This document summarizes research on adaptation in maize during and after domestication. It discusses how maize adapted through standing genetic variation, regulatory changes, and polygenic adaptation using multiple mutations and alleles from teosinte. It describes the origins and spread of maize from its ancestor teosinte in Mexico around 9,000 years ago to other regions of Mexico, South America, and beyond. The differences between lowland and highland maize populations are also summarized.
1. Adaptation in maize:
domestication and beyond
Jeffrey Ross-Ibarra
@jrossibarra • www.rilab.org
Dept. Plant Sciences • Center for Population Biology • Genome Center
University of California Davis
photo by lady_lbrty
18. Wang et al. 2005 Nature
1 2 3 4 5
6 7 8 9 10
Figure 1.
Phenotypes. a. Maize ear showing the cob (cb) exposed at top. b. Teosinte e
internode (in) and glume (gl) labeled. c. Teosinte ear from a plant with a m
introgressed into it. d. Close-up of a single teosinte fruitcase. e. Close-up o
teosinte plant with a maize allele of tga1 introgressed into it. f. Ear of maiz
(Tga1-maize allele) with the cob exposed showing the small white glumes a
of maize inbred W22:tga1 which carries the teosinte allele, showing enlarge
h. Ear of maize inbred W22 carrying the tga1-ems1 allele, showing enlarged g
NIH-PAAuthorManuscriptNIH-PAAuthorManuscriptNIH-P
tga1
19. Wang et al. 2005 Nature
1 2 3 4 5
6 7 8 9 10
Figure 1.
Phenotypes. a. Maize ear showing the cob (cb) exposed at top. b. Teosinte e
internode (in) and glume (gl) labeled. c. Teosinte ear from a plant with a m
introgressed into it. d. Close-up of a single teosinte fruitcase. e. Close-up o
teosinte plant with a maize allele of tga1 introgressed into it. f. Ear of maiz
(Tga1-maize allele) with the cob exposed showing the small white glumes a
of maize inbred W22:tga1 which carries the teosinte allele, showing enlarge
h. Ear of maize inbred W22 carrying the tga1-ems1 allele, showing enlarged g
NIH-PAAuthorManuscriptNIH-PAAuthorManuscriptNIH-P
tga1
22. 1 2 3 4 5
6 7 8 9 10
gt1 tga1
Wills et al. 2013 PLoS Genetics
5’ control region 3’ UTR
23. 1 2 3 4 5
6 7 8 9 10
tb1
Studer et al. 2011 Nat. Gen.; Vann et al. 2015 PeerJ
tga1gt1
24. 1 2 3 4 5
6 7 8 9 10
tb1
Studer et al. 2011 Nat. Gen.; Vann et al. 2015 PeerJ
tga1
GENETICS ADVANCE ONLINE PUBLICATION 3
nguish maize and teosinte. Both the maize and teosinte
s for the distal component repressed luciferase expression
luc
luc
luc
luc
luc
luc
Hopscotch
mpCaMV
M-dist
T-prox
M-prox
0 0.5 1.0 1.5 2.0
∆M-dist
∆M-prox
ProximalcontrolregionDistal
Constructs and corresponding normalized luciferase expression
nsient assays were performed in maize leaf protoplast. Each
is drawn to scale. The construct backbone consists of the
romoter from the cauliflower mosaic virus (mpCaMV, gray box),
ORF (luc, white box) and the nopaline synthase terminator
). Portions of the proximal and distal components of the
gion (hatched boxes) from maize and teosinte were cloned
ction sites upstream of the minimal promoter. “ ” denotes
on of either the Tourist or Hopscotch element from the maize
Horizontal green bars show the normalized mean with s.e.m.
onstruct.
relative expressionconstruct
gt1
25. 1 2 3 4 5
6 7 8 9 10
tb1
Figure 2 Map of parviglumis Populations and Hopscotch allele frequency. Map showing the frequency
of the Hopscotch allele in populations of parviglumis where we sampled more than 6 individuals. Size of
circles reflects number of individuals sampled. The Balsas River is shown, as the Balsas River Basin is
believed to be the center of domestication of maize.
as our independent trait for phenotyping analyses. SAS code used for analysis is available at
http://dx.doi.org/10.6084/m9.figshare.1166630.
