Fonticulida

''Fonticula'' is a genus of cellular slime mold which forms a fruiting body in a volcano shape. As long ago as 1979 it has been known to not have a close relationship with either the Dictyosteliida or the Acrasidae, the two well-established groups of cellular slime molds. In 1979, ''Fonticula'' was made a new genus of its own due to the unique characteristics of its fruiting body, with only one species: ''Fonticula alba''.

The life cycle of ''Fonticula alba'' alternates between an amoeboid vegetative stage and aggregative fruiting stage. The fruiting body of the genus has a unique shape, as its sorocarp resembles a volcano and sorus looks like a ball of hot lava emerging from that volcano.

Molecular phylogenies have found alignments in genes of ''Fonticula alba'' to subgroups in Opisthokonta. A 2009 study has found that ''Fonticula'' is the sister taxa to ''Nuclearia'', thus making it related to the kingdom Fungi.

''Fonticula'', ''Nuclearia'', and Fungi have been united into the Holomycota, which is sister to the Holozoa.

History and etymology

While working at the University of Wisconsin in 1979, Ann Worley, Kenneth Raper and Marianne Hohl discovered an organism that fit no recognized genus of the slime mold taxon Acrasiomycetes. At the time Acrasiomycetes could be divided into two classes: Acrasidae and Dictyostelidae. This classification was based on morphological characteristics, but it is now known that the two subgroups are not closely related. Moreover, ''F. alba'' did not truly fit into either of these subclasses but shared a few characteristics of both. Although ''F. alba'' shared characteristics of subclasses within Acrasiomycetes, Worley ''et al.'' (1979) were convinced that its best fit taxonomically would be in a new, undescribed family designated as ''Fonticulaceae'', which would then contain the genus ''Fonticula''. The new genus name ''Fonticula'' is a reference to the fruiting-body morphology: ''Fonti-'' from the Latin word ''Fons'' (''fountain'', "shape, form") and ''-cula'', from Latin diminutive ''culus'' (''little'', "size").

Opisthokonta is an exceptionally diverse eukaryotic group, containing shared ancestry between fungi, animals and even a few protists (Brown ''et al.'', 2009). In 2009 it was concluded that the genus ''Fonticula'' is part of the unranked group Opisthokonta. Brown ''et al.'' (2009) sequenced nuclear encoded genes of ''Fonticula alba'' for phylogenetic analysis and concluded that the genus was a sister group to the filose amoebae in the genus ''Nuclearia'' and that the ''Fonticula'' and ''Nuclearia'' clade are sister groups to fungi.

Description

Morphology and anatomy

The morphological characteristics of genus ''Fonticula'' are unlike those seen in slime mold subgroups Acrasidae or Dictyostelidae. Several studies have found that in its vegetative state, myxamoebae of ''F. alba'' are generally small and irregular in form, ranging from 8–12 x 6–10 μm in size. The myxamoebae have finger-like projections deemed as filose pseudopodia, which extent at the posterior or lateral ends of the cell. Worley ''et al.'' (1979) found that the myxamoebae had a distinguishable ectoplasm and endoplasm. The clear ectoplasm is on the outer edges, while the inner endoplasm is more granular. Vacuoles are also found in numerous digestion stages in actively feeding ''F. abla''. These small vacuoles contain bacteria. In active feeding stages, there is a slime coat surrounding the myxamoeba to which bacteria sink onto. The ultrastructure of ''Fonticula'' also includes small contractile vacuoles, which are mainly deposited towards the posterior end of the cell. An ultrastructural feature that is shared between ''Fonticula'' and certain Acrasidae is mitochondria with discoid cristae. The golgi apparatus lends a helping hand in the fruiting stage in ''Fonticula'' as numerous dictyosomes are involved in the sorogenesis process. Cells in the genus are generally uninucleate