shared by almost all genera that diverge before
the peronosporomycete/saprolegniomycete split
is their apparent lack of a sexual cycle involving
oospores (Beakes and Sekimoto 2009 ; Beakes
et al. 2012 ). As noted by Sparrow ( 1976 ), sexual
stages in these groups are probably not easily
recognized and had been overlooked until then.
This conclusion was given credence when
Schnepf et al. ( 1977 ,1978b) described an unusual
life cycle in the diatom parasiteLagenisma cosci-
nodisci.Inthisspecies,zoosporesresultingfrom
meiotic divisions (zoomeiospores) encyst on the
surface of their host. One cyst (the presumptive
male) produces a fertilization tube that fuses with
the adjacent (presumptive female) cyst and plas-
mogamy takes place, followed by nuclear fusion
and zygote formation (Schnepf et al. 1977 ).
Recently, a remarkably similar sequence was
observed in the basal speciesE. dicksoniiwhen
it infectedChoristocarpus(Gachon, pers. com-
mun.). Cysts settle on the host surface and fuse
to give rise to an enlarged zygote cell. This frag-
mentary evidence suggests sexual reproduction
by means of zoospores arising from meiosis
(zoomeiospores), and fusion of the resulting
cysts (or thalli) is the most likely form of sexual
reproduction in early-diverging oomycetes. A
similar pattern of sexual reproduction has also
been described inLagena radicola(Fig.3.14e, f)
(Barr and De ́saulniers 1990 ).Haptoglossais a
diverse genus, but there are still no reports of
where meiosis takes place, although fusion of
adjacent thalli has been illustrated inH. hetero-
spora(Karling 1981 ). SeveralHaptoglossaspecies
produce both single and binucleate aplanos-
pores, as described in H. heteromorpha
(Fig.3.7j, k) (Glockling and Beakes2000c)and
H. erumpens(Fig.3.7l) (Beakes and Glockling
2002 ), but how this is related to any sexual cycle
is not known.
FreshwaterOlpidiopsis spp. are the only
known basal clade representatives to form rec-
ognizable male and female thalli leading to the
formation of a thick-walled resting spore
(Fig.3.7p, q) (Karling 1981 ; Martin and Miller
1986 ). In Olpidiopsis varians, synchronous
gametangial meiosis takes place in adjacent
gametangial thalli (evidenced by the presence
of synaptonemal complexes) followed by
nuclear transfer via a fertilization tube to the
larger (presumptive female) thallus (Martin
and Miller 1986 ). The characteristic pattern of
sexual morphogenesis in oomycetes by means
of egg-containing oogonia with attached
antheridial cells was first observed in Leptomi-
tales (Fig. 3.8e, m, h, i) and Rhipidiales
(Fig.3.12a, b, c, e), which are the two orders
that lie at the cusp of saprolegniomycete/pero-
nosporomycete divergence. Some genera in the
~Leptomitales s. lat., such as Apodachlyella
(Fig.3.8h, i) (Longore et al. 1987 ) andEury-
chasmopsis(Canter and Dick 1994 ), produce
sporangium-like antheridia in which the cysts
act as individual antheridial cells, transferring
their nuclei to the eggs via a germ tube. In
Aphanomycopsis sexualisthe elongate anther-
idium is divided into compartments, each of
which forms a separate fertilization tube
(Fig.3.8m) (Martin 1975 ; Karling 1981 ). This
suggests that, around the time of the divergence
of the peronosporomycete line, oogonia and
antheridia evolved from zoomeiosporangia as
a result of the progressive suppression of cleav-
age and retention of the female gamete. Other
basal saprolegniomycete genera, such asChla-
mydomyzium(Fig.3.8t) (Glockling and Beakes
2006b), Cornumyces (Beakes unpublished
observations), and Ducellieria (Hesse et al.
1989 ), all seem to produce oospore-like resting
spores parthenogenetically without any appar-
ent involvement of antheridia. Sexual repro-
duction in the holocarpic lagenidiaceous
members of thePeronosporaless. lat., such as
Myzocytiopsis vermicola, is brought about by
the transfer of a nucleus between neighbouring
thallus compartments (Fig.3.14i). The receiv-
ing oosphere is differentiated from a peripheral
periplasmic layer, as expected in this order
(Glockling and Beakes2006a).
The morphology of the mature oospore has
also been an important taxonomic character in
oomycetes (Dick 1969 ,2001a,b; Johnson et al.
2002 ). In Saprolegniomycetes an outer primary
wall layer immediately forms around the naked
differentiated oospheres (Beakes 1981b), below
which the various mature oospore wall layers
are later accreted. In contrast, in all Peronos-
poromycetes the single oosphere is cleaved
from a layer of peripheral cytoplasm, the peri-
plasm (Fig.3.12b). This is one of the definingSystematics of the Straminipila: Labyrinthulomycota, Hyphochytriomycota, and Oomycota 81