* Corresponding author: Present address: Division of Molecular Epidemi- ology, National Center for Toxicological Research, 3900 NCTR Road, Jefferson, AR 72079. E-mail: jjchen@nctr.fda.gov 199 Genetic Variation in Mitochondrial DNA of North American Herring Gulls, Larus argentatus Junjian Z. Chen 1,* , Carole L. Yauk 2 , Craig Hebert 3 , and Paul D.N. Hebert 1 1 Department of Zoology University of Guelph Guelph, Ontario N1G 2W1 2 Department of Biology McMaster University Hamilton, Ontario L8S 4L8 3 National Wildlife Research Center Canadian Wildlife Service, Hull, Quebec K1A 0H3 ABSTRACT. The herring gull (Larus argentatus), which is one of the most abundant gulls in temperate North America, is used as an important bioindicator species, but little is known about its patterns of genetic variation. This study examines DNA sequence diversity in the cytochrome b gene of mitochondrial DNA (mtDNA) from herring gull colonies in the Great Lakes basin and from a site in the Maritimes. A 307-bp fragment of the gene was analyzed using a rapid mutation screening procedure coupling directed termination PCR (DT-PCR) with single-stranded conformation polymorphism (SSCP). Seven mtDNA haplotypes were revealed in 224 specimens with a common haplotype dominating all populations. Direct sequencing of a 1063-bp fragment of the cytochrome b gene identified 8 additional haplotypes, but the sequence divergence among all 15 haplotypes was very low (average 0.24%). These data suggest that herring gulls experienced a bottleneck during the last glaciation that is now reflected in their low levels of genetic divergence. The geographic distribution of mtDNA haplotypes indicates that gene flow among herring gull populations is sufficient to ensure genetic homogeneity in the Great Lakes, but insufficient to homogenize genetic variation between the Great Lakes and the Maritimes. Meanwhile, a comparison analysis of frozen egg specimens suggests a possible shift in the genetic composition of the Great Lakes populations over the last 15 years. INDEX WORDS: Genetic variation, DNA, cytochrome b gene, herring gulls, directed termination PCR, Great Lakes. J. Great Lakes Res. 27(2):199–209 Internat. Assoc. Great Lakes Res., 2001 INTRODUCTION The herring gull (Larus argentatus) is one of the most abundant gulls throughout the temperate re- gions of North America, reflecting, in part, its rapid increase in population size since the turn of the cen- tury (Kadlec and Drury 1968). Because it is a colo- nial nester, top predator, and year-round resident, the herring gull has been used as a sentinel species for studies of the biological effects of contaminant exposure in the Great Lakes region since the late 1960s (Mineau et al. 1984, Peakall and Fox 1987, Yauk and Quinn 1996). Although the herring gull’s behavior, migratory movements, and ecotoxicology have been studied extensively, little is known about its patterns of genetic variation. Yet, such informa- tion can shed new understanding of the extent of gene flow among populations and on the spatial and temporal patterning of genetic diversity in the species. This investigation of genetic diversity in herring gull populations has focused on examination of the mitochondrial genome, which has proven to be par- ticularly valuable in studies of population structure. Because of its rapid rate of sequence evolution, lack