Partial Male Sterility Research Articles

Identification and characterization of dwarf 62, a loss-of-function mutation in DLT/OsGRAS-32 affecting gibberellin metabolism in rice

A dwarf mutant, dwarf 62 (d62), was isolated from rice cultivar 93-11 by mutagenesis with γ-rays. Under normal growth conditions, the mutant had multiple abnormal phenotypes, such as dwarfism, wide and dark-green leaf blades, reduced tiller numbers, late and asynchronous heading, short roots, partial male sterility, etc. Genetic analysis indicated that the abnormal phenotypes were controlled by the recessive mutation of a single nuclear gene. Using molecular markers, the D62 gene was fine mapped in 131-kb region at the short arm of chromosome 6. Positional cloning of D62 gene revealed that it was the same locus as DLT/OsGRAS-32, which encodes a member of the GRAS family. In previous studies, the DLT/OsGRAS-32 is confirmed to play positive roles in brassinosteroid (BR) signaling. Sequence analysis showed that the d62 carried a 2-bp deletion in ORF region of D62 gene which led to a loss-of-function mutation. The function of D62 gene was confirmed by complementation experiment. RT-PCR analysis and promoter activity analysis showed that the D62 gene expressed in all tested tissues including roots, stems, leaves and panicles of rice plant. The d62 mutant exhibited decreased activity of α-amylase in endosperm and reduced content of endogenous GA(1). The expression levels of gibberellin (GA) biosynthetic genes including OsCPS1, OsKS1, OsKO1, OsKAO, OsGA20ox2/SD1 and OsGA2ox3 were significantly increased in d62 mutant. Briefly, these results demonstrated that the D62 (DLT/OsGRAS-32) not only participated in the regulation of BR signaling, but also influenced GA metabolism in rice.

Production of aborted pollen in marsh species of Chenopodiaceae: evidence of partial male sterility in Suadeaea and Salsoleae species

The main objective of this study is to establish the level of pollen abortion prior to its release from the anther in species of Chenopodiaceae in marsh habitats and to compare this level among the taxons, locations and positions of the flowers in the dichasium and the relative positions of the anthers. We established the level of nonfunctional pollen formation in 39 samples belonging to 10 species, using lactophenol cotton blue staining. Aborted pollen grains were found to be significantly smaller than normal ones. The results revealed differences in the percentage of normal grains among different species and tribes studied: Atripliceae (56.1 ± 27.1%), Salicornieae (80.36 ± 16.52%), Suaedeae (54.7 ± 31.2%) and Salsoleae (32.5 ± 25.7%). In some species there were significant differences among the populations studied, but in species of Saliconieae we found no differences either between different positions in the dicasia or in the anther. In the tribes Suaedeae and Salsoleae, we postulate the existence of a system that maintains different levels of partial male sterility among individuals in the populations, which would reduce the autogamous fruit set rate and favor the cross-pollination rates of sterile male individuals. We base this on high intrapopulation variability at those levels, on the constancy with which they are presented in the different populations studied, and on the lack of interannual differences.

BoPMEI1, a pollen-specific pectin methylesterase inhibitor, has an essential role in pollen tube growth

Pectin methylesterase (PME) is known to have important roles in pollen development and pollen tube growth. As pivotal regulatory factors in PME activity modulation, PME inhibitors (PMEIs) are thought to be key regulators of cell wall stability at the tip of the pollen tube. We report on the cloning and characterization of a novel B. oleracea PMEI gene, BoPMEI1. Heterologously expressed BoPMEI1 showed PMEI activity. RT-PCR studies of different tissues and promoter-GUS fusions confirmed that BoPMEI1 was specifically expressed in mature pollen grains and pollen tubes. Based on in vivo transient assays, we found that BoPMEI1 appears to be largely localized to the plasma membrane. Transgenic Arabidopsis plants expressing antisense BoPMEI1 under the control of the CaMV 35S promoter suppressed the expression of the orthologous gene At1g10770, which led to partial male sterility and decreased seed set by inhibition of pollen tube growth. Our study demonstrates the involvement of BoPMEI1 in pollen tube growth.

