Analysis of Similarity Pattern and Selective Constraint in C. elegans and C. briggsae Genomes

After an evolving lineage splits, independent mutation and random drift in the new lineages lead to divergence of their homologous DNA sequences. Different positive selection in the new lineages accelerates their divergence, while uniform negative selection due to retention of the same function in all lineages leads to selective constraint and slows the divergence down [2]. Protein-coding regions usually evolve slowly, indicating strong selective constraint, although some of them evolve very fast [1, 2]. Many instances of selective constraint were also detected in non-coding DNA [3], both transcribed and untranscribed, but there have been no estimates of the proportion of constrained nucleotides within the total non-coding DNA of any species. Such estimates are particularly important for multicellular eukaryotes, because non-coding DNA constitutes from ∼70% (C. elegans) to ∼95% (H. sapiens) of their genomes [4]. New data on C. briggsae DNA sequences provided the first opportunity to study the pattern of similarity and selective constraint between the genomes of two closely related species.