The taxonomy of Branta bernicla, commonly referred to as brant or brent
geese, has been highly controversial. It is generally accepted that there are
two or three groups of brant distinguished on the basis of color and distribution,
although they are similar in size and shape. Much of the confusion about the
identity of the subspecies is the result of intergradation of morphological
characters and interbreeding where their breeding ranges overlap. This study
dealt mainly with black brant (B.b. nigricans) and Atlantic brant (B.b.
hrota). Morphologically, these birds differ in that the black brant has
a darker abdomen and a necklace of white feathers which is complete ventrally,
whereas Atlantic brant have light colored abdomens and incomplete necklaces.
Furthermore, black brant winter on the Pacific coast of North America, south
to Baja California, and Atlantic brant winter on the Atlantic coast, south to
North Carolina.
Interestingly, the holotype for black brant was collected in 1846 by George
N. Lawrence at Egg Harbor, New Jersey. The occurrence of the holotype and other
dark-bellied brant on the east coast has been explained as a distinct population
of brant, as stragglers from western populations, or as intergrades resulting
from melanism or hybridization of the subspecies. Due to the controversy surrounding
the morphological identity of B. bernicla subspecies, this project sought
to use molecular techniques to investigate the genotypic identity of Lawrence's
holotype.
Using a variety of published and newly-designed oligonucleotide primers, sequences
were obtained for two fragments (307 and 162 base pairs) of the cytochrome b
gene of the mitochondrial DNA (mtDNA) for several fresh brant specimens collected
on their wintering grounds. Sequences for shorter fragments of mtDNA were obtained
for several museum specimens, but no sequence was obtained for Lawrence's holotype;
this probably is due to the age of Lawrence's specimen (150 years old) as opposed
to the other museum specimens (50 years old). Single-strand conformation polymorphism
(SSCP) successfully distinguished between the black and Atlantic brant for the
longer fragment. This variation corresponds to two apparently fixed polymorphisms
between the subspecies. Some SSCP variation within the black brant for this
same fragment, however, could not substantiated by sequencing data. Additionally,
sequences for this limited region of mtDNA contained more variation within the
subspecies than between the subspecies.
Three extraction protocols (two phenol/chloroform/isoamyl alcohol (PCI); and
one silica-based) were used for the majority of the museum specimens. An additional
Chelex-based method was used for the holotype. The PCI-based methods produced
amplifiable DNA from a variety of museum skins, whereas the other two methods
failed to yield amplifiable DNA. This suggests that PCI-based extractions are
more efficient for the extraction of DNA from bird museum skins, although the
silica method worked very well for a more recent (~5 years old) cowbird skin.
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