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us, the link is an evolutionary legacy, a remnant of the original archaebacterial-eubacterial connector. The modern organelles (i.e., centriolekinetosomes, untethered nuclei, Golgi, and axostyles) derive from what first ensured genetic continuity of the chimera's components: the karyomastigont, a structure that would have been much more conspicuous to Proterozoic investigators than to us.

We thank our colleagues Ray Bradley, Michael Chapman, Floyd Craft, Kathryn Delisle (for figures), Ugo d'Ambrosio, Donna Reppard, Dennis Searcy, and Andrew Wier. We acknowledge research assistance from the University of Massachusetts Graduate School via Linda Slakey, Dean of Natural Science and Mathematics, from the Richard Lounsbery Foundation, and from the American Museum of Natural History Department of Invertebrates (New York). Our research is supported by National Aeronautics and Space Administration Space Sciences and Comision Interministerial de Ciencia y Tecnologia Project No. AMB98-0338 (to R.G.).

TABLE 1. Karyomastigont distribution in unicellular protoctists

Archaeprotista*

Class

Karyomastigont

Kinetosome

Nucleus

Pelobiontids

+

-

+

Metamonads

+

+

+/-

Parabasalids

+

+

+

Trichomonads

+

+

+/-

Hypermastigids

-

+

-

Chlorophyta

Genus

Karyomastigont

Kinetosome

Nucleus

Chlamydomonas

+

+

-

Chlorella

Acetabularia

+

+

+

Ciliophora

Subphyla

Karyomastigont

Kinetosome

Nucleus

Postciliodesmatophora

-

+

+

Rabdophora

-

+

+

Cyrtophora

-

+

+

Discomitochondria

Class

Karyomastigont

Kinetosome

Nucleus

Amoebomastigotes

+

-

+/-

Kinetoplastids

-

-

-

Euglenids

-/?

+

-

Pseudociliates

-

+

-

Granuloreticulosa

Class

Karyomastigont

Kinetosome

Nucleus

Reticulomixids

-/+

-

+

Foraminiferans

+

-

+

Hemimastigophora

Genus

Karyomastigont

Kinetosome

Nucleus

Stereonema

-

+

+/-

Spironema

-

+

-

Hemimastix

-

+

-

Zoomastigota

Class

Karyomastigont

Kinetosome

Nucleus

Jakobids

?

-

-

Bicosoecids

+/?

+

-

Proteromonads

+

-

-

Opalinids

-

+

+

Choanomastigotes

+

-

-

* Bold entries are protoctist phyla. All species of Archaeprotists lack mitochondria. “Karyomastigont,” “kinetosome,” and “nucleus,” refer to relative proliferation of these organelles. Members of the phylum Archaeprotista group into one of three classes: Pelobiontid giant amoebae; Metamonads, which include three subclasses: Diplomonads (Giardia), Retortamonads (Retortamonas), and Oxymonads (such as Pyrsonympha and Saccinobaculus); and Parabasalia. The Class Parabasalia unites trichomonads, devescovinids, calonymphids, and hypermastigotes such as Trichonympha. The phylum Discomitochondria includes amoebomastigotes, kinetoplastids (Trypanosoma), euglenids, and pseudociliates (Stephanopogon). The Hemimastigophora comprise a new southern-hemisphere phylum of free-living mitochondriate protists (Foissner et al. 1988). Hemimastigophorans probably evolved from members of the kinetoplastid-euglenid taxon (Foissner and Foissner, 1993). If so, they represent a seventh example of release of the nucleus from the karyomastigont and subsequent kinetosome proliferation. The phylum Granuloreticulosa includes the shelled (Class Foraminifera) and unshelled (Class Reticulomyxa) foraminiferans. The phylum Zoomastigota includes five classes of single-celled, free-living and symbiotrophic mitochondriate protists; Jakobids, Bicosoecids, Proteromonads, Opalinids, and Choanomastigotes. Details of the biology are in the work by Margulis et al. (1993). A current phylogeny is depicted in Figure 2.

† Structure known but not demonstrated for all species at the electron microscopic level.



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