Maize evolution

1. “THE GENETICS OF MAIZE EVOLUTION”
MARUTHI PRASAD B. P.
PAMB-1066
Department of Genetics and Plant Breeding
1

2. INTRODUCTION
 Maize is an important cereal and staple food crop of the world.
 Chromosome number: 2n=2x=20
 Genome size: 2.3 gigabase
 C4 photosynthetic plant
 Photo-insensitive crop with high adaptability
 Vital source of proteins and calories to billions of people.
 A source of important vitamins and minerals to the human body.
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3. 3
Taxonomy of Maize
Kingdom Plantae (plants)
Subkingdom Tracheobionta (vascular plants)
Superdivision Spermatophyta (seed plants)
Division Magnoliophyta (flowering plants)
Class Liliopsida (monocotyledons)
Subclass Commelinidae
Order Cyperales
Family Poaceae (grass family)
Genus Zea
Species Zea mays ssp. mays

4. UTILIZATION OF MAIZE IN THE WORLD AND IN INDIA
Source: https://www.researchgate.net/profile/Shankar_Jat/publication/260094182/figure/fig12/AS:668222319767574@1536328024791/Current-utilization-pattern-of-maize-for-
different-purposes-in-India-and-in-Global-Maize.jpg
4

5. Area (m ha) Production (m t) Productivity (t ha-1)
Globally 193.7 1147.7 5.75
India 9.2 27.8 2.96
Karnataka 1.3 4.4 2.77
AREA, PRODUCTION AND PRODUCTIVITY OF MAIZE
Source: FAOSTAT, 2020,
https://iimr.icar.gov.in/,
Anon.,2016.
Source: https://iimr.icar.gov.in/wp-content/uploads/2020/05/img15-
1.jpg
5

6. Where does
the corn
comes
from?
6

7. The term ‘maize’ is derived from
the word ‘mahiz’ of Taino
language of the Caribbean islands,
which became ‘maiz’ in Spanish
(Oxford dictionary 2015 ).
Origin: Mexico and Central
America
Origin of Maize
7

10. • Unlike most crops,
maize does not have a
morphologically
similar wild progenitor.
• Particularly, maize has
no wild relative having
a cob-like pistillate
inflorescence (ear).
10

12. 12
Tripsacum
• Tripsacum dactyloides
• Common name: Eastern
gamagrass
• Chr. No.: x=18, 2n=36,72
• It is a grassy type
Teosinte
• Zea mays spp. parviglumis
• Common name: Balsas teosinte
• Chr. No.: x=10, 2n=20
• Morphology is similar to maize
but branchy type.

13. Theories of origin of maize
13
1. Tripartite hypothesis
2. Catastrophic sexual transmutation theory
3. Tripsacum-Zea diploperennis hypothesis
4. Teosinte hypothesis

14. Proposed by Mangelsdorf and Reeves (1938, 1939), and later elaborated by Mangelsdorf
(1974).
14
Mangelsdorf and Reeves, 1938

15. 15
States that, “Maize was domesticated from some unknown wild,
now extinct maize plant that had structures similar to the ear of
modern maize”. Mangelsdorf and Reeves, 1938
Tripsacum
TEOSINTE
MAIZE
Unknown wild maize from South America
(extinct or undiscovered)

16. The hypothesis comprised three parts;
1. The progenitor of maize was a wild maize prototype from
South America, which has become extinct or remained
undiscovered.
2. Teosinte is the offspring of a cross between maize and
Tripsacum.
3. Sections of Tripsacum chromosomes had contaminated maize
germplasm.
16
Mangelsdorf and Reeves, 1938

17. 17
Counter arguments for tripartite hypothesis (teosinte is the intermediate
between Tripsacum and corn)
1. Corn and Tripsacum have never been known to cross naturally, in spite of
the fact the they grow in close proximity over millions of acres. Man-made
crosses can be accomplished only with special techniques.
2. None of the 18 chromosomes of Tripsacum pair normally with any of the
10 chromosomes of corn.
3. The man-made crosses of corn and Tripsacum are completely male sterile.

