LOW DIVERSITY AND BIASED SUBSTITUTION PATTERNS IN THE MITOCHONDRIAL-DNA CONTROL REGION OF SPERM WHALES - IMPLICATIONS FOR ESTIMATES OF TIMESINCE COMMON ANCESTRY

The mitochondrial DNA (mtDNA) control region was sequenced in 37 sperm whales from a large part of the global range of the species. Nucleoti de diversity was several-fold lower than that reported for control reg ions of abundant and outbred mammals, but similar to that for populati ons known to have experienced bottlenecks. Relative rate tests did not suggest that the low diversity is due to a lower substitution rate in sperm whale mtDNA. Rather, it is more likely that demographic factors have reduced diversity. The pattern of nucleotide substitutions was e xamined by cladistic methods, facilitated by the apparent monophyly of lineages from the Southern Hemisphere, as defined by a single base pa ir deletion. Substitutions were nonrandom in nature, confined to a few ''hot spots,'' and parallel substitutions constituted a majority of t he inferred changes. The substitution pattern fitted a negative binomi al distribution better than a Poisson distribution, and the bias in nu mber of substitutions among sites was considerably higher than previou sly reported for the mtDNA control region of any species. A novel meth od of estimating time since common ancestry was developed, which utili zes the transition/transversion ratio R and the number of substitution s inferred from a parsimony analysis. Using this method, we estimated the age of sperm whale mtDNA diversity to be about 6,000-25,000 years, and when the uncertainty of R was accounted for, a range of about 1,0 00-100,000 years was obtained.