract

A study of the marine dinoflagellate Glenodinium foliaceum is presented which attempts to provide information on the process of host-endosymbiont integration in a unicellular system that may be of relevance to current hypotheses on chloroplast evolution. An experimental ultrastructural review of G. foliaceum supports the proposal that the chloroplasts and supernumerary nucleus belong to a morphologically reduced, endosymbiotic chromophyte. DAPI staining shows that the endosymbiont nucleus is fragmented in some cells and appears to be randomly divided by the host's cleavage furrow at cytokinesis. A quantification of the intensity of mithramycin fluorescence from single cells by flow cytometry suggests that both nuclei synthesize their ENA in synchrony. Protocols are outlined for isolating the chloroplasts and dinoflagellate nucleus from G. foliaceum and a method is given for fractionating whole cell lysates to prepare chloroplast, host nuclear, and endosymbiont nuclear DNAs. It is estimated that the symbiont and dinoflagellate nuclei contain about 34 pg and 40 pg of ENA respectively. There is no evidence of amplified genesized ENA molecules in the symbiont nucleus. The results of preliminary re-association experiments show only the presence of highly repeated ENA and sequences of intermediate repetitiveness in total cell ENA, which together comprise about half the genome. The chloroplast DNA is shown to a molecular weight of approximately 103 kb by restriction endonuclease analysis but an attempt to localize the rbc S gene in either the chloroplast or nuclear DNAs using a cloned pea ENA probe proved unsuccessful. Two other anomalously pigmented dinoflagellates, Gyrodinium aureolum and Gymnodinium galatheanum, are demonstrated to lack endosymbionts by electron and fluorescence microscopy, but the atypical ultrastructural or cytological characters of their chloroplasts are considered to indicate an endosymbiotic origin. The possibility that Glenodinium foliaceum could evolve into a similar uninucleate species is discussed in relation to the conservation of genetic information for nuclear-encoded, chloro-plast proteins.