A novel partial male sterility resource in bread wheat

A plant of the wheat cultivar Moulin grown in a greenhouse was found to be completely sterile. The study of the F1 and F2 offspring of the plant pollinated by Moulin showed that in field conditions the sterility was partial (fertility reduced by 91.6% with only a few completely sterile plants), recessive and monogenic. The F1 crosses of the mutant Moulin with Ms1b/ms1b Sirocco and Probus monosomic 4B showed that the gene for sterility is not located in chromosome 4B and hence cannot be allelic to the recessive allele ms1b which confers complete male sterility in wheat. The expression of sterility was demonstrated in wheat backgrounds with high levels of fertility. This is the first time partial-monogenic sterility is reported in wheat. This kind of sterility is of no interest for producing wheat F1 hybrids. However, it might be of interest for recurrent selection, or dynamic management of genetic resources where limited intermating is required, as well as in the study of genes involved in microsporogenesis.

The HD‐ZIP IV transcription factor OCL4 is necessary for trichome patterning and anther development in maize

Among the genes controlling the differentiation and maintenance of epidermal cell fate are members of the HD-ZIP IV class family of plant-specific transcription factors, most of which are specifically expressed in the epidermis of tissues. Here, we report the functional analysis of the maize HD-ZIP IV gene OCL4 (outer cell layer 4) via the phenotypic analysis of two insertional mutants, and of OCL4-RNAi transgenic plants. In all three materials, the macrohairs, one of the three types of trichomes present on adult maize leaf blades, developed ectopically at the margin of juvenile and adult leaves. Consistent with this phenotype, OCL4 is expressed in the epidermis of the leaf blade, with a maximum at the margin of young leaf primordia. Expression of OCL4 in the model plant Arabidopsis under the control of the GLABRA2 (GL2) promoter, a member of the Arabidopsis HD-ZIP IV family involved in trichome differentiation, did not complement the gl2-1 mutant, but instead aggravated its phenotype. The construct also caused a glabrous appearance of rosette leaves in transformed control plants of the Ler ecotype, suggesting that OCL4 inhibits trichome development both in maize and Arabidopsis. Furthermore, insertional mutants showed a partial male sterility that is likely to result from the presence of an extra subepidermal cell layer with endothecium characteristics in the anther wall. Interestingly, the epidermis-specific OCL4 expression in immature anthers was restricted to the region of the anther locule where the extra cell layer differentiated. Taken together these results suggest that OCL4 inhibits trichome development and influences division and/or differentiation of the anther cell wall.

Genetic and cytological analyses of the male sterility mutation induced in a sorghum tissue culture with streptomycin

Treatment of sorghum callus cultures with 500-1000 mg/l streptomycin led to a high regeneration frequency of plants with complete or partial male sterility (MS), up to 100% of all green regenerants. The induced MS mutation (ms-str) was preserved in the F1 and BC1 progenies and was genetically unstable: many families produced semisterile and fertile revertants, whose progenies again contained semisterile and sterile mutants. The ms-str mutation was maintained through eight generations via selection and self-pollination of semisterile plants. The mutation was inherited as a recessive nuclear mutation in tester crosses of sterile plants segregated in the progenies of fertile and semisterile revertants and was expressed only in single cases in a test cross for ms-str transfer through pollen of hybrids with restored male fertility. Recessive nuclear mutations determining a low plant height (dwarfness) and the lack of waxy bloom on the stem and leaves (bloomless) were found in male-sterile plants with the ms-str mutation. Cytological analysis of sterile plants reveal multiple alterations of various pollen development stages and tapetal cells: cytomyxis, defects of chromosome conjugation, distorted cytokinesis in meiotic division II, a lack of tetrad separation, a defective formation of the microspore coat, generation of microspores with two to four nuclei, and the formation of micronuclei and large vacuoles in tapetal cells. A possible transfer of the induced cytoplasmic MS mutation into the nuclear genome and the causes of the high genetic instability are discussed.

