High-rate spontaneous reversion to cytoplasmic male sterility in sugar beet: a characterization of the mitochondrial genomes

Summary Among the fertile sugar beet lines with nuclear sterility maintenance genes, rf, in a homozygous recessive state, sublines capable of reverting spontaneously at a high rate to sterility were identified. Of 24 related fertile sublines studied, 6 were found to spontaneously revert to sterility with a frequency of about 19%. Genetic analysis confirmed the cytoplasmic nature of spontaneously arising sterility. Reversion to sterility in these sublines was accompanied by alterations in the mitochondrial genome structure: loss of the autonomously replicating minicircle c (1.3 kb) and changes in the restriction patterns of high-molecular-weight mitochondrial DNA (mtDNA). Southern hybridixation analysis with cloned minicircle c as a probe revealed no integration of this DNA molecule into the main mitochondrial and nuclear genomes of the revertants. Comparative BamHI and EcoRI restriction analysis of the mtDNA from the sterile revertants and fertile parental subline showed that the spontaneous reversion is accompanied by extensive genomic rearrangement. Southern blot analysis with cloned -subunit of F₁-ATPase (atpA) and cytochrome c oxidase subunit II (COX II) genes as probes indicated that the changes in mtDNA accompanying spontaneous reversion to sterility involved these regions. The mitochondrial genomes of the spontaneous revertants and the sterile analogue were shown to be identical.     

Free-radical-induced mutation vs redox regulation: Costs and benefits of genes in organelles

The prokaryotic endosymbionts that became plastids and mitochondria contained genes destined for one of three fates. Genes required for free-living existence were lost. Most genes useful to the symbiosis were transferred to the nucleus of the host. Some genes, a small minority, were retained within the organelle. Here we suggest that a selective advantage of movement of genes to the nucleus is decreased mutation: plastids and mitochondria have high volume-specific rates of redox reactions, producing oxygen free radicals that chemically modify DNA. These mutations lead to synthesis of modified electron carriers that in turn generate more mutagenic free radicals—the “vicious circle” theory of aging. Transfer of genes to the nucleus is also advantageous in facilitating sexual recombination and DNA repair. For genes encoding certain key components of photosynthesis and respiration, direct control of gene expression by redox state of electron carriers may be required to minimize free radical production, providing a selective advantage of organelle location which outweighs that of location in the nucleus. A previous proposal for transfer of genes to the nucleus is an economy of resources in having a single genome and a single apparatus for gene expression, but this argument fails if any organellar gene is retained. A previous proposal for the retention of genes within organelles is that certain proteins are organelle-encoded because they cannot be imported, but there is now evidence against this view. Decreased free radical mutagenesis and increased sexual recombination upon transfer to the nucleus together with redox control of gene expression in organelles may now account for the slightly different gene distributions among nuclei, plastids, and mitochondria found in major eukaryote taxa. This analysis suggests a novel reason for uniparental inheritance of organelles and the evolution of anisogametic sex, and may also account for the occurrence of nitrogen fixation in symbionts rather than in nitrogen-fixing organelles. Correspondence to: J.F. Allen     

Cloning and characterization of a 70 kd heat shock cognate (hsc70) gene from the zebrafish (Danio rerio)

The heat shock 70 family of proteins is one of the most highly conserved among all species. The genes encoding these proteins have been cloned and sequenced from bacterial species to humans with a high degree of homology preserved throughout evolution. Here we describe the cloning and characterization of a cDNA encoding a 70 kd heat shock cognate (hsc70) gene from the zebrafish (Danio rerio). A high degree of conservation is observed among hsc70 genes of other species as shown by phylogenetic analysis. The characterization of a hsc70 gene in the zebrafish provides a marker for studying the role of a constitutively expressed member of the hsp70 family in an important developmental and evolutionary model system.     

Seasonal patterns of dehydrins and 70-kDa heat-shock proteins in bark tissues of eight species of woody plants

Although considerable effort has been directed at identifying and understanding the function and regulation of stress-induced proteins in herbaceous plants, reports concerning woody plants are limited. Studies with herbaceous crops have revealed similarities in the types of proteins that accumulate in response to a wide array of abiotic stresses and hormonal cues such as the accumulation of abscisic acid. Many of the identified proteins appear to be related to dehydrins (the D-11 subgroup of late-embryogenesis-abundant proteins). The objective of the present study was to determine if seasonal induction of dehydrins is a common feature in woody plants and to see if seasonal patterns existed for other stress-induced proteins. Bark tissues from eight species of woody plants were collected monthly for a period of 1.5 years. The species included: peach (Prunus persica) cv. Loring; apple (Malus domestica) cv. Golden Delicious; thornless blackberry (Rubus sp.) cv. Chester; hybrid poplar (Populus nigra); weeping willow (Salix babylonica); flowering dogwood (Cornus florida); sassafras (Sassafras albidum); and black locust (Robinia pseudo-acacia). Immunoblots of bark proteins were probed with a polyclonal antibody recognizing a conserved region of dehydrin proteins, and monoclonal antibodies directed against members of the HS70 family of heat-shock proteins. Some proteins, immunologically related to dehydrins, appeared to be constitutive; however, distinct seasonal patterns associated with winter acclimation were also observed in all species. The molecular masses of these proteins varied widely, although similarities were observed in related species (willow and poplar). Identification of proteins using the monoclonal antibodies (HSP70, HSC70, BiP) was more definitive because of their inherent specificity, but seasonal patterns were more variable among the eight species examined. This study represents only a precursory examination of several proteins reported to be stress related in herbaceous plants, but the results indicate that these proteins are also common to woody plants and that further research to characterize their regulation and function in relation to stress adaptation and the perennial life cycle of woody plants is warranted.     

