nal end of the posterior kinetosome to the ante-
rior kinetosome (Fig.3.4k) (Barr and Allan
1985 ; Beakes 1987 ). In zoospores ofOlpidiopsis
saprolegniaethree electron-dense props inter-
connect the two kinetosomes (Fig.3.16i), but
there is no SF (Bortnick et al. 1985 ). However,
SFs have been reported inHaptoglossaamongst
basal oomycete genera (Beakes et al. 2012 ) but
have not been found in either labyrinthulid or
hyphochytrid zoospores (Fig.3.16c) (Barr and
Allan 1985 ; Porter 1990 ). However, an almost
identical structure has been reported in bico-
soecids (inRictus lutensis) (Yubuki et al. 2010 ),
which are generally considered to be the earliest
diverging stramenopiles, suggesting that the
origins of this structure are deeply rooted.
Another feature associated with the flagel-
lum is thetransitional helix(TH), which occurs
in the transitional zone just above the basal
plate that separates the flagellum from the
main body of the zoospore (Fig.3.16i, k). This
is another structural feature that is unique to the
stramenopile clade and has been the subject of
much phylogenetic speculation (Beakes 1987 ;
Cavalier-Smith and Chao 2006 ; Dick2001a;
Guillou et al. 1999 ; Patterson 1989 ). THs are
found in slopalinids (proteromonads and opa-
linids as defined by Patterson 1989 ), some bico-
secids (Patterson 1989 ), hyphochytrids (Barr
and De ́saulniers 1989 ), most oomycetes (Dick
2001), and, in a single helical form, in many
ochrophyte groups (Cavalier-Smith and Chao
2006 ; Guillou et al. 1999). A single-helix variant
of the TH is also reported inOlpidiopsis sapro-
legniae (Bortnick et al. 1985 ). Members of
Labyrinthulomycota also do not have a typical
TH (Fig.3.16c), but it has been equated with a
similarly placed cone-like structure [see discus-
sion by Cavalier-Smith and Chao ( 2006 )]. In
most oomycetes the TH usually takes the form
of a double-stranded helical coil with 6 to 12
turns (Dick2001a). The TH is apparently absent
from zoospores of some early-diverging species,
such asEurychasma dicksoniiand Olpidium
porphyrae(Sekimoto et al.2008a,b), and from
Lagena(Fig.3.16j; Barr and De ́saulniers 1987 ).
So although it provides a useful general marker
for stramenopiles, this structure does seem to
have been lost or modified numerous times,
without apparently affecting flagellar function.
- Encystment/Adhesive Vesicles
Cortical vesiclesin the peripheral cytoplasm of
zoospores that are typically discharged upon
encystment have also been widely discussed in
relation to phylogeny (Beakes 1987 , 1989 ;
Beakes et al. 1995 ). In bicosoecid flagellates
homologous vesicles are described as extruso-
somes (Yubuki et al. 2010 ) but do not seem to
be present in zoospores of the Labyrinthulomy-
cota (Moss 1985 , 1986 ; Perkins 1976 ; Porter
1990 ). Both hyphochytrid (Fuller and Reichle
1965 ; Fuller 1990 ) and oomycete (Beakes 1987 ,
1989 ) zoospores have morphologically similar
cortical vesicles that upon encystment form the
outermost layer of the encysted spore coat. The
comparative structure of these vesicles and
their homologues has already been discussed
at length (Beakes 1987 , 1989 ; Beakes et al.
1995 ). Electron microscopy and, later, cyto-
chemistry revealed that the cortical vesicles in
oomycetes were made up of two related vesicle
populations (Gubler and Hardham 1988 ;
Beakes 1994 ; Burr and Beakes 1994 ; Robold
and Hardham 2004 , 2005 ; Beakes et al. 1995 ).
In Peronosporomycetes one vesicle type
[ventral small vesicles (vsv)] (Beakes et al.
1995 ; Gubler and Hardham 1988 ; Hardham
2005 ) is associated with the ventral flagellar
groove, whereas in Saprolegniomycetes the
homologous vesicles areK-bodies, sonamed
because of their close association with
kinetosomes (Holloway and Heath1977a). K-
bodies are distinguished by their relatively
large size (0.3–0.5mm) and distinctive micro-
structure, which includes an often crystalline
matrix (Beakes 1989 ) and tubular inclusions
(Holloway and Heath1977a; Lehnen and Powell
1989 ). Upon encystment both vsv and K-bodies
are discharged to form a ventral adhesive pad
that helps attach the encysted spore to the
substrate (Burr and Beakes 1994 ; Gubler
and Hardham 1988 ; Lehnen and Powell 1989 ).
The large structured K-bodies were considered
to be a marker for saprolegnialean oomycetes
because they had been observed inApodachlya
in the Leptomitaceae (Beakes 1987 ; Randolph
and Powell 1992 ) and in genera throughout
the Saprolegniales (Beakes 1987 , 1989 ).Systematics of the Straminipila: Labyrinthulomycota, Hyphochytriomycota, and Oomycota 79