RESULTS
Genotyping for the Hopscotch insertion
The genotype at the Hopscotch insertion was confirmed with two PCRs for 837 individuals
of the 1,100 screened (Table S1 and Table S2). Among the 247 maize landrace accessions
genotyped, all but eight were homozygous for the presence of the insertion Within
our parviglumis and mexicana samples we found the Hopscotch insertion segregating
in 37 (n = 86) and four (n = 17) populations, respectively, and at highest frequency
within populations in the states of Jalisco, Colima, and Michoac´an in central-western
Mexico (Fig. 2). Using our Hopscotch genotyping, we calculated diVerentiation between
populations (FST) and subspecies (FCT) for populations in which we sampled sixteen
or more chromosomes. We found that FCT = 0, and levels of FST among populations
within each subspecies (0.22) and among all populations (0.23) (Table 1) are similar to
genome-wide estimates from previous studies Pyh¨aj¨arvi, HuVord & Ross-Ibarra, 2013.
Although we found large variation in Hopscotch allele frequency among our populations,
BayEnv analysis did not indicate a correlation between the Hopscotch insertion and
environmental variables (all Bayes Factors < 1).
Studer et al. 2011 Nat. Gen.; Vann et al. 2015 PeerJ
tga1
GENETICS ADVANCE ONLINE PUBLICATION 3
nguish maize and teosinte. Both the maize and teosinte
s for the distal component repressed luciferase expression
luc
luc
luc
luc
luc
luc
Hopscotch
mpCaMV
M-dist
T-prox
M-prox
0 0.5 1.0 1.5 2.0
∆M-dist
∆M-prox
ProximalcontrolregionDistal
Constructs and corresponding normalized luciferase expression
nsient assays were performed in maize leaf protoplast. Each
is drawn to scale. The construct backbone consists of the
romoter from the cauliflower mosaic virus (mpCaMV, gray box),
ORF (luc, white box) and the nopaline synthase terminator
). Portions of the proximal and distal components of the
gion (hatched boxes) from maize and teosinte were cloned
ction sites upstream of the minimal promoter. “ ” denotes
on of either the Tourist or Hopscotch element from the maize
Horizontal green bars show the normalized mean with s.e.m.
onstruct.
relative expressionconstruct
gt1
26. hard sweep
M T N P H R L
GGTCGA ATG ACT GAT CCA CAT CGA CTG TAG
tga1
27. hard sweep
M T N P H R L
GGTCGA ATG ACT GAT CCA CAT CGA CTG TAG
tga1
gt1
tb1
Multiple
Mutations
Standing
Variation
M T G P H R L
GGTAAA ATG ACT GGT CCA CAT CGA CTG TAG
28. Vann et al. 2015 PeerJ
polygenic adaptation
30%
phenotypic
variance
0%
phenotypic
variance
29. Hufford et al. 2012 Nat. Gen.
Chia et al. 2012 Nat. Gen
13 teosinte
23 maize
~500 genes (2%)
11M shared SNPs
3,000 fixed
genomes:
30. Hufford et al. 2012 Nat. Gen.
Chia et al. 2012 Nat. Gen
13 teosinte
23 maize
genomes:
31. Swanson-Wagner et al. 2012 PNAS
E
Dom/Imp genes
(n=1761)
89
4644
1582
twork edges Maize network edges
D
GRMZM2G068436
GRMZM2G137947
GRMZM2G375302
Mb
Mb
s with altered expression or conservation and targets of selection during improvement and/or domestication. (A) Venn diagram
ween DE genes, AEC genes, and the genes that occur in genomic regions that have evidence for selective sweeps during maize
vement (Dom/Imp genes). (B) Teosinte coexpression networks for three genes (GRMZM2G068436, GRMZM2G137947, and
t) Edges that are maintained in maize coexpression networks are shown. Although the differentially expressed gene (red node) is
32. Beissinger et al. In Prep
nucleotidediversity
distance to nearest substitution (cM)
33. Beissinger et al. In Prep
nucleotidediversity
distance to nearest substitution (cM)
34. how to adapt: domestication
standing
variation
M T G P H R L
GGTAAA ATG ACT GGT CCA CAT CGA CTG TAG
polygenic adaptation
regulatory variation
teosinte
maize
36. Mexico highland6,000 BP
Mexico lowland
9,000 BP
Matsuoka et al. 2002; Piperno 2006
Perry et al. 2006; Piperno et al. 2009
37. Mexico highland6,000 BP
S. America
lowland
6,000 BP
Mexico lowland
9,000 BP
Matsuoka et al. 2002; Piperno 2006
Perry et al. 2006; Piperno et al. 2009
38. Mexico highland6,000 BP
S. America
lowland
6,000 BP
S. America
Highland
4,000 BP
Mexico lowland
9,000 BP
Matsuoka et al. 2002; Piperno 2006
Perry et al. 2006; Piperno et al. 2009
41. SA MEX SA MEX
SA MEX SA MEX SA MEX SA MEX
Ear Height Plant Height
Tassel Br. Number
TW
Days to Anthesis
SA MEX SA MEX
SA MEX SA MEX
LowlandHighland
42. differences between lowland and highland maize in terms of
heterozygosity and differentiation from parviglumis (Fig. S3).
Structure analysis (21) of all Mexican accessions lends support
for this magnitude of introgression (Fig. 2). The three subspecies
form clearly separated clusters, but evidence of admixture is
cluding mexicana, in which Mexican Highland maize is tied wit
the West Mexico group as the most ancestral population (Fig. 3B
To mitigate the impact of introgression, we used a slight
modified approach that excludes both parviglumis and mexican
and calculates genetic drift with respect to ancestral frequencie
inferred from domesticated maize alone. Because the genet
Fig. 1. (A) Map of sampled maize accessions colored by genetic group. (B) First three genetic PCs of all sampled accessions.
van Heerwaarden et al. PNAS | January 18, 2011 | vol. 108 | no. 3 | 108
van Heerwaarden et al. 2011 PNAS
43. differences between lowland and highland maize in terms of
heterozygosity and differentiation from parviglumis (Fig. S3).
Structure analysis (21) of all Mexican accessions lends support
for this magnitude of introgression (Fig. 2). The three subspecies
form clearly separated clusters, but evidence of admixture is
cluding mexicana, in which Mexican Highland maize is tied wit
the West Mexico group as the most ancestral population (Fig. 3B
To mitigate the impact of introgression, we used a slight
modified approach that excludes both parviglumis and mexican
and calculates genetic drift with respect to ancestral frequencie
inferred from domesticated maize alone. Because the genet
Fig. 1. (A) Map of sampled maize accessions colored by genetic group. (B) First three genetic PCs of all sampled accessions.
van Heerwaarden et al. PNAS | January 18, 2011 | vol. 108 | no. 3 | 108
van Heerwaarden et al. 2011 PNAS
48. Beissinger et al. In Prep Berg & Coop 2014 PLoS Genetics
Z =
LX
i=1
↵ipi
allele freq.population
breeding value
effect size
49. Beissinger et al. In Prep Berg & Coop 2014 PLoS Genetics
Z =
LX
i=1
↵ipi
allele freq.population
breeding value
effect size
relatednessdispersion
add. genetic var.