一个恢复力受单基因控制的水稻CMS育性回复突变体

利用￣（６０）Ｃｏ－γ射线对具有印尼水田谷细胞质的籼稻细胞质雄性不育系Ⅱ－３２Ａ干种子进行诱变处理,获得了一育性回复突变体Ｔ２４。育性基因未纯合的突变体分离出可育株和完全不育株,比例为３∶１;其与Ⅱ－３２Ａ和珍汕９７Ａ测交,Ｆ１代分离出１∶１的可育株和不育株。育性稳定株系与Ⅱ－３２Ａ和珍汕９７Ａ杂交,Ｆ２分离成３∶１的可育株和不育株。表明其育性回复是由一对基因显性突变所致。这一突变体对不育系的育性恢复机制不同于明恢６３、２０９６４等恢复系,后者表现为两对显性恢复基因作用。未观察到Ｔ２４与亲本Ⅱ－３２Ａ除育性以外的其他性状的差异,因而两者构成育性恢复基因的近等基因系。本文还对不育系育性回复类型和Ｔ２４的理论意义与育种价值进行了讨论。     

Induction of paternal genome loss by the paternal-sex-ratio chromosome and cytoplasmic incompatibility bacteria (Wolbachia): A comparative study of early embryonic events

Paternal genome loss (PGL) during early embryogenesis is caused by two different genetic elements in the parasitoid wasp, Nasonia vitripennis. Paternal sex ratio (PSR) is a paternally inherited supernumerary chromosome that disrupts condensation of the paternal chromosomes by the first mitotic division of fertilized eggs. Bacteria belonging to the genus Wolbachia are present in Nasonia eggs and also disrupt paternal chromosome condensation in crosses between cytoplasmically incompatible strains. Cytoplasmic incompatibility Wolbachia are widespread in insects, whereas PSR is specific to this wasp. PGL results in production of male progeny in Nasonia due to haplodiploid sex determination. The cytological events associated with PGL induced by the PSR chromosome and by Wolbachia were compared by fluorescent light microscopy using the fluorochrome Hoescht 33258. Cytological examination of eggs fertilized with PSR-bearing sperm revealed that a dense paternal chromatin mass forms prior to the first metaphase. Quantification of chromatin by epifluorescence indicates that this mass does undergo replication along with the maternal chromatin prior to the first mitotic division but does not replicate during later mitotic cycles. Contrary to previous reports using other staining methods, the paternal chromatin mass remains condensed during interphase and persists over subsequent mitotic cycles, at least until formation of the syncytial blastoderm and cellularization, at which time it remains near the center of the egg with the yolk nuclei. Wolbachia-induced PGL shows several marked differences. Most notable is that the paternal chromatin mass is more diffuse and tends to be fragmented during the first mitotic division, with portions becoming associated with the daughter nuclei. Nuclei containing portions of the paternal chromatin mass appear to be delayed in subsequent mitotic divisions relative to nuclei free of paternal chromatin. Crosses combining incompatibility with PSR were cytologically similar to Wolbachia-induced PGL, although shearing of the paternal chromatin mass was reduced. Wolbachia may, therefore, block an earlier stage of paternal chromatin processing in the fertilized eggs than does PSR. © 1995 Wiley-Liss, Inc.     

