Genetic diversity and population structure of wintering Western Sandpipers from the Sinaloa coast, Mexico

Although Western Sandpipers (Calidris mauri) are one of the best-studied shorebirds along the Pacific Flyway, their genetic identity and population structure is poorly known. We studied the genetic population structure of nonbreeding Western Sandpipers at Bahia Santa Maria in Sinaloa, northwestern Mexico. A 685-bp fragment of the mtDNA control region was sequenced for 162 individuals, resulting in 29 variable positions that defined 41 haplotypes. The most common haplotype (WESA01) occurred in 44% of all individuals, whereas 15 were unique to single individuals. Nucleotide diversity was low (p= 0.0030 +/- 0.0019 [SE]), but haplotype diversity was moderately high (h= 0.802 +/- 0.033 [SE]). The main two maternal lineages exhibited a slightly different local scale distribution that appeared to be related to migratory chronology. Lineage A represented 71% of the haplotypes and was evenly distributed across the nonbreeding season and habitat types, whereas Lineage B represented only 29% of the haplotypes and was disproportionately represented in January in some habitat types. Overall, the low level of nucleotide diversity, the star-shape of the haplotype tree, the mismatch distribution, and the significantly negative Tajima's D values suggest that Western Sandpipers underwent a recent demographic expansion. Although our results are based on a small sample size from one of several wintering sites along the Pacific Flyway, Santa Maria likely contains a relatively high proportion of the species genetic variability because it is the wintering ground of nearly 10% of the global population.