Hosts strongly influence parasite fitness. However, it is challenging to disentangle host effects on genetic vs plasticity-driven traits of parasites, since parasites can evolve quickly. It remains especially difficult to determine the causes and magnitude of parasite plasticity. In successive generations, parasites may respond plastically to better infect their current type of host, or hosts may produce generally 'good' or 'bad' quality parasites. Here, we characterized parasite plasticity by taking advantage of a system in which the parasite (the yeast Metschnikowia bicuspidata, which infects Daphnia) has no detectable heritable variation, preventing rapid evolution. In experimental infection assays, we found an effect of rearing host genotype on parasite infectivity, where host genotypes produced overall high or low quality parasite spores. Additionally, these plastically induced differences were gained or lost in just a single host generation. Together, these results demonstrate phenotypic plasticity in infectivity driven by the within-host rearing environment. Such plasticity is rarely investigated in parasites, but could shape epidemiologically important traits.

Transcription of genes coding for the small nuclear RNAs (snRNAs) is dependent upon a unique transcription factor known as the small nuclear RNA-activating protein complex (SNAPc). SNAPc binds to an essential proximal sequence element located about 40 -65 base pairs upstream of the snRNA transcription start site. In the fruit fly Drosophila melanogaster, DmSNAPc contains three distinct polypeptides (DmSNAP190, DmSNAP50, and DmSNAP43) that are stably associated with each other and bind to the DNA as a complex. We have used mutational analysis to identify domains within each subunit that are involved in complex formation with the other two subunits in vivo. We have also identified domains in each subunit required for sequence-specific DNA binding. With one exception, domains required for subunit-subunit interactions lie in the most evolutionarily conserved regions of the proteins. However, DNA binding by DmSNAPc is dependent not only upon the conserved regions but is also highly dependent upon domains outside the conserved regions. Comparison with functional domains identified in human SNAPc indicates many parallels but also reveals significant differences in this ancient yet rapidly evolving system. The small nuclear RNA (snRNA) 3 -activating protein complex (SNAPc) is a multisubunit protein required for transcription of genes that code for the spliceosomal (and certain other) snRNAs (1-4). SNAPc recognizes and binds specifically to a proximal sequence element (PSE) located about 40 -65 base pairs upstream of the transcription start site. SNAPc has also variously been called PSE-binding protein (5, 6) and PSE-binding transcription factor (1, 3, 7). In humans, SNAPc contains five distinct polypeptide chains (SNAP190, SNAP50, SNAP45, SNAP43, and SNAP19) named based upon the apparent molecular weights of these subunits (4,(7)(8)(9)(10)(11)(12). For the remainder of this article, the human protein and its subunits will be indicated by the prefix "Hs."In the fruit fly Drosophila melanogaster, DmSNAPc contains three distinct polypeptide chains that are orthologous to HsSNAP190, HsSNAP50, and HsSNAP43 (13,14). The three fly subunits, DmSNAP190, DmSNAP50, and DmSNAP43, are each present in a single copy in native DmSNAPc (15) and have calculated molecular masses of 84, 43, and 42 kDa, respectively. Interestingly, a homologous complex (tSNAPc) is required for transcription of the spliced leader snRNA in trypanosomes (16 -18). This indicates that a SNAP-like complex arose very early in eukaryotic evolution and continues to be essential for snRNA transcription in widely divergent contemporary eukaryotes. However, even within insects, snRNA gene promoter sequences recognized by SNAPc have diverged fairly rapidly (19).The subunits of eukaryotic SNAPc tightly associate with each other in solution even when the complex is not bound to DNA. The subunits co-purify through numerous chromatography columns (1-3, 16 -18, 20). Moreover, each of the three metazoan core subunits is essential for sequence-specific binding to the PSE as no...

These results suggest that mild acute intermittent hypoxia can elicit differential forms of respiratory plasticity in sham surgery versus contused animals, and may be a promising neurorehabilitation approach to improve respiratory function after cervical spinal cord injury.

The small nuclear RNA activating protein complex (SNAPc) is the major unique transcription factor required for transcription of genes coding for small nuclear RNAs (snRNAs). In the fruit fly Drosophila melanogaster, DmSNAPc contains three distinct subunits (DmSNAP190, DmSNAP50, and DmSNAP43) that form a complex before binding to an snRNA gene promoter. We have used mutational analysis to identify domains within each subunit of DmSNAPc that are required for complex formation with the other two subunits in vivo. Also, we mapped domains in each subunit that are required for the DNA‐binding activity of DmSNAPc. We have found that the most evolutionarily conserved regions of the proteins are involved in SNAP complex assembly. Nevertheless, we found that domains outside of the conserved regions are also important for the DNA binding activity of DmSNAPc, even though they are not required for subunit assembly. Comparing our findings with published results in the human system indicates not only many important similarities but also significant differences in this ancient though rapidly evolving system. This work is supported by National Science Foundation grants MCB‐0131151 and MCB‐0641350 and in part by the California Metabolic Research Foundation. M. T. is a recipient of an Arne N. Wick Pre‐doctoral Research Fellowship from the California Metabolic Research Foundation.