Two of the genera in Sigmoideomycetaceae, Sigmoideomyces and Thamnocephalis, have been included in either Zygomycetes or Deuteromycetes (Benny et al., 1992). Thaxter (1891a) originally included Sigmoideomyces in Deuteromycetes. Blakeslee (1905) described Thamnocephalis, stating that it was most closely related to Sigmoideomyces. These fungi all form unispored sporangiola. Thamnocephalis was included in various mucoralean families but this was not usually the case for Sigmoideomyces (Benny et al., 1992).
Cunninghamella (Mucoraceae, Mucorales), Sigmoideomyces, and Thamnocephalis (Sigmoideomycetaceae, Zoopagales) all produce pedicellate, unispored sporangiola from fertile vesicles. In Cunninghamella these vesicles are borne singly at the apex of the main sporophore or its branches whereas in Sigmoideomyces and Thamnocephalis the fertile vesicles are borne in pairs, on stalks, from the cells at the branching point of the fertile hyphae. The latter two genera produce fertile heads consisting of septate, dichotomously branched fertile hyphae that terminate in sterile spines, have pairs of stalked, fertile vesicles, and they are haustorial parasites of other fungi. Cunninghamella spp. do not produce fertile heads, the sporophores are coenocytic, sympodial and/or verticellately branched, and terminate in a single fertile vesicle not in a sterile spine, and are saprophytic (Benny et al., 1992).
Three species were described in Sigmoideomyces, S. dispiroides Thaxter, S. clathroides Bayliss Elliott, S. divaricatus McLean, and two in Thamnocephalis, T. quadrupedata, T. ovalisporus, before 1965. Two species were added by Benny et al. (1992), one in Thamnocephalis and the other in a new genus, Reticulocephalis, the latter characterized by the anastomoses of the tips of the sterile spines on the fertile heads. Sigmoideomyces clathroides was transferred to Reticulocephalis, as R. clathroides along with the new taxon, R. niveus. Sigmoideomyces spp. do not have anastomosing spine tips. Thamnocephalis spp. form their fertile heads on a stalk whereas Reticulocephalis and Sigmoideomyces lack a stalk (Benny et al., 1992).
All members of Sigmoideomycetaceae are haustorial parasites of other fungi (Chien, 1992, 2000), including Basidiobolus.
Sigmoideomycetaceae Benny, R.K. Benjamin & P.M. Kirk, 1992 (Mycologia 84: 620).
Fertile head sessile or stalked, composed of regularly septate, coiled fertile hyphae that are dichotomously branched and terminate in sterile spines. Cell at the base of each branch, except final 1 or 2, producing pairs of stalked fertile vesicles covered with pedicellate, unispored sporangiola. Probably parasitic and heterothallic. Zygospores and chlamydospores unknown.
Type genus: Sigmoideomyces Thaxter.
KEY TO THE GENERA OF SIGMOIDEOMYCETACEAE
A. Fertile heads formed on a stalk —— Thamnocephalis
AA. Fertile heads sessile —— B
B. Sterile spines adherent —— Reticulocephalis
BB. Sterile spine free —— Sigmoideomyces
Synopsis of genera
RETICULOCEPHALIS Benny, R. K. Benjamin & P. M. Kirk, 1992 (Mycologia 84: 633).
SIGMOIDEOMYCES Thaxter, 1891 [Bot. Gaz. (Crawfordsville) 16: 22).
THAMNOCEPHALIS Blakeslee, 1905 [Bot. Gaz. (Crawfordsville) 40: 165)].
Updated Feb 10, 2005