QX =
~Z0
T
F 1 ~Z0
2VA
53. domesticationhow to adapt: ——————————-
standing
variation
M T G P H R L
GGTAAA ATG ACT GGT CCA CAT CGA CTG TAG
polygenic adaptation
regulatory variation
local adaptation
57. Piperno 2006; Perry et al. 2006; Piperno et al. 2009
Mexico highland6,000 BP
domestication in
Mexico lowland
9,000 BP
Photo by Pesach Lubinsky
mexicanamaize
60. El Porvenir
Opopeo
Xochimilco
Puruandiro
Tenango del Aire
Ixtlan
Nabogame
Santa Clara
San Pedro
Allopatric
El Porvenir
Opopeo
Xochimilco
Puruandiro
Tenango del Aire
Ixtlan
Nabogame
Santa Clara
San Pedro
Allopatric
Hufford et al. 2013 PLoS Genetics
61. El Porvenir
Opopeo
Xochimilco
Puruandiro
Tenango del Aire
Ixtlan
Nabogame
Santa Clara
San Pedro
Allopatric
Inv4n
El Porvenir
Opopeo
Xochimilco
Puruandiro
Tenango del Aire
Ixtlan
Nabogame
Santa Clara
San Pedro
Allopatric
Hufford et al. 2013 PLoS Genetics
62. El Porvenir
Opopeo
Xochimilco
Puruandiro
Tenango del Aire
Ixtlan
Nabogame
Santa Clara
San Pedro
Allopatric
Inv4n
El Porvenir
Opopeo
Xochimilco
Puruandiro
Tenango del Aire
Ixtlan
Nabogame
Santa Clara
San Pedro
Allopatric
Hufford et al. 2013 PLoS Genetics
64. Lauter et al. 2004 Genetics
Hufford et al. 2013 PLoS Genetics
65. Lauter et al. 2004 Genetics
Inv4n
b1
Moose et al. 2004 Genetics
photobyEdCoe
mhl1
Hufford et al. 2013 PLoS Genetics
66. Lauter et al. 2004 Genetics
Inv4n
b1
Moose et al. 2004 Genetics
photobyEdCoe
mhl1
Hufford et al. 2013 PLoS Genetics
0 50 100 150 200 250 300
0.00.6
Chromosome 1
bp
proportionofpopulations
0
0.00.6
proportionofpopulations
Mb
B
gt1 tb1bif2
resistance
67.
68.
69.
70.
71. Sattah et al. 2011 PLoS Gen.
Williamson et al. 2014 PLoS Gen
Hernandez et al. 2011 Science
72. Sattah et al. 2011 PLoS Gen.
Williamson et al. 2014 PLoS Gen
Hernandez et al. 2011 Science
73. Sattah et al. 2011 PLoS Gen.
Williamson et al. 2014 PLoS Gen
Hernandez et al. 2011 Science
distance to nearest substitution (cM)
scaleddiversity
74. • Ne diploids
• µ beneficial mutations rate per haploid genome
• selection from standing variation when 2Neµ < 1
Messer and Petrov 2013 TIG
75. Sattah et al. 2011 PLoS Gen.
Williamson et al. 2014 PLoS Gen
Hernandez et al. 2011 Science
distance to nearest substitution (cM)
scaleddiversity
2Neµ > 1
2Neµ < 1
76. Sattah et al. 2011 PLoS Gen.
Williamson et al. 2014 PLoS Gen
Hernandez et al. 2011 Science
distance to nearest substitution (cM)
