Polyploidy and Contact Zone Dynamics in context of Molecular Evolution of
Stress Regulatory Pathways
Gaurav Moghe, Genetics Program
Michigan State University
Polyploidy leads to multiplication of genomic content, which in turn, provides raw material for
mutations and natural selection to act upon. A large number of angiosperms, including a majority
of the food crops, are believed or known to be neo-polyploids or paleo-polyploids (Otto and
Whitton, 2000). This observation has led to a multitude of questions regarding the significance of
polyploidy in the plant world. Although we have substantial knowledge on the effects of
artificially-selected polyploidy in crop plants, relatively fewer studies have looked into the range
of adaptive significance of this phenomenon under natural conditions. Also, large gaps exist in
our knowledge on the molecular aspects of the contribution of polyploidy in plant adaptation.
(reviewed in Adams, 2007; Comai, 2005).
It is indeed an intriguing question how polyploidy has contributed to the adaptive evolution of
morphological and physiological characters of plants. Polyploidization is expected to introduce
substantial perturbations in the genetic and metabolic networks of the cell. It is also well
established that polyploids are subject to minority cytotype exclusion, the frequency dependent
selection on propagation right after polyploidization. Dioecious polyploids simply cannot
propagate themselves, as the gametes they produce are either incompatible with their diploid
progenitors, or produce infertile offsprings. To be able to sustain themselves over evolutionary
time, the polyploids would either have to:
1)
Explore different geographical habitats or
2)
Explore different ecological features like flowering time, pollinator etc. or
3)
Reach a stable genomic state over time
Evolutionary dynamics resulting out of changes in the ploidy state can be studied at various
levels of organization, at the contact zones between polyploids and their diploid progenitors.
Indeed, such contact zones are regarded as natural laboratories for studying evolution in action
(Lexer and Loo, 2006). Quite a few studies have explored the different strategies employed by
polyploids and their diploid progenitors in the same environment. Thompson (2003), in his
studies on Heuchera grossulariifolia showed that the ploidy level can strongly affect plantanimal interactions. Buggs and Pannell (2007) explored the population dynamics of Mercuralis
annua at contact zones of polyploids-diploids in Spain. They found that the polyploid and
diploid populations were in competition with each other, with the diploids probably better
adapted to the environment. The suspected higher fitness among diploids likely allows them to
swamp the contact zones and capture territories where the polyploids were once in majority.
Also, lab investigations to test the effect of cytotype frequency on the survival of Chamerion
augustifolium populations (Husband, 2000) confirmed the theory that the minority polyploid
cytotype can be out-competed by pollen swamping. It was also observed that the presence of
other counteracting forces like pollinators can allow for the sustenance of polyploids at the
contact zones (Husband, 2000).
Although substantial efforts have been devoted to understand the ecological and reproductive
dynamics of diploid and polyploid individuals at contact zones, the whole range of attributes
over which differences exist between these individuals under natural conditions remains to be
discovered. Also, relatively few studies have looked at the molecular and physiological basis of
such dynamics (reviewed in Adams, 2007; Comai, 2005). An area that I am particularly
interested in is the differences in physiological responses, particularly stress tolerance – both
biotic and abiotic -- between polyploids and diploids at the contact zones. Although it is
hypothesized that increased heterozygosity and gene redundancy would confer a higher stress
tolerance or resistance to infections, it has been observed in at least one case that diploids are
more resistant to pest attack (Thomson and Janz, 2002). Also, a large number of studies have
shown that the fitness of any one cytotype is largely a function of its environment.
I plan to study the physiological differences in the context of stress response in Arabidopsis
ecotypes obtained from diploid-polyploid contact zones. In addition, I will also be examining the
stress regulatory pathways in these ecotypes using expression analyses. It has been proposed that
polyploidization results in drastic changes in gene expression patterns (Adams et al, 2003), and
epigenetic mechanisms are likely at play (Lee and Chen, 2001). I plan to use microarrays to
monitor the differences in expression between known stress regulatory genes and at the same
time, unravel new genes involved in the response.
Another area that interests me is the genome reorganization that occurs after polyploidization.
The case of preferential gene retention in paleo-polyploids has been well documented in the
context of Arabidopsis thaliana, where preferential retention of transcription factor and
developmental genes has been observed (Bodt et al, 2005). This process of diploidization,
resulting in multiple gene losses, functional overlap, functional divergence, expansion of gene
families etc. is a fascinating topic to study. I plan to probe into questions on the rate of gene loss
after polyploidization, the effect of polyploidy on cellular networks, patterns of functional
divergence between duplicated genes etc. using bioinformatic and comparative genomic
approaches.
References:
1)
Adams, K.L. et al, Genes duplicated by polyploidy show unequal contributions to the
transcriptome and organ specific reciprocal silencing, Proc.Natl.Acad.Sci., April 2003;
100(8):4649-4654
2)
Adams K., Evolution of duplicate gene expression in polyploid and hybrid plants, J.
Hered., 2007; 98(2): 136-141
3)
Bodt, S.D. et al, Genome Duplications and the origin of angiosperms, Trends in Ecol.
And Evol., Nov 2005; 20(11): 591-597
4)
Buggs R., Pannell J., Ecological Differentiation and diploid superiority across a moving
ploidy contact zone, Evol., Jan 2007; :125-140
5)
Comai L., The advantages and disadvantages of being polyploid, Nat.Rev.Gen., Nov
2005; 6: 836-846
6)
Husband, B.C., Constraints on polyploid evolution: a test of the minority cytotype
exclusion principle, Proc.R.Soc.Lond., 2000;267: 217-223
7)
Janz N., Thomson J., Plant polyploidy and host expansion in insect herbivore, Oecologia,
Feb 2002; 130(4): 570-575
8)
Lee, H., Chen, Z.F., Protein coding genes are epigenetically regulated in Arabidopsis
polyploids, Proc.Natl.Acad.Sci., June 2001; 98(12): 6753-6758
9)
Lexer C., Loo V.M., Contact Zones: Natural Labs For Studying Evolutionary Transitions,
Current Biology, 2006; 16(11):R407-R409
10)
Otto, S.P. and Whitton J., Polyploidy incidence and Evolution, Annu. Rev. Genet., 2000;
34: 401-437