Origin, Evolution and Cytogenetics of Maize

1. ORIGIN, EVOLUTION
AND
CYTOGENETICS OF MAIZE

2. INTRODUCTION ORIGIN
EVOLUTION CYTOGENETICS
CONCLUSION

3. INTRODUCTION
Maize (Zea mays L.) is a cereal crop which is cultivated widely throughout
the world.
Countries
Production in
thousand
metric tons
United States 370,960
China 215,891
Brazil 82,000
European Union 62,277
Argentina 32,000
India 28,720
Mexico 27,450
Ukraine 24,115
Canada 14,100
South Africa 13,525
https://www.statista.com/search/?q=maize

4. • d
https://www.nsf.gov/maize

5. TAXONOMY OF MAIZE
Kingdom Plantae
Sub kingdom Tracheobionta Vascular plant
Super division Spermatophyta Seed plant
Division Mangoliophyta Flowering plant
Class Liliopsida Monocotyledon
Subclass Commelinidae
Order Cyperales
Family Poaceae Grass family
Genus Zea L.
Species Zea mays. L

6. INTRODUCTION ORIGIN
EVOLUTION CYTOGENETICS
CONCLUSION

7. ORIGIN
• South central Mexico
• Archeological remains of the earliest maize cob, found at Guila Naquitz
in the Oaxaca Valley of Mexico (6250 years ago).

8. Contd..,
• Microfossil evidence suggesting
dispersal by 7,000 – 5,000 BP
• Estimation of domestication of
maize: 12,000-6,000 BP
• According to Vavilov maize was
originated in Mexico
Yam Kaax
The Mayan corn god

9. INTRODUCTION ORIGIN
EVOLUTION CYTOGENETICS
CONCLUSION

10. EVOLUTION OF MAIZE
Teosinte
Tripsacum

11. Contd..,
• Tripsacum : Tripsacum dactyloides
• Eastern gamagrass
• n=9, 2n=18,36,72
• It is grassy type
• Teosinte : Z. mays parviglumis
• Balsas teosinte
• n=10, 2n=20
• Similar in morphology but
branching type

12. Origin of Maize :Three Hypothesis
• Tripartite hypothesis
• Teosinte hypothesis
• Recombination hypothesis

13. Tripartite hypothesis

14. Contd..,
• The oldest remains of Zea mays are fossil pollen grains isolated from the Bellas Artes
drill core taken from below Mexico City at a depth of between 69 and 72 m. Related
Tripsacum pollens are found deeper than them.
(Barghooran et al ., 1954)
• They pointed to their own successful cross of maize and Tripsacum as validation for their
hypothesis.
• They were able to produce a few, largely sterile maize– Tripsacum hybrids.
• They also analyzed backcross populations of maize–teosinte hybrids and were able to
identify factors responsible for the morphological differences between maize and
teosinte.

15. • The earliest maize cobs uncovered in Tehuacan belonged to a wild maize. These
cobs are between 19 and 25 mm long, bisexual with a lower female part and apical
male spike, and correspond to a hypothetical wild maize reconstructed by
Mangelsdorf (1958).
• Counter arguments:
a) Differences in chromosome number and
constitution between Zea (10 chromosomes)
and Tripsacum (18 or 36 chromosomes).
b) Basing their assumption on a “missing wild maize”
Contd..,

16. Teosinte hypothesis
As stated by Beadle (1939)
• Ancient people cultivated teosinte because it provided a useful
food source
• During cultivation, mutations that improved teosinte’s usefulness to
humans arose and were selected.
• As few as 5 major mutations would be sufficient to convert teosinte
in a primitive form of maize.
• Over the course of time, humans selected additional major
mutations plus many minor ones

17. • The summary phylogeny for the genus Zea, based on chromosomal number and
morphology (Kato Y., 1976; Kato Y. and Lopez R., 1990)
• Chloroplast (Doebley et al., 1987)
• Ribosomal (Buckler and Holtsford, 1996)
• Isozyme (Doebleyet al., 1984)
• Simple sequence repeat
(Matsuoka et al., 2002)
Contd..,

