Fonticula

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.[2][1] The myxamoebae have finger-like projections deemed as filose pseudopodia, which extent at the posterior or lateral ends of the cell.[2] 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.[2] These small vacuoles contain bacteria. In active feeding stages, there is a slime coat surrounding the myxamoeba to which bacteria sink onto.[1] The ultrastructure of Fonticula also includes small contractile vacuoles, which are mainly deposited towards the posterior end of the cell.[2] An ultrastructural feature that is shared between Fonticula and certain Acrasidae is mitochondria with discoid cristae.[2] The golgi apparatus lends a helping hand in the fruiting stage in Fonticula as numerous dictyosomes are involved in the sorogenesis process.[1] Cells in the genus are generally uninucleate, however there have been cases of some cells containing two or even three nuclei.[2] The nucleus of F. alba cells have an inconspicuous nucleolus under the light microscope.[2] The fruiting body of F. alba contains an unbranched sorocarp, which is composed of upright tapered stalks which apically bear a round source containing spores. Stalks range from 200-500 μm in length. The sori on the fruiting body are white, and roughly 200-350 μm in diameter. Spores are cystic in shape, and roughly 5.0-6.0 μm in diameter.[2]

Multigene phylogenetic analysis was conducted in F. alba in 2009 which allowed it to be placed into Opisthokonta. The five nuclear encoded genes that were sequenced were: small subunit ribosomal RNA (SSU rRNA), actin, beta-tubulin, elongation factor 1-alpha (EF1-a) and heat shock protein 70 (HSP70). The study's results depicted that from 42 eukaryotic taxa, many alignments were made with a total of 2802 aligned sequences (Brown et al., 2009). Phylogenetic trees based on the molecular genetics sequenced in this study showed that the genus Fonticula was a sister taxon to the genus Nuclearia. Those two sister taxa as a clade are in turn a sister taxa to Fungi. Fonticula therefore represents the first evolution of an organism with a cellular slime mold-like morphology within the broad-group Opisthokonta (Brown et al., 2009); (Brown, 2010).

Another study done by Barlow et al. in 2014 traced the evolution of the five adaptor protein (AP) complexed in fungi, but also provided some insight onto Fonticula alba. The adaptor proteins investigated work in vesicular transport in eukaryotes, particularly in cargo-selection and coat-protein recruitment. The study found that F. alba had all five adaptor protein complexes present in its genome, whereas the kingdom Fungi only retained the first three adaptor protein complexes (Barlow et al., 2014). This study concluded that the last common ancestor of the sister groups Fungi, and Fonticula and Nuclearia (also known as the clade Holomycota) contained a complete set of all five adaptor protein complexes.

An amoeboid trophic phase alternating with an aggregating fruiting phase describes the life cycle of F. alba. Fruiting bodies are volcano-like structures that are unique to its genus.[1] Commencing of the fruiting stage occurs when trophic amoeba halt their feeding, and dense aggregations begin to form (Deasey, 1982). Over time, an envelope of a mucus-like substance begins to surround the aggregated amoebae.[2] The amoebae at the top of the aggregation begin to project upward, and a hyaline membrane is laid down onto the projection, as development occurs. Stalk material is secreted while the amoebae move upwards within the projection.

In early stages of orogenesis, the cells form a number of dictyosomes from the outer nuclear membrane in response to a certain stimuli.[1] The dictyosomes aid in the accumulation and deposition of stalk material. The golgi apparatus in F. alba swells up, instead of having small vesicles pinch off is cristae, which is unique to the genus.[1] The vesicles produce a thread-like material, within a mucous-matrix that lets the aggregated mound turn into a tapered stalk.[2] The vesicles then release their internal contents via fusing with plasma membrane of the sorogenic cells. The material made within the vesicles is mostly assembled at the thick base of the stalk, and is thought to provide structural support. Cells are amoeboid during stalk formation.

As the sorocarp reaches max height, orogenic cells begin to depict differential shapes and thus different functions. Amoeboid cells remain near the base, continuously producing stalk material through their numerous dictyosomes during spore formation. These cells continue producing stalk material even when spores have been released.[1] Cystic cells are above the amoeboid cells, these cells are preparing to flourish into spores and thus have less dictyosomes, known as "pre-spores".[1]

The ultrastructural comparison between the amoeboid cells and pre-spores is quite evident. The pre-spores have less dictyosomes which tend to be flattened. As the number of dictyosomes decreases, the cytoplasm takes up more room within the cell and the cell adapts a more round shape (Deasey, 1982). There is also a difference in the plasma membrane of amoeboid orogenic cells and pre-spore cells. The pre-spore cells plasma membrane appears to be invaginated, whilst the amoeboid cells PM looks smooth (Deasey, 1982). Elliptical cells, located in the upper neck of the sorocarp are a little further along into spore-development than the cyst cells. These cells lack dictyosomes, are thick-walled and thought to increase pressure within the neck of the stalk.[1] As pressure increases, the encysted cells will then come up from the apex and come together and form a large ball called the sorus.[2] The majority of the cells within the source have successfully developed into spores. When the sorus is developed, the neck of the fruiting body is almost fully lacking in sorogenis cells. Amoeboid cells remain at the base of the sorocarp.[1] Spores of F. alba release and flow as the stalk collapses as time goes on. The optimal pH for growth and fruiting-body development for Fonticula alba is a substrate with a near neutral pH. A lower pH may result in a poorly developed fruiting structure.[2]