genera, such as ~Lagenidiumand ~Myzocy-
tiopsis(Fig.3.6), that are parasites of arthro-
pods and algae (Karling 1981 ). It is thought that
the first land arthropods appeared around 500–
450 mya in the Cambrian period and the first
insects in the Devonian period around 400 mya.
Some oomycetes could have migrated into
freshwater ecosystems and onto land along
with their invertebrate hosts, such as nema-
todes and insect larvae. It also seems likely
that host-jumping from invertebrates to algae
occurred multiple times. The pathogenicity fac-
tors required for successful infection of animals
(Krajaejun et al. 2011 ; Phillips et al. 2008 ) and
algae (Grenville-Briggs et al. 2011 ) are very
different from those required to infect terres-
trial plants (Jiang and Tyler 2012 ).
It is still unclear whether the saprolegnio-
mycete/peronosporomycete schism took place
before or after oomycetes had migrated to the
land. The earliest diverging clade in the sapro-
legniomycete line is the Atkinsiellaceae, an
exclusively marine family (Cook et al. 2001 ).
However, the two earliest diverging peronos-
poromycete clades (the Rhipidiales and Albu-
ginales) are wholly freshwater or terrestrial.
Indeed, the evolution of peronosporomycete
characteristics, like thick-walled oospores,
might have been the direct result of oomycetes
adapting to life on land. However, it is becom-
ing increasingly apparent that there are many
marine peronosporomycete species, such as the
crustacean parasite Lagenidium callinectes
(Hatai 2012 ), pythiaceous parasites of red sea-
weeds, such asP. marinumandP. porphyrae,
and the saprotrophic saltmarsh genusSalispilia
(Beakes et al. 2009; Cook et al. 2001 ; Hulvey
et al. 2010 ). Just as is thought to have occurred
in the fungi (Richards et al. 2012 ), there have
probably been multiple transitions from land to
sea (and vice versa) within the oomycete line-
age. It does seem that the oomycetes were adept
at exploiting estuarine ecosystems and at cross-
ing between marine and freshwater environ-
ments.
The one out-of-place piece of the oomycete
evolutionary jigsaw puzzle seems to be the
Albuginales. The white blister rusts were origi-
nally considered to be a highly derived group of
obligate angiosperm pathogens (Beakes 1987 ),
and it was a surprise when they turned up
amongst the early-diverging peronosporomy-
cete clades (Hudspeth et al. 2003 ; Thines and
Spring 2005 ). We now know that the earliest
families to have eucarpic thalli and reproduce
sexually by means of morphologically distinct
oogonia and antheridia are the Leptomitaceae
in the saprolegnialean line and the Rhipidiaceae
and Albuginaceae in the peronosporomycete
line (Beakes et al. 2009, 2011). A critical evalua-
tion of the fossil evidence for ancient terrestrial
oomycetes is given in a recent review by Krings
et al. ( 2011 ). This paper describes microfossils
of what appear to be oogonium and antherid-
ium complexes in chert and coal ball samples
that provide very convincing physical evidence
for the appearance of terrestrial oomycetes to
support the molecular clock inferences. Stidd
and Consentino ( 1975 ) describe structures that
they consider to beAlbugooospores in the
megagametophyte seed tissue of the ancient
gymnosperm Nucellangium glabrum from
around 310 mya. However, the structures
described were not conclusively oospores of
anAlbugo(Krings et al. 2010 ). A far more
convincing, though still controversial,Albugo-
like microfossil appears to beHassiella mono-
spermafrom the 412 mya lower Devonian Rhy-
nie chert (Taylor et al. 2006 ). Structures
purported to be small oogonia in these fossils
actually look much more like the small globose
haustoria ofAlbugo, and the apparently amphi-
gynous antheridium purported to be at the base
of the verrucose oogonium could just be a
swollen oogonium stalk. If this fossil is
accepted as representing an obligately bio-
trophic pathogen of Rhyniophyte shoots,
related to the present-day Albuginales, it
implies that the evolution of obligate parasitism
by oomycetes of land plants can be traced back
400 mya. This, coincidently, is also the time
when the first Glomeromycota arbuscles have
been identified as having formed mycorrhizal
associations with similar plants (cited in Krings
et al. 2010 ). However, all extant white blister
rusts are obligate parasites of angiosperms, and
the latter only diversified from a common
ancestor ca. 150 mya, which is considerably
later than the apparent emergence of oospores
reminiscent of present-day Albuginales. How-Systematics of the Straminipila: Labyrinthulomycota, Hyphochytriomycota, and Oomycota 83