Donna L. Dittmann

Gene flow, refugia, and evolution of geographic variation in the song sparrow (Melospiza melodia)

We surveyed mtDNA restriction‐site variation in song sparrows taken from across their continental range. Despite marked geographic variation in size and plumage color, mtDNA variation was not geographically structured. Subspecies were not identifiable by mtDNA analysis. We suggest that postglaciation dispersal scattered mtDNA haplotypes across the continent, explaining the lack of mtDNA geographic patterns. Evolution of size and plumage coloration has probably proceeded faster than mtDNA evolution, leading to the well‐structured continental pattern of morphological variation. We suggest that the nonordered geographic distribution of haplotypes reflects the recency of population establishment following completion of range expansion. Dispersal distance was estimated from the mtDNA data at 6.1 km per generation, an order of magnitude greater than that (0.3 km) estimated from demographic data. Island samples were not especially different from continental ones. Rooting the haplotype cladogram with a putative primitive haplotype identified Newfoundland and the Queen Charlotte Islands as potential sites of recent refugia. We question whether study of geographic variation in song sparrows leads to insights concerning speciation.

Trans-Beringia comparisons of mitochondrial DNA differentiation in birds

We compared mitochondrial DNA (mtDNA) restriction fragment profiles from samples of 13 bird species that occur on both sides of Beringia. All but two species, Lapland Longspur (Calcarius lapponicus) and Green-winged Teal (Anas crecca), exhibited evidence of genetic differentiation, albeit at varying degrees. Several species exhibited mtDNA dif- ferentiation consistent with species-level distinctness: Marbled Murrelet (Brachyramphus marmoratus), Three-toed Woodpecker (Picoides tridactylus), Whimbrel (Numenius phaeo- pus), Mew Gull (tints canus), Black-billed Magpie (Pica pica), American Pipit (Anthus rubescens), and Rosy Finch (Leucosticte arctoa). Other species exhibited levels of mtDNA differentiation intermediate between populations and species: Barn Swallow (Hirundo rus- tica), Common Tern (Sterna hirundo), Common Snipe (Gallinago galhnago), and Pelagic Cormorant (Phalacrocoraxpelagicus). Because of small sample sizes, we do not recommend formal taxonomic changes, although our data could be combined with other data to raise several taxa to species level. Our data do not indicate a consistent level of mtDNA differ- entiation between putatively conspecific populations on different sides of Beringia, suggesting different times of colonization or cessation of gene exchange. Most comparisons of birds within continents exhibit less mtDNA differentiation than our tram-Beringia comparisons, suggesting limited gene flow between continents.

Evolutionary Genetics of Hawaiian Freshwater Fish

We surveyed mitochondrial (mt)DNA restriction site variation within each of the five native species of Hawaiian freshwater fishes: Lentipes concolor, Stenogobius hawaiiensis, Sicyopterus stimpsoni, Awaous guamensis, and Eleotris sandwicensis. MtDNA haplotypes did not vary geographically except among samples of S. hawaiiensis, where haplotypes from Maui clustered together. One haplotype in this cluster also was found on Kauai and on Oahu, and one haplotype was found only on Kauai. MtDNA data suggest gene flow among islands, consistent with interisland dispersal through amphidromous larvae. We hypothesize that dispersal has prevented further speciation in these fishes and that colonization of island freshwater habitats by larvae could be independent of natal island.

Evolution of brown towhees: mitochondrial DNA evidence

Mitochondrial DNA (mtDNA) differentiation was studied among members of the Brown Towhee complex, Pipilo albicollis, P. aberti, P. fuscus, and P. crissalis. Using 16 restriction endonucleases, 196 fragments and 119 sites were observed, and an average interspecific sequence divergence of 6.4% was estimated. There was little geographic differentiation between Baja California and California samples of crissalis or between California and Arizona samples of aberti. Using phylogenetic procedures, it was shown that aberti and crissalis are sister taxa, as predicted by Davis (1951). Allozyme and mtDNA data clearly support the species distinction of crissalis and fuscus. The relationship between albicollis and fuscus, viewed as sister taxa by Zink (1988) based on allozymes, is supported albeit not strongly by the mtDNA data. MtDNA and allozyme data reveal similar patterns of evolutionary history within this group.

