Transcription and Translation

CCP asks

What is the phylogenetic distribution of centrioles? Does it match that of cilia / flagella?

Just to summarize what all these cellular structures are, centrioles are distinct structures found in most eukaryotic cells. They are composed of nine microtubule triplets and two of these centrioles come together to form a centrosome, a structure which often sits at the microtubule organizing center (MTOC). Microtubules radiate out of the MTOC to form an aster (see images in this post). Remarkably, in most tissue culture cells, the MTOC sits near the cell centroid where as the nucleus lies beside the MTOC (yes the nucleus is rarely found at the cell centroid). In S phase, when DNA replicates, the two centrioles split apart and duplicate themselves. Whether centrioles undergo semi-conservative replication is a controversial issue (see yesterday’s post). Basal bodies are structures that are resemble centrioles and are thought to be made near, if not right off of, centrioles. Again the idea of whether basal bodies are the result of a semi-conservative replication step is controversial. From the basal bodies, cilia and motile flagella grow – but watch out flagella can mean many things, for example bacterial flagella are totally different from what is being discussed here. Cilia and related structures are long whiplike extensions that have axoneme microtubules in their shaft. Unlike centrioles and basal bodies, ciliary axonemes have nine microtubule doublets and two central microtubule “spokes”.

i-2fbaf0a6fce8a8c461d15892b9849fc4-basalbody copy.jpg

Cilia and flagella are used in two ways:

1) They can move in a whip like motion to either push extracellular items around – think of the ciliated epithelia in your throat that transports phlegm – or that push the cell around – think of how the pulsing flagella propels sperm.

2) They can act as sensory modules. In your rods and cones, cilia are fat and house most of the photoreceptors. In your kidney cilia found in the epithelia are thought to sense the serum filtrate. In fact individuals that have short cilia get a condition known as polycystic kidney disease.

So who has centrioles, basal bodies, sensory and motility cilia?

All eukaryotes had centrioles and basal bodies, then at some point between algae and plants, centrioles and basal bodies were lost. Yeast also lost centrioles and basal bodies but they have a somewhat related structure, the spindle pole body (see this post). To my knowledge all cilia and flagella (of the microtubule sort) have an associated basal body … but for a more detailed picture, here is a figure from a paper from the Zuker lab (ref bellow):

i-616c697b5cb028ddbb512134e8932bbe-cilia.jpg

Sensory Cilium Structure and Evolution(A) The sensory cilium is composed of a connecting cilium (CC) and the outer segment (OS). Cilia are anchored to the inner segment (IS) through the basal body (BB) and rootlet (R). M, mitochondria; AX, axoneme, MT, microtubules; D, discs; TB, tubular body; this is a rich array of tightly packed microtubules at the tip of the cilia. Diagram of Drosophila external chemosensory (B) and mechanosensory (C) organs, bristle socket (bs) and bristle shaft (sh).(D) Ciliary genes are selectively lost from nonciliated organisms but maintained in ciliated organisms. Eukaryote phylogeny showing the two main lineages: Bikonts (gray) and Unikonts (Stechmann and Cavalier-Smith 2002 and Stechmann and Cavalier-Smith 2003). The ancestral nature of the eukaryotic cilia is evident by its presence in organisms from both lineages: D. melanogaster (Dm), H. sapiens (Hs), T. brucei (Tb), and C. reinhardtii (Cr). In three independent events (indicated by red stars), cilia were lost in lineages leading to A. thaliana (At), D. discoideum (Dd), and S. cerevisiae (Sc). Similarly, compartmentalized cilia were lost in P. falciparum (Pf), while motile cilia were lost in C. elegans (Ce). Shown below the phylogenetic tree is an illustration of the various ciliary structures present in the different organisms used in the bioinformatics searches. Also shown is a summary of the screens, which provides the number of genes identified in each search (the numbers in parenthesis refer to previously known genes); Supplemental Figure S1 online shows a flowchart illustrating the four bioinformatics screens.

The first author of this study (Tomer Avidor-Reiss) started a lab in our department and is studying how sensory cilia are formed. To read more on how cilia are synthesized look up inter flagellar transport (IFT) in Pubmed.

Ref:
Tomer Avidor-Reiss, Andreia M. Maer, Edmund Koundakjian, Andrey Polyanovsky, Thomas Keil, Shankar Subramaniam and Charles S. Zuker
Defining Specialized Genes Required for Compartmentalized Cilia Biogenesis
Cell (04) 117:527-539 doi:10.1016/S0092-8674(04)00412-X

Comments

  1. #1 Anthony Oliver Morgan
    October 20, 2009

    What kind of force does it take to separate the sperm head from its tail at the centrioles? In what scale is this?

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