Two different genomes that produce the same result

Despite differences in genomic sequence, the developmental program of two distantly related Dictyostelium species is remarkably similar

Exploitation of Other Social Amoebae by Dictyostelium caveatum

Dictyostelium amoebae faced with starvation trigger a developmental program during which many cells aggregate and form fruiting bodies that consist of a ball of spores held aloft by a thin stalk. This developmental strategy is open to several forms of exploitation, including the remarkable case of Dictyostelium caveatum, which, even when it constitutes 1/10(3) of the cells in an aggregate, can inhibit the development of the host and eventually devour it. We show that it accomplishes this feat by inhibiting a region of cells, called the tip, which organizes the development of the aggregate into a fruiting body. We use live-cell microscopy to define the D. caveatum developmental cycle and to show that D. caveatum amoebae have the capacity to ingest amoebae of other Dictyostelid species, but do not attack each other. The block in development induced by D. caveatum does not affect the expression of specific markers of prespore cell or prestalk cell differentiation, but does stop the coordinated cell movement leading to tip formation. The inhibition mechanism involves the constitutive secretion of a small molecule by D. caveatum and is reversible. Four Dictyostelid species were inhibited in their development, while D. caveatum is not inhibited by its own compound(s). D. caveatum has evolved a predation strategy to exploit other members of its genus, including mechanisms of developmental inhibition and specific phagocytosis

Evidence that talin alternative splice variants from Ciona intestinalis have different roles in cell adhesion

BACKGROUND: Talins are large, modular cytoskeletal proteins found in animals and amoebozoans such as Dictyostelium discoideum. Since the identification of a second talin gene in vertebrates, it has become increasingly clear that vertebrate Talin1 and Talin2 have non-redundant roles as essential links between integrins and the actin cytoskeleton in distinct plasma membrane-associated adhesion complexes. The conserved C-terminal I/LWEQ module is important for talin function. This structural element mediates the interaction of talins with F-actin. The I/LWEQ module also targets mammalian Talin1 to focal adhesion complexes, which are dynamic multicomponent assemblies required for cell adhesion and cell motility. Although Talin1 is essential for focal adhesion function, Talin2 is not targeted to focal adhesions. The nonvertebrate chordate Ciona intestinalis has only one talin gene, but alternative splicing of the talin mRNA produces two proteins with different C-terminal I/LWEQ modules. Thus, C. intestinalis contains two talins, Talin-a and Talin-b, with potentially different activities, despite having only one talin gene. RESULTS: We show here that, based on their distribution in cDNA libraries, Talin-a and Talin-b are differentially expressed during C. intestinalis development. The I/LWEQ modules of the two proteins also have different affinities for F-actin. Consistent with the hypothesis that Talin-a and Talin-b have different roles in cell adhesion, the distinct I/LWEQ modules of Talin-a and Talin-b possess different subcellular targeting determinants. The I/LWEQ module of Talin-a is targeted to focal adhesions, where it most likely serves as the link between integrin and the actin cytoskeleton. The Talin-b I/LWEQ module is not targeted to focal adhesions, but instead preferentially labels F-actin stress fibers. These different properties of C. intestinalis the Talin-a and Talin-b I/LWEQ modules mimic the differences between mammalian Talin1 and Talin2. CONCLUSION: Vertebrates and D. discoideum contain two talin genes that encode proteins with different functions. The urochordate C. intestinalis has a single talin gene but produces two separate talins by alternative splicing that vary in a domain crucial for talin function. This suggests that multicellular organisms require multiple talins as components of adhesion complexes. In C. intestinalis, alternative splicing, rather than gene duplication followed by neo-functionalization, accounts for the presence of multiple talins with different properties. Given that C. intestinalis is an excellent model system for chordate biology, the study of Talin-a and Talin-b will lead to a deeper understanding of cell adhesion in the chordate lineage and how talin functions have been parceled out to multiple proteins during metazoan evolution

The Putative bZIP Transcripton Factor BzpN Slows Proliferation and Functions in the Regulation of Cell Density by Autocrine Signals in Dictyostelium

The secreted proteins AprA and CfaD function as autocrine signals that inhibit cell proliferation in Dictyostelium discoideum, thereby regulating cell numbers by a negative feedback mechanism. We report here that the putative basic leucine zipper transcription factor BzpN plays a role in the inhibition of proliferation by AprA and CfaD. Cells lacking BzpN proliferate more rapidly than wild-type cells but do not reach a higher stationary density. Recombinant AprA inhibits wild-type cell proliferation but does not inhibit the proliferation of cells lacking BzpN. Recombinant CfaD also inhibits wild-type cell proliferation, but promotes the proliferation of cells lacking BzpN. Overexpression of BzpN results in a reduced cell density at stationary phase, and this phenotype requires AprA, CfaD, and the kinase QkgA. Conditioned media from high-density cells stops the proliferation of wild-type but not bzpN− cells and induces a nuclear localization of a BzpN-GFP fusion protein, though this localization does not require AprA or CfaD. Together, the data suggest that BzpN is necessary for some but not all of the effects of AprA and CfaD, and that BzpN may function downstream of AprA and CfaD in a signal transduction pathway that inhibits proliferation

Figure 2

<p>Time-lapse microscopy of the development of <i>D. caveatum</i>/<i>D. discoideum</i> mixtures. Cells of control GFP-expressing <i>D. discoideum</i> populations (A) and GFP-expressing <i>D. discoideum</i> populations containing 1/10³ Cell Tracker Red-labeled <i>D. caveatum</i> (B) were allowed to develop for 36 hours and continuously observed by time-lapse video microscopy. A. The control <i>D. discoideum</i> cultures undergo development from aggregation to fruiting (collective circular motion within aggregates is observed; see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0000212#pone.0000212.s006" target="_blank">Movie S2A</a>). Three stages are shown here: beginning of development (before aggregation), the tight aggregate stage, and the slug stage. Time is in HH:MM and 00:00 corresponds to the beginning of recording, during the initiation of aggregation. Bar = 200 µm. B. Aggregates infected with <i>D. caveatum</i> are blocked at the aggregate stage and no collective circular motion is observed (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0000212#pone.0000212.s007" target="_blank">Movie S2B</a>). Finally <i>D. caveatum</i> amoebae emerge as slugs and fruiting bodies when all of <i>D. discoideum</i> amoebae are consumed (and the GFP signal disappears).</p

Range and specificity of inhibition and phagocytosis

<p>Four additional Dictyostelid species were tested according to the same protocol as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0000212#pone-0000212-g004" target="_blank">Figure 4A</a>. The table indicates the number of aggregates for each species as a percentage of the total number of developmental stages observed on filters after 24 hours of incubation (average (standard deviation)), as illustrated in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0000212#pone-0000212-g005" target="_blank">Figure 5</a>. In the controls, development at 24 h has passed the aggregate stage, while in the presence of <i>D. caveatum</i> (10⁸ cells/mL inside the dialysis membrane) development is blocked at this stage (compare first two columns). The ability of <i>D. caveatum</i> to phagocytose ameobae of these species, as assessed by live cell microscopy, is also indicated: all these Dictyostelid species are ingested by <i>D. caveatum</i> in the same manner as <i>D. discoideum</i> (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0000212#pone.0000212.s005" target="_blank">Movie S1D</a>).</p