18. 18
Iltis ( 1983 ) proposed that maize was originated due to a sudden sexual
transmutation that condensed the branches of teosinte and placed them in the
female expression area of the plant.
Iltis, 1983

19. 19
• It states that the ear of maize was derived from the central spike of the tassel of
teosinte.
• According to Iltis, this has happened due to a phenomenon known as ‘genetic
assimilation’. This resulted in substantial alterations in the nutrient distribution of
the plant and led to drastic morphological changes.
• Morphogenetic and structural imbalance possibly had led to the transformation into
primitive maize.
• During the late 1980s, teosinte hypothesis started gaining importance and the
catastrophic sexual transmutation theory became less convincing.
Iltis, 1983

20. 20
Tripsacum- Z. diploperennis hypothesis can be considered as a modern
version of the tripartite hypothesis and was given by Eubanks ( 1995 ).
Eubanks, 1995

21. 21
Eubanks, 1995
GAMA GRASS (Tripsacum sp.) TEOSINTE (Zea mays sp.)
MAIZE (Zea mays subsp. mays)
The Recombination Hypothesis

22. 22
Eubanks, 1995

23. 23
Counter arguments for recombinantion hypothesis
• Tripsacum and Z. diploperennis can not be hybridized successfully. The
chromosome number of both ‘Tripsacorn’ and ‘Sundance’ is 2n = 20. These
hybrids would be expected to have 28 or 46 chromosomes if Tripsacum (2n = 36
or 72) had indeed been one of the parents.
• Of the polymorphisms identified by RFLP data, ‘Tripsacorn’ and ‘Sundance’
shared four times as many bands with Z. diploperennis than with Tripsacum,
indicating a much closer relationship with teosinte than with Tripsacum .
• Besides, 23% of the molecular markers surveyed were not found in either of the
parents.
Eubanks, 1995

24. 24
• Proposed by George Beadle (1939)
• States that teosinte is the sole progenitor of maize.
• Beadle believed that missing ancestor is not needed to explain the origin. He
could obtain completely fertile hybrids between maize and teosinte.
Beadle, 1939

25. 25
Highlights
1. Teosinte provided a useful food source and ancient people cultivated it.
2. During the cultivation of teosinte, mutations that improved teosinte’s usefulness
to humans arose and were selected by people.
3. As few as five major mutations would be sufficient to convert teosinte into a
primitive form of maize.
4. Different mutations controlled different traits, viz., one mutation would have
converted the disarticulating ear-type of teosinte into the solid ear type of
maize.
5. Over the period of time, humans selected additional major mutations coupled
with many minor ones.
Beadle, 1939

26. 26
• Teosinte was placed under the genus Euchlaena.
• Beadle studied cytology and genetics of corn-teosinte crosses. He confirmed
the fertility of the cross and showed that the 10 chromosomes in the cell of
teosinte were highly compatible with 10 chromosomes of corn.
• The chromosomes paired normally during the formation of sex cells in the
crossed plants.
• He concluded that cytologically and genetically corn and Mexican teosinte
could even be considered as same species.
Beadle, 1980

27. 27
a) Teosinte plant architecture is branched, with multiple ears per plant.
b) Maize architecture is apically dominant, with side branches tipped by female inflorescence (ears)
Teosinte v/s Maize

28. 28
Teosinte v/s Maize
(Hossain et al., 2016)
1. Teosinte plants are
branched and produce many
ears
2. Terminal position of
primary branch bears a
tassel
3. The leaves along the
lateral branches are fully
formed and composed of
leaf blade and sheath
1. Maize plants produce a
single upright stem with one
or few ears
2. Primary branch is modified
into ears
3. Leaves of the lateral branch
are modified into husks which
cover the ear
4. Secondary lateral branches
are extremely rare
4. Secondary lateral branch
is modified into ears

29. 29
5. Ears are covered loosely
by a single or few husks
6. Each ear possesses only
two kernel rows (distichous)
5. Ears are covered tightly by
many husks
6.Each ear possesses about 8–
22 kernel rows (polystichous)
7. Ear bear about 250–500
kernels
8. Each kernel is sealed
tightly in a stony casing or
fruit case
8. Each kernel is naked and
not covered by any fruit case
Teosinte v/s Maize
7. Ear possesses about 10–12
kernels
(Hossain et al., 2016)

30. 30
9. During development, out
of two spikelets one is
aborted, hence each fruit
case holds a single-spikelet
10. At maturity, fruit case
having the kernel shatter
and become the dispersal
units
9. Maize evolution
involved the de-repression
of the second spikelet
primordium, hence there
are two mature spikelets
10. At maturity, kernels do
not shatter, and remain
attached with ears
11. Seeds of maize do not
possess dormancy
Teosinte v/s Maize
11. Majority of teosintes
possess varying degree of
seed dormancy
(Hossain et al., 2016)

