Publications

2010
Balakrishnan CN, Edwards SV, Clayton DF. The Zebra Finch genome and avian genomics in the wild. Emu. 2010;110 :233-241.Abstract

The Zebra Finch (Taeniopygia guttata) is the first species of passerine bird with a complete genome sequence, making it an exciting time for avian evolutionary biology. Native to Australia and the Lesser Sunda Islands, this species has long played an important role in the study of ecology, behaviour and neuroscience. With the sequencing of its genome, the Zebra Finch now also represents an important model system for evolutionary and population genomics. The production of a genome sequence for the Zebra Finch will have far-reaching impacts on the study of avian biology. Here we discuss the genomic resources available for the Zebra Finch, including the genome sequence itself, and some of the ways in which they will facilitate the study of avian diversity. We also highlight recent examples from the literature that have already begun to leverage Zebra Finch genomic tools towards the study of birds in nature.

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Balakrishnan CN, Ekblom R, Volker M, Westerdahl H, Godinez R, Kotkiewicz H, Burt DW, Graves T, Griffin DK, Warren WC, et al. Gene duplication and fragmentation in the zebra finch major histocompatibility complex. BMC Biology [Internet]. 2010;8 (29) :(1 April 2010)-(1 April 2010). Publisher's VersionAbstract
Background:
Due to its high polymorphism and importance for disease resistance, the major histocompatibility
complex (MHC) has been an important focus of many vertebrate genome projects. Avian MHC organization is of
particular interest because the chicken
Gallus gallus
, the avian species with the best characterized MHC, possesses a
highly streamlined
minimal essential
MHC, which is linked to resistance against specific pathogens. It remains
unclear the extent to which this organization describes the situation in other birds and whether it represents a
derived or ancestral condition. The sequencing of the zebra finch
Taeniopygia guttata
genome, in combination
with targeted bacterial artificial chromosome (BAC) sequencing, has allowed us to characterize an MHC from a
highly divergent and diverse avian lineage, the passerines.
Results:
The zebra finch MHC exhibits a complex structure and history involving gene duplication and
fragmentation. The zebra finch MHC includes multiple Class I and Class II genes, some of which appear to be
pseudogenes, and spans a much more extensive genomic region than the chicken MHC, as evidenced by the
presence of MHC genes on each of seven BACs spanning 739 kb. Cytogenetic (FISH) evidence and the genome
assembly itself place core MHC genes on as many as four chromosomes with TAP and Class I genes mapping to
different chromosomes. MHC Class II regions are further characterized by high endogenous retroviral content.
Lastly, we find strong evidence of selection acting on sites within passerine MHC Class I and Class II genes.
Conclusion:
The zebra finch MHC differs markedly from that of the chicken, the only other bird species with a
complete genome sequence. The apparent lack of synteny between
TAP
and the expressed MHC Class I locus is in
fact reminiscent of a pattern seen in some mammalian lineages and may represent convergent evolution. Our
analyses of the zebra finch MHC suggest a complex history involving chromosomal fission, gene duplication and
translocation in the history of the MHC in birds, and highlight striking differences in MHC structure and

organization among avian lineages

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2009
Shedlock AM, Edwards SV. Amniotes (Amniota). The timetree of life. 2009 :375-379.
Liu L, Yu L, Kubatko L, Pearl DK, Edwards SV. Coalescent methods for estimating phylogenetic trees. Mol Phylogenet Evol. 2009;53 :320-8.Abstract