Pre-dispersal seed predation reduces the reproductive compensatory advantage of thrum individuals in Erythroxylum havanense (Erythroxylaceae)

Male-sterility mutations in hermaphroditic species represent the first step in the evolution of gender specialization. Male-sterile individuals commonly compensate the loss of the male function by increasing the number or quality of seeds. Because the magnitude of compensation determines the maintenance of females within populations and the evolution of sexual specialization, plant-animal interactions may affect these evolutionary processes if animals are sensitive to such reproductive asymmetries. Here we explore the effect of seed predation on the reproductive compensation of Erythroxylum havanense, a distylous shrub with morph-biased partial male sterility, during two consecutive years. Seed predation reduced the compensatory advantage of thrums in 1987, but not in 1988. Annual differences in the intensity of seed predation seem to be accounted for differences in the onset and synchrony of flowering. Thus, although seed predators may reduce the compensatory advantage of thrums, their impact is modulated by the environmental cues triggering flowering and insect emergence.

The isolation and characterization of an Arabidopsis mutant mutagenized by low-energy ion beam

A screen for Arabidopsis fertility mutants, mutagenized by low-energy ion beam, yields two partial male-sterile mutants tc243- 1 and tc243- 2 that have similar phenotype. And tc243- 2 is investigated in detail. Gene mapping and complementation testing indicate that it is allelic to auxin-signaling mutant axr1. A repetitive insertion of “TTATA” in + 624 bp of tc243 leads to premature AXR1. tc243 has severe morphological defects in leaf, plant height, inflorescence and flower morphology. The developmental defects in stamen directly result in the partial male sterility in tc243. In addition, as a strong mutant allele of axr1, tc243 is more insensitive to auxin than axr1- 3, a weak allele of axr1. However, the root growth of tc243 is more inhibited by MeJA than that of axr1- 3. These results suggest that flower development and root growth can be regulated by different downstream pathways of AXR1-mediated jasmonate signaling, and that the inactivity of AXR1 can lead to the increasing efficiency of AXL1-mediated jasmonate signaling.

Symmetric pollen mitosis I and suppression of pollen mitosis II prevent pollen development in Brachiaria jubata (Gramineae)

Microsporogenesis and pollen development were analyzed in a tetraploid (2n = 4x = 36) accession of the forage grass Brachiaria jubata (BRA 007820) from the Embrapa Beef Cattle Brachiaria collection that showed partial male sterility. Microsporocytes and pollen grains were prepared by squashing and staining with 0.5% propionic carmine. The meiotic process was typical of polyploids, with precocious chromosome migration to the poles and laggards in both meiosis I and II, resulting in tetrads with micronuclei in some microspores. After callose dissolution, microspores were released into the anther locule and appeared to be normal. Although each microspore initiated its differentiation into a pollen grain, in 11.1% of them nucleus polarization was not observed, i.e., pollen mitosis I was symmetric and the typical hemispherical cell plate was not detected. After a central cytokinesis, two equal-sized cells showing equal chromatin condensation and the same nuclear shape and size were formed. Generative cells and vegetative cells could not be distinguished. These cells did not undergo the second pollen mitosis and after completion of pollen wall synthesis each gave rise to a sterile and uninucleate pollen grain. The frequency of abnormal pollen mitosis varied among flowers and also among inflorescences. All plants were equally affected. The absence of fertile sperm cells in a considerable amount of pollen grains in this accession of B. jubata may compromise its use in breeding and could explain, at least in part, why seed production is low when compared with the amount of flowers per raceme.

A plant plasma membrane Ca2+ pump is required for normal pollen tube growth and fertilization.

Ca(2+) signals are thought to play important roles in plant growth and development, including key aspects of pollen tube growth and fertilization. The dynamics of a Ca(2+) signal are largely controlled by influx (through channels) and efflux (through pumps and antiporters). The Arabidopsis genome encodes 14 Ca(2+) pumps, 10 of which belong to a family of autoinhibited Ca(2+) ATPases (ACA) that are predicted to be activated by Ca(2+)/calmodulin. Here, we show that isoform ACA9 is expressed primarily in pollen and localized to the plasma membrane. Three independent T-DNA [portion of the Ti (tumor-inducing) plasmid that is transferred to plant cells] gene disruptions of ACA9 were found to result in partial male sterility. Complementation was observed by using a ACA9-yellow fluorescence protein (YFP) fusion that displayed plasma membrane localization. Mutant aca9 pollen displayed a reduced growth potential and a high frequency of aborted fertilization, resulting in a >80% reduction in seed set. These findings identify a plasma membrane Ca(2+) transporter as a key regulator of pollen development and fertilization in flowering plants.

