(Kazama 1980 ; Perkins 1976 ), a feature also
found in haptophyte and synurophyte algae
(Beakes 1989 ). In secondary/principal type
oomycete zoospores both flagella are inserted
into a small boss, which is located midway
along a deep ventral groove (Fig.3.16r) (Dick
2001; Holloway and Heath1977a). Zoospores of
genera in the early-diverging oomycete clades
are usually small (mostly less than 5mm), ovoid
to pip shaped, and have flagella inserted sub-
apically rather than laterally (Figs.3.7dand
3.16h, l–p) (Dick2001a). In this respect they
more closely resemble the primary/auxilliary
type of zoospore of Saprolegnia (Holloway
and Heath1977a) or those of the unsual, possi-
bly pythiaceous, genusLagena(Fig.3.16j) (Barr
and De ́saulniers 1987 , 1989 ).
All stramenopile zoospores share the same
underlying flagellar rootlet system (Fig.3.16c, f,
i–k), which shows a remarkable degree of con-
servation throughout the entire lineage (Ander-
sen et al. 1991 ; Barr and De ́saulniers 1989 ; Dick
2001a,b). In common with most biflagellate
stramenopiles, zoospores have four rootlets
(two associated with each flagellum)
(Fig.3.16i, j, k), whereas the monoflagellate
hyphochytridshave just the twoanterior root-
lets(Fig.3.16f) (Barr and Allan 1985 ; Barr and
De ́saulniers 1987 ; Hardham 1987 ). Rootlet
notation relates to which flagellum (anterior
or posterior) the root is associated with and
the number of microtubules normally asso-
ciated with each rootlet (Fig.3.16c, f, i–k). In
zoospores with ventrally inserted flagella, the
two anterior and posterior rootlets align on
either side of the flagellar groove (Fig.3.16i,
k), whereas in species with more pyriform
zoospores, such asLagena radicola, the right-
hand AR3 rootlet curves around the anterior
kinetosome (Fig.3.16j) (Barr and De ́saulniers
1987 ). The most variable rootlet in oomycetes is
the larger, left-hand posterior PR8 rootlet [also
referred to as the multistranded rootlet in some
accounts, e.g. Barr and De ́saulniers ( 1989 )],
which can consist of anywhere between four
and eight microtubules depending on the
genus (Fig.3.16c, i–k). Although a full recon-
struction of theHaptoglossazoospore rootlet
has not been made, it has the full P8 rootlet
(Glockling and Beakes, unpublished observa-
tion), suggesting this is the basal condition.
The left-hand AR2 rootlet has a backing of
electron-dense material that is associated with
rib-like microtubules that give the zoospore its
characteristic morphology (Barr and De ́saul-
niers 1989 ; Holloway and Heath1977b). The
pip-shaped primary Saprolegnia zoospore
lacks both anterior rootlets (Holloway and
Heath1977b; Dick2001a), which may account
for the poor motility of these spores.
The two basal kinetosomes linked to the
flagella are also interconnected by structural
components (Andersen et al. 1991 ; Barr and
Allan 1985 ; Barr and De ́saulniers 1989 ; Beakes
1987 ), although these seem more variable in
nature than the rootlets themselves. In both
Saprolegniomycetes and Peronosporomycetes
astriate fan(SF) structure connects the termi-Fig. 3.16(continued) mycota zoospores. Schematic
drawing summarizing zoospore ultrastructure inLab-
ryrinthula(a) andThraustochytrium(b) showing lon-
ger anterior flagellum decorated with mastigoneme
hairs and shorter smooth posterior flagellum. From
Porter ( 1990 ). (c) Diagram summarizing flagellar root-
let organization inThraustochytrium showing two
rootlets (AR2, AR3) associated with anterior flagellum
and two rootlets (PR2, PR4+2) associated with poste-
rior flagellum. From Barr and Allan ( 1985 ). (d–g)
Hyphochytriomycota zoospores.(d, e) Drawings
made of shadowed whole mount preparation (d) and
longitudinal section (e)ofRhizidiomyces apophysatus
zoospores, together with a schematic diagram of flagel-
lum base and two anterior rootlets (AR1, AR2) (f).
Longitudinal profile zoospore ofHyphochytrium cate-
noidesshowing general arrangement of organelles and
rootlets (g). (d,e) From Karling ( 1977 ) based on paper
of Fuller and Reichle ( 1965 ), (f) from Barr and Allan
( 1985 ), (g) from Fuller ( 1990 ) based on Cooney et al.
( 1985 ). (h–r) Diagrams summarizing ultrastructural
characteristics ofOomycota zoospores. General zoo-
spore structure (h) and rootlet organization (i)inOlpi-
dium saprolegniaein which single helically coiled TH
(i) has been arrowed. From Bortnick et al. ( 1985 ).
Rootlet organization in zoospores ofLagena radicola
(j) andSaprolegniasp. (k) Note presence of striate fan
(SF) in latter. From Barr and Allan ( 1985 ) and Barr and
De ́saulniers ( 1989 ). Series of schematic diagrams and
one whole mount preparation (n) comparing zoospore
ultrastructure inEurychasma dicksonii (l), Hapto-
glossa dickii(m,n),Olpidiopsis porphyrae(o),O. bos-
trychiae (p), Haliphthoros milfordensis (q) and
Saprolegniasecondary-type zoospore (r). Diagrams
adapted from Sekimoto ( 2008 ) and whole mount (n)
from Beakes and Glockling ( 1998 )78 G.W. Beakes et al.