We review recent models to estimate phylogenetic trees under the multispecies coalescent. Although the distinction between gene trees and species trees has come to the fore of phylogenetics, only recently have methods been developed that explicitly estimate species trees. Of the several factors that can cause gene tree heterogeneity and discordance with the species tree, deep coalescence due to random genetic drift in branches of the species tree has been modeled most thoroughly. Bayesian approaches to estimating species trees utilizes two likelihood functions, one of which has been widely used in traditional phylogenetics and involves the model of nucleotide substitution, and the second of which is less familiar to phylogeneticists and involves the probability distribution of gene trees given a species tree. Other recent parametric and nonparametric methods for estimating species trees involve parsimony criteria, summary statistics, supertree and consensus methods. Species tree approaches are an appropriate goal for systematics, appear to work well in some cases where concatenation can be misleading, and suggest that sampling many independent loci will be paramount. Such methods can also be challenging to implement because of the complexity of the models and computational time. In addition, further elaboration of the simplest of coalescent models will be required to incorporate commonly known issues such as deviation from the molecular clock, gene flow and other genetic forces.

Liu L, Yu L, Pearl DK, Edwards SV. Estimating species phylogenies using coalescence times among sequences. Systematic Biology. 2009;58 :468-477.
Liu L, Yu LL, Pearl DK, Edwards SV. Estimating species phylogenies using coalescence times among sequences. Systematic Biology. 2009;58 :468-477.Abstract

The estimation of species trees (phylogenies) is one of the most important problems in evolutionary biology, and recently, there has been greater appreciation of the need to estimate species trees directly rather than using gene trees as a surrogate. A Bayesian method constructed under the multispecies coalescent model can consistently estimate species trees but involves intensive computation, which can hinder its application to the phylogenetic analysis of large-scale genomic data. Many summary statistics-based approaches, such as shallowest coalescences (SC) and Global LAteSt Split (GLASS), have been developed to infer species phylogenies for multilocus data sets. In this paper, we propose 2 methods, species tree estimation using average ranks of coalescences (STAR) and species tree estimation using average coalescence times (STEAC), based on the summary statistics of coalescence times. It can be shown that the 2 methods are statistically consistent under the multispecies coalescent model. STAR uses the ranks of coalescences and is thus resistant to variable substitution rates along the branches in gene trees. A simulation study suggests that STAR consistently outperforms STEAC, SC, and GLASS when the substitution rates among lineages are highly variable. Two real genomic data sets were analyzed by the 2 methods and produced species trees that are consistent with previous results.

Haussler D, O'Brien SJ, Ryder OA, Barker FK, Clamp M, Crawford AJ, Hanner R, Hanotte O, Johnson WE, McGuire JA, et al. Genome 10K: A proposal to obtain whole-genome sequence for 10 000 Vertebrate Species. Journal of Heredity. 2009;100 :659-674.Abstract

The human genome project has been recently complemented by whole-genome assessment sequence of 32 mammals and 24 nonmammalian vertebrate species suitable for comparative genomic analyses. Here we anticipate a precipitous drop in costs and increase in sequencing efficiency, with concomitant development of improved annotation technology and, therefore, propose to create a collection of tissue and DNA specimens for 10 000 vertebrate species specifically designated for whole-genome sequencing in the very near future. For this purpose, we, the Genome 10K Community of Scientists (G10KCOS), will assemble and allocate a biospecimen collection of some 16 203 representative vertebrate species spanning evolutionary diversity across living mammals, birds, nonavian reptiles, amphibians, and fishes (ca. 60 000 living species). In this proposal, we present precise counts for these 16 203 individual species with specimens presently tagged and stipulated for DNA sequencing by the G10KCOS. DNA sequencing has ushered in a new era of investigation in the biological sciences, allowing us to embark for the first time on a truly comprehensive study of vertebrate evolution, the results of which will touch nearly every aspect of vertebrate biological enquiry.

