Systematics and Evolution, Part A The Mycota

3.10, and3.11) of water moulds (Tables3.3,3.4,
and3.5) (Dick1973b; Sparrow 1960 ). Most are
general saprotrophs or opportunist colonizers
of damaged plant and animal tissues (Dick 1976 ;
Sparrow 1960 ), but they also include a number
of significant pathogens of plant roots (Gaulin
et al. 2007 ; Levenfors and Fatehi 2009 ), inverte-
brates such as crayfish (Cerenius et al. 1988 ) and
vertebrates such as fish, and amphibians and
their eggs (Lilley et al. 2003 ; van West 2006 ).
The most recent taxonomic synopsis of Sapro-
legniales was published online (Johnson et al.
2002 ), replacing earlier monographs. Although
the Saprolegniales forms a well-supported
monophyletic clade, the formal naming and
branching order of the family-level clades is
not fully resolved (Fig.3.5) (Dick et al. 1999 ;
Inaba and Tokumasu 2002 ;Le ́clerc et al. 2000 ;
Petersen and Rosendahl 2000 ; Riethmu ̈ller et al.
1999 ; Spencer et al. 2002 ). Traditionally, this

order contained just a single family, theSapro-

legniaceae (Table 3.3; Dick 1973b; Sparrow

1960 ). However, molecular studies suggest that

the earliest diverging clade within this order

encompasses those species with slender hyphae

and simple hypha-like sporangia, as exemplified

by the genusAphanomyces(Fig.3.9). We there-

fore include this genus, together with a small

number of related genera, in the amendedVer-

rucalvaceaefamily. Most remaining representa-

tives of this order (Fig.3.11) produce stout fast-

growing hyphae on which clavate, often prolif-

erative septum-delimited sporangia, are borne

(Fig.3.11). The dozen or more genera (Figs.3.9,

3.10, and3.11, Table3.3) have been primarily

defined on the basis of their different patterns of

asexual sporogenesis(Figs.3.10and3.11) (Dick

1973b; Johnson et al. 2002 ; Sparrow 1960 ). In the

genusSaprolegniatwo morphologically distinct

generations of zoospores, traditionally referred

Fig. 3.9 Morphology of Verrucalvaceae.(a–d) Draw-
ings of genusAphanomycesspp. Eucarpic thallus of
Aphanomyces amphigynuswithin infested algal fila-
ment showing an oogonium (a) and undifferentiated
hypha-like sporangia with short lateral discharge tubes
releasing naked aplanospores, which immediately
encyst to form balls of spores (b). Detail of papillate
and smooth single-oospored oogonia ofA. cladogynus
(c) andA. euteiches(d) showing attached antheridia
and clear ooplast vacuoles of varying size. From John-
son et al. ( 2002 ). (e,f) Drawings ofPlectospira myrian-
dra a soil-borne saprotroph. Swollen sporangium

segment with elongate discharge tube and cluster of

primary aplanospores (e) and single-oospored oogo-

nium surrounded by investing antheridial hyphae (f).

From photographs in Watanabe ( 1987 ). (g–i) Drawings

ofSommerstorffiainfecting rotifer bodies. Lobate thal-

lus and developing external trapping hypha (g). Thallus

with differentiated aplanospores (h) showing aphano-

mycoid release (i). From Johnson et al. ( 2002 ). (j, k)

Drawings of oogonia and oospores of two root-

infecting graminicolus pathogens,Pachymetra chau-

norhiza(j) andVerrucalvus calvatus(k), neither of

which has been reported to produce asexual spores

Systematics of the Straminipila: Labyrinthulomycota, Hyphochytriomycota, and Oomycota 61