A better understanding of the genetic control of the male gametophytic phase can be efficiently obtained on the basis of gametophytic mutants for pollen development within the anthers and for pollen function, i.e. pollen germination and tube growth.
We adopted an indirect approach to select viable mutants affecting the male gametophytic generation in maize, based on the observation that components of pollen development and function show positive correlation with endosperm development (Mulcahy, Science 171:1155, 1971, Ottaviano et al. TA.G.63:249, 1982) and that some alleles determining defective endosperm in maize have been found to be expressed in the male gametophyte (Jones, 1928). Thus we selected endosperm defective mutants (de) and tested them for gametophytic gene expression studying the distortion from the expected Mendelian segregation.
Heterozygous plants were selfed and the resulting F2 ears scored for de and normal phenotypes. In order to study the effect of de alleles on pollen tube growth rate the F2 ears were divided into 3 sectors of equal length to obtain the proportion of de kernels in the top, middle and base sectors. Heterogeneity between sectors was interpreted as due to differential tube growth rate between pollen carrying the de allele and pollen carrying the normal allele. Thirty-four endosperm defective viable mutants, introgressed in B37 genetic background, that complementation tests revealed affected different genes controlling endosperm development, were analyzed.
The study of the segregation in F2 revealed 4 classes of de mutants: 1) mutants in which the mutation does not affect either gametophytic development or function; 2) mutants in which the effect on the gametophyte regards pollen development processes; 3) mutants showing effects on both pollen development and function; and 4) mutants where only pollen tube growth rate is affected. Some of the mutants included in class 2) and 3) showed also reduced pollen size and increased pollen sterility.
Positive and negative interactions between pollen and style were detected by means of mixed pollination (pollen produced by de/de plants and pollen from an inbred line used as a standard and carrying genes for colored aleurone) on de/de and de/+ plants. Positive interactions were interpreted as metabolic complementation between defective pollen and normal styles.
We are currently involved in the characterization of some of those mutants at the biochemical level.
M. Enrico Pe and Ercole Ottaviano
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