MuDR-like elements in Zapalote chico maize
--Christine Warren and Virginia Walbot
MuDR is the new designation for the transposase-encoding master Mutator element previously designated as Mu9, MuR1 or MuA2. MuDR encodes two major sense transcripts that, when fully spliced, yield mRNAs of ~2.9 kb and ~1.0 kb. The transcripts are convergent, initiating on opposite strands, but in the same sequence, in the nearly identical terminal inverted repeat elements. Between the polyadenylation sites mapped from cDNAs, there is a several hundred base pair region containing several classes of short direct and inverted repeat elements.
In a Southern blot survey of inbred maize lines, we found no evidence for an intact MuDR-like element, using probes from both transcribed regions. The collection of exotic lines available from the Co-op was also checked, and Zapalote chico contained multiple (10 - 20) copies of a MuDR-like element. Detailed genomic Southern mapping indicates that all restriction sites expected from the sequenced MuDR (Hershberger et al., PNAS 88:10198, 1991) are present in the Zapalote chico element, however, the intergenic region is several hundred bases longer in Zapalote chico. This was confirmed by PCR amplification using 8 pairs of primers that span MuDR followed by diagnostic restriction digests of the PCR products: MuDR and the Zapalote chico elements are identical in 7 regions, but differ by several hundred bases in the product that spans the intergenic region. To test whether the Zapalote chico elements are "active," northern blot analysis was performed; there are weak signals for the 2.9 kb and 1.0 kb transcripts, but the level of transcript is about 1% of that found in a typical Mutator line.
The data available are sufficient to suggest that Mutator is a normal part of the maize genome--there is no need to invoke horizontal transmission from another species. The unusual distribution of MuDR-like elements in modern maize, in contrast to the widespread occurrence of Ac and En/Spm, may reflect stronger selection against Mutator activity resulting in element loss in most lines.
The single Zapalote chico line available from the Co-op was recovered during a collecting trip to Southern Mexico about 40 years ago. The line has been propagated by self- or sib-crossing since then. The seed is pure white, eliminating the easiest method to visualize transposable element activity, and early selection for vigor to overcome "problems" with an exotic line from a different latitude might also have effectively selected against Mutator activity in the line available. Questions under study include: (1) Is Zapalote chico maize still grown, and, if so, where and why? Zapalote chico turns out to be the "staff of life" of the Zapotec people, a group of about 300,000 native Americans living in Oaxaca, Mexico. The masa prepared from Zapalote chico flour is ideal for preparing totopos, a dry cracker baked in a clay oven. Unlike tortillas, totopos store well for up to 6 months. Totopos are a key element in the 3,500 year oral history of the Zapotecs, suggesting that this line is very ancient. (2) Is there evidence for instability of Zapalote chico in its local environment? The Zapotecs believe that thieves who steal their corn will suffer because the stolen corn will poison the fields when mixed with other corn. Mexican corn breeders have found that crosses between inbred lines and Zapalote chico yield hybrid dysgenesis rather than hybrid vigor, giving scientific support to the Zapotec legend. The instabilities observed in F1 hybrids are reminiscent of an active Mutator system. It has been impossible for Mexican breeders to establish stable selfed lines from the hybrid populations. Similarly, Georgia Davis and R. Kowles made crosses between Zapalote chico and Wilbur's Knobless Flint and noted that the hybrid and backcrossed lines were not vigorous (personal communication). We plan to check the hypothesis that the low level of MuDR transcripts typical of Zapalote chico is increased in the hybrids. (3) How widespread are MuDR-like elements in Zapalote chico? We have 55 new accessions of Zapalote chico, and a survey will be performed on these materials. (4) Can crosses with Zapalote chico reactivate quiescent Mu elements in inactive Mutator lines? Although Zapalote chico has clear plant and kernel phenotypes, the line is not homozygous for anthocyanin markers: most populations are r-g Bz2 Bz1 and segregating for dominant and recessive (low or non-functional) alleles of b, c1 and c2. Some individuals are r-r and have purple anthers. Reactivation tests are in progress with inactive lines carrying mutable alleles of bz2 and bz1. (5) What is the exact structure of the MuDR-like elements in Zapalote chico? This will require cloning and sequencing an element.
to the MNL 68 On-Line Index
Return to the Maize Newsletter Index
Return to the Maize Genome Database Page