Systematics and Evolution, Part A The Mycota

taxonomic framework for the Oomycota is out-
lined in Fig.3.6and Table3.5and is described
in what we perceive to be an ascending phylo-
genetic hierarchy from the basal clades to the
most species-rich crown groups.

E. Early Diverging Clades: ClassesIncertae
Sedis

Early molecular studies seemed to show that
the oomycetes fell into just two lineages (Hud-
speth et al. 2000 ;Le ́clerc et al. 2000 ; Petersen
and Rosendahl 2000 ; Riethmu ̈ller et al. 1999 ).
The first indication that this might be an overly
simplistic view came when marine genera were
included in analyses (Cook et al. 2001 ). This

revealed thatHaliphthorosand related genera

formed a well-supported clade that diverged

before the main Saprolegniomycete and

Peronosporomycete lines separated (Fig.3.5).

Subsequent studies revealed a number of addi-

tionalearly diverging generathat are predomi-

nantly marine organisms, and most are

parasites(Figs.3.6and3.7, Table3.5) (Beakes

and Sekimoto 2009 ; Beakes et al. 2012 ). Many of

these basal oomycetes haveholocarpic thalli

that develop initially fromunwalled plasmodia

(Sekimoto et al. 2008a). The hyperparasitic

generaPseudosphaerita(Anderson et al. 1995 )

andRozellopsis (Held 1981 ) and the diatom

parasitesLagenisma(Schnepf et al.1978a,b)

and Ectrogella (Raghukumar 1980 ) are also

reported to form plasmodial thalli in the early

Fig. 3.6 Schematic tree of Oomycotasummarizing
main order- and family-level clades as well as some
taxonomically undefined subclades.BSMbrassicolus

downy mildew,DMPHdowny mildew with pyriform

haustoria,DMCCdowny mildew with coloured conidia,

GDMgraminicolus downy mildew

54 G.W. Beakes et al.