Evolutionary /Genetic origin/Ancestral Origin of Maize
Most crop plants differ from their wild progenitor in having larger size of the economic product. Such a wild form could not be identified in maize. Hence, the nature of the wild progenitor of maize was a contentious and interesting topic in the early 20th century. Many hypotheses/theories have been proposed by different scientists to account for the origin of maize. Among them (i) Tripartite hypothesis (ii) Catastrophic sexual transmutation theory (iii) Tripsacum-Zea diploperennis hypothesis and (iv) Teosinte hypothesis were debated and discussed in detail by different scientists.
i) Tripartite hypothesis (Mangelsdorf and Reeves, 1938, 1939; Mangelsdorf, 1974)
Tripartite hypothesis stated that maize was domesticated from some unknown wild, now extinct maize plant that had structures similar to the ear of modern maize. The hypothesis comprised three parts: (i) the progenitor of maize was a wild maize prototype from South America, which is now either extinct or undiscovered. (ii) teosinte is the offspring of a cross between maize and Tripsacum (iii) sections of Tripsacum chromosomes had contaminated maize germplasm. Thus, Mangelsdorf and Reeves explained the extreme morphological differences between maize and teosinte by imagining a missing ancestor, while relied on Tripsacum to explain their similarities. The evidence/validation for this explanation came from their own experiment. Mangelsdorf and Reeves obatained few sterile maize–Tripsacum hybrids from the artificial crossing. They could identify a minimum of four factors with large effect that controlled defined morphological syndrome. Mangelsdorf and Reeves believed that each of these four factors with large effects represented a block of many linked genes. This interpretation was tied to their hypothesis that maize evolved in nature from a now extinct form of wild maize over a long evolutionary period. They believed that the differences between maize and teosinte were very large and hence it is not possible that teosinte was transformed into maize in the few thousand years during which virtually all crops were domesticated. Until the 1960s, the tripartite hypothesis was widely accepted. However, data were not sufficient and their hypothesis could not withstand the test of time (Doebley, 2001).
ii) Catastrophic sexual transmutation theory (Iltis, 1983)
The catastrophic sexual transmutation was proposed by Iltis (1983). Iltis 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. 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. The catastrophic sexual transmutation theory was fascinating in many ways. But it suffered criticism due to a misinterpretation of the genetic assimilation concept of Waddington (1975a) and the catastrophe theory (Thom, 1977). Actually, the genetic assimilation concept is the Darwinist version of acquired traits. Using this Iltis (1983) described transformation into primitive maize through a possible morphogenetic and structural unbalance in the development of teosinte. During the late 1980s, teosinte hypothesis started gaining importance and became the most accepted theory among the scientific and academic communities.
iii) Tripsacum – Zea diploperennis hypothesis (Eubanks, 1995)
Tripsacum–Z. diploperennis hypothesis can be considered as a modern version of the tripartite hypothesis. This was given by Eubanks(1995). It proposed that maize arose from the progeny of a cross between Zea diploperennis and Tripsacum dactyloides (Eubanks, 2001). This proposal was put forward with the observations on two putative hybrids originated from these two grasses, viz., Tripsacorn and Sundance. The rudimentary ear of these putative hybrids had exposed kernels attached to a central rachis, or cob. If such hybrids occurred naturally, then at least according to proponents of the hypothesis—the evolutionary puzzle of the origin of maize and its unparalleled architecture is solved. Thus, this hypothesis challenged the idea that maize is a domesticated form of teosinte. However, Tripsacum and diploperennis could 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 (Eubanks, 1997), Tripsacorn and Sundance shared four times as many bands with Z. diploperennis as with Tripsacum, indicating a much closer relationship with teosinte than with Tripsacum. Besides, 23% of the molecular markers surveyed were not found in either parent.
iv) Teosinte hypothesis (Beadle, 1939)
The teosinte hypothesis states that teosinte is the sole progenitor of maize. As proposed by Beadle (1939, 1972 and 1978), the teosinte hypothesis states that (i) teosinte provided a useful food source and ancient peoples cultivated it (ii) during the cultivation of teosinte, mutations that improved teosinte’s usefulness to humans arose and were selected by ancient people, (iii) as few as five major mutations would be sufficient to convert teosinte into a primitive form of maize, (iv) different mutations controlled different traits, e.g., one mutation would have converted the disarticulating ear-type of teosinte into the solid eartype of maize, and (v) over the course of time, humans selected additional major mutations plus many minor ones.
Beadle (1939) believed that missing ancestor is not needed to explain the origin. Beadle could obtain completely fertile hybrids between maize and teosinte. He interpreted that these two species were conspecific and that they had only recently diverged. Beadle actually used Mangelsdorf and Reeves’s own data against them and suggested that their four factors might correspond to four major genes. Each of these major genes controlled a single trait that transformed teosinte into maize.
In modern form of teosinte hypothesis, Zea mays ssp. Parviglumis (wild Mexican grass teosinte)has beenpin pointed as the likely progenitor since ssp. parviglumis is the closest living relative of maize. Further that maize arose through large changes in parviglumis —through artificial selection for specific traits. Most maize geneticists and evolutionists (Bennetzen et al., 2001) have accepted that maize is a domesticated derivative of parviglumis. However, the exact morphogenetic steps involved in transformation of wild teosinte into cultivated maize are yet to be known clearly.
Genetic evidences for teosinte hypothesis
- All Zea species and subspecies have 10 chromosomes (Kato, 1976; Kato and Lopez, 1990), with the sole exception of Z. perennis, (n= 20 chromosome doubling). However, most Tripsacum species have either 18 or 36 chromosomes (Mangelsdorf and Reeves, 1938, 1939).
- Chromosomes of maize and teosinte are cytologically similar and hybrid between these two exhibits normal meiosis and full fertility.
- Crossing-over between maize and teosinte chromosomes occurs at frequencies similar to those observed in hybrids of two varieties of maize. Later, Beadle suggested that maize and Mexican annual teosinte were members of the same species. It is expected if maize were merely domesticated teosinte (Emerson and Beadle, 1932).
- Chromosome arm lengths, centromere positions, and the sizes and positions of knobs in annual teosintes are identical to those of maize (Longley, 1941). Also annual teosintes possess at similar frequencies as that of maize (Kato, 1976).
- Isozyme allele frequencies in annual teosinte, Z. mays ssp. parviglumis or Balsas teosinte, are essentially indistinguishable from those of maize. These data suggest that Balsas teosinte is the likely progenitor of maize (Doebley et al., 1984).
- Divergence studies using 18 currently sequenced genes in maize and Tripsacum (Tenaillon et al., 2001; Whitt et al., 2002) indicated that maize and Tripsacum alleles diverged around 5.2 million years ago. The domestication of maize cannot be older than significant human migration to the new world which took place about 15,000 years back (Dillehay, 1989). Molecular dating by microsatellites data indicate that maize and Balsas teosinte diverged about 9000 years ago (Matsuoka et al. 2002). This agrees with archaeological evidence, (Piperno and Flannery, 2001).
- Phylogenetic analyses based on the microsatellite data strongly favour a single domestication which is derived from Balsas teosinte. The microsatellite data imply that the populations of Balsas teosinte in the central portion of its distribution (meeting region of Guerrero, Michoacan, and Mexico) are ancestral to maize.