Genetical Research

Pattern of selective constraint in C. elegans and C. briggsae genomes

a1 Section of Ecology and Systematics, Cornell University, Ithaca, NY 14853, USA


Similarity between related genomes may carry information on selective constraint in each of them. We analysed patterns of similarity between several homologous regions of Caenorhabditis elegans and C. briggsae genomes. All homologous exons are quite similar. Alignments of introns and of intergenic sequences contain long gaps, segments where similarity is low and close to that between random sequences aligned using the same parameters, and segments of high similarity. Conservative estimates of the fractions of selectively constrained nucleotides are 72%, 17% and 18% for exons, introns and intergenic sequences, respectively. This implies that the total number of constrained nucleotides within non-coding sequences is comparable to that within coding sequences, so that at least one-third of nucleotides in C. elegans and C. briggsae genomes are under strong stabilizing selection.

(Received July 6 1998)
(Revised November 9 1998)
(Revised December 30 1998)

c1 Corresponding author. Tel: +1 (607) 254 4221. Fax: +1 (607) 255 8088. e-mail:
p1 Permanent address: Institute of Mathematical Problems in Biology, Russian Acad. Sci., Pushchino, Moscow Region, Russia, 142292.