Today’s seminar speaker was Todd Castoe from
the University of Texas at Arlington where I did my Ph.D. While Todd wasn’t on my committee my office when
I was a grad student was directly across the hall from Todd, so it was great to
catch up with him. His talk was titled:
Genome-wide evidence for perturbed systems as hotspots for adaptation. Writ large this was a talk about convergence
and the topography of the adaptive landscape.
The idea of adaptive landscapes is perhaps one of the oldest
metaphors in evolutionary biology. For
Todd’s talk we will consider the high points in these landscapes as phenotypic
combinations that provide high fitness and the pathway followed to travel
between peaks as the evolutionary changes responsible for moving between peaks.
Two alternative adaptive landscapes:
Here the
adaptive landscape has many peaks and there are many potential pathways to
reach any of these many peaks
In contrast there
could be only one reasonable path between only a handful of local optima
We generate expectations for convergence under each of the
above conceptions of the adaptive landscape.
In the first we would expect convergence at the phenotypic level to be relatively
more common than the actual underlying molecular changes responsible for the
phenotypes. In contrast in the second
case some form of constraint limits the pathways that can be used to move from
one optima to another and we should expect phenotypic and molecular convergence
to be more common.
Todd began his talk by suggesting that increasing evidence
(including a great example of mitochondrial convergence in squamate reptiles)
points to an adaptive landscape that is highly constrained and more like the
second metaphorical picture.
Todd then showed us a variety of projects that his lab is
working on that may shed light on the nature of this landscape and the nature
of convergence. For the sake of brevity
I’m going to talk about just one of these.
This project is a story centered on the amazing physiology
of the Burmese python. In its natural
habitat the Burmese python often feeds very rarely but consumes meals that may
mass as much as 50% of the snake’s mass.
After these large feedings the snake may fast for many months. This pattern of large meals followed by long
fasting is not unique to Burmese pythons and is also present in some of the
large vipers. The amazing part of this
story though is the energy saving solution these large snakes have found. During fasting periods the snakes have
evolved to lose much of the physiological structure necessary for the digestion
of these meals. The snakes exhibit
massive losses in the size of organs for instance the heart, liver, and
kidneys, and they even lose some structure within organs. For instance, microvilli in the small
intestine are lost during fasting periods.
These adaptations allow the snake to achieve some of the lowest
metabolic rates measured in vertebrates during fasting periods, but return to
more typical metabolic levels after feeding.
The phylogenetic distribution of these traits suggests that some of the
necessary machinery for this physiological remodeling is likely quite ancient
and has been fine tuned or loss in contemporary clades.
The Castoe lab is now doing exciting work looking at the
evolution of introduced Burmese python populations in Florida. There are many differences in the environment
that the invasive populations are living in.
The obvious and well documented difference is low temperatures. For instance winter mortality appears to be
as high as 40-90% in the invasive population.
Because of this it was expected that genome scans would reveal selection
on genes important in cold tolerance. Genome
scans revealed that approximately 80 genes show strong signs of selection in
this invasive population. But, genes
important in cold tolerance only accounted for a fraction (approximately 10%)
of these 80. What did the rest of the
genes under selection look like?
Preliminary results suggest that the majority are those genes that have
already been identified as being important in the evolution of physiological
remodeling. Why would these genes
suddenly come under selection? It ends
up that in Florida prey items are abundant year round and snakes that do not
remodel their organs may have higher fitness.
The story then may be a particularly fascinating version of convergent
evolution where this introduced population is converging on a phenotype of its
ancestor. Returning to the analogy of
adaptive landscapes at the beginning of the post then we would be seeing something like this:
Todd covered an impressive body of work in his talk and any
mistakes or unjustified extrapolations are likely my own. Below are links to some of the papers from
his lab that focus on these topics:
Very good website. I liked it very much.
ReplyDelete3 Track Sliding Windows Bhilai