Sampling locality is more detectable than taxonomy or ecology in the gut microbiota of the brood-parasitic Brown-headed Cowbird (Molothrus ater)

Brown-headed Cowbirds (Molothrus ater) are the most widespread avian brood parasite in North America, laying their eggs in the nests of approximately 250 host species that raise the cowbird nestlings as their own. It is currently unknown how these heterospecific hosts influence the cowbird gut microbiota relative to other factors, such as the local environment and genetics. We test a Nature Hypothesis (positing the importance of cowbird genetics) and a Nurture Hypothesis (where the host parents are most influential to cowbird gut microbiota) using the V6 region of 16S rRNA as a microbial fingerprint of the gut from 32 cowbird samples and 16 potential hosts from nine species. We test additional hypotheses regarding the influence of the local environment and age of the birds. We found no evidence for the Nature Hypothesis and little support for the Nurture Hypothesis. Cowbird gut microbiota did not form a clade, but neither did members of the host species. Rather, the physical location, diet and age of the bird, whether cowbird or host, were the most significant categorical variables. Thus, passerine gut microbiota may be most strongly influenced by environmental factors. To put this variation in a broader context, we compared the bird data to a fecal microbiota dataset of 38 mammal species and 22 insect species. Insects were always the most variable; on some axes, we found more variation within cowbirds than across all mammals. Taken together, passerine gut microbiota may be more variable and environmentally determined than other taxonomic groups examined to date.

Mitochondrial DNA variation, population structure and evolution of the Common Grackle (Quiscalus quiscula)

The Common Grackle (Quiscalus quiscula) includes two phenotypically differentiated forms, the Purple Grackle (Q. q. quiscula), which breeds along the Atlantic and Gulf coasts of the United States, and the Bronzed Grackle (Q. q. versicolor), which occurs over the rest of eastern North America. These grackles meet and hybridize in a zone that stretches from northeastern to the extreme south central United States. We used 20 restriction endonucleases to survey the level and pattern of mitochondrial DNA (mtDNA) variation within and among 35 individuals taken from eight sites. To establish the phylogenetic position of Q. quiscula, we also studied the Greater Antillean Grackle (Q. niger), the supposed sister taxon of Q. quiscula, the Boat-tailed Grackle (Q. major), and the Great-tailed Grackle (Q. mexicanus). A relatively high number (29) of clones was detected in Q. quiscula, of which 23 occurred in single individuals. Using results of simulations by Avise et al. (1988) we found times to common ancestry of the 31 clones that are much lower than predicted by neutral theory; however, it is likely that long-term effective population sizes of grackles are much lower than current population size, which potentially explains the discrepancy. Sixty-six percent of the birds were genetically most similar to an individual in another population sample, suggesting that gene flow is high. Inspection of the distribution of individual fragment profiles, composite haplotypes (clones) and a maximum parsimony phylogenetic tree of clones reveals no evidence of geographic variation. Thus, the mtDNA gene tree is paraphyletic with respect to the “plumage” tree of Q. quiscula. The mtDNA data do not reflect the historical event that lead to the origin of either Purple or Bronzed grackles. The low diversity among clones (P = 0.28%) suggests a recent common ancestry of clones and recent colonization of North America, which we suggest explains the lack of geographic variation; plumage evolution must therefore have occurred rapidly, likely enhanced by sexual selection. The separation of quiscula and niger occurred l-2 MY ago, but it is not clear that these species are sister taxa, whereas it is likely that major and mexicanus are.

