Evolutionary patterns of salt tolerance in angiosperms
Date
2015
Authors
Moray, Camile
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Abstract
Global food production is threatened by increasing land
salinization triggered by climate change, land clearing, and
irrigation. Salinity is toxic to most plants, including most crop
species. A tremendous research effort has focused on
understanding how a rare set of naturally salt tolerant plants,
halophytes, are able to cope with soil salinity, as a model for
producing salt tolerant crops. One largely unexplored area of
research is the evolution of salt tolerance. Previous studies
show that salt tolerance has evolved multiple times across the
angiosperms, but little is known about the patterns and processes
that underlie the evolution of salt tolerance. In my thesis I
addressed several questions relating to the evolution of salt
tolerance in angiosperms using a broad-scale, macroevolutionary
approach.
I first used taxonomic and phylogenetic comparative techniques to
assess the evolutionary patterns of salt tolerance in
angiosperms. I found that over one-third of angiosperm families
contain halophytes and that salt tolerance appears to have
evolved hundreds of times in the angiosperms. In over half of the
family phylogenies analyzed, salt tolerance appeared
evolutionarily labile: the origins of salt tolerance were
scattered across phylogenies and generally gave rise to only one
or a few halophytes.
I also explored the association between salt tolerance and
another trait associated with anthropogenic environmental change,
heavy metal hyperaccumulation: the ability to accumulate high
concentrations of heavy metals/metalloids. Taxonomic and
physiological similarities suggest that salt tolerance may be
associated with hyperaccumulation. I test the suggested
relationship between these abilities using taxonomic and
phylogenetic analyses. Significantly more angiosperm families
contain both halophytes and hyperaccumulators and significantly
more species are reported as both halophytes and
hyperaccumulators than expected given the rarity of each trait.
In several families, halophytes and hyperaccumulators are more
closely related than expected if the two traits evolved
independently. These results support the observation that salt
tolerance and heavy metal hyperaccumulation are associated in
angiosperms.
Prolonged or repeated exposure to salinity can cause oxidative
stress that may lead to increased mutation rates. These mutations
may lead to increased substitution rates in halophytes compared
to non-salt tolerant relatives. We tested this idea by comparing
DNA sequences of multiple genes from the chloroplast,
mitochondrial, and nuclear genomes from several halophytes with
their non-salt tolerant relatives. We found that halophytes have
significantly increased total substitution rates compared to
their non-salt tolerant relatives in mitochondrial genes. This
finding provides evidence that environmental factors may be
associated with molecular rates.
The goal of developing salt tolerant crops has proved incredibly
difficult, which may be partly due to loss in genetic variation
associated with domestication. Yet several studies suggest that
domesticated animals and plants may have increased rates of
molecular evolution, which could lead to increased variation. We
test whether domesticates have consistently increased rates of
molecular evolution by comparing the mitochondrial genomes of
domesticated mammals and birds to their wild relatives. While a
few domesticates exhibited higher rates, in general we found no
consistent difference in mitochondrial rates of domesticated
animals compared to their wild relatives.
Keywords: comparative analysis, phylogenetics, domestication,
macroevolution, rates of molecular evolution, halophytes
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comparative analysis, phylogenetics, domestication, macroevolution, rates of molecular evolution, halophytes
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