Kearns AM, Joseph L, Edwards SV, Double MC. Inferring the phylogeography and evolutionary history of the splendid fairy‐wren Malurus splendens from mitochondrial DNA and spectrophotometry. Journal of Avian Biology. 2009;40 :7-17.
Edwards S, Bensch S. Looking forwards or looking backwards in avian phylogeography? A comment on Zink and Barrowclough 2008. Molecular Ecology. 2009;18 :2930-2933.
Brito PH, Edwards SV. Multilocus phylogeography and phylogenetics using sequence-based markers. Genetica. 2009;135 :439-455.
Edwards SV. Natural selection and phylogenetic analysis. Proceedings of the National Academy of Sciences of the United States of America. 2009;106 :8799-8800.
Edwards SV. Is a new and general theory of molecular systematics emerging?. Evolution. 2009;63 :1-19.
Balakrishnan CN, Edwards SV. Nucleotide Variation, Linkage Disequilibrium and Founder-Facilitated Speciation in Wild Populations of the Zebra Finch (Taeniopygia guttata). Genetics. 2009;181 :645-660.Abstract

The zebra finch has long been an important model system for the study of vocal learning, vocal production, and behavior. With the imminent sequencing of its genome, the zebra finch is now poised to become a model system for population genetics. Using a panel of 30 noncoding loci, we characterized patterns of polymorphism and divergence among wild zebra finch populations. Continental Australian populations displayed little population structure, exceptionally high levels of nucleotide diversity (pi = 0.010), a rapid decay of linkage disequilibrium (LD), and a high population recombination rate (rho approximate to 0.05), all of which suggest an open and fluid genomic background that could facilitate adaptive variation. By contrast, Substantial divergence between the Australian and Lesser Sunda Island populations (K-ST = 0.193), reduced genetic diversity (pi = 0.002), and higher levels of LD in the island Population suggest a strong but relatively recent founder event, which may have contributed to speciation between these populations as envisioned under founder-effect speciation models. Consistent with this hypothesis, we find that tinder a simple quantitative genetic model both drift and selection could have contributed to the observed divergence in six quantitative traits. In both Australian and Lesser Sundas populations, diversity in Z-linked loci was significantly lower than in autosomal loci. Our analysis provides a quantitative framework for studying the role of selection and drift in shaping patterns of molecular evolution in the zebra finch genome.

Liu L, Edwards SV. Phylogenetic analysis in the anomaly zone. Syst Biol. 2009;58 :452-60.
Janes DE, Ezaz T, Graves JAM, Edwards SV. Recombination and nucleotide diversity in the sex chromosomal pseudoautosomal region of the Emu, Dromaius novaehollandiae. Journal of Heredity. 2009;100 :125-136.Abstract

Pseudoautosomal regions (PARs) shared by avian Z and W sex chromosomes are typically small homologous regions within which recombination still occurs and are hypothesized to share the properties of autosomes. We capitalized on the unusual structure of the sex chromosomes of emus, Dromaius novaehollandiae, which consist almost entirely of PAR shared by both sex chromosomes, to test this hypothesis. We compared recombination, linkage disequilibrium (LD), GC content, and nucleotide diversity between pseudoautosomal and autosomal loci derived from 11 emu bacterial artificial chromosome (BAC) clones that were mapped to chromosomes by fluorescent in situ hybridization. Nucleotide diversity (pi = 4N(e)mu) was not significantly lower in pseudoautosomal loci (14 loci, 1.9 +/- 2.4 x 10(-3)) than autosomal loci (8 loci, 4.2 +/- 6.1 x 10(-3)). By contrast, recombination per site within BAC-end sequences (rho = 4Nc) (pseudoautosomal, 3.9 +/- 6.9 x 10(-2); autosomal, 2.3 +/- 3.7 x 10(-2)) was higher and average LD (D') (pseudoautosomal, 4.2 +/- 0.2 x 10(-1); autosomal, 4.7 +/- 0.5 x 10(-1)) slightly lower in pseudoautosomal sequences. We also report evidence of deviation from a simple neutral model in the PAR and in autosomal loci, possibly caused by departures from demographic equilibrium, such as population growth. This study provides a snapshot of the population genetics of avian sex chromosomes at an early stage of differentiation.

