The elongate method of generating tetraploid maize stocks, revisited

--Philip S. Stinard

D. E. Alexander, DE, in a frequently overlooked 1957 paper (Agron. J. 49:40-43), describes a method of inducing autotetraploidy in maize using the elongate (el) gene. Plants homozygous for el produce both reduced and unreduced eggs; the 2n constitution of the unreduced eggs is probably due to the omission of the second meiotic division (Rhoades, MM and Dempsey, E, Genetics 54:505-522). This unusual meiotic behavior can be exploited in order to first transfer a mutant allele of interest from a diploid background into a diploid homozygous el background, followed by recovery of the mutant in a tetraploid background following pollination of the mutant el el line by a previously established tetraploid line. We used this method to generate tetraploid stocks carrying the dominant mutant amylose-extender allele, Ae-5180.

A homozygous el plant was pollinated by a heterozygous, diploid Ae-5180 plant (Ae-5180 Ae) in our 1989 summer nursery. The resulting ear segregated for plump wildtype kernels (with Ae Ae El el embryos), plump glassy kernels (with Ae-5180 Ae El el embryos), and shriveled, defective (triploid or aneuploid embryo) kernels. Plump Ae-5180 kernels were planted in our 1989-1990 winter nursery, and the resulting plants self-pollinated. The selfed ears segregated in a 3:1 ratio of Ae-5180 phenotype : wildtype kernels. Among the selfed progeny kernels, one fourth would be expected to have el el embryos and thus give rise to plants producing unreduced (2n) eggs. Progeny kernels with Ae-5180 phenotype were planted in our 1990 summer nursery, and the resulting plants pollinated by tetraploid maize lines obtained from the Maize Stock Center. Of 27 successful pollinations, two ears segregated for both plump (putative tetraploid embryo) and shriveled (triploid embryo) kernels, and thus must have been borne on el el plants. The remaining 25 ears had only shriveled (triploid embryo) kernels.

Putative tetraploid embryo kernels with Ae-5180 phenotype were planted in our 1990-1991 winter nursery, and the resulting plants pollinated by a tetraploid W23 stock obtained from the Maize Stock Center. The four ears obtained from these crosses were scored for the number of Ae-5180 and wildtype kernels (Table 1). Three of the four ears had ratios of Ae-5180 : wildtype that approximated 1:1 ratios (one of these ears differed from 1:1 at the p < 0.05 level). The remaining ear had a 5:1 ratio of Ae-5180 : wildtype, a classical tetraploid ratio. These ratios can be explained if the three 1:1 ears came from plants with the tetraploid genotype Ae-5180 Ae Ae Ae (the product of a single crossover between Ae-5180 and the centromere during meiosis I in the el el parent), and the 5:1 ear came from a plant with the genotype Ae-5180 Ae-5180 Ae Ae (no crossover between Ae-5180 and the centromere). We conclude that Ae-5180 behaves as a dominant mutant allele in tetraploid as well as in diploid backgrounds.

Table 1. Counts of Ae-5180 and wildtype kernels on ears from the cross of tetraploid plants carrying Ae-5180 by a tetraploid W23 (wildtype) stock.
 
Female Parent Ae-5180 wildtype chi-square (ratio)
90-91-8525-1 136 162 2.268 (1:1)
-8525-4 153 115 5.388 (1:1)*
-8525-5 154 140 0.667 (1:1)
-8526-8 261 52 0.001 (5:1)
* Chi-square for 1:1 ratio significant at p < 0.05 level.


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