The history of maize has been difficult to trace as its fruiting cob and monoe- cious nature are unique in the Gramineae. In other grasses the sexual parts are in close proximity rather than isolated at different locations in the plant, and the grains are protected individually by glumes rather than being naked.
The closest relatives of maize are a few species of Tripsacum and a number of teosintes (Table 8.1), which were for a long time considered to be a separate genus, Euchlaena. The teosintes have a habit that resembles that of maize, but their tassels are not on a central spike like maize and their ears are much less complex (Figs 8.3 and 8.4). The Tripsacumspecies have pistillate and staminate flowers that are borne separately, but, unlike maize, they are adjacent on the spike. Their seeds are embedded in segments of the rachis and scatter when ripe.
Table 8.1. Maize and teosinte taxonomy according to Iltis and Doebley (1980).
Chromosome
Species and subspecies number (2n) Synonyms Cultivated maize
Zea maysL. subsp. mays 20 Z. mays
Teosinte 20 Zea(or Euchlaena) mexicana
Z. mayssubsp. mexicana(Shrader) Iltis 20 Z.(or E.) mexicana Z. mays subsp. parviglumisIltis & Doebley 20 Z. (or E.) mexicana Z. mayssubsp. diploperennisIltis, 20 Zea(or Euchlaena)
Doebley & Guzmán diploperennis
Z. mayssubsp. perennis(Hitchc.) Reeves 40 Zea(or Euchlaena) perennis
& Mangelsdorf
Zea luxurians(Durieu & Ascherson) Bird 20 Euchlaena luxurians
MI PLI
SLI PLB
PLI
SLI
Teosinte Maize
Fig. 8.3. Differences in fruiting structure of maize and teosinte. Note that tassels and ears are on the same fruiting stalk of teosinte, while on maize they are segregated on different spikes. MI, main inflorescence; PLI, primary lateral inflorescence; SLI, secondary lateral inflorescence; PLB, primary lateral branch.
(Used with permission from J. Doebleyet al., 1990, Proceedings of the National Academy of Sciences USA87, 9888–9892.)
All the native Zea species have very restricted ranges in Mexico and Central America, and they carry the same chromosome number, 2n = 20, except for Z. perennis, which is tetraploid. The Tripsacum species all have multiples of x = 18. Diploid teosinte crosses readily with maize, and recipro- cal introgression may occasionally occur today in Mexico and Guatemala, where teosinte grows adjacent to cultivated maize, although evidence of modern gene flow is minimal (Doebley et al., 1984; Doebley, 1990a).
Tripsacum can be crossed with maize (Eubanks, 2001a,b) and a natural hybrid species has been identified (Talbert et al., 1990), but the sterility bar- riers are sufficient to greatly limit natural introgression (Newell and deWet, 1973; James, 1979; deWet et al., 1984a).
The high chromosome number of maize suggests that it might be an ancient polyploid formed from two diploids with 2n = 10. Several independent lines of evidence support this conjecture (Molina and Naranjo, 1987):
(i) haploid maize shows many chromosome associations (Ting, 1985); (ii) nor- mal taxa display secondary associations of bivalents (Vijendra Das, 1970); (iii) chromosomes of Zea maysform four subsets of five chromosomes in somatic metaphase cells rather than two sets of ten (Bennett, 1984); (iv) maize carries a high number of isozyme and restriction fragment length polymorphism (RFLP) duplications (Stuber and Goodman, 1983; Helentjaris et al., 1988); (v) DNA sequence data have shown numerous duplications (Gaut and Doebley, 1997);
and (vi) distant relatives Coixand Sorghumhave a haploid number of 5.
Fig. 8.4. Grain-bearing inflorescences of maize and its relatives. Left to right:
Tripsacum dactyloides, Zea mexicanaandZea mays.
The lack of fossil evidence of a prototype maize plant has led to three major hypotheses concerning the origin of maize: (i) maize, teosinte and Tripsacum were separate lineages that evolved from a common, unknown ancestor (Weatherwax, 1954); (ii) an interspecific hybridization between two or more native grasses produced maize (Mangelsdorf and Reeves, 1939;
Eubanks, 2001a,b); and (iii) maize evolved directly from teosinte (Beadle, 1939; Galinat, 1973; Dorweiler and Doebley, 1997; Iltis, 2000).
In a complex scenario, Mangelsdorf and Reeves (1939) initially sug- gested that modern maize arose through a series of interspecific crosses, and in fact was the progenitor of teosinte. In their ‘tripartite hypothesis’ they envisioned that now extinct races of pod corn were introduced into Mexico and Central America and subsequently hybridized with Tripsacumto form teosinte. This new teosinte then hybridized with maize to produce superior races. Mangelsdorf (1974) later altered this hypothesis and considered teosinte to be a mutant derivative of maize.
