Duplicated parts of chromosomes in relation to the origin of maize

The replicated patterns being revealed by studies of comparative DNA sequencing have revived interest in a question that was first phrased by Rhoades (Amer. Nat. 75:105-110, 1951) as follows: "That the architecture of the germplasm of maize contains duplicated regions can hardly be doubted but whether or not they represent vestiges reflecting an ancient amphidiploid origin or represent later occurring duplications cannot be decided at this time." Now 35 years later we still do not have the answer but the new technologies may yet provide one.

It is generally agreed that the genomes of the various races of both corn and teosinte are more or less completely homologous despite their differences in chromosome knobs and allelic frequencies (see pg. 21 and 22 of my review: Chapter 1, The Origin of Corn. G. F. Sprague (Ed.) 1977. Corn and Corn Improvement.) It is, therefore, assumed that no traditional gross translocations have been involved in moving parts of chromosomes around that might result in duplications, at least in recent millennia. If maize (or teosinte) had an ancient amphidiploid origin, the base genomes would have only five pairs of chromosomes. This number does not occur in any of the genera of possible close relatives of maize (Chandravadana and Galinat, 1976) as they fit into a morphological sequence leading to formation of the cupulate fruit case of teosinte (Galinat, 1956). Millions of years back near the beginning of this sequence, the genus Elyonurus had a species (E. argenteus) with this base number of five pairs (R.P. Celarier, 1957, Bull. Torrey Bot. Club 84:157-162). Celarier describes its chromosomes as being long, two of which have terminal knobs as well as knobby regions throughout the length of the chromosomes. In these respects, it has more similarities to maize-teosinte chromosomes than do the tiny knobless chromosomes of Manisuris (n=9). Although native to South Africa and undoubted only remotely related to maize, E. argenteus deserves some experimental work including some comparative DNA sequencing with the American Maydeae.

Some of the duplications in the maize-teosinte genome may have been important during the separation of maize from teosinte by domestication and thereby be relatively recent (ca. 8000 years old). The key traits that separate maize and teosinte are each controlled at two loci although usually only a one-allele change is necessary for the switch in F2 segregations. If maize originated several times from independent domestications, sometimes one pair was the key, sometimes it was the other.

A duplication of loci was probably an important factor in evolution of the diverse races of maize. Alternative loci provide double pathways and, thereby, allow the accumulation of genetic variability in the redundant system. With genetic drift into isolated niches, then divergence during adaptation to these different niches followed still later by reconvergence, there is a sudden release of recessives due to recombination between races that depended upon different alternative systems. The double systems may also serve a developmental function in which one replicate has a regulatory action and/or an amplifying effect on the genetic signals from the other (see Galinat, 1982, in Maize for Biological Research, Sheridan, pg. 333, 334).

Walton C. Galinat

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