News

I will be away from Blog until mid-August.

COI barcodes aim to enable identification of species, assigning unknown specimens to known species, and helping flag genetically divergent organisms that may represent new species. Might barcodes also help understand relationships among species?

Here I look at one example from birds, comparing differences among COI barcodes to a recently revised phylogeny of terns (subfamily Sternini). According to American Ornithologists’ Union Check-list of North American Birds Supplement 47 (2006), “the data show that the genus Sterna as currently defined…is paraphyletic.”…”[W]e follow the recommendation of Bridge et al 2005 to resurrect four generic names currently placed in synonomy with Sterna.” The figure at left, taken from the 2005 paper by Bridge, Jones, and Baker, shows phylogenetic relationships based on 2800 bp of mtDNA from 33 species of terns (Bayesian tree with ML distances and ML boostrap support indices), and is juxtaposed to an NJ tree of COI barcodes from 29 of the same 33 taxa. The figure is colored according to the revised generic assignments (AOU 2006).

The topology of the COI NJ tree is similar to the larger data set tree, including that all currently recognized genera are reciprocally monophyletic, and most show similarly high boostrap values as in the Bayesian/ML analysis based on the larger data set. 

Of course mitochondrial DNA is widely used in analyzing relationships among animal species, including birds. Most of these studies are focused on relatively small groups of species, such as the tern study cited here. With growing DNA barcode libraries it will be increasingly possible to get at least a preliminary look at genetic relationships for large numbers of species (so far 2,393 avian species (24% of world birds) have barcode records in BOLD). This could be exciting! 

The DNA barcode for animals is a 648 base pair (bp) fragment from the 5′ end of mitochondrial gene cytochrome c oxidase subunit I (COI).  Does this relatively short mitochondrial sequence contain enough information to make evolutionary inferences about species limits, or is it a more of a rough survey method that needs to be confirmed by more data including from nuclear genes? 

In April 2008 Mol Ecol researchers from University of Minnesota and American Museum of Natural History, New York, analyze utility of mitochondrial as compared to nuclear DNA for inferring recent evolutionary history. Zink and Barrowclough first apply population genetic theory and then look at real data from bird species. 

Based on mathematical population genetics, they find “mitochondrial loci are generally a more sensitive indicator of population structure than are nuclear loci,” primarily due to much smaller effective population size (Ne) for mitochondrial as compared to nuclear markers, which leads to more rapid sorting of differences among genetically isolated populations. Analysis of real-world data in 45 studies of differences among and within avian species confirms this expectation, ie either the patterning is consistent between mitochondrial and nuclear genes, or there are shallow mtDNA trees which are not yet reflected in nuclear genes. Reanalysis of one study, which appears to show a split in nuclear but not mitochondrial markers, suggests possible misinterpretation. Regarding other factors that could potentially lead to mistaken inferences about species limits based on mitochondrial DNA (NUMTs, sex-biased gene flow, introgression), experimental data suggests these are rarely important, at least in birds. The authors conclude “mtDNA patterns will prove to be robust indicators of population history and species limits.” Nuclear markers ARE important for deep gene trees, for detecting hybrids, and for “quantitative estimates…of rates of population growth and values of gene flow.”  

Regarding length of mitochondrial sequence, this only has to be long enough to capture differences among closely-related species. Most populations that we recognize as species differ from their closest relatives by >1% in mitochondrial coding regions (corresponding to about 0.5 million years or more of reproductive isolation). At this level, even 100 bp is generally sufficient to distinguish most closely-related species, and a 648 bp COI barcode sequence should generally allow resolution of populations/species which have been reproductively isolated for much shorter periods of time. 

In July 2008 Wildlife Management researchers from Smithsonian Institution report on identifying otherwise unidentifiable remnants from bird-aircraft collisions (hereafter birdstrikes). Authors Dove et al point out “birdstrikes are a serious safety hazard and a major expense for the industry”. The US Federal Aviation Agency Wildlife Mitigation site shows about 600 incidents a month over the past year, peaking in late summer and early fall, presumably coincident with fall migration. The Smithsonian Institution has been identifying birdstrike species for military and civil aviation industries since the 1960s, analyzing specimens which range from whole carcasses to bits of feathers, tissue, or blood. Prior to availability DNA testing, identifications have relied on expert examination of detailed feather morphology with comparisons to Smithsonian’s vast bird specimen collection.

Of 1,715 birdstrike samples sent to Smithsonian Insitution during 4 months in fall 2006, 821(47.9%) contained only blood or tissue. Of these, 554 (67.5%) had amplifiable mtCOI DNA, and 535 (96.6%) with DNA led to species-level identifications based on reference sequences in Barcode of Life Database (BOLD). DNA barcoding identified 128 species representing 14 orders of birds, plus 2 bat species. 19 cases were deemed inconclusive as DNA barcode matched to a set of 2 or more closely-related species with overlapping barcodes, or the recovered sequence did not meet their 98% match criteria when compared to BOLD.

