In situ visualization of a simple bipartite kinetochore with a single microtubule attachment in Giardia intestinalis (Metamonada)
Abstrakt
To understand general features in evolution of kinetochore organization, investigating a wide range of mitotic mechanisms in various non-model eukaryotes is necessary. A binucleate flagellate Giardia intestinalis is a representative of highly divergent eukaryotic lineage of Metamonads.
FIB/SEM tomography was used to investigate ultrastructural details of its mitotic architecture, including kinetochores. Giardia undergoes semiopen mitosis, with the nuclear envelope remaining intact except for polar fenestrae, allowing microtubules to enter the nucleoplasm.
At the onset of mitosis, the nuclear envelope bends inward, forming a concave depression at the spindle poles. Spindle microtubules emanate from a cytoplasmic fuzzy microtubule organizing center near the flagellar basal bodies.
Kinetochoral microtubules enter the nucleoplasm and bind to kinetochores. A small bipartite kinetochore composed of a dense inner disk, approximately 46 nm in diameter, and a two-armed outer fork, is attached to just one microtubule.
To our knowledge, this is the first in situ evidence of a one-microtubule attachment to a kinetochore, which could represent a basic eukaryotic situation.