33. 33
https://i.pinimg.com/originals/b0/43/94/b04394965852d49c56d5f935e635d781.jpg

39. 39
Zea mays ssp. parviglumis

40. 40
Classification of genus Zea (include wild taxa, known as
teosinte and domesticated corn)
Phylogeny of the genus Zea Buckler and Stevens, 2006

45. 45
• In modern form of teosinte hypothesis, Z. mays ssp. parviglumis (wild Mexican
grass teosinte) has been pinpointed as the likely progenitor of maize.
• Further, maize arose through large changes in parviglumis– through artificial
selection for specific traits.
• Most maize geneticists and evolutionists have now accepted that maize is a
domesticated derivative of parviglumis.
Beadle, 1980

46. 46
1. Studies on chromosome number and morphology
• Most Zea species and subspecies, including maize, have 10 chromosomes with the
sole exception of Z. perennis, which has 20—clearly an example of a complete,
duplicated set of chromosomes. On the other hand most Tripsacum species have
either 18 or 32 chromosomes.
• Study of chromosome morphology among teosinte plants, Focusing on
chromosomal knobs, revealed that certain grasses such as Tripsacum and several
Zea species had terminal knobs only, whereas others, including three subspecies of
Zea mays, displayed interstitial knobs.
(Kato T. A., 1984; McClintock et al., 1981)
Evidences supporting teosinte hypothesis

47. 47
2. Iso-enzymne studies: Zea can be divided into 2 major groups:
1. Sect. luxuriants, including Z. perennis, Z. diploperenneis, and Z. luxurians.
2. Sect. Zea, including Z. mays subsp. mays, subsp. parviglumis, and subsp.
Mexicana.
• Zea mays var. huehuetenangenesis is iso-enzymatically distinct from both sections,
but show its closest relationship to Z. mays var. parviglumis of sect. Zea.
• Population of Z. mays subsp. mexicana and var. parviglumis grade iso-
enzymatically from one into other without any clear break, but without any overlap
either.
Doebley et al., 1984

48. 48
• Five population of Z. mays subsp mays are all iso-enzymatically very similar to
Zea mays var parviglumis.
• The iso-enzyme data are consistent with the theory that Mexican annual teosinte
is the ancestor of maize.
• The levels of variation within and among population of Zea taxa varies
considerably.
• Zea taxa seems to have more variation than most other plant species for which
iso-enzyme data are available
Doebley et al., 1984

50. Methods
To study the meiotic behaviour of the Zea perennis Zea mays ssp
Analyzing meiotic configurations in the hybrid - genomic source of each
chromosome
GISH and FISH – To established the genomic affinities between the parental
species
Materials
Plant material
Parents - Zea mays ssp. mays (race Amarillo Chico)
Zea perennis
F1 Hybrid- Zea mays ssp. mays x Zea perennis

51. Cytological analysis
Panicles from Zea mays ssp. mays, Zea perennis and their F1 hybrids were fixed
in 3:1 (absolute alcohol:acetic acid) solution
The pairing configurations were determined at diakinesis-metaphase I
GISH and FISH
Genomic DNA probes were isolated from adult leaves of Zea mays ssp. mays and
Zea perennis
The pTa 71 plasmid, containing the 18S-5.8S-25S ribosomal sequences from
Triticum aestivum (Gerlach & Bedbrook 1979), was used as a probe

52. Results
Meiotic behaviour
Zea mays ssp. mays (2n = 20,
genomic formula AmAmBmBm)
shows regular meiosis, forming
10 bivalents (II) in metaphase I

53. Zea perennis (2n = 40,
ApApA¶pA¶p Bp1Bp1Bp2Bp2) is an
amphioctoploid showing a IV
(tetravalent) range from 2 to 6
The most frequent configuration being
5 IV + 10 II

54. The hybrid between Zea perennis and Zea mays ssp. mays (2n = 30,
ApA¶pAmBmBp1Bp2)
Five trivalents (III) + five bivalents (II) + five univalents (I) as the most
frequent configuration
The trivalents have the Frying pan shape and the bivalents are
homomorphic

