INTRODUCTION TO THE KICKXELLOMYCOTA

Adl et al. (2012) and Hibbett et al. (2007) describe the two groups in this phylum as symbionts of of many taxa, which range from protists to many groups of arthropods. Some also are parasites of other fungi, and many are free-living in soil and dung. Though filamentous, their hyphae variable: coenocytic to septate. Asexual reproduction occurs mainly by conidia, but they show many variations on the conidial theme. The most notable are the taxa that once were placed in the class of Zygomycota, the conjugating fungi, called the ‘trichomycetes’, the hair or thread fungi. These were single spores on the ends of elongates filaments. Both groups in this phylum are conjugating fungi, most of which produce zygospores that are distinctive for their particular lines, though sexual reproduction seems to have been lost in a few of the lines. In general, these fungi are small because they are poorly-known, not because they have low diversity.

KICKXELLOMYCOTINA

This is a fairly large group of group of fungi that can be saprobic or form haustoria (feeding hyphae) that penetrate the tissues of the host taxa: fungi and arthropods. The mycelium consists of hyphae (branched or unbranched) that are septate (Hibbett et al. 2007). The crosswalls have distinctive pores that are plugged. The asexual sporangia usually have one or two spores and the sexual spores (zygospores) are variously shaped.

Dimargaris (Figure 1) is specialized as a parasite of other fungi. They have hyphae with septae (text with tooltip) Septate means walled or partitioned. (crosswalls), and they produce asexual spores that are paired (bispores).

Kickxella (Figure 2) is a parasite of other fungi. Their hyphae have septa with distinctive lens-shaped cavities. Asexual reproduction is by sporangiola (a merosporangium of single spores; Figure 3). They seem to be related to or associated with the Harpellales.

Harpella (Figure 3) inhabits the gut of insects where the small mycelium attaches to the lining of the hind gut. These are especially common in the guts of black fly larvae, which at times of high rates of infection, can reduce the black fly considerably. The vegetative phase is reduced but variable. The name ‘trichomycetes’, which is sometimes used to describe them, comes from the distinctive hair-like asexual spores.

ZOOPAGOMYCOTINA

Zoopagomycotina, a name which literally means the animal-attacking fungi, all seems to be parasites of soil fungi, invertebrates, or amoebae. Asexual spores may be reduced to a few to many in branched chains (Hibbett et al. 2007). Some make sporangia comparable to those of the Mucoromycota.

Amoebophilus (Figure 4) infects the very small, much like a chytrid. As its name implies, it feeds on amoebae as do most members of its order. However, some feed on or parasitize nematodes. Typically, the hyphae are coiled, which can suddenly release, scrambling the insides of the organism that they have invaded.

FIGURE 1. Dimargaris showing bisporic merosporangia. Image from http://microfungi.truman.edu/old/secure/Anamorphic-fungi/Dimargaris/Dimargaris-sp.html	FIGURE 2. The sporangium (text with tooltip) Sporangia are spore-bearing structures. of Kickxella. Image from http://www.bioimages.org.uk/HTML/P4/P46250.HTM	FIGURE 3. Hypha of Harpella with the characteristic hair-like branches. Taken from the gut of an insect. Image from http://www.scielo.br/img/revistas/mioc/v98n6/484136.jpg	FIGURE 4. Amoebophilus with sporangial hyphae emerging from an ameba. Image from http://www.uoguelph.ca/~gbarron/MISC2004/amoeboph.htm

FIGURE 5. A cladogram showing the relationships between subphyla of the Kickxellomycota (taxa in the shaded box) and the other fungal taxa. The topology is supported by James et al. (2006) and Porter et al. (2011).

SYSTEMATICS OF THE KICKXELLOMYCOTA

The zygospore-producing fungi, formerly placed in the phylum Zygomycota, are basal groups, and, like many other basal groups with diversity in form and taxa, are paraphyletic. Indeed, the main defining synapomorphy, the zygospore, appears to be primitive in the fungi that do not produce motile cells. This was one of the results of Lutzoni et al. (2004) in their comprehensive systematic study of the Fungi. Increasingly, molecular work has confirmed that the Zygomycota is not monophyletic. Schussler et al. (2001) removed the arbuscular mycorrhizal (AM) fungi from the Zygomycota and defined them as a separate phylum, the Glomeromycota. Lutzoni et al. (2004) in a SSU and LSU r-RNA study of the Kingdom Fungi show that the existing orders of Zygomycota occupy at least 4 different monophyletic lines. Also, White et al. (2006), in an analysis of the r-RNA operon, confirm the paraphyletic nature of the traditional groups of Zygomycota. In their analysis, the zygomycotes could be divided into three independent clades, Thus, the group could be fragmented into at least three (White et al. 2006) or four (Lutzoni et al. 2004) more phyla. The organization of the conjugating fungi in our system is an interpretation of White et al. (2006) and Lutzoni et al. (2004), but informed by Tanabe et al. (2005) and Hibbett et al. (2007) and separates the former Zygomycota into the following higher taxa: Mucoromycota, Glomeromycota, Entomophthoromycota, and Kickxiellomycota.

The zygospore-producing fungi are taxa that superficially resemble some of the conjugating algae. They clearly are fungi in which the the dikaryotic phase does not go beyond the bounds of the zygospore. For many taxa, the vegetative filaments do not have septa and are haploid. Bruns et al. (1992) in their examination of 18S rRNA nucleotide sequences, confirmed that the chytrids and the conjugating fungi were basal groups within the clade of the Kingdom Fungi. Mainly, the conjugating fungi, like the rest of the fungi, never produce flagella at any stage in their life history. More recently, further molecular evidence (see Tudge, 2000 and Patterson, 1999 for a synopsis) has confirmed their position near the root of the fungi [see also Lang, The Fungal Mitochondrial Genome Project], which is part of a larger clade called the opisthokonts, a group that includes the choanoflagellates and the metazoans (Patterson, 1999). Margulis and Schwartz (1998) still maintain that the fungi are a kingdom of conjugating taxa, and, therefore, include the zygomycota.

Because Entomophthoromycota is a phylum as defined by Humber (2012) and confirmed by Gryzanskyi et al. (2013), taxa within Kickxellomycotina and Zoopagomycotina are orphaned. Thus, Kickxellomycota as a phylum was was our interpretation of James et al. (2006) and Porter et al. (2011; see Figure 5). The relationships of the various orders in this group are problematic. White et al. (2006) in a Bayesian analysis of the 18S,+28S+5.8S rDNA genes found that the Kickxellales+Harpellales+Dimargaritales+Zoopagales clade was monophyletic, but far removed from the Amoebidales+Eccrinales clade. Tanabe et al. (2000, 2004, and 2005) suggest problems with the relationships, particularly the association of the Zoopagales with the other taxa. Thus this system is tentative only.