scaleddiversity
Ne ~ 150,000 Ne ~ 10,000*
Ne ~ 2,000,000 Ne ~ 600,000
2Neµ > 1
2Neµ < 1
78. M T G P H R L
ATG ACT GGT CCA CAT CGA CTG TAG
M T N P H R L
ATG ACT GAT CCA CAT CGA CTG TAG
79. M T G P H R L
ATG ACT GGT CCA CAT CGA CTG TAG
M T N P H R L
ATG ACT GAT CCA CAT CGA CTG TAG
80. M T G P H R L
ATG ACT GGT CCA CAT CGA CTG TAG
M T N P H R L
ATG ACT GAT CCA CAT CGA CTG TAG
x x x
81. M T G P H R L
ATG ACT GGT CCA CAT CGA CTG TAG
M T N P H R L
ATG ACT GAT CCA CAT CGA CTG TAG
x xx x
82. M T G P H R L
ATG ACT GGT CCA CAT CGA CTG TAG
M T N P H R L
ATG ACT GAT CCA CAT CGA CTG TAG
x xx x
83. Sattah et al. 2011 PLoS Gen.
Williamson et al. 2014 PLoS Gen
Hernandez et al. 2011 Science
distance to nearest substitution (cM)
scaleddiversity
Ne ~ 150,000 Ne ~ 10,000*
Ne ~ 2,000,000 Ne ~ 600,000
2Neµ > 1
2Neµ < 1
84. Sattah et al. 2011 PLoS Gen.
Williamson et al. 2014 PLoS Gen
Hernandez et al. 2011 Science
distance to nearest substitution (cM)
scaleddiversity
Ne ~ 150,000 Ne ~ 10,000*
Ne ~ 2,000,000 Ne ~ 600,000
µ ∝ 130 x 106bp µ ∝ 220 x 106bp
µ ∝ 2,500 x 106bp µ ∝ 3,100 x 106bp
2Neµ > 1
2Neµ < 1
95. Rayburn et al. 1994 Plant Breeding
Francis et al. 2008. Ann. Bot.
excluded. Indeed, if we ignore the marked dis
of the y-axis caused by their inclusion, then the n
effect is strong for all species regardless of phyl
test the rigour of these hypotheses would requi
plug the gap between Trillium grandiflorum
majority of C-value/cell cycle times analysed he
Separate plots for diploids and polyploids show
nucleotypic effect on CCT in diploids (Fig. 3;
Removing the five diploid outliers (.25 pg) re
slope (b ¼ 0.27) by approximately four-fold
regression continued to be significant (P , 0.
the polyploids, a nucleotypic effect on CCT
detected (Fig. 3; Table 2); however, removing the
ploid outliers rendered the regression non-signifi
0.03x 2 13.5). This confirms previous work in
slope/rate of increase in CCT with increasing
higher in diploids than in autopolyploids (Eva
1972). With the exception of Scilla sibirica, CC
FIG. 3. DNA C-value (pg) and cell cycle time (h) in the roo
istem of a range of diploid and polyploid angiosperms. See
regression analyses.
2. DNA C-value (pg) and cell cycle time (h) in the root apical mer-
m of a range of (A) eudicots and monocots (n ¼ 110), and (B) eudicots
(n ¼ 60). See Table 2 for regression analyses.
LE 2. Regression analyses of all data presented in
s. 2–4 together with the percentage variance accounted
by the regression (R2
), the level of probability (P) for
each regression
Francis et al. 2008.Ann. Bot.
0
10
20
30
100 105 110
DNA
plants
cycle
0
6
late flowering
early flowering
96. • “Soft sweeps” and polygenic selection predominate in
maize
• Gene flow may provide a novel source of adaptive alleles
• Both population size and mutational target contribute
• Large, complex genomes may mean more targets and
more soft sweeps in plants
• Genome size itself may be adaptive
Concluding Thoughts
97. Acknowledgments
Maize Diversity Group
Peter Bradbury
Ed Buckler
John Doebley
Theresa Fulton
Sherry Flint-Garcia
Jim Holland
Sharon Mitchell
Qi Sun
Doreen Ware
Collaborators
Jim Birchler
Jeremy Berg
Graham Coop
Nathan Springer
Lab Alumni
Tim Beissinger (USDA-ARS, Mizzou)
Matt Hufford (Iowa State)
Tanja Pyhäjärvi (Oulu)
Shohei Takuno (Sokendai)
Joost van Heerwaarden (Wageningen)