Claessens LP, Edwards SV, Martinez R, Krzyzak M, Eckardt M, Leslie G, Marcucci M, Neabore S, Vrcek I, Moss S, et al. Aves 3D: A new online resource for avian skeletal anatomy. Integrative and Comparative Biology [Internet]. 2009;49 :E213-E213. Publisher's VersionAbstract
P2.108
CLAESSENS, L.P.*; EDWARDS, S.V.; MARTINEZ, R.; KRZYZAK,
M.; ECKARDT, M.; LESLIE, G.; MARCUCCI, M.; NEABORE, S.; VRCEK, I.;
MOSS, S.; GRASSI, K.; College of the Holy Cross, Worcester, MA, Harvard
University, Cambridge, MA;
Aves 3D: A new online resource for avian skeletal anatomy
Here we report the launch of a new National Science Foundation funded
online resource for avian skeletal anatomy, www.Aves3D.org, which will offer
three-dimensional models of a wide selection of skeletal elements of extant
and extinct birds. The Aves 3D database is produced through non-contact
laser scanning of skeletal material from, amongst others, the Harvard
Museum of Comparative Zoology and the Yale Peabody Museum of Natural
History. Database growth is fueled by undergraduate student research
projects, which involve a variety of functional and phylogenetic studies
supervised by both affiliated and external post-graduate students and
researchers. The online database will also serve as a digital archive for the
collections of contributing museums and allows for rapid global dissemination
of 3D digital data on common as well as rare and potentially fragile
specimens, in a format ready for a wide range of two and three-dimensional
computational analyses. The size of the database is currently limited, and
opportunities exist for initiating new student-based projects that will

contribute to database growth

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Chapus C, Edwards SV. Genome evolution in Reptilia: in silico chicken mapping of 12,000 BAC-end sequences from two reptiles and a basal bird. Bmc Genomics [Internet]. 2009;10. Publisher's VersionAbstract

Background

With the publication of the draft chicken genome and the recent production of several BAC clone libraries from non-avian reptiles and birds, it is now possible to undertake more detailed comparative genomic studies in Reptilia. Of interest in particular are the genomic events that transformed the large, repeat-rich genomes of mammals and non-avian reptiles into the minimalist chicken genome. We have used paired BAC end sequences (BESs) from the American alligator (Alligator mississippiensis), painted turtle (Chrysemys picta) and emu (Dromaius novaehollandiae) to investigate patterns of sequence divergence, gene and retroelement content, and microsynteny between these species and chicken.

Results

From a total of 11,967 curated BESs, we successfully mapped 725, 773 and 2597 sequences in alligator, turtle, and emu, respectively, to sites in the draft chicken genome using a stringent BLAST protocol. Most commonly, sequences mapped to a single site in the chicken genome. Of 1675, 1828 and 2936 paired BESs obtained for alligator, turtle, and emu, respectively, a total of 34 (alligator, 2%), 24 (turtle, 1.3%) and 479 (emu, 16.3%) pairs were found to map with high confidence and in the correct orientation and with BAC-sized intermarker distances to single chicken chromosomes, including 25 such paired hits in emu mapping to the chicken Z chromosome. By determining the insert sizes of a subset of BAC clones from these three species, we also found a significant correlation between the intermarker distance in alligator and turtle and in chicken, with slopes as expected on the basis of the ratio of the genome sizes.

Conclusion

Our results suggest that a large number of small-scale chromosomal rearrangements and deletions in the lineage leading to chicken have drastically reduced the number of detected syntenies observed between the chicken and alligator, turtle, and emu genomes and imply that small deletions occurring widely throughout the genomes of reptilian and avian ancestors led to the ~50% reduction in genome size observed in birds compared to reptiles. We have also mapped and identified likely gene regions in hundreds of new BAC clones from these species.

