Polyploids and
domesticated species
Natalia Alvarez
UW Madison
March 20th, 2007
Polyploidy and its effects
It is estimated that 70% of the flowering plants has
polyploidy in their history (Masterson,1994).
• Increased cell size and gigas effect in some organs.
• Changes in shape and texture of organs.
• Greater ability to colonize new habitats than diploid
ancestors.
• Reduction in fertility and seed production
(Stebbins,1971)
The role of polyploidy in crop
improvement
• Gene buffering: Slower response to selection but more
adaptive potential.
• Dosage effect: additive effect of the alleles increases the
number of phenotypes.
• Increased allele diversity and heterozygosis: more possible
allele combinations and opportunities for breeding.
• Novel phenotypic variation: genome interactions and
changes in gene expression in new synthesized
allopolyploids.
Did polyploidy confer advantages
for plant domestication..?
• Survey of 244 crops species belonging to 11 monocot
and 48 dicot families. Chromosome number obtained
from literature
• Neopolyploids determined by comparing with the
smallest chromosome number in its respective genus.
• Determination of Paleopolyploids. Two criteria:
(Goldblatt,1980), n = 11 and (Grant,1963), n = 13.
• The frequency of polyploids in crops was compared
with estimates for angiosperms, monocots and dicots.
(Hilu,1993)
Did polyploidy confer advantages
for plant domestication..?
If yes, polyploids frequency should be higher for crops
than for angiosperms.
Angiosperms
estim.
Total crops
# of
species
counted
% Polyploidy
n = 13
% Polyploidy
(Grant, 1963)
(Goldblatt,1980;
Lewis, 1980b)
-
47
75
244
55
75
n = 11
Not significant difference was found. Therefore,
domestication would not favored polyploids over diploids.
Comparing at the family level
Polyploids frequency was not statistically different in 5 of
the selected families, except in Dioscoreaceae.
(Hilu,1993)
Are polyploids more frequent in
perennial plants?
% Polyploidy
# of
n = 13
species
counted (Grant, 1963)
% Polyploidy
n = 11
(Goldblatt,1980;
Lewis, 1980b)
Angiosperms estim.
-
47
75
Total crop annuals
76
46
68
Total crop perennials
146
60
76
The frequencies of annual vs. perennial polyploid crops
were statistically similar, contrasting with the proposed
idea that perennial polyploids have a selection advantage.
(Hilu,1993)
What happens after polyploid
formation?
In autopolyploids
• Genomes “act independently”
• Gene expression
– Dosage effect (linear relationship between gene
expression and number of gene copies).
– Non-dosage regulation (over/under-regulation).
What happens after polyploid
formation?
In allopolyploids
• Genomic changes
– Diploidization and structural evolution
– Intergenomic colonization.
– Nuclear-cytoplasmic interactions.
– Rapid genome changes
• Gene changes
– Divergence
– Silencing
– Intergenomic gene conversion
– Differential rate of evolution
Bringing the story to the cotton…
Parental genomes from different
continents:
• Maternal A-genome from Africa
• Paternal D-genome from the New
World
Polyploidization ~1.5 Mya
Origin of 5 Allopolyploid species
http://www.athenapub.com/nwdom1.htm
New World
The progenitors
Africa
G. raimondii
G. arboreum
G. herbaceum
2n= 2x = 26
The descendants
2n = 4x = 52
The domesticated
species
G. darwinii
G. tomentosum
G. mustelinum
G. hirsutum
G. barbadense
G. hirsutum
G. barbadense
G. arboreum
G. herbaceum
http://www.eeob.iastate.edu/faculty/WendelJ/fiberevolution.htm
Domestication in the New World
• Archaeological reports of cotton
fabrics found in prehistoric ruins in
Arizona.
• Gossypium hirsutum evolved in
Mexico. The oldest archaeological
specimens were found in Tehuacan,
and are tentatively dated at 3400 to
2300 B.C.
• Gossypium barbadense, is the
second species of New World
cotton. Peruvian archaeological
excavations found cotton textiles of
~2500 B.C.
http://www.mayanindians.com/mayan-weavers.html
http://www.hno.harvard.edu/gazette/2002/01.24/09-textile.html
What happens in the nucleus of
the polyploid cotton…?
Gene and genome evolution hypothesis
Adams and Wendel, 2004
Genomic interactions
• Intergenomic colonization
Repetitive sequences
specific from A-genome
are found in the D-genome
in Gossypium polyploids.
Transposable elements
might be related.
(Zhao et al. 1998)
Genomic interactions
• Rapid genomic changes and silencing:
Immediate consequences of allopolyploidization
seem to occur in evolutionary timescale.
Near-complete genomic stasis across generations
of synthetic allopolyploids is observed. It contrast
with evidence from other synthetic allopolyploids.
Similar gene silencing within synthetic
allopolyploids and respect to the natural
allotetraploid of G. hirsutum. (Adams et al.,2003)
Evolution of duplicated genes
• Biased expression toward one homeologue or the
other .
• Some genes show organ-specific, reciprocal
silencing.
adhA gene in
G. hirsutum
Transcript
level (%)
Adams et al. 2003)
Evolution of duplicated genes
• Interlocus concerted
evolution:
Sequences of ITS regions
and 5.8S ribosomal gene
in the AD-genome species
and their diploid
progenitors show
homogeneity. Four of the
5 allopolyploids
homogenized the 4 loci to
the D-like form and one
to the A-like form.
Gene tree
Wendel et al.(1995)
Evolution of duplicated genes
• Differential rate of evolution
Nucleotide diversity at homeologous locus
of adhA and gene in G. hirsutum and G.
barbadense was higher in the D-genome
than in the A-genome of the allopolyploids.
The results were observed also for adhC
gene (Small et al.,1999; Small and Wendel,2002).
References
• Adams,K and Wendel, J. (2004) Exploring the genomic mysteries of
polyploidy. Biol. Journal of the Linnean Society 82: 573-581.
• Hilu, K. (1993) Polyploidy and the evolution of domesticated plants.
Amer. Journal of Botany 80(12): 1494-1499.
• Stebbins, G. (1971) Chromosomal evolution in higher plants. Ch. 5.
Edward Arnold, London
• Udall, J. and Wendel, J. (2006) Polyploidy en crop improvement. The
Plant Genome (A supplement to Crop Science), Nov. 2006, No. 1.
• Wendel, J. (2000) Genome evolution in polyploids. Plant Molecular
Biology 42: 225-249.