There is a discrepancy between the number of identified gametophyte mutants and the number expected based on the number of genes expressed in gametophytes. One explanation for this phenomenon is that many such mutants are gametophyte lethal. Gametophyte lethals are quickly removed new mutations from the test population because they produce no seed. If the gametophytes fail prior to competent gamete formation, mutant heterozygotes either are never produced or cannot be maintained. However, if the new mutation causes 100% gametophyte lethality, a heterozygous seed is never produced. Similarly, if a mutation is 90% lethal it is ten times less likely to pass to the next generation than a mutation with no effect. Consequently, we are taking advantage of maize genetics to identify mutations in essential genes by generating them in the presence of a genetic duplication. We are taking advantage of the translocations of arms from the 10 normal A chromosomes to the supernumerary B chromosome. This system can be used to generate tertiary trisomic stocks in which the chromosome arm in question is linked to the B centromere and inherits independently of the A chromosome, frequently producing gametophytes with two copies of a chromosome arm. Mutagenesis of the normal chromosome with the regionally biased Activator transposable element can be used to generate lethal mutants that are covered by the duplication in some gametophytes and therefore viable. Activator elements distributed throughout the genome by the laboratory of Thomas Brutnell are being used for this mutagenesis.

Figure 13. Screen for gametophyte lethal mutants. The short arm of chromosome 9 is shown for example. B chromosome sequences are shown in blue. wx1=waxy1 locus. fgl=female gametophyte lethal. Ac=Activator transposable element. Tertiary trisomic stocks carrying an Ac element linked to a recessive marker and covered by a tertiary trisome have been made. Seeds are selected that carry the trisome and a new Ac element (based on increased Ac dosage). Plants are grown and tested for inheritance of the Ac element. If the Ac is inserted into a gametophyte lethal locus, it can only be transmitted in the presence of the trisome, which provides wild type gene function.