E. Ottaviano

The extent of gametophytic-sporophytic gene expression in maize

SummaryTo determine the extent of gametophytic gene expression and the type of transcription, haploid or haplo-diploid, of the genes, isozymes were used as genetic markers. Fifteen enzymatic systems, including thirty-four isozymes, were studied. The determination of the type of expression of genes coding for multimeric enzymes was based on the comparison of electrophoretic patterns of pollen and of sporophytic tissues from plants heterozygous for electrophoretic mobility: if gene expression in pollen is of a gametophytic (haploid) origin, pollen, unlike the sporophyte, would reveal only the parental homomultimeric bands. The enzymes analyzed can be grouped in three categories according to type of gene expression: i) enzymes present in both pollen and sporophyte, coded by the same gene with haplo-diploid expression; ii) enzymes controlling analogous functions in pollen and sporophyte, coded by different genes, expressed in only one of the two phases; iii) enzymes present in two or more forms in the sporophyte and only in one form in the gametophyte. The data allow the proportion of haplo-diploid gene expression in the loci examined to be estimated at 0.72; 0.22 and 0.06 being the proportions attributable to the sporophytic and gametophytic domains, respectively.

Angiosperm pollen and ovules.

The biology of pollen development and the phenomenon of self-incompatibility has long been a subject of great interest to plant biologists. Methods in molecular genetics are now facilitating the isolation of genes responsible for controlling pollen development and pollen-stigma interactions, resulting in a revolution in our understanding of these crucial events in plant reproduction. The contributions in this volume present current research on the physiology, molecular genetics and biotechnology of pollen and ovule development and interactions in angiosperms.

Similarity of maize and sorghum genomes as revealed by maize RFLP probes

SummaryDensely saturated genetic maps of neutral genetic markers are a prerequisite either for plant breeding programs to improve quantitative traits in crops or for evolutionary studies. cDNA and genomic clones from maize were utilized to initiate the construction of a RFLP linkage map in Sorghum bicolor. To this purpose, an F2 population was produced from starting parental lines IS 18729 (USA) and IS 24756 (Nigeria) that were differentiated with regard to many morphological and agronomical traits. A total of 159 maize clones were hybridized to the genomic DNA of the two parents in order to detect polymorphism: 154 probes hybridized to sorghum and 58 out of these were polymorphic. In almost all of the cases hybridization patterns were similar between maize and sorghum. The analysis of the segregation of 35 polymorphic clones in an F2 population of 149 individuals yielded five linkage groups. The three principal ones recall regions of maize chromosomes 1, 3 and 5: in general, colinearity was maintained. A possible inversion, involving a long region of maize chromosome 3, was detected. Simulations were also performed to empirically obtain a value for the lowest number of individuals of the F2 population needed to obtain the same linkage data.

Male gametophytic selection in maize

SummaryThere is evidence that male gametophyte selection is a widespread phenomenon in higher plants. The pollen tube growth rate is one of the main components of gametophyte selective value; genetic variability for this trait, due to the effect of single genes or to quantitative variation, has been described in maize. However, indication of gametophytic selection has been indirectly obtained; its effect was revealed by the positive relation observed between gametophyte competitive ability and sporophyte metrical traits.This paper considers the results of selection applied to gametophyte populations produced from single plants. The competitive ability of the lines was evaluated in comparison with that of a standard line by means of the pollen mixture technique. Sporophytic traits were measured in the hybrid progeny obtained by crossing selected S3 and S4 families with an unrelated single cross and an inbred line. Gametophyte selection produced inbred lines with high gametophyte competitive ability. In view of the selection procedure adopted, this result was interpreted as an indication of haploid expression of genes involved in the control of pollen tube growth. Moreover, this gametophytic trait was positively correlated with sporophytic traits (seedling weight, kernel weight and root tip growth in vitro), indicating that both groups of characters have a common genetic basis.

Molecular markers (RFLPs and HSPs) for the genetic dissection of thermotolerance in maize.

SummaryCellular membrane stability (CMS) is a physiological index widely used to evaluate thermostability in plants. The genetic basis of the character has been studied following two different approaches: restriction fragment length polymorphism (RFLP) analysis, and the effects of segregating heat shock protein (HSP) loci. RFLP analysis was based on a set of recombinant inbreds derived from the T32 × CM37 F1 hybrid and characterized for about 200 RFLP loci. Heritability of CMS estimated by standard quantitative analysis was 0.73. Regression analysis of CMS on RFLPs detected a minimum number of six quantitative trait loci (QTL) accounting for 53% of the genetic variability. The analysis of the matrices of correlation between RFLP loci, either within or between chromosomes, indicates that no false assignment was produced by this analysis. The effect of HSPs on the variability of the CMS was tested for a low-molecular-weight peptide (HSP-17) showing presence-absence of segregation in the B73 × Pa33 F2 population. Although the genetic variability of the character was very high (h2=0.58) the effect of HSP-17 was not significant, indicating either that the polypeptide is not involved in the determination of the character or that its effect is not statistically detectable.

