The evolution of morphological complexity in the Dictyostelids

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 are sacrificed to form a structure that aids spore dispersal. 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 100.000 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. Because its genome has been sequenced, we have access to all the genes that control these processes. D.discoideum uses cyclic AMP (cAMP) as the major signal molecule for cell-cell communication. It acts as a chemoattractant to bring starving cells together. It then continues to guide cells to move coherently and build a fruiting body. cAMP also decides which cells should differentiate into spores. D.discoideum is one of 75 known social amoeba species. These species display large differences in the size and shape of their fruiting structures. To understand how these species gradually became mor complex and different from each other, we first need to know how they are related to each other and to their ancestors, the solitary amoebas. In previous research we used DNA data to construct a family tree of the social amoebas and we now know that there are four major groups of social amoebas. D.discoideum belongs to the most evolved group 4. We also found that many of the genes that are necessary for cAMP signalling are present in all four groups. However, between species, there are differences in the stage of development at which these genes are active. In addition, some genes have duplicated and started to assume novel roles. In this project we will reconstruct in what order all changes in shape and size occurred during social amoeba evolution. We will do this by measuring a large number of characters that determine the typical size and shape of all 75 species and by plotting these characters on the family tree. This will allow us to conclude which character was there first and how it gradually changed into greater or sometimes lesser complexity. We will also plot the presence of specific cAMP signalling genes to the family tree and the changes in these genes. This allows us to conclude whether a specific change in a gene was accompanied by a specific change in character. By manipulating the gene in question and observing its effect on species character we will be able to prove that a particular genetic change was the actual cause for a specific change in character. In this manner we will be able to unravel the genetic mechanisms that have been used by evolution to generate species diversity and complexity.

We use the Dictyostelids as a model system to understand the genetic mechanisms that generated diversity and morphological complexity during the evolution of multicellular organisms. The project is based on recent construction of a molecular phylogeny of the Dictyostelids and on current data that a deeply conserved mechanism for cAMP signalling regulates cell-type diversification and coordinated cell movement in the Dictyostelids. We will use the following strategies to achieve three objectives: Objective I. Reconstruct the evolution of morphological complexity in the Dictyostelids The entire repertoire of morphological traits of all known Dictyostelid species will be mapped tot the molecular phylogeny in order to infer the basal state of each trait and its history of change. We will use gross morphological traits that are available from species descriptions, such as the size and shape of growing amoebae, spores, stalks and spore heads, the habit and branching patterns of fruiting structures and the morphology and phototactic behaviour of slugs. Where necessary we will curate and complement these data by microphotography and morphometric quantitation. In addition, we will obtain detailed information on the major hallmarks of multicellular life i.e. the traits that mark cell-type divergence and the coordination of cell movement. To do so we will visualize the presence of a prestalk-prespore tissue pattern in all species with an antispore antibody. We will measure proportions of prestalk-, prespore- and anterior-like cells in dissociated structures. The latter cell-type is the progenitor of multiple support structures in derived species. We will also visualize the dynamics of cell movement. cAMP oscillations mediate aggregation in D.discoideum and are also emitted by organizer regions in multicellular structures. The oscillations give rise to complex spiral cell movement patterns which are considered to shape slugs and fruiting bodies. We will use time-lapse videomicroscopy to record cell movement during and after aggregation in a large number of representative species and we will plot the presence and dynamics of cell movement waves to the phylogeny. Objective II. Develop genetic tractability for representative species. The ability to modify or disrupt genes is essential to be able to establish causality between genetic and morphological change. For the model system D.discoideum, mutants are available that can grow in liquid media. Under these conditions modified genetic material can readily be introduced using plasmids that are either of bacterial or D.discodeum origin. Transfection procedures have been developed for species, such as P.pallidum that grow on the natural food source, bacteria. We will test and optimize these procedures for transfection of one or two species from each of the four taxon groups. Objective III. Determine whether the evolution of specific morphological traits is caused by modifications in cAMP signalling genes. We will use both PCR and biochemical assays to test whether the cAMP signalling genes ACA, PdsA, PdeE and cAR have been conserved throughout the phylogeny. We will study the spatio-temporal expression pattern of conserved genes to assess whether gene regulation has been altered during evolution. For one species from each taxon group, we will obtain full length coding sequence for conserved genes by library screens or inverse PCR to complement a null mutant in its D.discoideum ortholog. This will tell us whether gene function is altered. By plotting the observed changes in gene function and/or regulation to the phylogeny we can infer whether the history of morphological change is correlated with the history of change in cAMP signaling genes. By disrupting the function of the altered genes in a genetically tractable species we will establish whether the observed correlation results from causality.