cally, their thallus organization, with mono-
centric (Fig. 3.4a–i) and polycentric thalli
(Fig.3.3j–k) with rhizoids, resembles that of
chytrids, with which they were once placed
(Sparrow 1960 ,1973a). Hyphochytrids share a
number of biochemical similarities with oomy-
cetes, including using thea-e-diaminopimelic
acid pathway for lysine biosynthesis (Vogel
1964 ). As with saprolegnialean oomycetes,
they are able to endogenously synthesize sterols
(Fuller 1990 , 2001 ). Hyphochytrids have both
chitin and cellulose in their cell walls
(Bartnicki-Garcia 1970 ; Clay et al. 1991 ; Fuller
1990 , 2001 ), which is similar to leptomitalean
oomycetes. Hyphochytrids are primarily
saprotrophs colonizing plant and animal
debris, and they have the capacity to withstand
conditions of drought and temperature
extremes (Gleason et al. 2009 ). Genera such as
Rhizidiomycescommonly parasitize Glomero-
mycote azygospores (Sparrow 1977 ) and oomy-
cete oogonia (Fig.3.4i) and oospores (Ayers
and Lumsden 1977 ; Karling 1981 ; Wynn and
Epton 1979 ). Some have also been reported to
be pathogens of crustaceans, although there is
uncertainty as to whether the causal agents
described were hyphochytrids (Fuller 1990 , 2001 ).
Traditionally the hyphochytrids have been
divided into three families, the monocarpicRhi-
zidiomycetaceae(Fig.3.4a–f), the polycarpic
Hyphochytriaceae(Fig.3.4j–l), and the endobi-
otic Anisolpidiaceae (Fig. 3.4m–q) (Canter
1950 ; Fuller 1990 ; Karling 1981 ). To date, SSU
rRNA sequences for two species, Hyphochy-
trium catenoides(Fig.3.4j–l) andRhizidomyces
inflatus(Fig.3.4a–h), confirm that the Hypho-
chytriomycota form a statistically well-
supported monophyletic clade (Fig.3.1b) nested
between the Ochrophyta and Oomycota clades
(Hausner et al. 2000 ; Van der Auwera et al.
1995 ). Dick (2001a), however, considered that
the Anisolpidiaceae, which are holocarpic endo-
parasites of algae (Fig.3.4m–r) (Canter 1950 ;
Ku ̈pper and Mu ̈ller 1999 ), ought to be placed
in their own order (Anisopidiales) of uncertain
affiliation. Unpublished sequence data forAni-
solpidium ectocarpi(Fig.3.4m–r) support the
exclusion of this genus from the Hyphochytrio-
mycota s. str. (Table3.2) and may actually fall
within the Oomycota (C Gachon, K Fletcher and
F Kupper, pers. commun.). However, until more
of these genera and species are sequenced, it is
impossible to know how robust the two remain-
ing family groupings will be and where the
other, so far unsequenced, genera (Table3.2)
will fit in. Over 20 environmental clones origi-
nating from the Antarctic, Mediterranean, and
North Atlantic ocean fell in the hyphochytrid
clade, and all were distinct from the two
sequenced species (Die ́z et al. 2001 ). Hyphochy-
trids, in common with most other marine stra-
menopiles (Richards et al. 2012 ), seem to be
widespread in marine ecosystems, although the
niches they occupy remain to be discovered.Astonishingly, a little-known monotypic biflagellate
phagotrophic parasitoid of marine diatoms,Pirsonia,
apparently formed the sister clade (Fig.3.1a) to the two
sequenced hyphochytrid species (Ku ̈hn et al. 2004 ).
However, in more recent analyses, they were shown as
separate but closely related clades (Yubuki et al. 2010 ).
For the time being,Pirsoniais consideredincertae sedis
but closely related to the hyphochytrids (Table3.2).Table 3.2Taxonomic classification of Hyphochytriomycota (Dick 2001; Fuller 2001 )
Kingdom: STRAMINIPILA
Phylum: Hyphochytriomycota
Order: Hyphochytriales
Family:Hyphochytriaceae Canteriomyces,Cystochytrium,Hyphochytriuma
Family: Rhizidiomycetaceae Latrostium, Reesia,Rhizidiomyces
Phylum and order incertae sedis: Pirsoniales
Family:Pirsoniaceae Pirsonia
Phylum and order incertae sedis: Anisopidiales
Family:Anisolpidiaceae Anisolpidium
aGenera in bold have published sequences
48 G.W. Beakes et al.