Male gametophytic selection in maize

Summary There 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.     

Male gametophyte in maize: II. Pollen vigor in inbred plants

Summary The competitive ability of pollen from inbred plants in mixed pollinations in this study is not merely maintained but enhanced through successive generations of selfing. The data presented suggest two conclusions: 1) the possible existence of pollen-stylar interactions during successive selfings, which select for certain pollen genotypes, those best suited for rapid growth through self styles; and 2) the presence of sporophytic vigor in the heterotic F₁ sporophyte, or its absence in the depressed F₇ sporophyte, is not necessarily demonstrated in the gametophytic generation, perhaps because it can be overwhelmed by other factors, e.g. gametophytic response to selection.     

gaMS-1: A gametophytic male sterile mutant in maize

Several pollen-specific genes from different species have been isolated and characterized at the molecular level, but the precise role of most of them is unknown. Mutant analysis represents a direct approach to uncovering gene function, but the paucity of available mutants affecting pollen development and/or function and the poor characterization of the known mutants have so far limited the exploitation of this approach. Here we present the cytological characterization ofgametophytic male sterile-1 (gaMS-1), a maize mutant that we identified in a program of transposon insertion mutagenesis for the production of mutations in gametophytically acting genes involved in microsporogenesis.gaMS-1 is expressed during or immediately after the first microspore division and leads to the production of immature, nonfunctional pollen grains. The mutation appears to affect the events leading to the developmental switch that follows the first microspore mitosis.     

Fertilization in maize indeterminate gametophyte1 mutant

Summary. Mature embryo sacs of the maize mutant indeterminate gametophyte1 displayed different cellular patterns compared to those of the wild type. About 40% of the ig1 embryo sacs contained three or more synergids and two or more egg cells at the micropylar end. During fertilization in embryo sacs with two synergids, both of them frequently degenerated and were penetrated by two pollen tubes. 75% of the embryo sacs containing three or more synergid cells were penetrated by two or more pollen tubes, although most of them had only one degenerated synergid. Multiple fusions between the sperm cells and eggs frequently occurred in the same embryo sac, which subsequently generated multiple embryos. There were two or more central cells in about 33% of ig1 embryo sacs. The largest central cell was usually adjacent to the egg apparatus and contained two unfused polar nuclei, while those extra central cells located at the chalazal end usually had a single nucleus. Fertilization occurred only between the male gamete and the largest binucleate central cell. The extra central cells eventually degenerated after fertilization.Present address: GI Basic Research Center, Mayo Clinic, Rochester, Minnesota, U.S.A.Correspondence and reprints: State Key Laboratory of Plant Physiology and Biochemistry, College of Biological Science, China Agricultural University, Beijing 100094, Peoples Republic of China.     

A multi-nucleate male-sterile mutant of maize with gametophytic expression

 In order to dissect the complex genetic system that controls pollen development, we have undertaken a program of transposon insertion mutagenesis, with the purpose of producing mutations in gametophytically acting genes that are important for this process. The present work reports the developmental cytology of one of the mutants isolated, gaMS-2 (gametophytic male sterile-2). A peculiar feature of the mutant grains was lack of differentiation between the vegetative and the generative nuclei, leading to alteration in number, conformation and placement of nuclei. At anthesis, the grains carrying the mutant allele are about 40% of the normal grain size, contain a very reduced amount of starch and exhibit various nuclear abnormalities. Received: 31 May 1996 / Revision accepted: 26 August 1996     

Male and female gametophyte selection of barley for salt tolerance

The contribution of the gametophyte in the inheritance of salt tolerance was studied by crossing F3 and BC1 hybrids of the tolerant cultivars Rannii 1 and Pirkka with the sensitive cultivar K-30356. The third generation shows that the progenies of heterozygous plants grown in saline conditions show elevated salinity tolerance. Again, a comparison of the BC1 hybrid progenies shows that the male and female gametophytes contribute to the inheritance of salt tolerance. Gametic selection is maximally efficient during the formation of the female gametophyte and the germination of pollen grains on the stigma.     

