Molecular mechanisms for the evolution of multicellular complexity in social amoebas

Biologists want to understand how complex multicellular organisms like ourselves have evolved from their simple single-celled ancestors. We know in theory how this happened: Spontaneous mutations in the genes of earlier organisms caused small changes in the developmental program of their offspring. This sometimes resulted in an improved adult that more successfully reproduced, and therefore gradually replaced the earlier form. However, to really understand this process and prove that it actually occurred, we have to trace back which genes were mutated and how this mutation changed gene function. We also need to know which developmental mechanisms were regulated by the mutated genes and how the altered developmental mechanism eventually produced the improved adult form. Because it is not possible to obtain such detailed information for highly evolved animals like ourselves, we investigate this problem in the social amoebas. These organisms live as single cells when they are feeding. However, when starved, they come together and form a multicellular fruiting body, in which a proportion of cells is preserved as spores. The other cells form a stalk and other structures to support the spore mass. This life style depends on mutual collaboration and specialization of cells. In the course of evolution the social amoebae have progressed from basal species that formed structures with 10-100 cells and only two cell-types, to species that form large complex structures with over 1 million cells and up to five cell types. One species, D.discoideum, is used by many laboratories as a model system to understand how cells move, eat, propagate and communicate with each other. Over a 100 known social amoeba species have been isolated worldwide. To understand how these species gradually became more complex and different from each other, we first used DNA data to construct a family tree of the social amoebas. This tree showed that there are four major groups of social amoebas and that D.discoideum is a member of the group that was formed most recently. We next measured a large number of characters that determine the typical size and form of all 75 species. By plotting these characters on the family tree we can trace back which character came first and how it gradually changed into greater complexity. Most recently we were involved in sequencing the genomes of species that represent each of the four groups. These project are almost completed and offer us enormous opportunities to study how genes have changed during evolution. In this study we will combine a detailed analysis of changes in genes with our earlier analysis of the changes in form that occurred during evolution. This should give us indications which changes in the genes might have been responsible for the appearance of novel forms. By manipulating the gene in question and observing its effect on the form of the species, we will be able to prove that a particular genetic change was the actual cause for a specific change in form. In this manner we will be able to unravel the genetic mechanisms that have been used by evolution to generate the enormous variety of forms that we see in modern multicellular organisms.

A major goal of biology is to understand how complex multicellular organisms evolved from unicellular life forms. This is ultimately caused by natural selection acting on genetic variation, but it has proven to be difficult to identify the gene modifications that actually caused the evolution of novel multicellular forms. Dictyostelid social amoebas or are uniquely suited to resolve this problem. They display conditional multicellularity, where cells aggregate to form motile slugs and architecturally complex fruiting structures. Several Dictyostelia show excellent experimental and genetic tractability and one species, D.discoideum, is a widely used model for studying problems in cell- and developmental biology. In previous research we constructed a robust molecular phylogeny of all Dictyostelia. We plotted an extensive range of species traits to the phylogeny, providing information about the order in which these traits evolved. Most recently we participated in an international effort to sequence the genome of at least one species in each of the four major groups of Dictyostelia. In addition to the completed D.discoideum genome, the genomes of D.fasciculatum and P.pallidum are now almost completely assembled, while draft sequences of two more genomes are being finalized. Combined with the map of trait evolution, the completed genomes offer a tremendous opportunity to understand how changes in genes and genomes caused the appearance of novel phenotypes. We will firstly identify genes with core developmental roles in all social amoebas and when possible to retrace the original function of these genes to their solitary ancestors. Secondly, we will map how key regulatory genes have changed during evolution and correlate these changes with the evolution of morphological features. Selected genes will be subjected to allelic replacement with ancestral orthologs to confirm that the genetic change caused the phenotypic innovation.