Interplay Between Spindle Architecture and Function - International Review of Cell and Molecular Biology - page 107

5.1. Spindle features and phylogeny
In small and genetically tractable organisms such as yeast, spindle morphol-
ogy and molecular mechanisms can more easily be correlated because serial
section electron microscopy and electron tomography have allowed detailed
reconstructions of whole spindles in the evolutionarily divergent fungi
S. cerevisiae , Schizosaccharomyces pombe , and Ashbya gossypii ( Ding et al.,
1993; Gibeaux et al., 2012; Winey et al., 1995 ). Interestingly, S. cerevisiae
has one MT per kinetochore, A. gossypii has two, and S. pombe has two
to four MTs per kinetochore. One might imagine that these differences
could help elucidate important mechanisms controlling the number of
MT-kinetochore connections and test hypotheses as to whether certain fac-
tors correlate with particular aspects of spindle architecture. However, mor-
phological similarity does not necessarily indicate common mechanisms.
S. cerevisiae and A. gossypii both possess point centromeres in which a
DNA-binding sequence is necessary and sufficient to drive kinetochore
assembly ( Meraldi et al., 2006 ). In contrast, other fungi, plants, and mammals
form regional centromeres that usually occupy repetitive DNA and are
inherited epigenetically ( Burrack and Berman, 2012b ). Nevertheless, the
functional MT-binding unit of the kinetochore is conserved and allows
remarkable flexibility to maintain accurate chromosome segregation, despite
differences in centromeric sequences, kinetochore protein levels, and the
number of MT-binding sites ( Burrack and Berman, 2012a ).
Recently, by comparing genetic interactions within S. cerevisiae and
S. pombe , it has been possible to confirm widespread conservation of genetic
relationships and also to identify repurposing of orthologous complexes that
have evolved divergent functions and partnerships that alter mitotic mech-
anisms ( Frost et al., 2012 ). For example, despite striking morphological dif-
ferences between the yeast spindle pole body and mammalian centrosome,
the endosomal sorting complex required for transport (ESCRT) plays a role
in their duplication in both S. pombe and in human cells ( Frost et al., 2012 ).
However, this ESCRT function is not conserved in S. cerevisiae .
Figure 3.6 — Cont'd for positioning cues. (E) Plant spindles, like this example from
Arabidopsis thaliana, are acentrosomal and typically take on a barrel-shaped architec-
ture with less-focused poles and no astral MTs. Plant spindle image adapted from
Smertenko et al. (2008) ,( www.plantcell.org ) . Copyright American Society of Plant Biologists.