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Janes DE, Organ CL, Edwards SV. Variability in Sex-Determining Mechanisms Influences Genome Complexity in Reptilia. Cytogenetic and Genome Research [Internet]. 2009;127 (2-4) :242-248. Publisher's VersionAbstract

In this review, we describe the history of amniote sex determination as a classic example of Darwinian evolution. We suggest that evolutionary changes in sex determination provide a foundation for understanding important aspects of chromosome and genome organization that otherwise appear haphazard in their origins and contents. Species with genotypic sex determination often possess heteromorphic sex chromosomes, whereas species with environmental sex determination lack them. Through a series of mutations followed by selection at key genes, sex-determining mechanisms have turned over many times throughout the amniote lineage. As a consequence, amniote genomes have undergone gains or losses of sex chromosomes. We review the genomic and ecological contexts in which either temperature-dependent or genotypic sex determination has evolved. Once genotypic sex determination emerges in a lineage, viviparity and heteromorphic sex chromosomes become more likely to evolve. For example, in extinct marine reptiles, genotypic sex determination apparently led to viviparity, which in turn facilitated their pelagic radiation. Sex chromosomes comprise genome regions that differ from autosomes in recombination rate, mutation rate, levels of polymorphism, and the presence of sex-determining and sexually antagonistic genes. In short, many aspects of amniote genome complexity, life history, and adaptive radiation appear contingent on evolutionary changes in sex-determining mechanisms.

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2008
Janes DE, Ezaz T, Graves JAM, Edwards SV. Characterization, chromosomal location, and genomic neighborhood of a ratite ortholog of a gene with gonadal expression in mammals. Integrative and Comparative Biology. 2008;48 :505-511.Abstract

A locus that we name SubA was discovered during large-scale sequencing and characterization of a bacterial artificial chromosome library from an emu, Dromaius novaehollandiae. This locus yields a significantly negative Tajimas D in emus and is conserved across emu, chicken, mouse, and human. Expression of SubA orthologs has been reported in human ovaries and in mouse testes, but remains unknown in emus. The locus was physically mapped onto a pair of microchromosomes in emus by fluorescent in situ hybridization and also in chicken as previously reported. By characterizing emu SubA in this article, we aim to improve current descriptions of the cascade of genes associated with avian sex differentiation. Future experimentation will report the expression of SubA in ratites, other birds, and nonavian reptiles.

Brumfield RT, Liu L, Lum DE, Edwards SV. Comparison of Species Tree Methods for Reconstructing the Phylogeny of Bearded Manakins (Aves: Pipridae, Manacus) from Multilocus Sequence Data. Systematic Biology. 2008;57 :719-731.Abstract

Although the power of multi-locus data in estimating species trees is apparent, it is also clear that the analytical methodologies for doing so are still maturing. For example, of the methods currently available for estimating species trees from multiocus data, the Bayesian method introduced by Liu and Pearl (2007; BEST) is the only one that provides nodal support values. Using gene sequences from five nuclear loci, we explored two analytical methods (deep coalescence and BEST) to reconstruct the species tree of the five primary Manacus OTUs: M. aurantiacus, M. candei, M. vitellinus, populations of M. manacus from west of the Andes (M. manacus (w)), and populations of M. manacus from east of the Andes (M. manacus (e)). Both BEST and deep coalescence supported a sister relationship between M. vitellinus and M. manacus (w). A lower probability tree from the BEST analysis and one of the most parsimonious deep coalescence trees also supported a sister relationship between M. candei and M. aurantiacus. Because hybrid zones connect the distributions of most Manacus species, we examined the potential influence of post-divergence gene flow on the sister relationship of parapatrically distributed M. vitellinus and M. manacus (w). An isolation-with-migration (IM) analysis found relatively high levels of gene flow between M. vitellinus and M. manacus (w). Whether the gene flow is obscuring a true sister relationship between M. manacus (w) and M. manacus (e) remained unclear, pointing to the need for more detailed models accommodating multispecies, multilocus DNA sequence data.

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