Transcript Unexpected Resilience of TSD species at the

Unexpected Resilience of
TSD species at the
Cretaceous-Palaeogene
boundary
Summary
● Meteor strike event in Cretaceous-Palaeogene boundary (K-Pg) caused
increase in global temperature
o volcanic eruptions- lots of CO2
o lots of minerals evaporated-more solar radiation
● GSD vs TSD- how is survival affected
o GSD-independent evolving chromosomes could have adaptive benefit
o TSD-if extreme temperature changes would extinction result?
● Hypothesized that GSD had more survival than TSD, but actually the
opposite!
o could be because short term temperature change shifted to more
females than more males, or maybe there was actually less climate
change than expected, or maybe egg-laying TSD resulted in more
protection and less sex skewing than we think...
Connections to other readings
● Skinks are also TSD
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cooler = males
warmer = females
● article hypothesized warming from meteors/ volcanoes
to cause extinction
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imbalanced sex ratios cause extinction, but in short term w/ more
females, could be beneficial
Did your reading clarify any
points/ideas?
In the instance of the k-pg, TSD seems to have
been the advantageous trait over GSD.
Whether it is male or female favored sex ratio
variance is not known, but most likely femalefavored sex variation is better.
Other interesting aspects
● TSD vs. GSD are more resilient in the short-term
● There isn’t a great way to recreate the climate change
at the K-Pg boundary
● This is a new area of research! Lots of cool new ideas!
Summary
● Previously thought that TSD species would be negatively
affected by climate change
o Explanation for the extinction of dinosaurs?
● KT boundary: massive climate change due to volcanic
eruptions and/or meteor
● 14/16 TSD species survived the KT boundary, while 61%
GSD species went extinct
● Think that TSD allowed species to either maintain their
sex ratio or shift towards more females (more beneficial
for population)
● Maybe the skinks will adapt easily to climate change
● ESD vs GSD
● Global Warming may have begun from CO2 release in
India!
● Article clarifies how TSD could be adaptive
● TSD is an example of phenotypic plasticity
Y degeneration
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A single chromosome mutated so all
inheritance were male.
Y chromosomes can’t line up with X to
recombine so they have been evolving
separately for millions of years
Connects to reading transition of
environmental sex determination to genetic
sex determination.
Losing genes but also gaining genes so is it
really disappearing?
Decay seems to have come to a halt.
Connects to reading transition of
environmental sex determination to genetic
Y-Chromosome Article
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Theory that the Y chromosome might disappear altogether from the genome as it keeps losing genes and does not have a backup
copy like other chromosomes
Arguments against this theory:
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Hasn’t lost any genes within the past 44 million years
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Remaining genes may be too essential to lose
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More responsibilities than simply determining maleness- New genes play a role in sperm production
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There has been addition of genes to the Y chromosome
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Loss of the Y chromosome in some tissues has already shown to be detrimental
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Men who lose Y chromosome in bone marrow are at higher risks for cancer and have been shown to die younger
Graves in support of her theory:
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The Y chromosome has already disappeared in some mammals
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Just because the chromosome has gone so long without losing genes doesn’t mean it couldn’t lose them tomorrow
Other interesting facts:
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The Y chromosome regulates genes differently than the X chromosome which can lead to important consequences in disease
prevalence between males and females
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61 of the human Y chromosome’s 78 genes were not present before the first inversion
Connection with other material:
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This article relates to the figure from the article on sex determination that shows how during the evolution of different
species there has been a consistent loss of swappable genes between the X and Y chromosomes with less recombination
being seen in the Y chromosome.
Avian sex determination
• Sex chromosomes: males: ZZ; females: ZW.
– Called female heterogamety
• In mammals (female XX; male XY) sex determination carried on Y
chromosome, expressed in the embryonic male gonad to trigger
testis formation. Testosterone relased from the male gonad
masculinizes the tissues.
• Mechanism for avian sex determination unknown
• Might be determined by
– Dosage of a Z-linked gene (males have 2, so a larger dosage)
– Dominant ovary-determining gene carried on the X sex chromosome
– Or both
Other things
• Fun fact: in chickens, the Z chromosome has has
over 350 genes, and the W chromosome has
probably less than 20.
• This article supplemented on the article we read
for Tuesday, Nov. 18th
– Sex determination
• Mechanisms
– Female heterogamety
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birds have genetically determined sex
males (ZZ) females (ZW)
ZZ→ male or W→ female?
ratites are homomorphic (same # genes on sex chromosomes) indicating earlier
in evolutionary lineage. (sex chromosomes shortening over evolutionary time)
● triploidy ZZW→shows male and female characteristics
● W chromosome carries female determinant (regardless of number of Z
chromosomes)→ implies that W not dominant, can be overridden by ZZ
chromosomes so Z dosage and dominant W hypotheses are viable