features of this class and is among the features
indicating that members of the Rhipidiales,
such asAraiospora(Fig.3.12b, c) (Sparrow
1973a), should be in the peronosporomycete
lineage. The mature oospores of the latter
have complex outer oospore wall layers which
appear to be derived from the periplasm
(Fig.3.12b, c), a situation also found in Albugi-
nales, such asAlbugo(Beakes 1981b; Tewari
and Skoropad 1977 ). ManyMyzocytiopsisspe-
cies, such asM. vermicola(Fig.3.14i, j) (Glock-
ling and Beakes 2006a), have complex,
scalloped, multilayered oospore walls, although
these differentiate without apparent periplas-
mic involvement. Beakes ( 1987 ) considered
complex multilayered walls to be a derived fea-
ture, but it now seems more likely that such
walls are associated with early-diverging mem-
bers of the peronosporomycete line.
IV. Evolutionary Timeline and the
Fossil Record
Stramenopiles form a well-supported mono-
phyletic clade that is sister to the alveolates
(Keeling 2009 ). Hyphochytrids and oomycetes
are part of the lineage that shares a common
ancestor with the photosynthetic ochrophyte
stramenopiles (Yubuki et al. 2010 ; Riisberg
et al. 2009 ; Tsui et al. 2009 ). It was recently
estimated that the stem origin of the Ochro-
phyta was around571 million years ago(mya)
(a mean of estimates ranging from 735 to 434
mya) (Brown and Sorhannus 2010 ). Hyphochy-
trids and oomycetes presumably evolved after
the ochrophyte line diverged, that is later than
570 mya. Previous molecular clock estimates
had also placed the origins of the oomycetes
at between 1,000 and 524 mya (Bhattacharya
et al. 2009 ). Previously it had been claimed
that fossil oomycete-like organisms were pres-
ent in Precambrian deposits, but the evidence
for this was considered by many to be uncon-
vincing (Dick 2001; Johnson et al. 2002 ). The
Labyrinthulomycota are part of a parallel clade
that presumably evolved around the same time
or even earlier than the other osmotrophic stra-
menopiles. The evidence is now overwhelming
that all osmotrophic stramenopiles had their
origins in the sea, even though Dick (2001a)
concluded that both hyphochytrids and oomy-
cetes were of terrestrial freshwater origin.
The closest known relatives of hyphochytrids
(Pirsonia) and oomycetes (Developayella
and MAST-1 clade members) are marine organ-
isms (Fig.3.1) (Kuhn et al. 2004 ; Leipe et al.
1994 ; Moreira and Lo ́pez-Garcia 2002 ;
Yubuki et al. 2010 ), as indeed are the majority
of early diverging oomycete genera (Fig.3.6)
(Cook et al. 2001 ; Sekimoto 2008 ; Sekimoto
et al. 2007 ,2008a,b, 2009 ). The unexpectedly
close relationship between the uniflagellate
hyphochytrids and the phagotrophic marine
biflagellate protistPirsonia(Ku ̈hn et al. 2004 )
perhaps gives some indication of the type of
ancestor that might have given rise to the
osmotrophic stramenopiles. Like Pirsonia,
many simple holocarpic oomycete genera,
such as Ectrogella (Raghukumar 1980 ) and
Lagenisma(Schnepf et al.1978a), are also para-
sites of marine diatoms. Diatoms, however,
evolved no earlier than 240 mya (Sims et al.
2006 ), which is considerably later than the
oomycetes, if the timeline suggested previously
is accepted. The same logic also applies to
primitive oomycetes, such as Eurychasma
(Ku ̈pper et al. 2006 ; Sekimoto et al. 2008), as
their phaeophyte hosts did not appear until the
Triassic period, approximately220 mya. How-
ever, it has also been reported thatEurychasma
species infect red algae (Sparrow 1960 ), which
are much more ancient in origin. It seems more
likely that the earliest oomycetes were morpho-
logically simple (holocarpic) necrotrophs of
marine nematodes, crustaceans, and possibly
algae, all of which would have been present in
the Cambrian period around 550–500 mya.
Thraustochytrids similarly also are parasites
of molluscs and gastropods. Oomycetes, which
opportunistically parasitize animal tissues, pos-
sess a huge array of proteinase and glucanase
genes that were probably part of the ancestral
make-up of this lineage (Jiang and Tyler 2012 ;
Krajaejun et al. 2011 ).
Even after the two main eucarpic classes
had diverged, there are many saprolegniomy-
cete genera, such asAtkinsiella,Crypticola, and
Chlamydomyzium, and peronosporomycete82 G.W. Beakes et al.