Evolutionary patterns of morphometrics, allozymes, and mitochondrial DNA in thrashers (genus Toxostoma)

We examined patterns of variation in skeletal morphometrics (29 characters), allozymes (34 loci), mitochondrial DNA (mtDNA) restriction sites (n = 74) and fragments (n = 395), and mtDNA sequences (1,739 bp from cytochrome b, ND2, ND6, and the control region) among all species of Toxostoma. The phenetic pattern of variation in skeletal morphometrics generally matched traditional taxonomic groupings (based on plumage patterns) with the exceptions of T redivivum, which because of its large size clusters outside of its proper evolutionary group (lecontei), and T. occelatum, which did not cluster with T. curvirostre. Skull characters contributed highly to species discrimination, suggesting that unique feeding adaptations arose in different species groups. Although genetic variation was detected at isozyme loci (average heterozygosity = 3.6%), these data yielded little phylogenetic resolution. Similarly, mtDNA restriction sites were relatively uninformative; hence, phylogenetic conclusions were based on sequence data. Phylogenetic analyses confirmed the monophyly of these traditionally recognized assemblages: rufum group (T. rufum, T. longirostre, and T. guttatum), lecontei group (T lecontei, T crissale, and T. redivivum), and cinereum group (T. bendirei and T. cinereum). The cinereum and lecontei groups appear to be sister lineages. Monophyly of the curvirostre group (which also includes T. occelatum) was not confirmed. Sequence data suggest that T. occelatum and T. curvirostre, which differ by 7.7% sequence divergence, are probably most closely related to the rufum group. Toxostoma rufum and T. longirostre have similar external appearances and differ by 5.0%. Toxostoma guttatum is restricted to Cozumel Island and often is considered a subspecies of T. longirostre; it differs by more than 5% from the other two members of the rufum group and is a distinct species constituting the basal member of this group. The phenotypically distinctive T. bendirei and T. cinereum differ in sequence divergence by only 1.6%. Overall, mtDNA distances computed from coding genes (mean 8.5%) exceeded distances computed from the control region (mean 7.6%), contrary to expectation. Because neither allozymes nor mtDNA could unambiguously resolve the placement of T. occelatum and T. curvirostre, a scenario involving contemporaneous speciation is suggested. Application of a molecular clock suggested that most speciation occurred in the late Pliocene or early Pleistocene.

Willet be one species or two? A genomic view of the evolutionary history of Tringa semipalmata

ABSTRACT The Willet (Tringa semipalmata; Scolopacidae) is composed of 2 morphologically and vocally distinct subspecies with broadly disjunct breeding distributions in North America. Nominate T. s. semipalmata breeds in coastal salt and brackish marshes along the Atlantic and Gulf coasts of eastern North America and the West Indies, and T. s. inornata breeds in wet grasslands and prairies in the northwestern interior of North America. To assess divergence and test for hybridization between the 2 subspecies, we sampled breeding and wintering populations and collected morphological data, mitochondrial DNA sequences from the ND2 locus, and nuclear DNA sequences from genomic libraries enriched for ultraconserved elements (UCEs). Mitochondrial haplotypes were reciprocally monophyletic between the 2 subspecies and indicated divergence approximately 700,000 yr ago. The UCE dataset included 4,635 loci containing 19,322 single nucleotide polymorphisms (SNPs), and, based on these data, individuals clustered by subspecies with no evidence of admixture between them and no substructure within subspecies. We identified 42 nuclear loci that contained SNPs fixed for alternate alleles between the 2 subspecies. Of the 42 loci with fixed differences, a statistically disproportional 17 were Z-linked, indicating a role for sexual selection in the divergence of the 2 subspecies. Genetic, morphological, ecological, and behavioral differences suggest that the 2 Willet subspecies may merit treatment as separate species. Further studies are needed to determine the presence of pre- or post-mating reproductive isolation.

Diagnoses of hybrid hummingbirds (Aves: Trochilidae). 17. Documentation of the intrageneric hybrid ( Archilochus colubris × Archilochus alexandri )

Abstract Although the breeding ranges of Archilochus alexandri (Black-chinned Hummingbird) and Archilochus colubris (Ruby-throated Hummingbird) are narrowly parapatric in central Texas and central and southern Oklahoma, there have been few reports of hybridization in the literature and no well-documented hybrid specimens. Here we provide a comprehensive assessment of two male hybrids collected, respectively, in Grayson County, Texas, and East Baton Rouge Parish, Louisiana. As has been the pattern in other hummingbird hybrids, both specimens exhibit a blended mosaic of plumage characters of the parental species. Sequence for mitochondrial cytochrome c oxidase subunit 1 (CO1) for the hybrid specimen from East Baton Rouge Parish (LSU 182,831) confirms A. colubris as the female parent.