EVOLUTION OF INCOMPATIBILITY-INDUCING MICROBES AND THEIR HOSTS

In many insect species, males infected with microbes related to Wolbachia pipientis are “incompatible” with uninfected females. Crosses between infected males and uninfected females produce significantly fewer adult progeny than the other three possible crosses. The incompatibility-inducing microbes are usually maternally transmitted. Thus, incompatibility tends to confer a reproductive advantage on infected females in polymorphic populations, allowing these infections to spread. This paper analyzes selection on parasite and host genes that affect such incompatibility systems. Selection among parasite variants does not act directly on the level of incompatibility with uninfected females. In fact, selection favors rare parasite variants that increase the production of infected progeny by infected mothers, even if these variants reduce incompatibility with uninfected females. However, productivity-reducing parasites that cause partial incompatibility with hosts harboring alternative variants can be favored once they become sufficiently abundant locally. Thus, they may spread spatially by a process analogous to the spread of underdominant chromosome rearrangements. The dynamics of modifier alleles in the host are more difficult to predict, because such alleles will occur in both infected and uninfected individuals. Nevertheless, the relative fecundity of infected females compared to uninfected females, the efficiency of maternal transmission and the mutual compatibility of infected individuals all tend to increase under within-population selection on both host and parasite genes. In addition, selection on host genes favors increased compatibility between infected males and uninfected females. Although vertical transmission tends to harmonize host and parasite evolution, competition among parasite variants will tend to maintain incompatibility.     

Molecular evolution of the HSP70 multigene family

Eukaryotic genomes encode multiple 70-kDa heat-shock proteins (HSP70s). The Saccharomyces cerevisiae HSP70 family is comprised of eight members. Here we present the nucleotide sequence of the SSA3 and SSB2 genes, completing the nucleotide sequence data for the yeast HSP70 family. We have analyzed these yeast sequences as well as 29 HSP70s from 24 additional eukaryotic and prokaryotic species. Comparison of the sequences demonstrates the extreme conservation of HSP70s; proteins from the most distantly related species share at least 45% identity and more than one-sixth of the amino acids are identical in the aligned region (567 amino acids) among all proteins analyzed. Phylogenetic trees constructed by two independent methods indicate that ancient molecular and cellular events have given rise to at least four monophyletic groups of eukaryotic HSP70 proteins. Each group of evolutionarily similar HSP70s shares a common intracellular localization and is presumed to be comprised of functional homologues; these include heat-shock proteins of the cytoplasm, endoplasmic reticulum, mitochondria, and chloroplasts. HSP70s localized in mitochondria and plastids are most similar to the DnaK HSP70 homologues in purple bacteria and cyanobacteria, respectively, which is consistent with the proposed prokaryotic origin of these organelles. The analyses indicate that the major eukaryotic HSP70 groups arose prior to the divergence of the earliest eukaryotes, roughly 2 billion years ago. In some cases, as exemplified by the SSA genes encoding the cytoplasmic HSP70s of S. cerevisiae, more recent duplication events have given rise to subfamilies within the major groups. The S. cerevisiae SSB proteins comprise a unique subfamily not identified in other species to date. This subfamily appears to have resulted from an ancient gene duplication that occurred at approximately the same time as the origin of the major eukaryotic HSP70 groups. Correspondence to: E.A. Craig     

Phylogenetic analysis of the stress-70 protein family

The eukaryotic cyto-/nucleoplasmatic 70-kDa heat-shock protein (HSP70) has homologues in the endoplasmic reticulum as well as in bacteria, mitochondria, and plastids. We selected a representative subset from the large number of sequenced stress-70 family members which covers all known branches of the protein family and calculated and manually improved an alignment. Here we present the consensus sequence of the aligned proteins and putative nuclear localization signals (NLS) in the eukaryotic HSP70 homologues. The phylogenetic relationships of the stress-70 group family members were estimated by use of different computation methods. We present a phylogenetic tree containing all known stress-70 subfamilies and demonstrate the usefulness of stress-70 protein sequences for the estimation of intertaxonic phylogeny. Correspondence to: S.A. Reusing     

Cell Size and the Heat-Shock Response in Rat Brain

Abstract: The expression of mRNAs encoding two members of the heat-shock protein 70 family, the constitutively-expressed heat-shock cognate (hsc70) mRNA and the strictly heat-inducible (hsp70) mRNA, was quantitated in cerebellar and hippocampal cells of rats 3 h after amphetamine-induced or heat-induced hyperthermia. Intracellular heat-shock mRNA levels in specific cell types were compared with those of total polyadenylic acid [poly(A)] mRNA or 18S rRNA in the same cell type. Levels of poly(A) mRNAs, 18S rRNAs, and hsc70 mRNAs were highest in large neurons and lowest in glia. hsp70 mRNAs were also present at highest levels in large neurons, suggesting that hsp70 mRNAs accumulated as rapidly in these cell types as they did in small neurons and glia. However, compared with levels of intracellular poly(A) mRNAs or levels of rRNAs, large neurons contained two- to 12-fold lower levels of hsp70 mRNAs than neurons of intermediate size and five- to 30-fold lower levels than glia. These results suggest that hsp70 mRNAs accumulated as rapidly in large neurons as in small neurons and glia, but that the large size of these neurons precluded intracellular hsp70 mRNA concentrations increasing as quickly. The susceptibility of large neurons to stress-induced cell death could be due, in part, to their inability to synthesize rapidly hsp70 in sufficient amounts to protect these cells from the initial molecular consequences of stress.