Protistology 5 (2/3) 146–155 (2007/8)
The flagellar apparatus structure of Apusomonas proboscidea and apusomonad relationships
Sergey A. Karpov
Biological Faculty, Herzen State Pedagogical University, St. Petersburg, Russia
Summary
Apusomonads represent one of the most enigmatic groups among free-living heterotrophic flaagellates, as
in spite of having knowledge on their morphology and molecular phylogeny we still cannot find their nearest
relatives. The cytoskeleton, particularly the flagellar apparatus, is regarded as an evolutionary conservative
structure and often helps to show the relationships and taxonomic position of a group. The flagellar apparatus
structure of Apusomonas proboscidea was investigated by serial sections and a 3D reconstruction is presented
here for the first time. Ap. proboscidea has 2 smooth heterocont flagella without paraxonemal rods and
with short simple transition zones. Both kinetosomes have a thin cylinder and a cartwheel structure. The kinetosome
of the anterior flagellum associates with a broad right microtubular root (RM), a dorsal left root of 2 microtubules
and with a striated fibrillar root. The kinetosome of the posterior flagellum associates with a rhizostyle (RH).
Both kinetosomes are connected to a multilayered fibrillar structure (MFS), located at the apical part of proboscis.
This MFS initiates RH and RM. Each kinetosome has prominent lateral feet, which fuse apart from them, and form a
thick electron dense axis. The common features of the apusomonad cytoskeleton have been established and compared
with the flagellar apparatuses of other protists. Some cytoskeletal elements of apusomonads were found in other
protists, but there are no protists with the same set of roots. The most similar groups in this respect are
cryptomonads and myxogastrids; Multicilia and excavates have fewer common features. This description of
common cytoskeletal characters for apusomonads confirms their monophyly, as revealed by molecular data, but
still shows their isolated position among the eukaryotes.
Key words: Apusomonas proboscidea, apusomonads, flagellar apparatus, phylogeny, taxonomy,
|