Systematics and Evolution, Part A The Mycota

Fo ̈rster et al. 1990 ). While it was obvious that
posteriorly uniflagellate “phycomycetes” were
allied with true fungi, molecular phylogenetic
studies immediately detected a large phyloge-
netic distance between the blastoclads and the
chytrids (Bruns et al. 1992 ; Nagahama et al.
1995 ). Despite an uncertain phylogenetic place-
ment, the composition of blastoclads has
remained largely consistent over time (with
the exception of the genusPhysoderma), and
it has been accepted as monophyletic because
of the presence of several distinctive character-

istics. The most consistent characteristic of the

group is reproduction by a zoospore with a

prominent nuclear cap that in hyphal species

often demonstrates bipolar germination

(Fig.7.1a, b). Another distinguishing feature is

the thick-walled, darkly pigmentedresting spo-

rangiumthat is often ornamented with pits,

ridges, or spines (Fig.7.1c–f) and undergoes

germination by the cracking of an outer wall

through which an endosporangium protrudes.

The resting sporangium is sometimes referred

to as a resting spore; however, because the

Fig. 7.1Characteristic stages of blastoclads include
zoospores and bipolar germination of encysted spores.
(a) Uniflagellate zoospore ofAllomyces javanicus. ffla-
gellum,nnucleus,ncnuclear cap,lglipoid granules. (b)
Cyst (c) demonstrating bipolar germination with one
end enlarging to form a hypha and the other end com-
posed of thin rhizoids involved in substrate attachment
and resource extraction. (c) Scanning electron micro-
graph of resting sporangia ofCoelomomyces iliensis

var.iliensis, a parasite of the mosquitoCulex antenna-

tus[photo from Couch and Bland ( 1985 ), used with

permission from Elsevier]. (d) Spiny resting sporangia

ofCatenaria spinosa, a parasite of the midgeChirono-

mus decorus.(e) Cluster of resting sporangia ofAllo-

myces moniliformisdemonstrating pitted outer wall.

(f) Germinating resting sporangium ofA. javanicus

releasing meiospores. Scale bar in A–C¼ 10 mM; D,

F¼ 30 mM; E¼ 20 mM

178 T.Y. James et al.