This ppt describes in brief how maize evolved from its ancestor teosinte

1. Evolutionary Significance
of tb1 locus in Maize
Aditi Sarkar
L-2019-A-82-M
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2. Evolution and Domestication
Evolution
•Change in heritable
characteristics of biological
populations over time
Domestication
•Process of human
interference to direct crop
evolution
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3. Stages of Domestication and Diversification
Onset of domestication
Increase in frequency of desirable alleles
Formation of cultivated populations that are
adapted to new environments and local
preferences
Deliberate breeding
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4. Domestication
Syndrome
Properties that
distinguish a certain crop
from its wild progenitor
• Larger fruits or grains
• Increase in apical
dominance
• Reduced shattering
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5. Maize (Zea mays L.)
3rd staple food crop
Poaceae family
Genus- Zea, group of annual and
perennial grasses
Monoecious, protandrous
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6. Geographical origin of maize
• According to Vavilov, origin of maize
is South Central Mexico
• It was supposed to be first
domesticated in the Tehuaccan valley
of Mexico and the original wild form
has been extinct since many years
• Archaeological remains of earliest
maize cob, found at Guila Naquitz
Cave in Oxaca valley of Mexico
(6250 years ago).
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7. Evolution of Maize: Three Hypothesis
Tripartite Hypothesis
Recombination Hypothesis
Teosinte Hypothesis
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9. • Argument
Differences in
chromosome
number and
genetic
constitution
between Zea
and Tripsacum
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10. Recombination Hypothesis (Eubanks,1995)
Tripsacum
dactyloides
Zea
diploperennis
Zea
mays
Putative Hybrids
Tripsacorn and
Sundance
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11. Teosinte Hypothesis
(Beadle 1939)
• Teosinte cultivated by ancient
people.
• Mutations arose and were selected
• 5 major mutations
• Over the course of time, additional
major plus minor mutations
selected
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12. What is teosinte?
• Teosinte is a wild grass in the
Poaceae family that includes
the species Zea mays (in
addition to other wild grass
species: Z.diploperennis, Z.
perennis, and Z. luxurians)
• Z. mays encompasses several
subspecies among which Zea
mays ssp. parviglumis and ssp.
mexicana share a close genetic
relationship with maize.
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13. Why teosinte and not tripsacum?
• Teosinte is having same chromosome number and nearly similar
genetic constitution as that of maize
• Similar in morphology to maize except that its branching type
• Tripsacum differs in both chromosome number and genetic
constitution.
• It is grassy type
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14. Difference between
maize and teosinte
• Difference in
plant architecture
• Difference in
inflorescence
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16. Evolution and Domestication of Maize
• The origin of maize was a mystery
• George Beadle suggested that teosinte is the ancestor of maize when
he observed that teosinte and maize have nearly identical
chromosomes
• But many botanists doubted Dr. Beadle’s conclusion because maize
and teosinte have many physical differences.
• Molecular analyses identified one form of teosinte (Zea mays ssp.
parviglumis) as the progenitor of maize
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17. Contd..,
• This teosinte grows commonly as a wild plant in the valleys of Southern Mexico
• It is also know as Balsas teosinte
• The domestication of crop plants has often involved an increase in apical
dominance
• Maize exhibits a striking example of this phenomenon, there is a profound
increase in apical dominance compared with teosinte.
• Differences in the morphologies of maize and teosinte were the result of human
selection during the domestication process
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18. Cytogenetics and molecular evidence to
evolution
Chromosome number of cultivated maize (Zea mays ssp. mays)
2n = 2x = 20
Chromosome number of teosinte (Zea mays ssp. mexicana)
2n = 2x = 20
Chromosome number of tripsacum (Tripsacum. dactyloides)
2n = 4x = 36
• Mexicana- complete chromosomal pairing and full fertility
• Parviglumis- similar isozyme allelic constitution
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19. Genetic evidence of maize domestication
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Experiment conducted by Dr Beadle in
the 1970s
Teosinte × Maize F1 F2
Prediction of number of genes

20. Genetic evidence of maize domestication
(Contd..)
• From these data and his mathematical model, he
concluded that four or five genes were responsible for
the differences between teosinte and maize
• In the 1990s, Dr. Doebley and Dr. Stec identified five
genetic regions, that together account for most of the
variation between maize and teosinte, further
supporting Dr. Beadle’s hypothesis.
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21. Key domestication genes cloned in maize
Gene Phenotype
tb1 Plant architecture
gt1 Plant architecture
tru1 Plant architecture
tga1 Hardened fruit case
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22. Key domestication genes cloned in maize (Contd..)
Gene Phenotype
ZmSWEET4c Seed filling
UB3 Kernel row number
ZmSh1-1 Shattering
ra1 Inflorescence architecture
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23. Teosinte glume architecture (tga1) and teosinte branched one (tb1)
• regulatory genes
• transformation (Teosinte  maize)
• tga1  expression of the traits associated with the seeds.
• tb1  expression of the traits associated with branching and
inflorescence.
• Difference in expression pattern between teosinte and maize
version
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25. Teosinte branched 1 (tb1)
was identified as a major QTL involved in apical dominance
•is a member of the TCP family of transcriptional regulators
•rice homolog of tb1- OsTB1
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26. Contd..,
tb1 also targets other domestication loci in maize
involved in regulation of some phytohormones
also modulates FT1 activity
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27. Contd..,
tbl-ref - recessive, null or loss of function mutant that
produces plants with long lateral branches tipped by tassels
at upper nodes and tillers at basal nodes
Tb1+maize - dominant maize allele, produces short
lateral branches tipped by ears at upper nodes and few or
no tillers at basal nodes
tb1+ teosinte – weak function, a partial teosinte
branched phenotype
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28. A Model for tb1 (Doebley et al 1995)
• Plants respond to
environmental conditions by
altering their architecture.
• Teosinte also exhibits a
plastic response to local
environment
• tb1 is involved in regulating
the plasticity response
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29. A Model for tb1 (Contd..,)
Favorable environmental conditions
Tb1+ teosinte turned off
Axillary meristems develop
fully into tillers or long lateral
branches tipped by tassels
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30. A Model for tb1 (Contd..)
Unfavorable environmental conditions
Tb1+ teosinte turned on
Few or no tillers and only
short lateral branches
tipped by ears
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31. The Role of tb1 in Maize Evolution
In teosinte, tb1 should be off or expressed at low levels in the
primordia that form the primary branches
tb1 should also be off (or at low levels) in the inflorescence
primordia, so that tassels would be formed rather than an ear
Thus, the evolution of maize required an increase in tb1
expression
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32. Data supporting the tb1 model
Maize allele of tb1 is expressed at about twice the level of teosinte allele in
immature axillary branches and the inflorescence primordia
In-situ hybridization in teosinte showed no sign of tb1 expression
in axillary buds, where maize shows strong expression
No fixed amino acid differences between maize and teosinte
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33. Quantification of tb1 mRNA levels
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34. Conclusion
• Present races of maize was achieved through selection for
overexpression of the teosinte branched1 (tb1) transcription factor.
• Based on the study of genomic changes during maize domestication, it
was found that many traits are affected by limited number of genes.
• Thus, it is possible that we could improve specific maize traits by
manipulating only a handful of genes to meet modern demands of
diverse maize types.
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35. THANK YOU
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