1. Hirundo rustica: a model for the genetic basis of continuous phenotypic
variation and speciation
Ryan Higgins Undergraduate BA, University of Colorado Department of Molecular, Cellular, and Developmental Biology
Joanna K. Hubbard Ph.D. Candidate, University of Colorado Department of Ecology and Evolutionary Biology
Introduction
Pigmentation genes play an important role in the generation
of phenotypes (what something looks like). In many cases,
distinguishing between two species based on color patterning
can be simple. Take for example a female lion and a female
tiger; the morphological characteristics are very similar, but
their coloration makes it easy to elucidate which is the lion
and which is the tiger. However, within a species certain
traits, including pigmentation, can become selected for and
more pronounced in some populations and not others: an
evolutionary process known as Cladogenesis. Environment
and sexual selection are two strong mechanisms that can
mediate phenotypic variation within a species. Over 150
genes have been found to affect animal coloration and
patterning, and many of these genes have also been found to
be strongly conserved in the vertebrate taxa. Here we explore
coloration in barn swallows (Hirundo rustica) from North
America and the United Kingdom at the molecular level by
looking at genetic variations in TYR, a gene coding for
Tyrosinase: an enzyme that oxidizes tyrosine residues that
then undergo reactions and yield eumelanin, which
contributes to dark coloration. Phenotypically barn swallows
in the United Kingdom have lighter breast plumage than
those in Colorado, and it is likely the UK population will
share characteristic mutations that decrease Tyrasinase
activity, while those in Colorado will share mutations that
increase tyrosinase activity.
Methodology
• Sampled adult barn swallows in the UK and Colorado
in 2008
• Took breast feathers from each bird
• Measured with photospectrometer to get an objective
measure of color
• Sequenced 48 males from each population – the 24
darkest and 24 lightest males
• Tyrosinase – UK and Colorado
• DNA sequence analysis
• Looked for Single Nucleotide
polymorphisms (SNP) in each gene
• Scored genotype in each bird
• Determined whether genotypic is
synonymous or non-synonymous
Results
Conclusion
Figure 1: NA population shows darker coloration than UK population
Figure 2: Relation of genotype and color in NA Figure 3: Relation of genotype and color in UK
Figure 4: Relation of genotype and color in both populations
While this study is not comprehensive in melanogenisis, as tyrosinase
is just one component, it has produced some interesting and useful
information for future use. Figure 1 shows that barn swallow
populations in North America have a darker breast coloration
compared to ones in the United Kingdom. Figures 2, 3, (NA and UK
respectively) and 4 (NA and UK) elucidate what mutations in
tyrosinase may cause the distinct phenotypes of each population. The
most notable variable sites (indicated as base pair position) are 501
and 873. At 501 it appears the replacement of threonine (T) residues
with Serine (S) results in darker coloration. Notice how only the NA
population has homozygous mutations at that site. At site 873 the
replacement of a tyrosine (Y) with an asparagine (N) appears to cause
a lighter coloration, and thus, decreased enzymatic activity. Notice
how this mutation only occurs in the UK population. Thus, presence of
S and N at 501 and 873 respectively, demonstrate Figure 1 at the
molecular level. It is important to note that there is variation within
each population at these sites, and thus, this is the underlying theme
of continuous phenotypic variation. Why one phenotype is
predominant over another, and whether mechanisms in the
environment influence a certain phenotype, are still being studied;
these will give us a much better understanding of evolutionary
mechanisms.
United Kingdom (UK)
North America (NA)
Figure 5: Amino acid variation at each site
NA
UK