St. Paul, Minnesota
University of Minnesota
Madison, Wisconsin
University of Wisconsin
Evidence for genetic variation for phenotypically invariant traits in teosinte --Lauter, N. and Doebley, J. Many of the plant and inflorescence morphologies in teosinte are discretely different from maize, and vary little or not at all within and among teosinte populations. For example, teosinte ears always have exactly two ranks of cupules that each bear only one spikelet, while maize ears have many ranks of cupules that bear paired spikelets. While the lack of phenotypic variation for such traits in teosinte may suggest an absence of underlying genetic variation, it could also be the case that teosinte harbors genetic variation for such traits that is not phenotypically expressed in the teosinte genetic background.

Among F1 hybrids derived by crossing teosintes to maize inbred lines, we have often observed a broad range of phenotypes for otherwise discrete traits, suggesting that genetic variation for these traits may exist in teosinte. If this variation is genetic, then F1 hybrids of different teosintes with the same maize inbred should have distinct phenotypes. If this variation is strictly environmental in origin, then all maize-teosinte F1 hybrids with the same inbred should be equivalent. In order to assess to what extent phenotypic variation among maize-teosinte hybrids depends on the teosinte parent (i.e. is genetic), we examined plant and inflorescence morphologies in 12 F1 families derived from crosses between maize inbred W22 and 12 teosinte plants, each of which came from a different natural population in Mexico. So that we could also compare the contributions of the two subspecies of Mexican annual teosinte, we used five Zea mays ssp. mexicana and seven Zea mays ssp. parviglumis plants as teosinte parents (Table 1).

Five traits were measured two to three weeks post-anthesis on the uppermost primary lateral branch and on the inflorescence at its tip. These traits were chosen because they define the key morphological differences between maize and teosinte (Doebley and Stec 1993, Genetics 134: 559-570). Because teosinte has a long lateral branch and maize a short one, we measured the lateral branch length and divided this by the number of internodes in the branch to determine the lateral branch internode length (LBIL). The inflorescence at the tip of the primary lateral branch is a branched male inflorescence (tassel) in teosinte and an unbranched female inflorescence (ear) in maize. To record variation in the sex of this inflorescence, we counted the number of inflorescence internodes with male spikelets and the number with female spikelets, and computed the percentage of the total that were male or staminate (STAM). To record variation in inflorescence branching, we counted the number of branches on the primary lateral inflorescence (LIBN). The ears of teosinte bear single sessile spikelets on each internode, while those of maize have paired spikelets on each internode, one sessile and one pedicellate. To record variation in this trait, the percentage of female internodes lacking the pedicellate spikelet was computed and termed PEDS. Finally, teosinte bears its spikelets in two ranks on opposite sides of the ears, while maize has its spikelet pairs in four or more ranks and thus around the entire circumference of the ear. To record variation in this trait, the number of ranks of spikelets (or spikelet pairs) was counted for the primary lateral inflorescence. Since RANK can vary over the length of the inflorescence in teosinte-maize hybrids, it was calculated as a weighted sum.

There was considerable phenotypic variation among the maize-teosinte F1 families for all five traits (Table 2). Family means ranged from 13.5% to 89% for PEDS, from 20% to 69% for STAM and from 2.5 cm to 5.7 cm for LBIL. It is possible that this wide phenotypic variation is exclusively environmental in origin; however, we think this unlikely, since all twelve families were grown together in the same nursery and the data suggest that there are significant differences among families (Table 2). Since the maize parent of the F1 families was an inbred and therefore isogenic, it could not have been a source of genetic variation for differences among the F1 families. Thus, we suspect that genetic variation affecting these traits existed among the 12 teosinte plants used as parents, even though they each exhibited the same fixed-in-teosinte phenotypes for them.

