RESEARCH REVIEW INTERNATIONAL MICROBIOLOGY (2006) 9:163-172 www.im.microbios.org Jan Sapp Two faces of the prokaryote concept Department of Biology, York University, Toronto, Ontario, Canada Summary. Bacteria had remained undefined when, in 1962, Roger Y. Stanier and C.B. van Niel published their famed paper “The concept of a bacterium.” The articulation of the prokaryote–eukaryote dichotomy was a vital moment in the his- tory of biology. This article provides a brief overview of the context in which the prokaryote concept was successfully launched in the 1960s, and what it was meant to connote. Two concepts were initially distinguished within the proka- ryote–eukaryote dichotomy at that time. One was organizational and referred to comparative cell structure; the other was phylogenetic and referred to a “natural” classification. Here, I examine how the two concepts became inseparable; how the prokaryotes came to signify a monophyletic group that preceded the eukaryotes, and how this view remained unquestioned for 15 years, until the birth of molecu- lar evolutionary biology and coherent methods for bacteria phylogenetics based on 16S rRNA. Today, while microbial phylogeneticists generally agree that the Address for correspondence: Department of Biology prokaryote is a polyphyletic group, there is no agreement on whether the term York University should be maintained in an organizational sense. [Int Microbiol 2006; 9(3):163- 4700 Keele Street 172] Toronto, Ontario M3J 1P3, Canada Tel. +1-416-7362100. Fax +1-416-7365698 Email: [email protected] Key words: prokaryote · eukaryote · phylogeny · evolution · taxonomy acknowledged that one of their predecessors, Edouard The scandal of bacteriology Chatton (1883–1947) [47 bis)] had also recognized that the cell structure of bacteria and blue-green algae was different In their famed paper of 1962, “The concept of a bacterium”, from that of other organisms: Roger Y. Stanier (1916–982) and Cornelis B. van Niel (1897–1985) emphasized that the nature and relationships of “It is now clear that among organisms there are two differ- bacteria remained unresolved: “the abiding intellectual scan- ent organizational patterns of cells, which Chatton (1937) dal of bacteriology has been the absence of a clear concept of (sic) called, with singular prescience, the eukaryotic and a bacterium” [51]. Structurally speaking, there had been sev- prokaryotic type. The distinctive property of bacteria and eral enduring issues. Was it true that bacteria lacked a nucle- blue-green algae is the prokaryotic nature of their cells. It is us? How could one distinguish bacteria from viruses? What on this basis that they can be clearly segregated from all about blue-green algae? Did they also lack a nucleus? Stanier other protists (namely, other algae protozoa and fungi), and van Niel explained that a satisfactory description of the which have eukaryotic cells” [51]. difference between bacteria and blue-green algae on the one hand and viruses and nucleated cells on the other could be Since that time, Chatton’s “prophetic insight” has been articulated only after the advances in microscopy, molecular often repeated, and indeed mythologized at the font of the biology, and genetics following the World War II. They prokaryote–eukaryote dichotomy both as an organizational 164 INT. MICROBIOL. Vol. 9, 2006 SAPP and a taxonomic distinction at the highest level [43]. As tals. Nay, even the difference between unnucleated monera recent scholarship has shown, Chatton did not articulate the (as cytodes) and the real nucleated cells may fairly be regard- distinction (he wrote very little about it) and he certainly did ed as greater still” [21]. not see it as the basis for taxa, nor did Stanier and van Niel Bacteria had been considered to be plants since the 1850s. when they revitalized the dichotomy in 1962 [43]. Moreover, They were often called Schizomycetes (fission fungi), and many microscopists since the turn of the 20th century had when they were grouped with blue-green algae they were also distinguished bacteria and blue-green algae from other called Schizophyta (fission plants), as Ferdinand Cohn organisms that possessed a nucleus. The nature of their argu- (1828–1898) named them in 1875 [8,22]. Haeckel argued ments and the significance of the distinction differ between that bacteria had nothing in common with fungi, and that the writers; it is important to differentiate them. only real comparison between chromacea and plants was with the chromatophores (chromatella) (chloroplasts). Thus, he suggested that chloroplasts had evolved as “a symbiosis The meanings of Monera between a plasmodomonous green and plasmophagus not- green companion” [21]. Ernst Haeckel (1834–1919) was perhaps the first biologist to Did