Approximating genealogies for partially linked neutral loci under a selective sweep.

Consider a genetic locus carrying a strongly beneficial allele which has recently fixed in a large population. As strongly beneficial alleles fix quickly, sequence diversity at partially linked neutral loci is reduced. This phenomenon is known as a selective sweep. The fixation of the beneficial allele not only affects sequence diversity at single neutral loci but also the joint allele distribution of several partially linked neutral loci. This distribution can be studied using the ancestral recombination graph for samples of partially linked neutral loci during the selective sweep. To approximate this graph, we extend recent work by Etheridge et al. (Ann Appl Probab 16:685-729, 2006) and Schweinsberg and Durrett (Ann Appl Probab 15:1591-1651, 2005) using a marked Yule tree for the genealogy at a single neutral locus linked to a strongly beneficial one. We focus on joint genealogies at two partially linked neutral loci in the case of large selection coefficients alpha and recombination rates rho = theta(alpha/log alpha) between loci. Our approach leads to a full description of the genealogy with accuracy of theta((log alpha)(-2)) in probability. As an application, we derive the expectation of Lewontin's D as a measure for non-random association of alleles.

Analysis of cyclin B1 and CDK activity during apoptosis induced by camptothecin treatment.

We have studied the role of cyclins and cyclin-dependent kinase (CDK) activity in apoptosis induced by camptothecin (CPT). In this model, 22% of the cells stain for annexin-V at 24 h and then proceed to be 93% positive by 72 h. This time window permits the analysis of cyclins in cells that are committed to apoptosis but not yet dead. We provide evidence that cyclin protein levels and then associated kinase levels increase after CPT treatment. Strikingly, cyclin B1 and cyclin E1 proteins are present at the same time in CPT treated HT29 cells. Although cyclin B1 and E1 CDK complexes are activated in CPT treated cells, only the cyclin B1 complex is required for apoptosis since reduction of cyclin B1 by RNAi or roscovitine treatment reduces the number of annexin-V-stained cells. We have detected poorly organized chromosomes and phosphorylated histone H3 epitopes at the time of maximum cyclin B1/CDK kinase activity in CPT-treated cells, which suggests that these cells enter a mitotic catastrophe. Understanding which CDKs are required for apoptosis may allow us to better adapt CDK inhibitors for use as anti-cancer compounds.