There are also clearly different requirements for the assembly
of cilia in different cell types within a single organism.
In C. elegans , the different sensory cilia have dramatically different
morphologies, with some having elaborate branched or
fanlike structures and others having more canonical cylindrical
cilia shapes. In addition to these structural differences, the different
types of cilia also differ in their requirement for different
parts of the IFT machinery. Two different kinesins, kinesin-II
and OSM-3, normally cooperate to build sensory cilia, with
OSM-3 specifi cally required to build the distal half of the
cilium, which contains only singlet microtubules and is presumably
specialized for sensory functions ( Ou et al., 2005 ).
Interestingly, in the cilia of the AWB neurons, the two motors
are no longer coupled, and, unlike in the other ciliary types previously
analyzed, OSM-3 is no longer required to build the distal
segment ( Mukhopadhyay et al., 2007 ). In an even more
extreme case, some ciliary structures such as sperm fl agella in
Drosophila melanogaster do not require IFT at all for their assembly
( Han et al., 2003 ). In diatoms, the lack of retrograde IFT
motor along with IFT complex A and Bardet-Biedl syndromeproteins ( Scholey, 2008 ) from the genome suggests that these
parts of the IFT systems are dispensable in some cases ( Merchant
et al., 2007 ). If a similar variability in the requirement for IFT is
seen between different cell and tissue types in humans, one
could imagine that genetic defects in different components of
the IFT machinery might have more severe ciliary defects in
some cell types than in others.