Comparison of conserved DNA or protein sequences is a more direct and reliable method to establish genetic relationships. Two family trees of the social amoebas were constructed by comparing the DNA sequences of their small subunit ribosomal RNA (SSU rRNA) gene on one hand and the amino-acid sequences of their a-tubulin protein on the other. Both trees show that the similarities in fruiting body architecture only partially reflect an underlying genetic similarity. (24) Instead, both the a-tubulin tree and the SSU rRNA tree shown here (Fig. 3 ) subdivide the 75 known species of social amoebas into four major groups. There are dictyostelids in all four groups. The acytostelids and all white polysphondylids are members of group 2, but the purple polysphondylid P. violaceum occupies a position between groups 3 and 4, and forms a small clade with the dictyostelid D. laterosorum. This indicates that at least two out of the three previously proposed genera are polyphyletic. Multiple origins for the polyspondylids were also predicted by a family tree that was based on 18 combined morphological traits. (29) The DNA-based family tree was subsequently used to investigate trends in the evolution of morphology. The multicellular stages of different species of Dictyostelia show a large variety of shapes and sizes, which have been carefully quantitated and noted in the original species diagnoses, along with differences at the cellular level (see Refs 3, 30 for overviews). Species use different chemoattractants and aggregate as single cells or as inflowing streams (Fig. 4) . Once formed, aggregates may either produce one or several organizing tips, giving rise to solitary or clustered fruiting bodies. Secondary tips may appear in characteristic positions on rising sorogens, giving rise to secondary body axes and a range of different fruiting body architectures. Fruiting body stalks may develop a variety of support structures, such as discs, crampons or triangular supporters, while their tips can vary from thinly pointed to bulbous. Many species form motile slugs, which may optionally form a stalk while migrating. At the cellular level, spores can be round or oblong and, in the latter case, display conspicuous granules at their poles, which are either loosely grouped or consolidated. Some species have retained the ancestral survival strategy of encystation, or display the capacity to mate and form sexual macrocysts. A mapping of all these characters to the molecular tree indicates which characters are shared between close relatives, and yields information about the order in which characters evolved (Fig. 5) . Amoeba size shows no strong group-specific trend, but spores are consistently smaller in the most basal group 1. Of all traits, spore morphology follows the phylogeny most strongly. Spores of the evolutionary youngest group 4 species have no polar granules, in group 2, polar granules are loosely grouped, and in groups 1 and 3, they are consolidated. The acellular stalk evolved only once. The shape of the stalk tip also marks relatedness; species in group 2 usually have pointy or blunt tips, while in groups 1 and 4 stalk tips tend to be club-or head-shaped. Species with similarly coloured stalks and spore heads are often related, but no color is specific for any of the four major groups.