Pollen competitive ability in maize: within population variability and response to selection

SummaryMale gametophytic selection can play a special role in the evolution of higher plant populations. The main assumption — gametophytic-sporophytic gene expression of a large portion of a plants genes — has been proven by a number of studies. Population analyses have revealed a large amount of variability for male gametophytic fitness. However, the data available do not prove that at least a portion of this variability is due to postmeiotic gene expression. This paper reports the analysis of a synthetic population of maize based on a gametophytic selection experiment, carried out according to a recurrent scheme. After two cycles of selection, the response was evaluated for gametophytic and sporophytic traits. A parameter representing pollen viability and time to germination, although showing a large amount of genetic variability, was not affected by gametophytic selection, indicating that this variability is largely sporophytically controlled. Pollen tube growth rate was significantly affected by gametophytic selection: 21.6% of the genetical variability was released by selection. Correlated response for sporophytic traits was observed for mean kernel weight: 15.67% of the variability was released. The results are a direct demonstration that pollen competitive ability due to pollen tube growth rate and kernel development are controlled, to a considerable extent, by genes expressed in both tissues. They also indicate that gametophytic selection in higher plants can produce a higher evolution rate than sporophytic selection; it can thus serve to regulate the amount of genetic variability in the populations by removing a large amount of the genetic load produced by recombination.

Genetic Variability of Gametophyte Growth Rate in Maize

SummaryIn order to measure differences in the pollen growth rate of numerous lines of maize and to investigate the main features of their genetic control, gametophyte growth was studied in vitro and in vivo. In vitro pollen tube growth of twenty inbred lines and seven hybrids was measured; a remarkable variability was observed in the growth rate of the inbred lines examined: most lines were distinct, showing different levels of growth.Analysis of frequency distribution of pollen tube lengths for pairs of inbred lines and their F1′ s revealed greater variance among lengths of F1 pollen tubes, presumably indicating the segregation of genetic factors expressed in the gametophyte.Similar frequency distributions of tube lengths in pollen produced by two pairs of reciprocal hybrids virtually excluded the presence of a cytoplasmic component. In vivo competitive ability of pollen tubes was measured as the increase in relative fertilization frequency from apex to base of the ear. Mixtures were made using two types of genetically distinguishable pollen, and were applied to a female common parent. Nine pairs of inbred lines furnished the pollen for the mixtures. In all cases where the B14 line was involved, this pollen type fertilized nearly all the ovules, perhaps indicating the presence of a gametophytic factor. When other lines were compared, the ears contained mixtures of the two possible seed types, the relative proportions of which indicated the differential competitive abilities of the two pollen tube types.A comparison between in vitro and in vivo behavior was made for some genotypes. In vivo results generally agreed with in vitro results. The degree of the differences between lines however was changed, presumably because pollen-style or pollen-pollen interactions are absent in vitro.Differing growth patterns between lines were also revealed in vivo by direct observation of fluorescent pollen tubes within the silks, a finding which may be useful in further studies.

Herbicide-tolerant corn by pollen selection

SummaryMaize pollen was exposed to the herbicide Chlorsulfuron (CS), and segregation for tolerance was observed. The resulting plant generation exhibited significantly greater tolerance to CS than other (control) progeny. Since the increase in tolerance occurred after only one generation of pollen exposure, this result demonstrates that pollen selection can be used to develop herbicide-resistant crop species, even when the species are not amenable to cell culture. It also suggests a possible mechanism for the rapid evolution of herbicide tolerance in weeds.

Gametophytic expression of genes controlling endosperm development in maize

SummaryAn indirect approach was adopted to select viable mutants affecting the male gametophytic generation in maize. This approach consists of a selection of endosperm defective mutants followed by a test for gametophytic gene expression, based on the distortion from mendelian segregation and on the measurement of pollen size and pollen sterility. The material used consisted of 34 endosperm defective viable mutants introgressed in B37 genetic background. Complementation tests indicated that the mutation in the collection of mutants affected different genes controlling endosperm development. The study of the segregation in F2 revealed four classes of de (defective endosperm) mutants: (1) mutants in which the mutation does not affect either gametophytic development or function; (2) mutants in which the effect on the gametophyte affects pollen development processes; (3) mutants showing effects on both pollen development and function, and (4) mutants where only pollen tube growth rate is affected. 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 methabolic complementation between defective pollen and normal styles.

Male Gametophytic Selection in Higher Plants

Gametophytic selection refers to the differential gene transmission during the haploid phase in higher plants, which begins with the meiosis product and ends with fertilization. The special features of the male gametophyte (large population size, independence of the maternal plant, direct exposure to environmental stresses, competition within the same style) suggest that selection in this phase may be expected to act with greater intensity then in the female gametophyte.