INTRODUCTION

The Chytridiomycota are considered to be primitive organisms whose affinities with all other fungi are debatable (Bowman et al. 1992; Powell 1993; Barr 2001; Lutzoni et al. 2004). They are regarded as basal to the remaining fungi based on their simple thalli and the formation of flagellated spores (Alexopoulos et al. 1996; Paquin et al. 1997; Webster and Weber 2007). Molecular phylogenetic studies using protein sequences suggest that they originated in the Precambrian, 1400-1600 million years ago (Heckman et al. 2001). An ancient origin for the group is indicated by the diversity of life forms (parasites, saprotrophs, and mutualists), the variety of modes of sexual reproduction, the characteristic flagellar apparatus, the simple but diverse thallus morphology, and the strict dependence on moist conditions (Sparrow 1960; Karling 1977; Powell 1993). In addition, a number of studies have shown that chytridiomycetes are polyphyletic, which is consistent with the morphological and ecological diversity of its members (James et al. 2000; Lutzoni et al. 2004).

Chytridiomycetes comprise five orders based on both morphological and molecular characters (Barr 1980, 2001; James et al. 2000). Barr (1980) introduced zoospore ultrastructure as the main diagnostic character for chytridiomycete phylogeny. Other morphological features include the presence or absence of a lid-like operculum, zoospore ultrastructure and size, and flagellum length (Barr et al. 1987; Fuller and Clay 1993). Sexual reproduction is also valuable in understanding the phylogeny and taxonomy of chytridiomycetes (Barr 1978; James et al. 2000); however, this has only been rarely observed in members of this group (Powell 1993).

Chytridiomycetes produce a globose or filamentous coenocytic thallus, which includes thin- and/or thick-walled sporangia, and zoospores (Barr 1992). The zoospore, which constitutes the initial means of chytridiomycetes attachment to its substrate (Lozupone and Klein 2002), is characterized by either a posteriorly directed whiplash flagellum or, as in the Neocallimastigales, numerous flagella are present (Li and Heath 1992). The morphologically simpler chytridiomycetes live entirely within host cells (endobiotic). Other species live either on the surface of the host or substrate (epibiotic), or within the host or substrate (interbiotic). Individuals whose entire thallus is converted into one or more reproductive structures are termed "holocarpic." Only a portion of the thallus is converted into a reproductive structure in "eucarpic" species. Another character of systematic importance is type of thallus development (Barr 2001). In the endogenous type the encysted zoospore may give rise to a single sporangium (monocentric) or to a number of sporangia (polycentric). When the nucleus of the encysted zoospore migrates to originate the sporangia, the development is termed exogenous (monocentric or polycentric). Within the exogenous polycentric development, the nucleus may divide with each new nucleus originating a prosporangium that collectively form a cluster of sporangia (colonialist type); alternatively, each new nucleus may move into a germ tube and divide, with the resulting nuclei migrating into an independent system of rhizoids (filamentous type).

In spite of the simplicity of the thallus and associated morphological structures, several fossil fungi have been assigned to extant chytridiomycetes (e.g., Renault 1895; Bradley 1967; Taylor and Stubblefield 1987). Included are epibiotic and endobiotic zoosporangia on cordaitean pollen grains (Millay and Taylor 1978) and several stages of the life cycle of holocarpic and eucarpic forms (Taylor et al. 1992). The objective of this paper is to report the presence of well-preserved zoospores, and thin-walled and thick-walled sporangia comprising several stages of the life cycle of a fungus that is morphologically and developmentally assignable to endobiotic chytridiomycetes, particularly Synchytrium.