spread in natural host populations and
the pathogen can enter into the generative
organs of their hosts (Ploch and Thines 2011 ).
Theindependent evolutionofobligate biotro-
phy in this group of Peronosporomycetes is
also reflected by the fact thatAlbugo laibachii,
the white blister rust pathogen ofArabidopsis
thaliana(Thines et al.2009a), has a much smal-
ler genome compared with obligate parasites in
the Peronosporales (Kemen and Jones 2012 ;
Kemen et al. 2011 ) and has evolved a novel
group ofCHxC/CxHC effectorsthat are unique
to this clade (Kemen et al. 2011 ). Therefore,
based on the mostly restricted host range of
the currently described species, it can be
assumed that the 50 or so white blister rust
species currently known probably represent
only a fraction of the total diversity of white
blister rusts (Ploch and Thines 2011 ; Ploch et al.
2010 ;VoglmayrandThines 2009 ).
- Peronosporales s. lat.
Members of the Peronosporales s. lat. as
defined here (Fig. 3.14) are found in both
marine and terrestrial ecosystems and include
saprotrophs, opportunistic and obligate plant
pathogens, and animal parasites (Dick2001a;
Hatai 2012 ;Le ́vesque et al. 2010 ; Mendoza 2009 ;
Schurko et al. 2004 ; Sparrow1973c). Thallus
morphology can range from holocarpic
(Fig.3.14a–j) to eucarpic (Fig.3.14k), but in
most (with the exception of derived plant
pathogens), the final differentiation of their
zoospores occurs within anexternal sporangial
vesiclethat forms at the neck of the discharge
tube or the exit pore of the sporangium
(Fig.3.14c, h, l, n). Even those genera, such as
Halopthytophthora (Fig. 3.15d) and Phy-
tophthora (Fig.3.15h), that form their fully
differentiated zoospores intrasporangially dis-
charge their spores into a transient evanescent
vesicle (as shown in Fig.3.15e).Peronosporales
s. lat. contains around 30 genera (Table3.3) and
1,000 species. Dick (2001a) split his order
Pythiales (equivalent to the ~Pythiaceae in
this review) into two families, thePythiogeto-
naceae(Voglmayr et al. 1999 ) and the much
largerPythiaceae(Table3.4). However, there
is no molecular support for separating the
genusPythiogetoninto its own family because
it forms a monophyletic clade nested within
one of thePythiumclades (Huang et al. 2012 ).
Until more robust molecular markers are devel-
oped, it seems unwise to create lots of new
families and genera within the Pythiaceae
s. lat., although it may ultimately be necessary.a) Salisapilaceae
A recent study of saprotrophs of saltmarsh lit-
ter identified a novel pythiaceous clade which
was located basal to the main pythiaceous and
peronosporaceaous clades (Hulvey et al. 2010 ).
Members of this family had been originally
assigned to the genusHalophytophthorawith
which they share some morphological similari-
ties, such asvesiculate zoospore release(Hul-
vey et al. 2010 ). They have mycelial thalli and
formovoid papillate sporangiaandsmooth-
walled oogoniawith large translucent ooplast
vacuoles (Hulvey et al. 2010 ). This clade was
given the family designationSalispilaceae(Hul-
vey et al. 2010 ) and is retained as a separate
family within Peronosporales s. lat. (Table3.5).
However, preliminary unpublished work sug-
gests that some holocarpic lagenidiaceous spe-
cies may diverge before the Salisapilaceae, and
the delineation of families within the pythiac-
eous oomycetes will undoubtedly need to be
reviewed.b) Pythiaceae s. lat.
The ~Pythiaceae s. lat.(Fig.3.14) is a large
complex family that contains a number of
well-documented subclades (Fig. 3.5b) (Le ́v-
esque and de Cock 2004 ). Limited molecular
studies have shown that genera such as ~Lagen-
idiumand ~Myzocytiopsisspp. are paraphyletic
but share a common ancestor with ~Pythium
(Fig.3.5b) (Beakes et al. 2006 ; Schroeder et al.
2012 ).It is likely that the holocarpic lagenidiac-
eous parasites of freshwater algae, which Dick
(2001a) placed in the generaMyzocytiums. str.
andSyzygangia(Table3.4), as well as the gra-
minicolous root pathogen Lagena radicola
(Fig.3.14a–g), may also be part of this assem-
blage, although whether all of these genera
(including Lagenidium) will remain valid isSystematics of the Straminipila: Labyrinthulomycota, Hyphochytriomycota, and Oomycota 71