Molecular systematics of dictyostelids: a case for single genus

The cellular slime moulds or dictyostelids, are a major group of edaphic (soil-dwelling) unicellular eukaryotic micro-organisms which feed on bacteria in the humus layer of tropical and temperate soils. The extraordinary behaviour of dictyostelids where solitary, predatory amoebae become social, forming a multi-cellular spore-producing ‘organism’ when their food supply becomes limited has fascinated generations of researchers. Since their discovery the taxonomy of the cellular slime moulds has been largely based on morphological features, revised many times, but still remains unresolved and controversial. One focus of discussion is the controversial relationship between the two principal genera of Dictyostelium and Polysphondylium, which by virtue of a common amoeba type are placed in the same family - Dictyosteliaceae. Despite the current distinction between Dictyostelium and Polysphondylium it is impossible to definitively resolve these genera, or indeed the broader classification of cellular slime moulds, using only morphological features. Given that dictyostelids are both of major ecological importance and often used as a model eukaryotic system, it has become imperative to resolve these taxonomic uncertainties and to finally establish the validity of one or two genera. This study used two PCR based techniques: (1) direct sequencing of the ITS 1 and ITS 2 regions of the rDNA complex together with (2) ISSR-PCR, a standard molecular technique but rarely applied to the dictyostelids. The latter requiring the development of a working protocol before it could be implemented. The sequence data from 51 different dictyostelid species and isolates was aligned with CLUSTAL and analysed via PAUP. This study clearly demonstrates that molecular markers cannot distinguish Dictyostelium (sensu stricto) from Polysphondylium (sensu stricto): indeed it presents evidence to support the existence of a single genus. The implications of these results are unequivocal: the current systematics of cellular slime moulds, based on morphological characters, must be revised.