Linkage group XIX of Chlamydomonas reinhardtii has a linear map

The Centriole Mystique

Centrioles organize the microtubule network and mitotic spindle and, as basal bodies, nucleate cilia and flagella. They undergo a beguiling process in which one appears to give rise to another and at a baffling orthogonal geometry. Nucleic acid-based replication has been pondered during cycles of zeniths and nadirs of plausibility, the latter now the state. Centrioles can also arise de novo, and thus the longstanding focus on centriole 'replication' may have led us astray from ground truth. We are in an era in which the assembly pathways of most intracellular machines are becoming understood in considerable detail. But apart from knowing the structure and parts list, little in our extant knowledge conveys how centrioles arise. Here the matters at hand are summarized, and a siren call is sounded.

Centriole evolution

Centrioles are cylindrical structures found at the core of the mitotic spindle pole, which also act as basal bodies to nucleate the formation of cilia. Centrioles have a complex, ninefold symmetric structure, and reproduce by an intriguing duplication process. The complexity and apparent self-reproduction of centrioles raises the question of how such a structure could have evolved, making them a favorite topic for theological speculation by 'intelligent design' creationists. In fact, centrioles are capable of robust self-assembly and can tolerate dramatic perturbations while still maintaining basic functionality. Far from being irreducibly complex, centrioles appear to be based on a rather minimal underlying core structure requiring only a handful of genes to construct.

Molecular map of the Chlamydomonas reinhardtii nuclear genome

We have prepared a molecular map of the Chlamydomonas reinhardtii genome anchored to the genetic map. The map consists of 264 markers, including sequence-tagged sites (STS), scored by use of PCR and agarose gel electrophoresis, and restriction fragment length polymorphism markers, scored by use of Southern blot hybridization. All molecular markers tested map to one of the 17 known linkage groups of C. reinhardtii. The map covers approximately 1,000 centimorgans (cM). Any position on the C. reinhardtii genetic map is, on average, within 2 cM of a mapped molecular marker. This molecular map, in combination with the ongoing mapping of bacterial artificial chromosome (BAC) clones and the forthcoming sequence of the C. reinhardtii nuclear genome, should greatly facilitate isolation of genes of interest by using positional cloning methods. In addition, the presence of easily assayed STS markers on each arm of each linkage group should be very useful in mapping new mutations in preparation for positional cloning.

CHLAMYDOMONAS AS A MODEL ORGANISM

The unicellular green alga Chlamydomonas offers a simple life cycle, easy isolation of mutants, and a growing array of tools and techniques for molecular genetic studies. Among the principal areas of current investigation using this model system are flagellar structure and function, genetics of basal bodies (centrioles), chloroplast biogenesis, photosynthesis, light perception, cell-cell recognition, and cell cycle control. A genome project has begun with compilation of expressed sequence tag data and gene expression studies and will lead to a complete genome sequence. Resources available to the research community include wild-type and mutant strains, plasmid constructs for transformation studies, and a comprehensive on-line database.

Chlamydomonas reinhardtii: biological rationale for genomics

Chlamydomonas reinhardtii has been the subject of genetic, biochemical, cytological, and molecular analyses for over 50 years. It is an ideal model system for the study of flagella and basal bodies as well as the study of photosynthesis and chloroplast biogenesis, cell-cell recognition and fusion, phototaxis, and secretion. It is clear that many of the genes identified in Chlamydomonas have homologs in land plants as well as animals. Thus, a genomic approach in Chlamydomonas will provide another important avenue for the understanding of important biological processes.

The UNI3 gene is required for assembly of basal bodies of Chlamydomonas and encodes delta-tubulin, a new member of the tubulin superfamily

We have cloned the UNI3 gene in Chlamydomonas and find that it encodes a new member of the tubulin superfamily. Although Uni3p shares significant sequence identity with alpha-, beta-, and gamma-tubulins, there is a region of Uni3p that has no similarity to tubulins or other known proteins. Mutant uni3-1 cells assemble zero, one, or two flagella. Pedigree analysis suggests that flagellar number in uni3-1 cells is a function of the age of the cell. The uniflagellate uni3-1 cells show a positional phenotype; the basal body opposite the eyespot templates the single flagellum. A percentage of uni3-1 cells also fail to orient the cleavage furrow properly, and basal bodies have been implicated in the placement of cleavage furrows in Chlamydomonas. Finally when uni3-1 cells are observed by electron microscopy, doublet rather than triplet microtubules are observed at the proximal end of the basal bodies. We propose that the Uni3 tubulin is involved in both the function and cell cycle-dependent maturation of basal bodies/centrioles.

Basal body-associated DNA: in situ studies in Chlamydomonas reinhardtii

We have explored the localization of the uni chromosome (LG XIX) of Chlamydomonas reinhardtii using the technique of in situ hybridization. Using standardized methods of cell fixation together with large chromosome-specific probes we have studied the position of uni DNA sequences in metaphase and interphase cells. We find that in dividing cells uni probes identify a condensed metaphase chromosome that shows no specialized orientation. In interphase cells uni hybridization signals occur on the anterior edge of the nucleus at a position where basal bodies are normally associated with the nuclear envelope. These data reveal an underlying spatial organization of uni chromosomal DNA within the interphase nucleus that may be significant in terms of the fact that this chromosome encodes numerous functions affecting basal body and flagellar assembly.