Montana State University


University of Wisconsin

Genomic diversity detected by transposable element probes is correlated with heterosis --L. E. Talbert, W. F. Tracy and J. Gerdes Although the transposable elements Mu, Spm, and Ac are detectably active only in certain maize stocks, multiple sequences hybridize to probes for the elements in all stocks. Sequences that hybridize to Spm and Ac tend to be deletion derivatives of the intact elements (Doring and Starlinger, Annu. Rev. Genet. 20:175-200, 1986). There is no evidence for activity of the Mu hybridizing sequences. In fact, a Mu4 element was shown to be in the same genomic location in diverse Zea taxa, suggesting an ancient and stable insertion (Talbert et al., J. Mol. Evol. 29:28-39, 1989).

Approximately 20-40 separate hybridizing bands may be visualized in all maize lines on Southern blots using probes for Mu, Ac, and Spm. Assuming that these hybridizing sequences are not actively transposing, this suggest that a single Southern hybridization with a transposable element probe may be used to monitor genomic variation at several genomic segments. Heterosis in maize is correlated with genomic diversity of parents. Thus, we were interested to determine whether genomic diversity of parents as measured by transposable element probes is correlated with heterosis of F1 progeny.

Initial experiments suggested that genomic diversity as monitored by transposable element probes may be too great among unrelated maize lines to make meaningful comparisons regarding relative diversity. Thus, we monitored seven sublines of the maize inbred line P39 for transposable element variation. A difference among parents was measured as the presence of a hybridizing band in one subline not found in the other line. A total of 56 hybridizing bands were observed on EcoRI/HindIII-digested DNA from the seven sublines using Mu, Ac, and Spm. Differences among the 21 possible pairwise comparisons ranged from 0-16, with an average number of differences of 7.6 between sublines.

The seven sublines were crossed in a diallel fashion to provide 21 F1 hybrids. These were yield tested in Wisconsin in 1988. Parental diversity of parents as measured by the three transposable element probes was positively correlated with ten-ear weight (Table 1). Thus, parents that showed the greatest genomic diversity tended to produce the highest yielding F1 progeny. These results suggest that probes for the maize transposable elements may be efficient tools for monitoring useful genetic variation among closely related maize lines.

Table 1. Correlation between transposable element differences among parents and yield of F1 progeny for seven P39 sublines. Yield was measured as dry weight of ears from ten plants per plot in ten reps in Wisconsin in 1988.

Previous work has shown that probes for Mu and Spm clearly delineate the Zea and Tripsacum genomes, as hybridizing sequences were observed in Zea but not Tripsacum. This fact has been used to determine that T. andersonii is a natural hybrid between Tripsacum and Zea (Talbert and Doebley, MNL 63:45, 1989; Talbert et al., Amer. J. Bot., 1990, in press). Thus, transposable elements are not only interesting in their own right, but may also be exploited to assess relatedness for either taxonomic or plant breeding purposes.

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