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Phylogeography and Population Genetic Structure of Least Terns (Sterna antillarum): Conservation Genetics of Least Terns
Completed
The taxonomic status of Least Terns is a highly contentious issue. This has implications for setting conservation priorities at erroneous levels of taxonomic distinctness. In addition, Least Terns have a wide ranging, highly mobile, colonial nesting life history which can affect taxonomic groupings and population structure. Implementing molecular methodologies can resolve taxonomic confusion and identify patterns of population genetic structure in Least Terns.
Least Terns and the Old World Little Tern (S. albifrons) were thought to be the same species until their separation in 1983 based on differences in vocalizations and behavior (AOU 1983, USFWS 1990, Massey 1998). At least five subspecies of Least Tern have been described based on morphological characteristics (S. a. antillarum [Lesson 1847], S. a. athalassos [Burleigh & Lowery 1942], S. a. browni [Mearns 1916], S. a. mexicana Van Rossem & Hachisuka 1937], and S. a. staebleri [Brodkorb 1940]). Three subspecies in the United States (S. a. antillarum, S. a. athalassos, and S. a. browni) are recognized by the American Ornithologists Union (AOU 1998). The taxonomic status of the two subspecies described from Mexico (S. a. mexicana and S. a. staebleri) is uncertain (Garcia and Ceballos 1995, Patten and Erickson 1996). Although the U.S. Fish and Wildlife Service does not recognize subspecies of Least Terns as distinct population segments, it has listed the California and Interior populations as endangered under the ESA due to population declines as a result of habitat loss (USFWS 1980, USFWS 1985, Whittier 2001).
There is general disagreement surrounding descriptions of the recognized Least Tern subspecies. Most criticisms have centered on classifications based on vocalizations, behavior, and morphological characteristics such as plumage. Three separate studies {Burleigh & Lowery (1942), Massey (1976) and Thompson et al. (1992)} found no differences between the Eastern and California subspecies based on such characteristics. However, using refined colorimetry, Johnson et al. (1998) found validation for the three subspecies using lightness and hue of feathers on the dorsum and hind neck.
Previous molecular work revealed little genetic differentiation among Least Tern subspecies. Using 50 (12 polymorphic) allozyme loci, Thompson et al. (1992) found no genetic differentiation between the Interior and Eastern subspecies. However, these results should be taken with caution due to small sample size of the Interior subspecies (n = 4) and all samples originating from Texas, a possible subspecies hybrid zone. To further address the subspecies issue, Whittier (2001) sequenced a portion of the mitochondrial cytochrome-b region and two nuclear intron genes for individuals from the U.S. subspecies (Eastern: n = 17, Interior: n = 22, California: n = 14). MtDNA analyses revealed no genetic differentiation between the three subspecies but one intron gene indicated differences between the California and Interior breeding populations. It should be noted however, that both genetic markers revealed low amounts of genetic variation (mtDNA haplotypes = 3, nuclear intron = 3 alleles). To date, no comprehensive molecular work has been done on Least Terns using the mitochondrial control region which has been shown to be highly informative at the population level (Baker & Marshall 1997).
Understanding population structure and taxonomy is critical for successful conservation of Least Terns. Lack of clarity regarding the relationship between regional populations and definition of appropriate conservation units need to be addressed. Thus, using mitochondrial and microsatellite DNA, objectives for this study are the following:
1. Clarify the taxonomic status of Least Terns in North America.
I will test the hypothesis that the genetic structure conforms to current taxonomic boundaries.
2. Examine phylogeographic patterns over the species range.
I will investigate geographic patterns of genetic variation within and among populations and examine the relative importance that historical and contemporary factors play in shaping phylogeographic structure. If data are available, I will then compare Least Tern data with phylogeographies of Piping Plovers (Charadrius melodus) and Snowy Plovers (Charadrius alexandrinus) two species that occupy breeding ranges and breeding habitat similar to Least Terns.
3. Quantify within and among population genetic diversity in Least Terns.
I will estimate genetic diversity within and among populations. I will infer levels of relatedness, natal philopatry and breeding site fedelity, among individuals and between males and females.
Tissue in the form of blood, feather, or embryo was during 2001-2003 from breeding populations throughout the United States and Caribbean (Table 1). In addition, I will also be contacting other sources (e.g., Louisiana State University Museum of Natural Science, Missouri River Keeper Study, USFWS) for additional genetic material. For field sampling protocol see Haig Lab Conservation Genetics website (http://fresc.usgs.gov/staff/haig/conservation/). To date I have samples from 4 Eastern breeding areas (more to be collect over next field season), 2 Interior breeding area, and 4 California areas. Samples will be divided up into populations, regions and subspeices (Figure 2).DATA ANALYSES
Mitochondrial DNA--ARLEQUIN version 2.0 (Schneider et al. 2000) will be used for all statistical tests unless otherwise stated. Analysis of the total number of mtDNA haplotypes, polymorphic sites, haplotype diversity (h), nucleotide diversity (p), and tests of assumptions of neutrality for mtDNA mutations by Tajima’s D (Tajima 1989) will be performed. Population differentiation will be calculated between populations and subspecies.
Aligned mtDNA sequences will be imported into program PAUP 4.0 b10 (Swofford 2002) for phylogeographic analysis. Using the hierarchical nest likelihood-ratio test in program MODELTEST 3.06 (Posada & Crandall 1998), the best DNA substitution model will be used to implement maximum parsimony, maximum-likelihood, and distance tree reconstructions. Support for internodes will be performed using bootstrap percentages (Felsenstein 1985).
Molecular variance analysis will be performed using separate analyses of molecular variance (AMOVA; Excoffier et al. 1992) at different hierarchical levels (within, among population, regions and subspecies). Mitochondrial DNA population structuring will be assessed by calculating FST values based on pairwise sequence divergence. Association between population Euclidean geographical and pairwise genetic distances will be assessed by nonparametric Mantel tests (Mantel 1967).
Nested clade analysis will also be performed by estimating statistical haplotype tree network with 95 percent parsimonious connection on the program TCS 1.13 (Clement et al. 2000). The tree will be converted into a series of nested clades following the nested rules in Templeton & Sing (1993). Ambiguities in the network will be resolved using guidelines provided in Templeton & Sing (1993). Program GEODIS 2.0 (Posada et al. 2000) will calculate two nested clade statistics: 1) Clade distance (DC) which represents the geographical spread of the clade and 2) Nested clade distance (DN) which represents distance of the clade from the geographical center of the clade. In addition, an interior-tip statistic will be estimated within each nested category as the average interior distance minus the average tip distance, which represents an approximation of the distribution of young versus old haplotypes. Clades will be tested against their geographical location using permutational contingency analysis; with a null hypothesis of random distribution of all clades within the nested clade.
Microsatellite DNA--All statistical tests for microsatellite genetic analyses will be performed using genetic analysis software GENEPOP (Raymond & Rousset 1995) and F-STAT (Goudet 2001). Multi-scale genetic diversity will be measured as mean number of alleles per locus (A), and observed (HO) and expected (HE) heterozygosities (Nei 1978). Deviations from Hardy-Weinberg equilibrium (HWE) and linkage disquilibria over all and within populations will be tested. Evidence of recent small effective population size will be evaluated using program BOTTLENECK (Piry et al. 1999).
Draheim, H.M., 2006, Phylogeography and Population Genetic Structure of Least Terns (Sterna antillarum): Corvallis, OR, Oregon State University--M.S. Thesis, 96 p. [FullText] Catalog No: 1567
Haig, Susan M. - Supervisory Research Wildlife Biologist
Phone: 541-750-7482
Email: susan_haig@usgs.gov
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