18. INTRODUCTION ORIGIN
EVOLUTION CYTOGENETICS
CONCLUSION

19. Cytogenetics and evidence to evolution
• Chromosome number of cultivated maize 2n=2x=20
• Chromosome number of teosinte (Z. diploperennis) 2n=2x=20
• Z. luxurians, has chromosomes that are cytologically distinct from
those of maize, and that maize-Z. luxurians hybrids exhibit two or
more unpaired chromosomes during metaphase and partially sterile.
• Z. mays ssp. mexicana, has chromosomes that are cytologically
similar to those of maize, and its hybrids with maize exhibit complete
chromosomal pairing and full fertility.

20. Molecular Evidence
• Isozyme allele frequencies of teosintes such as Z. luxurians, Z.
diploperennis, and Z. perennis (the latter two perennials) are strongly
differentiated from those of maize.
• Allele frequencies of one Mexican annual teosinte, Z. mays ssp.
mexicana, are more maize-like, although still distinct from maize.
• Allele frequencies of another Mexican annual teosinte, Z. mays ssp.
parviglumis or Balsas teosinte, are essentially indistinguishable from
those of maize.

21. Recombination hypothesis
(Tripsacum – Z. diploperennis Hypothesis )

22. • RFLP molecular analysis for these hybrids were done.
• Overlapping regions of the Venn diagrams correspond to the number of shared bands between parent
and putative offspring, whereas the numbers that appear in a single circle represent unique RFLP bands.

23. • The genera Zea and Tripsacum cross readily when they are not isolated by
gametophytic barriers, and it has been postulated that intergeneric introgression
played a role in the evolution of maize.
• The basic x = 9 Tripsacum and x = 10 Zea genomes have little cytological affinity
for each other in hybrids that combine 10 Zea with 18 Tripsacum chromosome.

24. Contd..,
• One to four Tripsacum chromosomes sometimes associate with Zea chromosomes
in hybrids between Zea mays (2n=20) and T. dactyloides (2n=72).
T. dactyloides x Zea mays
2n=72 2n= 20
2n=46 ( 10 Zm + 36 Td )
• These hybrids with10 Zea and 36 Tripsacum chromasomes frequently produce
functional female gametes with 36 Tripsacum chromosomes only.

25. • In these individuals, intergenome pairing is the rule, and when they are pollinated
with maize, their offspring have 36 Tripsacum and 10, 12, 14, 16, 18 or 20 Zea
chromosomes.
• Successive backcrosses with maize selectively eliminate Tripsacum chromosomes,
and eventually plants with 2n = 20 Zea chromosomes are recovered.
• Attempts by Mangelsdorf and Reeves (1939) to transfer Tripsacum genes to
modern maize met with little success.
Contd..,

26. • The hybrids between tetraploid Tripsacum 2n= 36 and maize 2n=20 were female
sterile and when backcrossed with maize, the cytologically unreduced female
gamete functioned sexually to produce offspring with 20 maize and 18 Tripsacum
chromosomes.
• Hybrids between octaploid T. dactyloides (2n = 72) and maize, produced by were
completely sterile
• The Tripsacum chromosomes in most hybrids with 36 Td and 10 Zm
chromosomes associate into bivalents during meiotic prophase, with the Zea mays
chromosomes present as univalents.
Contd..,

27. • Hybrids combining 10 Zea mays (Zm) chromosomes with 18, 36 and 72 T.
dactyloides (Td) chromosomes, and hybrids combining 20 Zm with 36 Td
chromosomes were studied cytogenetically (Dewet et al. 1972a, Dewet et al.
1972b).
• All hybrids resemble Tripsacum more than Zea with respect to gross inflorescence
morphology, no matter how many haploid Zea and Tripsacum genomes are
involved ( Newell and Dewet, 1 973).
• Plants with 36 Tripsacum and 20 Zea chromosomes behave cytologically as
alloploids, although the Tripsacum genome is contaminated with maize, and one
basic maize genome is contaminated with Tripsacum genetic material.
Contd..,

28. INTRODUCTION ORIGIN
EVOLUTION CYTOGENETICS
CONCLUSION

29. CONCLUSION