The introduction of foreign DNA into cells and organisms has facilitated much of modern biological research, and it promises to become equally important in clinical practice. Locating sites of foreign DNA incorporation in mammalian genomes has proven burdensome, so the genomic location of most transgenes remains unknown. To address this challenge, we applied nanopore sequencing in search of the site of integration of Tg(Pou5f1-EGFP)2Mnn (also known as Oct4:EGFP), a widely used fluorescent reporter in mouse germ line research. Using this nanopore-based approach, we identified the site of Oct4:EGFP transgene integration near the telomere of Chromosome 9. This methodology simultaneously yielded an estimate of transgene copy number, provided direct evidence of transgene inversions, revealed contaminating E. coli genomic DNA within the transgene array, validated the integrity of neighboring genes, and enabled definitive genotyping. We suggest that such an approach provides a rapid, cost-effective method for identifying and analyzing transgene integration sites.

Gene conversion is GC-biased across a wide range of taxa. Large palindromes on mammalian sex chromosomes undergo frequent gene conversion that maintains arm-to-arm sequence identity greater than 99%, which may increase their susceptibility to the effects of GC-biased gene conversion. Here, we demonstrate a striking history of GC-biased gene conversion in 12 palindromes conserved on the X chromosomes of human, chimpanzee, and rhesus macaque. Primate X-chromosome palindrome arms have significantly higher GC content than flanking single-copy sequences. Nucleotide replacements that occurred in human and chimpanzee palindrome arms over the past 7 million years are one-and-a-half times as GC-rich as the ancestral bases they replaced. Using simulations, we show that our observed pattern of nucleotide replacements is consistent with GC-biased gene conversion with a magnitude of 70%, similar to previously reported values based on analyses of human meioses. However, GC-biased gene conversion since the divergence of human and rhesus macaque explains only a fraction of the observed difference in GC content between palindrome arms and flanking sequence, suggesting that palindromes are older than 29 million years and/or had elevated GC content at the time of their formation. This work supports a greater than 2:1 preference for GC bases over AT bases during gene conversion and demonstrates that the evolution and composition of mammalian sex chromosome palindromes is strongly influenced by GC-biased gene conversion.