because in some trees, it groups with species
considered by Dick (2001) to be in theSapro-
legniaceae(Fig.3.10) (Arcate et al. 2006 ; Inaba
and Tokumasu 2002 ;Le ́clerc et al. 2000 ; Peter-
sen and Rosendahl 2000 ).
b) Achlyaceae
We have introduced a new family, the “Achlya-
ceae” (Table3.5), for the “achlyoid/thraustothe-
coid clade” with eccentric oospores (Fig.3.10),
which encompasses four genera:Achlyas. str.
(Fig.3.11a, b) (Spencer et al. 2001),Brevilegnia
(Fig.3.11f, g),Dictyuchus(Fig.3.11h–j), and
Thraustotheca(Fig.3.11c–e). All have oogonia
containing either single (Fig.3.11g, i) or multi-
ple (Fig.3.11b, e) oospores withstrongly eccen-
tric oospores, usually with just a single
coalesced lipid droplet (Figs.3.10and3.11b, e,
g, h). Diplanetism is not observed in this clade,
although there is diversity in the pattern of
zoosporogenesis (Figs.3.10and3.11a, c, f, h).
Species that discharge their primary aplanos-
pores from the sporangium to form a ball of
spores constitute the genus Achlya s. str.
(Fig. 3.11a). Genera such as Brevilegnia
(Fig.3.11f) andThraustotheca(Fig.3.11c) retain
their primary cysts (aplanospores) within the
sporangium, which are released by the general
or localized rupture of the sporangium wall
(Fig. 3.11c, f). In contrast, in Dictyuchus
(Fig.3.11h), individual primary zoospores are
discharged from lateral papillae, which rupture
the sporangium wall and leave a distinctive net
of angular aplanospore walls behind. The sec-
ondary cysts of genera within this clade are
either smooth or, as inDictyuchus, decorated
by stout tapering spines (Fig.3.11j).
c) Saprolegniaceae
The family Saprolegniaceae s. str. is now
reserved for the saprolegnioid, achlyoid, apla-
noid, calypralegnoid, and leptolegnoid clades
(Fig.3.10) and contains species with centric/
subcentric oospores (Fig.3.11n, t, v, x). This
revised family includes the generaAplanes,
Aplanopsis,Calyptralegnia(Fig.3.11u, v),New-
bya(Achlya-like sporulation pattern, but with
centric oospores) (Fig.3.11k–n),Protoachlya,
Pythiopsis(Fig.3.11w, x), and the majority of
described ~Saprolegnia spp. (Fig. 3.11o–t,
Table3.5). All have oogonia that are predomi-
nantlymulti-oosporedwithcentric/subcentric
ooplast vacuoles(Figs.3.10and3.11n, t,u,v,
x), which usually contain granules that are in
Brownian motion (Dick 1969 ). Zoosporogen-
esis results in either the production of dipla-
netic zoospores (as in Fig.3.11q) or, depending
on the genus, the formation of intra- and extra-
sporangial primary spores (aplanospores). All
primary aplanospores or cysts release reniform
secondary-type zoospores with laterally
inserted flagella (as in Fig.3.11m).
The secondary cyst cases of genera in this
clade are often decorated by slender boat-hook
spines (Beakes 1983 ; Hallett and Dick 1986 ).
Such spines are particularly large inS. parasi-
tica(Fig.3.11s), which is an important patho-
gen of salmonids and other fish and poses a
serious threat to both wild and reared fish
stocks [see reviews by van West ( 2006 ) and
Phillips et al. ( 2008 )]. This species was recently
shown to produce effector-like proteins that are
selectively translocated to fish cells, where they
modulate the host immune response (Wavra
et al. 2012 ). Molecular studies seem to suggest
that many currently defined species of ~Sapro-
legniaare paraphyletic (Hulvey et al. 2007 ) but
may be the result of misidentified taxon names
in genetic databases. The precise placement of
the genusLeptolegniawithin the Saprolegniales
is also problematic. LSU sequence data place it
firmly in the Saprolegniaceae s. str. (Fig.3.10)
(Inaba and Tokumasu 2002 ; Petersen and
Rosendahl 2000 ), although it has a number of
anomalous morphological traits for this family,
such as relatively slender hyphae and single
oospored oogonia.G. PeronosporomycetesThe Peronosporomycete class (Figs.3.12,3.13,
3.14, and 3.15) represents a major derived
offshoot of the main oomycete line (Beakes
and Sekimoto 2009 ; Beakes et al. 2012 ; Jiang
and Tyler 2012 ; Thines and Kamoun 2010 ).
Members of this class requireexogenous ster-
ols to complete oogenesis (Kerwin and
Washino 1983 ). The saprotrophic members of
this group are able to utilize various would be
better nitrogen sources and sulphate (Dick
2001a,b; Gleason 1976 ). PeronosporomycetesSystematics of the Straminipila: Labyrinthulomycota, Hyphochytriomycota, and Oomycota 65