David M. Larson
Yellow-rumped Warblers (Setophaga coronata) are widespread breeders in North America. While we are most familiar with the Myrtle Warbler here in New England, Yellow-rumps constitute a species complex consisting of four taxa (subspecies, according to the American Ornithologists Union): Myrtle (S. c. coronata) breeds in the boreal forests of North America and winters in eastern North America, Central America, and the Caribbean; Audubon’s (S. c. auduboni) breeds west of the Rockies and winters in the southwestern United States, Mexico, and Central America; Black- fronted (S. c. nigrifrons) is resident in Mexico; and Goldman’s (S. c. goldmani) is resident in Guatemala. The fact that some of these subspecies migrate—Myrtle and most Audubon’s—while others do not—some Audubon’s and all Black-fronted and Goldman’s—provides an interesting experimental model for studies on the development of migration and on what happens in hybridization zones. Toews et al. (2013) have investigated the genetics, biochemistry, and physiology of Myrtle, Audubon’s, and Black-fronted warblers, concentrating on the areas of geographic overlap and particularly on birds from the hybridization zone in the southwestern United States.
Most animal cells contain two types of DNA. Nuclear DNA (nDNA) is the genetic material in the nucleus and is derived from both parents of the organism (half from the sperm, half from the egg). This is what we usually consider as the main getriphosphate (ATP), the small molecule that provides energy for biochemical processes throughout each cell. If there is a functional link between mitochondrial genes and migration, then the prediction would be that the migratory behavior should sort with the mtDNA type— migratory individuals should have the Myrtle-type mitochondria and nonmigratory individuals should have Black-fronted mitochondria. They tested this prediction by genetic analysis of birds captured along transects through the transition zone and by estimating the individual’s migratory movements through analysis of stable hydrogen isotopes in feathers. A second prediction was that changes in the mtDNA type should result in changes in mitochondrial function. They tested this prediction by genetic analysis of mtDNA from individual birds and testing the biochemical capabilities of mitochondria from the muscle tissue of the same birds.
Migration and mtDNA Type
Over the range of 32-40º N latitude, the more southerly Audubon’s Warblers were not significantly migratory based on feather isotope analysis while the more northerly birds tended to be migratory with an estimated migration netic coding. However, cells contain mitochondria and these small organelles have their own DNA. All of the mitochondria and hence all of the mitochondrial DNA (mtDNA) in an animal is maternally inherited from the egg. So nDNA and mtDNA can provide different windows into the evolution of a species.
Looking only at nDNA, there are three distinct groups of Yellow-rumps: Myrtle, Goldman’s, and a separate cluster containing Audubon’s and Black-fronted. The Audubon’s/Black-fronted group shows gradual changes in nDNA sequence and in phenotype (morphology) over the geographic overlap range from 40o to 30o N latitude. However, mtDNA sorts differently. North of the Utah-Arizona border (37o N), Audubon’s warblers have mtDNA similar to that of Myrtles. Near that border, there is a dramatic transition to an mtDNA type characteristic of Black-fronted Warbler. That means that some of the Audubon’s Warblers in Arizona and New Mexico have the Black-fronted Warbler mtDNA and some have Myrtle mtDNA. Could the differences in mtDNA type be functionally related to the different migratory behaviors of Audubon’s Warblers?
The authors tested the linkage between the geographic transition from the Black- fronted to Myrtle mtDNA and the cellular metabolic needs of migration. Mitochondria are the power plants in cells. They use nutrients to produce adenosine distance of 4-10º. The change in migration behavior paralleled the change in mtDNA phenotype as predicted.
mtDNA Sequencing and Mitochondrial Function
DNA sequencing showed a 4.1% divergence between Myrtle and Audubon’s/Black-fronted mtDNA and substantial differences in the composition of some of the enzymes important in mitochondrial energy production. While most of the functional tests showed no difference between the individuals, the authors found significant mtDNA-correlated differences in the efficiency of coupling within one crucial part of the ATP production pathway. The differences in coupling suggest that the Myrtle-type mitochondria from the northern, more migratory birds are more efficient than those from the southern, nonmigratory individuals.
Summary
Yellow-rumped Warblers underwent complex speciation events during the last glacial period, resulting in four recognizable subspecies. In the United States, the Myrtle and Audubon’s types diverged in nDNA and in appearance. Despite the differences in nDNA, most Audubon’s Warblers contain Myrtle mtDNA. However, the southernmost populations of Audubon’s Warblers—in the transition zone to Black- fronted Warbler—have Black-fronted mtDNA. The mtDNA types track with the migratory behavior of the populations. Myrtle and Audubon’s are migratory; southern Audubon’s and Black-fronted are not. Furthermore, Toews et al. (2013) have suggested that mitochondrial efficiency may be higher in Myrtle-type mitochondria, potentially an advantage in migratory populations. Although largely correlative, these results suggest that further examination of this Yellow-rumped Warbler complex could shed light on speciation and on hybridization at the nDNA, mtDNA, and functional and behavioral levels.
References
- Toews, D. P. L., M. Mandic, J. G. Richards, and D. E. Irwin. 2013. Migration, Mitochondria, and the Yellow-rumped Warbler. Evolution 68(1): 241-255.
David M. Larson, PhD, is the Science and Education Coordinator at Mass Audubon’s Joppa Flats Education Center in Newburyport, the Director of Mass Audubon’s Birder’s Certificate Program and the Certificate Program in Bird Ecology (a course for naturalist guides in Belize), a domestic and international tour leader, Vice President of the Nuttall Ornithological Club, and a member of the editorial staff of Bird Observer.