The College of Wooster


The University of Western Ontario

Stage-specific expression of small hsp gene RNA in anthers --R. A. Bouchard and D. B. Walden Two previous reports (MNL 60:71-72; MNL 62:114) described the recovery of a maize genomic clone showing homology to small hsp (heat shock protein) genes, based on its homology to cDNA clones for a major class of transcripts developmentally expressed during prophase of meiosis in lily. Evidence of sequence homology (P. S. Dietrich, R. A. Bouchard, E. M. Silva and R. M. Sinibaldi, J. Cell. Biol. 105:245a, 1987) and transient expression of reporter genes (Silva et al., J. Cell. Biol. 105:245a, 1987) both indicate that the isolate actually represents a maize small hsp gene, as does heat-induced accumulation of transcripts in seedling tissues (MNL 62:88). The question therefore arises as to whether this gene, or close relatives, might be developmentally expressed during the meiotic interval in maize in the absence of heat stress, just as corresponding genes are in lily, as well as yeast and fruitflies (Lindquist and Craig, Annu. Rev. Genet., 1988).


We therefore isolated RNAs from staged premeiotic tassels, from anthers containing meiotic and haploid microsporocytes and microspores (see the accompanying report for procedures used to obtain this material), and from anthers containing mature pollen as well as from shed pollen. RNA was also prepared from spikelets, representing somewhat coarser staging. We have performed preliminary RNA-Dot and Northern hybridization studies on these RNAs to detect transcripts of the 18kd clone gene or closely related sequences. While further work will be required for precise kinetics, it is evident that transcripts homologous to the small hsp clone are absent in premeiotic tassels, accumulate substantially during the period of meiotic prophase with peak abundance during late prophase, and are virtually undetectable by the time mature pollen is present. These patterns are evident in the RNA-Dot analyses of Seneca 43 spikelet RNAs and Ohio 43 anther RNAs, and the accompanying Northern analyses of Ohio 43 RNAs only, as shown in the accompanying figure. While developmental expression of small hsp gene in the absence of stress certainly occurs in anthers containing PMCs of the appropriate stages, it cannot yet be associated with the prophase meiocytes unequivocally. Further work is planned to refine our picture of the timing of developmental expression, to identify whether developmental expression is a property of all or only some of the maize small hsp genes, and ultimately to determine whether developmental expression is in fact localized in the meiocyte.

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