One of the enduring mysteries in the centrosome field has been how centrioles duplicate so that only one procentriole is assembled at, but slightly separated from, the proximal end of each mother centriole. This numerical and spatial specificity has, in the past, led workers to embrace the possibility that a ‘template’ on the mother centriole provides a unique site for procentriole assembly. Indeed, the notion of ‘templated duplication’ is still part of the contemporary lexicon. There are two key facets to the template hypothesis: first, there is only one unique site on the mother centriole that can seed the assembly of the new procentriole, and second, only one template is formed per mother centriole in each cell cycle. There must also be a reduplication block to prevent a new template from being formed on the daughter centriole in that cell cycle. Together, these characteristics provide the essential numerical and spatial control over procentriole assembly.
Although the term ‘template’ has often been used, the concept(s) behind this term have typically not been clearly spelled out. One of the most comprehensive discussions of the many possible meanings of this term can be found in the thoughtful review of Fulton (1971)
. The most literal interpretation of the term implies the existence of a preformed structure that directly patterns the nine triplet microtubules and perhaps the cartwheel structure at the proximal end of the procentriole. Such a ‘rubber stamp’ in the parlance of Fulton has derived support from exacting ultrastructural studies of basal body duplication. In Paramecium
, there is a plaque next to the parent basal body upon which the barrel of triplet microtubules progressively assembles (Dippell, 1968
), and in Chlamydomonas
, there is a looped fibre at the mother basal body containing nine densely staining foci that later elaborate into triplet microtubules [Gould, 1975
; reviewed by Beisson and Wright (2003)
]. However, it has never been certain whether these structures are the proposed template on the mother basal body or alternatively are the early assembly intermediates of the forming daughter basal bodies that were seeded by some other mechanism. Also, careful ultrastructural characterization of gamma-tubulin distribution at centrioles led to the proposal that the centriole duplication starts with the formation of a single focus of gamma tubulin on the wall of the mother centriole, which forms a template from which the microtubules of the procentriole grow (Fuller et al., 1995
). Another interesting proposal holds that fibers, which link the proximal ends mother and daughter centrioles, remain attached to both at the time of centriole disjoining in mitosis. When centriole duplication begins in the next cell cycle, these attached fibers form a locus on the proximal end of each parent centriole that serves as the unique site for the self assembly of the initiating cartwheel structure of the procentriole (Salisbury, 2008
Another version of the concept was outlined by Mazia et al. (1960)
who postulated that the centriole contains a self-reproducing seed of molecular dimensions that provides the specific site for the self-assembly of the procentriole. They propose that this self-reproducing seed (a ‘self-replicating entity’ or SRE in the words of Fulton, 1971
) organizes a copy of itself at each cell cycle to provide the assembly site for the next generation centriole. In effect, there is a semiautonomous organelle whose duplication is distinct from the visible centriolar structures. Though nobody has found a morphological, biochemical or genetic correlate for this SRE; a recent study has provided functional observations consistent with the existence of such an entity (Collins et al., 2010
). These workers report that when microtubule assembly is blocked (not only cytoplasmic microtubules, but also centriolar triplet microtubules), over time there is the accumulation of ‘cryptic’ centriole precursors that soon elaborate into centrioles after the microtubule inhibitor is washed out.
Students of the centriole have also long recognized the possibility that a new centrioles or basal body simply self-assembles in the vicinity of the mother centriole from subunit pools without a need for some entity to be transferred from the mother to the daughter structure [reviewed by Fulton (1971)
; Rodrigues-Martins et al, 2007
]. Left unanswered in this proposal, however, were the important questions of why the procentriole normally forms only in close spatial proximity to its parent centriole and more puzzling why only one daughter centriole is self-assembled. One possible answer is that the mother centriole provides a special and unique site or singularity that serves as a focus for the assembly of a macromolecular complex that, in time, seeds the progressive assembly of the familiar barrel of triplet microtubules and other structures (Rogers et al., 2009
; Guichard et al., 2010
). Presumably, this proposed singularity would have to be under tight numerical control to ensure that only one is present at each mother centriole at each cell cycle.
The appeal of the template or singularity hypothesis is, in part, driven by observations that, under normal circumstances, procentriole copy number is under tight control despite abundant and complete cytoplasmic pools of centriolar subunits. For example, early sea urchin and frog zygotes duplicate centrioles in proper copy number at each cell cycle despite the fact that they have complete pools of subunits on hand at fertilization to make many centrioles (Gard et al., 1990
; Sluder et al., 1990
). Early Drosophila
embryos properly duplicate centrioles in the face of subunit pools sufficient to assemble 2 × 1013
centriole pairs (Rodrigues-Martins et al, 2007
). Even mammalian somatic cells contain enough centriolar subunits to assemble multiple procentrioles within a single cell cycle. When Plk4 kinase is overexpressed, multiple daughter centrioles assemble around each mother, forming what looks like a rosette around the centrally located mother centriole () (Habedanck et al., 2005
; Kleylein-Sohn et al., 2007
). A similar phenotype is induced by overexpression of SAS-6, a central cartwheel protein needed for the initiation of procentriole assembly (Strnad et al., 2007
; Duensing et al., 2009
). In this case, there are adequate pools of all other centriolar subunits present to match the overexpressed SAS-6 in assembling multiple procentrioles. Given these observations, having the number of procentriole assembly sites (templates or singularities) strictly limited has been an attractive way to explain why only one procentriole is assembled at each mother centriole.
Over the years, the nature and composition of this proposed singularity or SRE has been the subject of much speculation. The seemingly semiconservative nature of centriole duplication and separation of mother–daughter pairs to sister centrosomes has obvious parallels to DNA replication. Naturally, this similarity inspired workers more than 40 years ago to propose that centrioles could be semiautonomous organelles with their own DNA, much like mitochondria. DNA replication is a modern paradigm for a templated reproductive process in which information and copy number are under rigid control. A variant on this theme was the hypothesis that centrioles, like ribosomes, contain RNA that serves a structural role during their assembly. These possibilities inspired numerous studies, the vast majority of which concentrated on trying to demonstrate the existence of centriole-/basal body-specific DNA or RNA under the assumption that presence implies function. All of this work was fraught with serious technical problems and ultimately produced inconclusive observations [reviewed by Fulton (1971)
; Marshall and Rosenbaum, 2000
]. Presently, there is no compelling evidence for the direct involvement of DNA or RNA in the duplication of centrioles or basal bodies.