Transcript Rapid micro-evolution and loss of chromosomal diversity in

Rapid micro-evolution and loss
of chromosomal diversity in
Drosophila in response to
climate warming
A study by Francisco
Rodriguez-Telles and Miguel A.
Rodriguez
The Problem:
• Global warming is
introducing
ecological
conditions that
organisms have
never before
encountered
• Empirical evidence: bacteria and other
simple organisms can evolve quickly in
response to ecological pressure
– Short generation time
– Large, varied populations
– Frequent mutation
• Can more complex organisms with smaller
population sizes and longer generation
times evolve rapidly enough to meet
pressures due to higher temperatures?
Study population:
Drosophila subobscura
–Drosophila is isothermal – habitat determines
body temperature
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The O chromosome
• The O chromosome is linked to thermotolerance traits
– Hsp70 gene
• Inversions of at least one gene on the O
chromosome cause measurable
phenotypic differences among individuals
– 15 arrangements exist in the population
• Inversion arrangements affect physiology
and behavior
Common arrangements
• OST, O3+4, O3+4+7, O3+4+8
• Climate affects geographic distribution of
arrangements
• Frequency of arrangements fluctuates with the
seasonal cycles
• This fluctuation parallels changes in temperature
Methods
• Drosophila were collected in spring, early
summer, late summer, and autumn from
1976-80 and 1988-91 in a southern
Palearctic habitat
• Frequencies of common O chromosomal
inversion arrangements were recorded in
each year
• Chromosomal diversity was calculated:
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IFR = Index of free recombination
Number of inversions
Frequency of inversions
Length of inversions
Amount of euchromatin not involved in the
inversion loops
• Low IFR value means population is
becoming more heterogeneous -- more
arrangements are present in the
population
Climate
• Statistical tests used
to determine:
– change in climate
over the study period
– effect of each variable
and combinations of
climactic variables on
distribution of
arrangements
Results
• Decrease in:
– Relative humidity
– Annual precipitation
• Temperature increased
linearly
• Temperature had strongest
correlation to shifts in
frequency of arrangements
From top to bottom: summer, spring, autumn, winter
Arrows indicate years in which sampling occurred
Overall, 18.3% loss of diversity in chromosome arrangements
OST arrangement
• Decreased in frequency 40.7%
– Negative correlation with year
• Frequency depends on temperature
• Decreased most in late summer and early
fall when temperatures were highest
• Strong correlation with latitude
– Present more in latitudes with colder
temperatures
O3+4+8 arrangement
• Decreased in frequency in the population
– Negatively correlated with year
O3+4 arrangement
• Frequency increased over 16-year-period
– Positively correlated with year
• Shift in frequency due to change in
temperature and humidity
O3+4+7 arrangement
• No significant shift in frequency
– No correlation with year
• Inferred advantage:
– Increased temp led to increased
frequency of arrangement O3+4
– O3+4 is present in highest frequencies in
warmer parts of the habitat
Therefore, O3+4 is advantageous in
warmer climates
– Hypothesis: Selection for this
arrangement in warmer temps
• Inferred disadvantage:
– Increased temperature led to decrease in
frequency of OST and O3+4+8
– OST and O3+4+8 arrangements more prevalent
in colder parts of habitat
– Individuals with OST arrangement are least
active during the hottest parts of the day
– OST and O3+4+8 are advantageous in colder
climates
– Hypothesis: Selection against these
arrangements in warmer temperatures
Supporting Studies
• Gilchrist and Huey, 1999
– Temperature sensitivity is heritable
– Correlation between increase in preferred
temperature and decreased genetic variation (graph)
• Levitan and Etges
– Identified “southern” chromosome arrangements that
have increased in frequency in northern populations
in recent years
– Frequency of “northern” arrangements has decreased
“almost to the point of extinction”
Implications
• Effect of increased global temperature on
Drosophila models possible impact on humans
and other complex organisms
• Warmer temperatures lead to a decrease of
genetic variety in populations
• If trend continues, directional selection could
move some arrangements to fixation and result
in the loss of others
• Currently, Drosophila is not experiencing
any negative effects from the loss of
certain chromosomal inversion
arrangements
• In fact the population is responding well to
selection pressure
• However, the cost of rapid evolution may be a
decrease in variation of arrangements
• Decreased genetic variation decreases species’
capacity to respond to future selection pressure,
retain fitness, and avoid extinction
• Natural selection depends on:
– Variation in traits
– Differential reproductive success
– Homogeneity inhibits response to selection
What if we didn’t believe in
evolution?
• Could not attribute large-scale loss of
genetic diversity to many small changes in
each generation that have accumulated
• Could not explain how organisms adapt
and evolve in response to conditions they
have never before encountered, since ID
believes they originated as they are now
and have not changed over time
References
• Gilchrist, George W. and Huey, Raymond B. 1999. The
direct response of Drosophila melanogaster to selection
on knockdown temperature. Nature, 15-29.
• Levitan, Max and Etges, William J. 2005. Climate
change and recent genetic flux in populations of
Drosophila robusta. BMC Evolutionary Biology, 5:4.
• Rodriguez-Trelles, Francisco and Rodriguez, Miguel A.
1998. Rapid micro-evolution and loss of chromosomal
diversity in Drosophila in response to climate warming.
Evolutionary Ecology, 829-838.