stages of infection and will probably fall
amongst these basal clades (Table3.5). In the
absence of sequence data they are listed as
incertae sedis, but we place them to the extent
possible where we think they are most likely to
belong (Table3.5).
- Eurychasmales
The order Eurychasmales was first introduced
by Sparrow ( 1976 ) to encompass two small
monotypic marine families, theAtkinsiellaceae
andEurychasmaceae, which are parasites of
crustaceans and seaweeds respectively
(Table 3.3). However, molecular data have
shown that Eurychasma and Atkinsiella are
not closely related (Cook et al. 2001 ), and there-
fore this order is now restricted to just a single
family, theEurychasmaceae.Eurychasma dick-
sonii(Figs.3.5aand3.7a–d) is an obligate par-
asite of wide geographic distribution that
infects a broad range of filamentous brown
seaweeds, such asChoristocarpus,Ectocarpus,
andPylaiella(Gachon et al. 2009 ;Ku ̈pper and
Mu ̈ller 1999 ; Sekimoto et al.2008a; Strittmatter
et al. 2009 ). This pathogen causes host cells to
become greatly enlarged without adversely
affecting cytoplasmic integrity, at least during
the early stages of infection (Sekimoto et al.
2008a). The flask-shaped thallus (Fig.3.7a–c)
forms one or more domed papillae that
rupture the algal wall, allowing the pyriform
zoospores to escape (Fig.3.7d). The character-
istic feature of this genus is that the primary
cysts (aplanospores) line the sporangium wall
and release their zoospores internally. This
gives the mature sporangium a distinctivenet-
like appearance(Fig. 3.7c) (Sekimoto et al.
2008a; Sparrow 1960 ). Phylogenetic analyses
based on SSU rRNA (Ku ̈pper et al. 2006 ), LSU
rRNA (Fig.3.5), andcox2 genes (Sekimoto et al.
2008a) all show thatEurychasmaforms theear-
liest diverging clade in the oomycete tree
(Fig. 3.5b). A prelimary genomic study
(Grenville-Briggs et al. 2011 ) revealed that this
pathogen has a number of unique pathogenicity
factors, including many putative algal-cell-wall-
degrading enzymes, which are very different
from those described in higher plant
pathogens. The unsequenced parasite of red
seaweeds Eurychasmidium is also placed in
this family (Dick2001a; Karling 1981 ).- Haptoglossales
The monotypic order (familyHaptoglossaceae)
was originally included by Karling ( 1981 ) in the
Lagenidiales.Haptoglossa(Fig.3.7e–l) contains
more than ten species that are parasites of
nematodes and rotifers (Beakes and Glockling
1998 , 2000 , 2002 ; Glockling and Beakes2000a,
b,c, 2001 ; Hakariya et al. 2002 , 2007 , 2009 ;
Karling 1981 ). Dick (2001a) speculated that
because of functional similiarities in their infec-
tion cells (Fig.3.7l)Haptoglossamight be more
closely related to plasmodiophorids than to
oomycetes. He placed the genus in a monotypic
order, the Haptoglossales, incertae sedis
(Table 3.4) (Dick 2001a). This genus forms
unsegmented sausage-like thalli with a distinc-
tive refractile cytoplasm (Fig.3.7e, i). Mature
thalli form exit tubes that rupture the nematode
cuticle (Fig.3.7i). The genus includes both apla-
nosporic (Fig.3.7i) and zoosporic (Fig.3.7e, f)
species (Karling 1981 ). Some species, such as
Haptoglossa heteromorpha(Fig.3.7i–k) (Glock-
ling and Beakes2000c),Haptoglossa polymor-
pha (Glockling and Beakes 2001 ), and
Haptoglossa erumpens(Beakes and Glockling
2002 ), produce two or three different types of
infection cell (Fig.3.7j, k), which suggests they
infect multiple, as yet uncharacterized, hosts.
The ultrastructure of these intricate infection
cells (gun cells) has been described for several
species (Fig.3.7l) (Beakes and Glockling 1998 ,
2002 ; Glockling and Beakes2000c; Robb and
Barron 1982 ) and has revealed they contain an
inverted tube containing a needle-like structure
(Fig.3.7g, h, l). Upon contacting a host the tube
explosively everts, forcing the needle through
the nematode cuticle and instanteously inject-
ing an infectivesporidiuminto the host body
cavity (Glockling and Beakes2000b; Robb and
Barron 1982 ).
Incox2-based trees (Hakariya et al. 2007 ,
2009 ; Sekimoto et al.2008a),Haptoglossaforms
a monophyletic clade, which diverges immedi-
ately after Eurychasma, whereas in SSU andSystematics of the Straminipila: Labyrinthulomycota, Hyphochytriomycota, and Oomycota 55