The Dictyostelia (social amoebae) are common soil dwelling amoeba most often isolated from the leaf litter decomposition zone of forest soils [1
]. The first known dictyostelid was isolated by Brefeld [2
], but by 1940 still only ten species were recognized. Now, largely due to the work of a handful of people, this number exceeds 100 [3
]. Dictyostelia are perhaps best known for the model organism Dictyostelium discoideum
, and the fact that their home clade, the Amoebozoa is the sister group to the Opisthokonts (holozoa + holofungi) [4
The dictyostelids possess an unusual life cycle, known almost exclusively from laboratory observations [1
]. Throughout most of the asexual cycle individual dictyostelid amoebae feed upon bacteria and multiply by binary fission. When food becomes scarce, amoebae aggregate by the tens of thousands gradually forming a multicellular entity with differentiated cell types. The aggregate then develops into a slug or pseudoplasmodium, a true multicellular polarized unit. The slug moves as a more or less coherent unit, with a head region seeking environmental conditions suitable for the formation of fruiting bodies [5
]. Fruiting bodies vary widely in morphology but essentially consist of one or more stalks, in most cases composed of dead cells supporting one or more distinct spore masses (sori) in a variety of arrangements [1
]. The entire process is coordinated by the production of chemo-attractants, a process that is well characterized in the model organism D. discoideum
but mostly unknown in other species [3
]. Thus, in the dictyostelid life cycle growth and development are completely separate; growth occurs only in the unicellular amoebae, and once aggregation starts, cell division ceases and differentiation and morphogenesis can begin [5
Less is known about the sexual cycle of dictyostelids, which culminates in the formation of a macrocyst. This cycle can be between both homothallic (self-fertile) and heterothallic forms [1
]. A recent study identified the mating-type locus for the model species Dictyostelium discoideum
]. As with fruiting bodies, macrocyst formation begins with an aggregative process. However, instead of forming a slug, two of the aggregating amoebae fuse, consume the remaining cells and then encyst. Macrocysts were only recognized as the sexual stage of Dictyostelia in the l960s [9
], and although they were probably present in the last common ancestor of the taxon [11
], there are many species for which this stage has yet to be observed. Macrocysts also serve as a resistant stage for surviving sub-optimal growth conditions [3
]. Thus, dictyostelids have three ways of dealing with unfavourable conditions: (a) encystment of individual amoebae (microcysts), (b) formation of sexual macrocysts and (c) formation of multicellular fruiting bodies containing spores.
Traditionally, classification of dictyostelids has been based on morphology [1
]. The characters used range from aspects of the initial aggregation stage including the overall pattern (mound, radiate) and type of aggregative signalling molecule (acrasin: cAMP, glorin, folate, etc), to structural features of the final fruiting body. The latter include features such as the type of growth (clustered, gregarious, coremiform or solitary) and branching pattern, characteristics of the spores such as their overall shape (round or elliptical) and the presence or absence of polar granules inside them, etc [1
]. Based on these characters, the three traditional genera of Dictyostelia were defined--Acytostelium
] with acellular stalks, Dictyostelium
] with cellular stalks and mostly unbranched or sparsely branched fruiting bodies and Polysphondylium
] with regularly spaced whorls of lateral branches on cellular stalks.
However, a cladistic study of dictyostelia morphology first suggested [14
] and molecular phylogeny later confirmed [11
] that this traditional morphology based taxonomy is deeply flawed. Instead, molecular phylogeny grouped nearly all of the species known at the time into four major clades. None of these major clades correspond to traditional genera indicating that there is far less pattern in dictyostelia morphological evolution at the deepest taxonomic levels than initially thought [11
]. SSU rDNA analyses also depicted a very deep phylogeny with many large gaps (long unbroken branches), suggesting large numbers of missing species and/or highly uneven SSU rDNA evolutionary rates [11
]. Meanwhile finer level studies including multiple isolates of several "species" showed that these are not always molecularly similar. Such morphologically similar but phylogenetically distinct taxa are referred to individually as cryptic species [16
] and together as species complexes. Species complexes are scattered throughout the dictyostelid tree, and are found in all four major groups [16
]. The combination of a deep taxon littered with cryptic species suggests that much dictyostelid diversity remains to be described. Filling in these gaps should help us to develop a better understanding of evolutionary trends across the group.
During the period of 2000 to 2009, the number of described species of Dictyostelia essentially doubled. This was primarily the result of the "Global Biodiversity Survey of Eumycetozoans" project (PBI), mandated to investigate the diversity of dictyostelids and other eumycetozoans throughout the world. Soil samples were collected at various localities, emphasizing (a) regions of the Southern Hemisphere where there were few or no previous records and (b) areas of the Northern Hemisphere that have received relatively little study. Together these include Australia (the mainland and Tasmania), New Zealand, South Africa, Patagonia, northern Thailand, Laos and East Africa. Additional samples were obtained from localities in Central America, Alaska and several islands (Ascension Island, Cocos Island, Puerto Rico and Madagascar). The range of vegetation types sampled included grassland, savanna, shrubland, southern beech forest, Eucalyptus forest, lowland tropical rain forest, montane tropical forest and tropical monsoon forest.
We have determined complete SSU rDNA sequences from all new species isolated from PBI project samples, as well as additional isolates of previously described species. The new species and isolates are dispersed across the tree, filling in many gaps and indicating that there are eight major divisions of Dictyostelia rather than the previously recognized four. Most importantly, the new species expand the known morphological diversity of all four previously recognized major clades, indicating that morphology in Dictyostelia is even more plastic than previously realized. At the same time, some patterns at higher taxonomic levels are beginning to emerge.