There was much better success recovering DNA from dry samples (70%) than from samples collected with a wet paper towel (about 23%), which had been the standard method, pointing the way toward improving yield of DNA-based ID. The authors conclude with a call for applying “a combination of morphological and molecular methods such as DNA barcoding for efficient, cost-effective birdstrike identifications”.

Just as in CSI television series, DNA-based identification can make possible what would otherwise be impossible; in this case, identifying birds from bits of tissue and blood and making birdstrike identifications available to those without access to Smithsonian’s experts or vast collections. In addition to helping airlines, birdstrike ID will inform our knowledge of bird migration routes. There are many exciting discoveries ahead.

Like the creatures Sigourney Weaver battles in Alien, parasitoids are organisms whose larva develop in other species, usually leading to the death of the host. Insect parasitoids are widely used as biological control agents; sometimes these efforts go awry, threatening non-pest species in local ecosystems. Widespread introduction of tachnid fly parasitoid Compsilura concinnata has failed to control Gyspy moth Lymantra dispar outbreaks in eastern US, but has led to dramatic declines in large, showy Silk moths including the beautiful Luna moth Actias luna (Elkinton and Boettler. 2004). 

About 10% of named insect species are parasitoids, mostly wasps, but recognizing these often minute insects can be tricky. In November 2007 Conservation Genetics researchers from Czech Academy of Sciences and University of South Bohemia, Czech Republic; Natural History Museum, London; and Imperial College London apply COI DNA sequencing to identify wasps parasitizing Canary Islands Large White butterfly Pieris cheiranthi, which is restricted to local endemic ecosystem of relict laurel forests. Lozan et al reared 55 P. cheiranthi caterpillars from 2 Canary Island sites, and found half of the larva from forest margin and none from central forest were parasitized with what appeared to be Cotesia glomerata, native to Europe and introduced elsewhere as biocontrol agent although not in Canary Islands. 

3 of 600 C. glomerata-like adult wasps reared from Canary Island White larvae and 2 of 700 C. glomerata reared in Czech Republic from European Large White P. brassicae larvae were analyzed and found to have identical 5′ COI DNA sequences (this is the same region selected as a DNA barcode for animals). The authors conclude that European C. glomerata has been accidentally introduced to Canary Islands and is threatening a local endemic butterfly already under pressure from habitat loss. Without mentioning DNA barcoding by name, the authors conclude with a call for “increased effort to sequence morphological Costesia spp. from a broad geographical range…enabling the regular testing of species hypotheses…and the incorporation of all life stages using a single character set”. I hope that the authors can join forces and enable their sequences and associated metadata (eg collection location, specimen photographs, voucher information) from this and future Cotesia spp work to be usefully combined with growing COI barcode database (>415,000 COI barcode records from >41,000 species in BOLD so far, including 514 records from 89 named and provisional Cotesia spp). Looking ahead, routine application of DNA-based identification to parasitoids will help establish host ranges of potential biocontrol agents and detect inadvertent introduction of broad-range parasitoids that damage local ecosystems.

In June 2008 PLoS ONE, thirteen researchers from nine Canadian universities, museums, and federal agencies report on mtDNA sequences from 1360 individuals representing 195 (95%) of Canada’s 205 freshwater fish species. Hubert et al follow “best practices” established for DNA barcode records (similar criteria would enhance the value of other genetic reference data as well), namely each sequence is derived from a vouchered specimen and the barcode record includes:

• “Bi-directional sequences of at least 500 base-pairs from the approved barcode region of COI, containing no ambiguous sites
• Links to electropherogram trace files available in the NCBI Trace Archive
• Sequences for the forward and reverse PCR amplification
• Species names that refer to documented names in a taxonomic publication or other documentation of the species concept used
• Links to voucher specimens using the approved format of institutional acronym:collection code:catalog ID number”

The researchers analyzed an average of 7.6 specimens/species, with an effort to sample across species ranges. A first pass look at genetic distances among and within Canadian freshwater fish shows results similar to those of other animal groups: average variation within species, 0.3%; average minimum distance between congeneric species (nearest neighbor), 8.3%; species with overlapping mtDNA sequences, 7% (4 species pairs and 1 flock of 5 species; one of the overlapping species pairs represents probable introgression. ) Five species showed divergent clusters differing by 1-2% in different parts of their geographic ranges, and 2 species showed larger divergences (3%, 7%); some or all of these might represent distinct species. 

A challenge for science publishing is disseminating the large data sets that are increasingly generated. Restricting publication to only those studies with novel findings can lead to a kind of distortion, sometimes with serious consequences. The bias against negative studies, for example, is one factor contributing to the misculation of risks of medicines. As biodiversity genetics moves forward, we need ways to ensure high-quality work, receive appropriate academic credit, and disseminate results in a timely manner.  PLoS ONE describes itself as “an international, peer-reviewed, open-access, online publication…that welcomes reports on primary research from any scientific discipline.” It seems to me that this sort of forum with a focus on quality rather than novelty is needed as a home for publication of large genetic data sets including DNA barcode records. Making this information available in a timely manner will in turn help drive development of analytic and display tools and enable scientific applications, such as identification of fish eggs and larva shown above.