Male gametophyte development inPlumbago zeylanica: cytoplasm localization and cell determination in the early generative cell

Summary The behavior of the generative cell during male gametophyte development inPlumbago zeylanica was examined by epifluorescence microscopy and electron microscopy with organelle nucleoid as a cytoplasm marker. When the thin sections stained with 4,6-diamidino-2-phenylindoIe (DAPI) were observed under an epifluorescence microscope, two types of fluorescence spots were detected in the cytoplasm of the pollen cells before the second mitosis. The spots emitting stronger fluorescence were confirmed as plastid nucleoids and those emitting dimmer fluorescence were mitochondrial nucleoids. Before the first mitosis, both plastid and mitochondrial nucleoids distributed randomly in the cytoplasm of the microspore. A small lenticular generative cell formed with attachment to the interior of the intine after the mitosis. Small vacuoles were found in the lenticular cell. In the cytoplasm of the lenticular cell, both plastid nucleoids and the small vacuoles were distributed randomly at the very beginning but began to migrate in opposite directions immediately. Plastid nucleoids aggregated to the side of the cell that faces the pollen center and the small vacuoles aggregated to the side of the cell that attaches to the inline. As the result, the lenticular generative cell appeared highly polarized in cytoplasm location soon after the first mitosis. In accordance with the definition of the cytoplasm polarization, the primary wall between the generative and the vegetative cells began to flex and the lenticular generative cell started to protrude towards the pollen center. When the generative cell peeled away from the inline, it was spherical in shape with the pole that aggregated plastids towards the vegetative nucleus. But the cell direction appeared to be transformed immediately. The pole that aggregated small vacuoles turned to the position towards the vegetative nucleus and the pole that aggregated plastid nucleoids turned to the position countering to the vegetative nucleus. A cellular protuberance formed at the edge of the pole that aggregated small vacuoles and elongated into a tapered end that got into contact with the vegetative nucleus. The polarization of the cytoplasm kept constant throughout the second mitosis. The small vacuoles that apportioned to the sperm cell which attached the vegetative nucleus (the leading sperm cell) disappeared during sperm cell maturation. Plastid nucleoids were apportioned to the other sperm cell (the trailing sperm cell) completely. Mitochondrial nucleoids became undetectable after the second mitosis.     

Actin coronas in normal and indeterminate gametophyte1 embryo sacs of maize

 The actin cytoskeletal organization and nuclear behavior of normal and indeterminate gametophyte1 (ig1) embryo sacs of maize were examined during fertilization. After pollination, during degeneration of one of the synergids and before arrival of the pollen tube, the cytoskeletal elements undergo dramatic changes including formation of the actin coronas at the chalazal end of the degenerating synergid and at the interface between the egg cell and central cell. The actin coronas are present only for a limited period of time and their presence is coordinated with pollen tube arrival and fusion of the gametes; they disappear before the zygote divides. This allows us to estimate the frequency of fertilized ovules along the ear. Up to 88% of the ovules on an ear contain actin coronas in the embryo sacs when observed 16–19 h after pollination, indicating the high frequency of fertilizing kernels along the ear at this stage. In the ig embryo sacs, two or more degenerated synergids containing actin coronas at their chalazal ends receive multiple pollen tubes for gametic fusion and can consequently give rise to twin or polyembryos. These findings with the monocot maize are consistent with previous reports on the dicots Plumbago and Nicotiana, suggesting that the formation of actin coronas in the embryo sac during fertilization is a universal phenomenon in angiosperms and is part of a mechanism of interaction between gametic signaling and actin cytoskeleton behavior which appears to precisely position and facilitate the access of male gametes to the egg cell and central cell for fusion. Received: 15 May 1998 / Revision accepted: 17 August 1998     

Male genotype affects female longevity in Drosophila melanogaster

Several recent studies suggest that interactions with conspecific males can reduce the longevity of female Drosophila melanogaster or support the idea that male and female fitness components are involved in antagonistic interactions. Here we report that males from third-chromosome isogenic lines demonstrated significant genetic variation in male reproductive performance and in the longevity of their mates. Increased male performance was marginally significantly associated with one measure of increased female survival rate. However, there was no indication of tradeoffs or negative correlations between male reproductive success and female survival. We discuss alternative hypotheses for the cause of the induced variation in female longevity.     

