Systematics and Evolution, Part A The Mycota

alternation of generations in which the gameto-
phyte thallus is reduced to a single-celled cyst
(Figs.7.5 and 7.6d). The sporophytic thalli of
Cystogenesspecies are like those ofEuallomyces
in structure and size, and meiosis takes place in the
resistant sporangium (Emerson and Wilson 1949 ;
Olson 1980 ;Wilson 1952 ). However, inCystogenes
isolates meiosis is followed by a pairing of haploid
nuclei prior to cleavage into spores. Meiospores
exit the resistant sporangium as binucleate cells
that move sluggishly as biflagellate motile cells or
as nonflagellated amoeboid cells before rapid
encystment. The two cyst nuclei undergo a single

mitotic division to produce four haploid cells (iso-

gametes) that exit upon deliquescence of the single

papilla. The fate of the quartet of cells emerging

from the cyst was unknown to Emerson; however,

the cells were shown to function as uniflagellate or

aflagellate amoeboid isogametes that fuse to form

zygotes (McCrainie 1942 ;Teter 1944 ).

Brachyallomyces Isolates in which the motile

spores from the resistant sporangia regularly gave

rise to asexual (sporophytic) rather than sexual

(gametophytic) thalli were placed in a new species,

A. anomalus, and included in the subgenus

Fig. 7.4(a–m) Life cycle ofAllomyces arbuscula.(a)
Germinating resistant sporangium with ruptured outer
wall and swollen inner wall with discharge papilla. (b)
Release of uniflagellate meiospores. (c–e) Stages in
growth of young thallus (c,d) into mature gameto-
phytic thallus bearing papillate male () and female ()
gametangia (e). (f) Release of male and female gametes
from paired gametangia. (g) Syngamy of anisogametes.
(h) Biflagellate planozygote. (i) Germination of

encysted planozygote. (j, k) Young thallus (j) develops

into mature sporophytic thallus (k) bearing resistant

sporangia (R) and thin-walled zoosporangia (Z). (l)

Release of zoospores from thin-walled zoosporangia.

(m) Germination of encysted zoospore to form young

thallus (j). Drawing from Emerson ( 1941 ), used with

permission from Lloyd Library and Museum; American

Society of Pharmacognosy

188 T.Y. James et al.