Nuclear-cytoplasmic male-sterility in diploid dandelions.

Male-sterility was found in diploid dandelions from two widely separated populations from France, and its inheritance was analysed by crossing a diploid male-sterile dandelion to diploid sexuals and triploid apomicts. Nuclear genetic variation, found in full-sib families, segregated for male-fertility, partial male-sterility, and full male-sterility, and also segregated for small-sized versus normally sized pollen. The crossing results are best explained by a cytoplasmic male-sterility factor in combination with two dominant restorer genes. Involvement of the cytoplasmic male-sterility factor was further investigated by chloroplast haplotyping. Male-sterility was exclusively associated with a rare chloroplast haplotype (designated 16b). This haplotype was found in seven male-sterile plants and one (apparently restored) male-fertile individual but does not occur in 110 co-existing male-fertile plants and not in several hundreds of individuals previously haplotyped. Apomicts with cytoplasmic male sterility were generated in some test crosses. This raises the question as to whether the male sterility found in natural dandelion apomicts, is of cytoplasmic or of nuclear genetic nature. As many breeding systems in Taraxacum are involved in shaping population structure, it will be difficult to predict the evolutionary consequences of nuclear-cytoplasmic male-sterility for this species complex.

A model for chloroplast capture.

Chloroplast capture, the introgression of a chloroplast from one species into another, has been frequently suggested as the explanation for inconsistencies between gene trees based on nuclear and cytoplasmic markers in plants. We use a genetic model to determine the conditions for capture to occur, and we find that they are somewhat more general than those given in earlier verbal arguments. Chloroplast capture can occur if cytoplasm substitution provides an advantage in seed production. This can happen through reallocation to the female function when cytonuclear incompatibilities cause partial male sterility, but also under more general conditions. Capture is promoted by nuclear incompatibilities between the two genomes (or a low heterosis in F1 hybrids) and by partial selfing when hybridization causes a decrease in the selfing rate and inbreeding depression is strong. We discuss empirical predictions that can be used to test this mechanism.

A MODEL FOR CHLOROPLAST CAPTURE

Chloroplast capture, the introgression of a chloroplast from one species into another, has been frequently suggested as the explanation for inconsistencies between gene trees based on nuclear and cytoplasmic markers in plants. We use a genetic model to determine the conditions for capture to occur, and we find that they are somewhat more general than those given in earlier verbal arguments. Chloroplast capture can occur if cytoplasm substitution provides an advantage in seed production. This can happen through reallocation to the female function when cytonuclear incompatibilities cause partial male sterility, but also under more general conditions. Capture is promoted by nuclear incompatibilities between the two genomes (or a low heterosis in F1 hybrids) and by partial selfing when hybridization causes a decrease in the selfing rate and inbreeding depression is strong. We discuss empirical predictions that can be used to test this mechanism.

Partial male sterility and the evolution of nuclear gynodioecy in plants.

Gynodioecy, a genetic dimorphism of females and hermaphrodites, is pertinent to an understanding of the evolution of plant gender, mating and genetic variability. Classical models of nuclear gynodioecy attribute the maintenance of the dimorphism to frequency-dependent selection in which the female phenotype has a fitness advantage at low frequency owing to a doubled ovule fertility. Here, I analyse explicit genetic models of nuclear gynodioecy that expand on previous work by allowing partial male sterility in combination with either fixed or dynamically evolving mutational inbreeding depression. These models demonstrate that partial male sterility causes fitness underdominance at the mating locus, which can prevent the spread of females. However, if partial male sterility is compensated by a change in selfing rate, overdominance at the mating locus can cause the spread of females. Overdominance at introduction of the male sterility allele can be caused by high inbreeding depression and a lower selfing rate in the heterozygote, by purging of mutations by a higher selfing rate in the heterozygote, and by low inbreeding depression and a higher selfing rate in the heterozygote. These processes might be of general importance in the maintenance of mating polymorphisms in plants.