Localization and fine mapping of gaMS-1, a male gametophytic mutant of maize

Post-meiotic mutants affecting pollen development are fundamental tools for defining the genetic program controlling microsporogenesis and pollen function. An example of such mutants is gametophytic male sterile-1 (gaMS-1). Heterozygous plants gaMS-1/+ that have a normal phenotype and are female fertile, segregate 1:1 normal:sterile pollen grains and their selfed progeny segregates 1:1 normal:semi-sterile plants. With the final aim of isolating the gene, a positional cloning strategy was adopted. In this paper, we report the results of fine mapping GaMS-1 by different types of molecular markers. Two back crosses were used as mapping populations. They were obtained by crossing the line carrying the mutation with the inbred lines Mo17 and WF9, used as recurrent male parents. Linkage disequilibrium analysis allowed assigning GaMS-1 to the short arm of chromosome 2.By the combined use of SSR, AFLP, PCR markers and ESTs a region of 1 cM containing GaMS-1 was delimited. Received: 15 November 2000 / Revision accepted: 24 May 2001     

Synthesis and accumulation of ribosomes in individual cells of the female gametophyte of maize during its development

Summary The pattern of synthesis of ribosomes during three stages of development of the female gametophyte of maize has been studied by in situ hybridization using a ribosomal RNA probe. Changes in volume of individual cells of the embryo sac during its maturation have been determined by confocal microscopy. These data have permitted us to calculate the relative numbers of ribosomes in the cells of the embryo sac at different stages of their maturation. The egg apparatus and the central cell at all stages of development contain several fold greater numbers of ribosomes than are present in the antipodal cells or cells of the surrounding nucellus. The accumulation of ribosomes during embryo sac maturation appears to proceed at a constant and high rate, with the rate being highest in the developing central cell.     

Male genotype influences female reproductive investment in Drosophila melanogaster

In many species, males can influence the amount of resources their mates invest in reproduction. Two favoured hypotheses for this observation are that females assess male quality during courtship or copulation and alter their investment in offspring accordingly, or that males manipulate females to invest heavily in offspring produced soon after mating. Here, we examined whether there is genetic variation for males to influence female short-term reproductive investment in Drosophila melanogaster, a species with strong sexual selection and substantial sexual conflict. We measured the fecundity and egg size of females mated to males from multiple isofemale lines collected from populations around the globe. Although these traits were not strongly influenced by the male's population of origin, we found that 22 per cent of the variation in female short-term reproductive investment was attributable to the genotype of her mate. This is the first direct evidence that male D. melanogaster vary genetically in their proximate influence on female fecundity, egg size and overall reproductive investment.     

Comparison between responses to gametophytic and sporophytic recurrent selection in maize (Zea mays L.)

Summary Experiments were conducted to determine the chromosomal location of the gene conditioning overproduction of a methionine-rich, 10-K zein in maize kernels of line BSSS53. In addition, the chromosomal location of the structural gene encoding the overproduced protein was determined. Whereas the structural gene, designated Zps10/(22), was found to be located on the long arm of chromosome 9 near the centromere, the locus regulating overproduction of the zein protein was mapped to the short arm of chromosome 4. This regulatory gene has been designated Zpr10/(22). Regulation of 10-K zein production by Zpr10/(22) is, therefore, via a trans-acting mechanism.     

Nuclear behavior is defective in the maize (Zea mays L.) lethal ovule2 female gametophyte

It has long been known that the maize lethal ovule2 mutation results in ovule abortion but has a much smaller effect on pollen development or function. The behavior of the nuclei, the microtubular cytoskeleton and other events were examined in normal and lo2 mutant female gametophytes in order to obtain an understanding the role of this gene in embryo sac formation. The effect of the lo2 mutation is manifested following meiosis. When the surviving single megaspore carries the mutant lo2 allele, often both the megaspore and its nucleus greatly enlarge, but the nucleus either fails to divide or divides only once or twice. Micronuclei are frequently present, nuclei are often clustered and the abundance and patterns of microtubules are abnormal in the mutant embryo sacs. The mutant female gametophytes are blocked at the one-, two- or four-nucleate stage. Nearly all the embryo sacs containing the lo2 allele fail to function as evidenced by the failure of transmission of closely linked loci. When mutant female gametophyte development is arrested, the immature embryo sac degenerates. This mutation appears to identify a gene that is essential in the female gametophyte for normal nuclear division and migration and the normal accompanying tubulin cytoskeleton behavior.