No abstract

The great majority of metazoans belong to bilaterian phyla. They diversified during a short interval in Earth's history known as the Cambrian explosion, ∼540 million years ago. However, the genetic basis of these events is poorly understood. Here we argue that the vertebrate genome organizer CTCF (CCCTC-binding factor) played an important role for the evolution of bilaterian animals. We provide evidence that the CTCF protein and a genome-wide abundance of CTCF-specific binding motifs are unique to bilaterian phyla, but absent in other eukaryotes. We demonstrate that CTCF-binding sites within vertebrate and Drosophila Hox gene clusters have been maintained for several hundred million years, suggesting an ancient origin of the previously known interaction between Hox gene regulation and CTCF. In addition, a close correlation between the presence of CTCF and Hox gene clusters throughout the animal kingdom suggests conservation of the Hox-CTCF link across the Bilateria. On the basis of these findings, we propose the existence of a Hox-CTCF kernel as principal organizer of bilaterian body plans. Such a kernel could explain (i) the formation of Hox clusters in Bilateria, (ii) the diversity of bilaterian body plans, and (iii) the uniqueness and time of onset of the Cambrian explosion.

SummaryThe problem of genetic hitch-hiking in a geographically subdivided population is analysed under the assumption that migration rates among populations are relatively small compared with the selection coefficient for a newly arising advantageous allele. The approximate method used in the paper is valid when the number of emigrants per generation (Nm) is less than one. The approximate analysis shows that hitch-hiking can result in substantial differences among populations in the frequencies of neutral alleles closely linked to the advantageous allele. Thus, in cases for which genetic hitch-hiking is thought to be responsible for low levels of genetic variability in regions of the genome with restricted crossing over, it might be possible to find confirmatory evidence for that hypothesis by finding unusual patterns of geographic differentiation in the same regions of the genome.