Clustering and methylation of repeated DNA: persistence in avian development and evolution.

In the chicken genome, clusters of repeated DNA sequences occur which have alternate arrangements of the component sequence elements. Many of these clustered, repeated sequences are extensively methylated. We have established that both their arrangement and their methylation are invariant regardless of the source of chicken DNA. Comparisons included DNA from sperm, from a series of embryonic stages, from tissues of single adult individuals, and from thirty individual chickens of two strains. These same sequences are found in the DNA of some avian species related to chickens, and there they show the same clustered, methylated form. In related species, some of the arrangements found in chicken DNA are different or missing.