Sterility of males determined by functional features of the mouse spermatozoa bearing t-complex

The mechanisms underlying normal spermatogenesis and its pathology expressed as male sterility determined by t-complex located on chromosome 17 in mice are considered in this review. t-Complex is a very convenient model with diverse markers of expression of the genes involved in development of the functional features of the spermatozoa bearing t-complex. These features include defects of mobility, capacitation, and acrosome reactions, which determine full or partial male sterility. It has been proposed that the defects of capacitation are also inherent in humans and affect male fertility. This homology is confirmed by the presence of the male gene Tcp11 in humans and demonstration of the fact that the protein TCP11 plays a leading role in modulation of the capacitation of murine spermatozoa. Hence it follows that the defects of human genes leading to incomplete binding of the fertilization promoting peptide could play a certain role in a decreased male fertility. All this is essential not only for deeper understanding of the biology of spermatozoa, but also for development of new therapeutic methods of finding and treating the semen pathology.

Molecular Evolution of Tandemly Repeated Heterochromatic Gene Clusters Involving a Switch of Their Function in the Genome of Drosophila melanogaster

Molecular evolution and divergence of paralogous tandem heterochromatic repeats Stellate and Su(Ste) located on the X and Y chromosomes, respectively, are discussed. These repeats appear to emerge as a result of amplification of a unique autosomal euchromatic gene encoding the regulatory β-subunit of the CK2 protein kinase. The autosomal gene and the clusters of heterochromatic repeats are transcribed in testes. A high level of the Stellate expression leads to partial male sterility and abnormal meiosis. The Stellate expression and its adverse effects are suppressed by homologous Su(Ste) repeats. In genome evolution, the open reading frames (ORF) of Stellate and Su(Ste) putative ancestor were maintained by translational selection. Then the coding Su(Ste) function has been damaged and switched to a new one resulting in antisense Su(Ste) transcription and silencing of theStellategenes. Symmetrical (sense and antisense) Su(Ste) transcription causes the formation of a double-strand RNA, which by interference (selective elimination of the homologous gene expression) suppresses the Stellate genes. The biological significance and the driving forces of evolution of the species-specific balanced interaction of Stellateand Su(Ste) repeats remain mysterious.

Evidence for Dobzhansky-Muller incompatibilites contributing to the sterility of hybrids between Mimulus guttatus and M. nasutus.

Both chromosomal rearrangements and negative interactions among loci (Dobzhansky-Muller incompatibilities) have been advanced as the genetic mechanism underlying the sterility of interspecific hybrids. These alternatives invoke very different evolutionary histories during speciation and also predict different patterns of sterility in artificial hybrids. Chromosomal rearrangements require drift, inbreeding, or other special conditions for initial fixation and, because heterozygosity per se generates any problems with gamete formation, F1 hybrids will be most infertile. In contrast, Dobzhansky-Muller incompatibilities may arise as byproducts of adaptive evolution and often affect the segregating F2 generation most severely. To distinguish the effects of these two mechanisms early in divergence, we investigated the quantitative genetics of hybrid sterility in a line cross between two members of the Mimulus guttatus species complex (M. guttatus and M. nasutus). Hybrids showed partial male and female sterility, and the patterns of infertility were not consistent with the action of chromosomal rearrangements alone. F2 and F1 hybrids exhibited equal decreases in pollen viability (> 40%) relative to the highly fertile parental lines. A large excess of completely pollen-sterile F2 genotypes also pointed to the segregation of Dobzhansky-Muller incompatibility factors affecting male fertility. Female fertility showed a pattern similarly consistent with epistatic interactions: F2 hybrids produced far fewer seeds per flower than F1 hybrids (88.0 +/- 2.8 vs. 162.9 +/- 8.5 SE, respectively) and either parental line, and many F2 genotypes were completely female sterile. Dobzhansky-Muller interactions also resulted in the breakdown of several nonreproductive characters and appear to contribute to correlations between male and female fertility in the F2 generation. These results parallel and contrast with the genetics of postzygotic isolation in model animal systems and are a first step toward understanding the process of speciation in this well-studied group of flowering plants.