We compared the group of 35 F1 plants that had a ssp. parviglumis parent to the group of 31 plants which had a ssp. mexicana parent in order to investigate whether or not one subspecies consistently harbors alleles that make the plants more maize-like (Table 2). For all traits except PEDS, t-tests indicated that the contributions made by the two classes of parents were statistically equivalent. For PEDS, the F1 plants that had a ssp. mexicana parent had a lower percentage of female cupules that lacked the pedicillate spikelet (P=0.002). This difference could be attributable to introgression of maize germplasm, since ssp. mexicana plants grow largely as a weed in Mexican maize fields. It could also be the case that these differences simply represent natural differences in allele frequencies that exist between the two subspecies. In either case, our data suggest that teosinte populations contain hidden genetic variation for morphological traits for which they exhibit no phenotypic variation.

Table 1. Accession data for the teosinte parents of the F1 families
 
Family # Collector and number Location
1 Benz 967 El Rodeo, Mexico
2 Beadle and Kato site 1 Mazatlan, Guerrero
3 Beadle and Kato site 2 Chilpancingo, Guerrero
4 Beadle and Kato site 4 Palo Blanco, Guerrero
5 Beadle and Kato site 6 Valle de Bravo, Guerrero
6 Beadle (no number) El Salado, Guerrero
7 Kato site 4 Palo Blanco, Guerrero
8 Iltis et al. 28622 Chalco, Mexico
9 Iltis et al. 28620 Texcoco, Mexico
10 Puga 11066 Degollado, Jalisco, Mexico
11 Wilkes 45461 Panindicuaro, Michoachan
12 Beadle (no number) Nobogame, Chihuahua

The family numbers cross-reference the accession data with the phenotype data in Table 2. The location of the populations where the teosintes came from are listed according to the nearest town/city, state.

Table 2. Average phenotypes of inbred maize by teosinte F1 families
 
Ssp Family # n LBIL LIBN PEDS RANK STAM
p 1 4 4.2 ± 1.1 1.0 ± 0.6 18.7 ± 6.2 4.0 ± 0.0 27.2 ± 14.6
p 2 4 4.4 ± 0.8 0.5 ± 0.5 78.1 ± 18.5 3.3 ± 0.4 55.0 ± 16.7
p 3 5 2.5 ± 0.3 0.0 ± 0.0 89.1 ± 6.0 3.6 ± 0.2 19.6 ± 5.0
p 4 4 5.5 ± 1.2 0.8 ± 0.5 56.4 ± 10.0 2.7 ± 0.4 69.3 ± 12.2
p 5 6 3.1 ± 0.5 0.0 ± 0.0 45.5 ± 13.8 2.6 ± 0.3 55.6 ± 6.4
p 6 6 3.4 ± 0.9 0.0 ± 0.0 46.0 ± 11.9 3.4 ± 0.3 58.8 ± 7.0
p 7 6 5.7 ± 1.3 0.0 ± 0.0 83.0 ± 4.9 3.2 ± 0.3 66.6 ± 7.6
  Average   4.1 ± 0.5 0.3 ± 0.2 60.2 ± 5.3 3.3 ± 0.1 51.2 ± 4.5
               
m 8 5 3.6 ± 1.0 1.0 ± 0.8 13.5 ± 6.2 3.1 ± 0.3 35.6 ± 9.6
m 9 7 5.3 ± 1.0 0.4 ± 0.3 49.5 ± 9.2 3.5 ± 0.3 50.3 ± 6.5
m 10 8 5.2 ± 1.4 1.1 ± 0.6 26.9 ± 10.7 2.4 ± 0.2 62.0 ± 11.0
m 11 4 5.4 ± 1.5 0.0 ± 0.0 57.4 ± 20.7 4.0 ± 0.0 34.0 ± 11.2
m 12 7 5.3 ± 1.4 0.3 ± 0.3 32.3 ± 12.1 3.7 ± 0.3 38.7 ± 11.4
  Average   5.0 ± 0.5 0.6 ± 0.2 35.0 ± 5.7 3.3 ± 0.1 46.2 ± 4.8

The subspecies of the teosinte parent is listed as p for ssp. parviglumis and m for ssp. mexicana. The family number cross-references the phenotype data with the accession data in Table 1. n is the number of plants per family. The family averages and their standard errors are listed in trait units: cm, branches, %, cupule columns, %, respectively.
 
 


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