56.  The association of homologous or homoeologous chromosomes during meiosis
reveals the relative affinities between the parental genomes of the hybrids and
polyploid species.
 These meiotic configurations detect chromosomal rearrangements that may act as
reproductive isolation mechanisms.
 They did this type of analysis on Zea species, and on artificial hybrids between
species with equal and different ploidy levels, we could deduce their polyploid
nature and the genomic formulae of all species (Poggio et al. 2005).
 Accordingly, two different genomes were postulated to occur in these cryptic
polyploids, each with x = 5 chromosomes, which were arbitrarily named FA_ and
FB_. The hypothetical formula proposed for 2n = 20 species was AxAxBxBx, and
for Zea perennis (2n = 40) ApApA¶pA¶pBp1Bp1Bp2Bp2 (Naranjo et al. 1994).

57. Meiotic analysis of the hybrid Zea perennis Zea mays ssp. mays,
whose putative genomic formulae is ApA¶p Am Bp1 Bp2Bm
Where ApA¶p & Bp1 Bp2 –From Zea perennis
Am & Bm – From Zea mays ssp. Mays
This hybrid formed 5 III + 5 II + 5 I, as the most frequent
configuration at metaphase I. It would not be possible to recognize
reliably the parental source of the chromosomes involved in each
meiotic configuration (i.e. III, II, I) using classical plant chromosome
staining methods

58. In-situ hybridization experiments
 In-situ hybridization experiments targeted mitotic chromatin of Zea mays ssp. mays and Zea perennis
 Total DNA of Zea perennis was hybridized as a probe onto Zea mays ssp. mays chromosomes
 The fluorescence signal was absent from at least two pairs of metacentric chromosomes and from all
heterochromatic (DAPI-positive) knobs of maize
 A dispersed signal was observed in the rest of the chromosomes.

59.  Labelled maize DNA was hybridized to
maize chromosomes competitively with
unlabelled total DNA from Zea perennis.
 They observed strong differential
fluorescence on all DAPI-positive knobs in
maize
 On the other hand, total labelled DNA of
Zea mays ssp. mays hybridized to Zea
perennis chromosomes yielded a
hybridization signal uniformly dispersed
across the whole complement

60. GISH was carried out on meiotic chromatin
of the hybrid Zea perennis Zea mays ssp.
mays (2n = 30)
In this case chromosomes were blocked with
unlabelled Zea perennis genomic DNA and
probed with labelled total genomic DNA
from Zea mays ssp. Mays
This resulted in a fluorescence signal on all
the univalents, but on none of the bivalents.
Further indicating their homomorphic
composition
 Trivalents, where observed, showed a strong
fluorescence signal on the F handle_ of the
Ffrying pan_ configurations

61. Inference:
Trivalents are formed by autosyndetic pairing (pairing of chromosomes coming from the
same parental gametes) of genomes ApA¶p from Zea perennis and by allosyndetic pairing
(pairing of chromosomes coming from different parental gametes) of genomes Am from
maize
Bivalents result from autosyndetic pairing of genomes Bp1 and Bp2 from Zea perennis
Univalents correspond to genome Bm of Zea mays ssp. mays. Similar results were obtained
by Poggio et al. (2000) when analysing the hybrid Zea luxurians Zea perennis
Conclusion:
conclude that the formation of bivalents and univalents is not random, and that the FA_
genome of 2n = 20 species is more homologous to the FA_ genomes of Zea perennis than to
its own FB_ genome, strongly suggesting a hybrid origin for the genus, with a common
progenitor for both taxa
These results reinforce the hypothesis of the amphiploid origin of Zea perennis, and would
indicate that the chromosomes with divergent repetitive sequences both in maize and Zea
luxurians could be remnants of a relict parental genome not shared with Zea perennis

62. FISH experiments
Carried out using the pTa71 probe (45S rDNA from Triticum aestivum), which labels
the nucleolar organizer regions.
The pTa71 probe was hybridized to Zea mays ssp. mays and Zea perennis mitotic cells,
two and four signals were detected respectively.

63. The rDNA probe was hybridized
to meiotic cells of the Zea
perennis Zea mays ssp. mays
hybrid
Three fluorescence signals were
observed on a single trivalent
(80% of 50 cells analysed)
Two signals on a bivalent plus
one on a univalent (20% out of
50 cells analysed)