Leaf and Flower Development in Pea (Pisum sativum L.): Mutants cochleata andunifoliata

The stipule mutant cochleata(coch) and the simple-leaf mutantunifoliata (uni) are utilized to increase understanding of the control of compound leaf and flower development in pea. The phenotype of the coch mutant, which affects the basal stipules of the pea leaf, is described in detail. Mutant coch flowers have supernumerary organs, abnormal fusing of flower parts, mosaic organs and partial male and female sterility. The wild-type Coch gene is shown to have a role in inflorescence development, floral organ identity and in the positioning of leaf parts. Changes in meristem size may be related to changes in leaf morphology. In the coch mutant, stipule primordia are small and their development is retarded in comparison with that of the first leaflet primordia. The diameter of the shoot apical meristem of the uni mutant is approx. 25% less than that of its wild-type siblings. This is the first time that a significant difference in apical meristem size has been observed in a pea leaf mutant. Genetic controls in the basal part of the leaf are illustrated by interactions between coch and other mutants. The mutantcoch gene is shown to change stipules into a more ‘compound leaf-like’ identity which is not affected by thestipules reduced mutation. The interaction of coch and tendril-less(tl) genes reveals that the expression of the wild-type Tl gene is reduced at the base of the leaf, supporting the theories of gradients of gene action.

Sterility of Drosophila with mutations in the Bloom syndrome gene--complementation by Ku70.

The Drosophila Dmblm locus is a homolog of the human Bloom syndrome gene, which encodes a helicase of the RECQ family. We show that Dmblm is identical to mus309, a locus originally identified in a mutagen-sensitivity screen. One mus309 allele, which carries a stop codon between two of the helicase motifs, causes partial male sterility and complete female sterility. Mutant males produce an excess of XY sperm and nullo sperm, consistent with a high frequency of nondisjunction and/or chromosome loss. These phenotypes of mus309 suggest that Dmblm functions in DNA double-strand break repair. The mutant Dmblm phenotypes were partially rescued by an extra copy of the DNA repair gene Ku70, indicating that the two genes functionally interact in vivo.

EVIDENCE FOR DOBZHANSKY-MULLER INCOMPATIBILITES CONTRIBUTING TO THE STERILITY OF HYBRIDS BETWEEN MIMULUS GUTTATUS AND M. NASUTUS

Both chromosomal rearrangements and negative interactions among loci (Dobzhansky-Muller incompatibilities) have been advanced as the genetic mechanism underlying the sterility of interspecific hybrids. These alternatives invoke very different evolutionary histories during speciation and also predict different patterns of sterility in artificial hybrids. Chromosomal rearrangements require drift, inbreeding, or other special conditions for initial fixation and, because heterozygosity per se generates any problems with gamete formation, F1 hybrids will be most infertile. In contrast, Dobzhansky-Muller incompatibilities may arise as byproducts of adaptive evolution and often affect the segregating F2 generation most severely. To distinguish the effects of these two mechanisms early in divergence, we investigated the quantitative genetics of hybrid sterility in a line cross between two members of the Mimulus guttatus species complex (M. guttatus and M. nasutus). Hybrids showed partial male and female sterility, and the patterns of infertility were not consistent with the action of chromosomal rearrangements alone. F2 and F1 hybrids exhibited equal decreases in pollen viability (> 40%) relative to the highly fertile parental lines. A large excess of completely pollen-sterile F2 genotypes also pointed to the segregation of Dobzhansky-Muller incompatibility factors affecting male fertility. Female fertility showed a pattern similarly consistent with epistatic interactions: F2 hybrids produced far fewer seeds per flower than F1 hybrids (88.0 ± 2.8 vs. 162.9 ± 8.5 SE, respectively) and either parental line, and many F2 genotypes were completely female sterile. Dobzhansky-Muller interactions also resulted in the breakdown of several nonreproductive characters and appear to contribute to correlations between male and female fertility in the F2 generation. These results parallel and contrast with the genetics of postzygotic isolation in model animal systems and are a first step toward understanding the process of speciation in this well-studied group of flowering plants.Corresponding Editor: S. Tonsor