In support of Mangelsdorf ’s hypotheses, pod corn has been found among the fossils of ancient communities that lived in New Mexico 4000 to 3000 BP and even older ears have been located in the Tehuacan Valley, Mexico, which appear to have traits of both popcorn and pod corn.
Mangelsdorf (1958) also crossed modern races of popcorn and pod corn and obtained a hybrid that had a combination of grass and maize character- istics. However, there is no hint of where the pod corn came from.
Eubanks (2001a,b) has suggested that maize was derived from a hybridization between Tripsacum dactyloides and Zea diploperennis.
Support for this hypothesis has come from her generating recombinant progeny that have maize-like flowering spikes and RFLP data where modern maize appears to carry a combination of fragments from a limited sample of native Tripsacumand Zea.
The most overwhelming support has been garnered for the teosinte hypothesis through molecular and isozyme studies (Bennetzen et al., 2001; Smith, 2001). No intermediate forms between maize and teosinte have been found in the archaeological record and there is little evidence that humans ever cultivated teosinte (Galinat, 1973; Mangelsdorf, 1974, 1986), but a punctuated change could have occurred in the ear or tassel that led to a dramatically different crop (Iltis, 1983, 2000). In an extensive analysis of electrophoretic variation in native populations, one variety of Z. mays subsp. parviglumis was found to have a high genetic identity of 0.92 with maize, and the two were tightly grouped when the data were subjected to a principal component analysis (Doebley et al., 1987). This suggests that they are directly related and are part of the same lineage.
The close similarity between maize and Z. mays subsp. parviglumis has also been documented using complementary DNA (cDNA) restriction frag- ments (Doebley, 1990c) and ribosomal internal transcribed space (ITS) sequences (Buckler and Holtsford, 1996). Most recently, Matsuoka et al.
(2002) were able to trace the origin of maize, using single sequence repeat
(SSR) markers, to a single domestication of subsp. parviglumisin southern Mexico about 9000 years ago. A paradox still remains in that the oldest evidence of maize cultivation at Guilá Naquitz Cave falls outside the cur- rent geographical range of Z. mays subsp. parviglumis (Benz, 2001;
Piperno and Flannery, 2001). However, there is no assurance that this was indeed the first place that maize was domesticated, and the range of Z. mays subsp. parviglumi could have been very different when maize emerged (Smith, 2001).
Even though maize and teosinte are separated by numerous polygenic traits, the punctuated change in maize morphology was probably facilitated by there being only a few major loci involved in the evolutionary change.
Iltis (1983) suggested that the emergence of maize may have been due pri- marily to a feminization of the tassel; however, John Doebley’s laboratory has found several key quantitative trait loci (QTL) that separate teosinte from maize through regulation of glume toughness, naked kernels, sex expression and the number and length of internodes in both lateral branches and inflorescences of maize (reviewed in Chapter 7).
Maize became an integral part of a Mesoamerican crop assemblage that included beans and squash (Smith, 2001). From its early cultivation in south-western Mexico before 8000BP, maize spread throughout Mexico, Central America and into the south-western USA over a period of 3000 years. It is likely that subsequent hybridization with teosinte played a role in the early development of maize, as the modern crop appears to contain as much as 77% of the landrace’s diversity (Eyre-Walker et al., 1998; Tenaillon et al., 2001). Maize arrived in eastern North America through the south- western states about 2000 years ago (Smith, 1998). It appeared in South America by 6000BP(Bush et al., 1989) and was introduced from there into Florida via the Caribbean.
By the time the Europeans arrived in the Americas in the 1500s, maize was an important staple across a vast area from Argentina to Canada.
Indians from all over North and South America had developed countless varieties of maize, many of which still exist in Mesoamerica (Doebleyet al., 1985; Bretting and Goodman, 1989). The primary forms that were devel- oped were: (i) popcorn, which has extremely hard seeds that explode when heated; (ii) flint maizes, which are composed of hard starch; (iii) flour maizes, which have soft starch that can be ground into flour; (iv) dent maizes, which have soft starch at the top of the kernel and hard starch below; and (v) sweet corn, which is eaten as a sugary vegetable (Fig. 8.5).
Within all these groups there exists tremendous variation for kernel colour, ear size, maturation dates and overall plant habit. The efforts of various primitive peoples represent a remarkable example of the changes possible under domestication. Heiser (1990) suggests that at least part of this diver- sity was produced by seeds being planted individually instead of being broadcast like the other grain species. This practice made people more aware of individual variation.