SET domain proteins modulate chromatin domains in eu- and heterochromatin

The SET domain is a 130-amino acid, evolutionarily conserved sequence motif present in chromosomal proteins that function in modulating gene activities from yeast to mammals. Initially identified as members of the Polycomb- and trithorax-group (Pc-G and trx-G) gene families, which are required to maintain expression boundaries of homeotic selector (HOM-C) genes, SET domain proteins are also involved in position-effect-variegation (PEV), telomeric and centromeric gene silencing, and possibly in determining chromosome architecture. These observations implicate SET domain proteins as multifunctional chromatin regulators with activities in both eu- and heterochromatin – a role consistent with their modular structure, which combines the SET domain with additional sequence motifs of either a cysteine-rich region/zinc-finger type or the chromo domain. Multiple functions for chromatin regulators are not restricted to the SET protein family, since many trx-G (but only very few Pc-G) genes are also modifiers of PEV. Together, these data establish a model in which the modulation of chromatin domains is mechanistically linked with the regulation of key developmental loci (e.g. HOM-C).     

Chaos in a long-term experiment with a plankton community

Mathematical models predict that species interactions such as competition and predation can generate chaos. However, experimental demonstrations of chaos in ecology are scarce, and have been limited to simple laboratory systems with a short duration and artificial species combinations. Here, we present the first experimental demonstration of chaos in a long-term experiment with a complex food web. Our food web was isolated from the Baltic Sea, and consisted of bacteria, several phytoplankton species, herbivorous and predatory zooplankton species, and detritivores. The food web was cultured in a laboratory mesocosm, and sampled twice a week for more than 2,300 days. Despite constant external conditions, the species abundances showed striking fluctuations over several orders of magnitude. These fluctuations displayed a variety of different periodicities, which could be attributed to different species interactions in the food web. The population dynamics were characterized by positive Lyapunov exponents of similar magnitude for each species. Predictability was limited to a time horizon of 15-30 days, only slightly longer than the local weather forecast. Hence, our results demonstrate that species interactions in food webs can generate chaos. This implies that stability is not required for the persistence of complex food webs, and that the long-term prediction of species abundances can be fundamentally impossible.