bacteria and cyanophyceae possess a nucleus? Much group bacteria and blue-green algae together as organisms depended on the definition of nucleus; and even observations that lacked a nucleus. In his Generelle Morphologie der were far from certain. Bacterial anatomical detail fell below Organismen (1866), he proposed that life-forms without a the resolution of the light microscope of the early twentieth nucleus, which he called “monera,” were at the lowest stage century. Although it did seem apparent that bacteria and of a third kingdom, Protista [20]. In biology textbooks today, cyanophyceae lacked a nucleus enclosed by a membrane, both Protista and Monera are often presented as kingdoms. there were bacterial granules that stained with the same dyes However, although the kingdom Monera is attributed to used to stain chromatin of other cells. Thus, many research- Haeckel, his concept differed from today’s in two fundamen- ers insisted, in direct opposition to Haeckel’s views, that bac- tal ways: Haeckel was uncertain whether that group of organ- teria possessed scattered nuclei, comparable to chromosomes isms had a single origin or whether it originated independent- of higher organisms. “If this identification is correct”, Ed- ly several times; and his monera were conceived of as precel- mund Beecher Wilson (1856–1939) wrote in 1900, in The lular entities supposed to lack all trace of the hereditary Cell in Development and Inheritance, “such forms probably determinants present in other organisms. give us the most primitive condition of the nuclear substance, The monera were born of Haeckel’s monist philosophy, which only in higher forms is collected into a distinct mass which demanded the removal of explanatory boundaries enclosed by a membrane” [62]. between life and non-life. Monera complied as organisms Still, these matters remained unsettled [13,44,57]. In that would have arisen spontaneously from inorganic materi- 1927, Edwin Copeland (1873–1964) argued that bacteria als. Most of the organisms Haeckel assigned to the group in deserve a kingdom of their own when he wrote that “there is 1866 were later shown to be either non-existent or belie the no other one thing so important in systematic biology as the definition of monera as organisms without nuclei. fact that the grouping of organisms reflects and expresses In his The Wonders of Life, published in 1904, Haeckel their true relationships” [9]. In 1938, his son, Herbert Cope- emphasized that bacteria and chromacae (blue-green algae) land (1902–1968) proposed that a kingdom be named with were, however, true to the definition [21]. “The whole vital Haeckel’s term Monera, based on two assumptions: (1) that activity of the simplest monera”, he wrote, “especially the bacteria and blue-green algae are “the comparatively little chromacae is confined to their metabolism, and is therefore a modified descendants of whatever single form of life purely chemical process, that may be compared to the catal- appeared on earth, and (2) that they are sharply distinguished ysis of inorganic compounds” [20]. Bacteria and chromacae from other organisms by the absence of nuclei” [10]. He simply lacked all traces of what Haeckel regarded as “the therefore proposed four kingdoms: Monera, Protista, Planta, first, oldest, and most important process of division of and Animalia. labour”, the nucleus, which “discharges the functions of Stanier’s and van Niel’s views were inconsistent. In 1941, reproduction and heredity, and the cytoplasm of the cell body they followed Copeland and assigned bacteria and blue-green [which] accomplishes the metabolism, nutrition and adapta- algae to the kingdom Monera [50]. They also expanded the tion” [21]. The difference between monera and any higher characterization of the group by adding two additional and organism, he said was “greater in every respect than the dif- equally negative criteria: the absence of plastids, and the ference between the organic monera and the inorganic crys- absence of sexual reproduction. Then, in 1955, van Niel PROKARYOTE VS. EUKARYOTE INT. MICROBIOL. Vol. 9, 2006 165 renounced the Monera on the very same three grounds. New The organizational difference between prokaryote and evidence based on electron microscopy seemed to suggest eukaryote was made unequivocal; but, as they and other that bacteria possessed a nucleus, and that the photosynthet- authors recognized, the prokaryote was still defined in nega- ic bacterium Rhodospirillum rubrum possessed plastid-like tive terms [43]. Eukaryotes had a membrane-bound nucleus entities [57]. Moreover, he argued, demonstrations of recom- that divided by mitosis, a cytoskeleton, an intricate system of bination in mixed cultures of bacteria by Lederberg and internal membranes, mitochondria