Organelle inheritance in plant cell division: the actin cytoskeleton is required for unbiased inheritance of chloroplasts, mitochondria and endoplasmic reticulum in dividing protoplasts

Nuclear inheritance is highly ordered, ensuring stringent, unbiased partitioning of chromosomes before cell division. In plants, however, little is known about the analogous cellular processes that might ensure unbiased inheritance of non-nuclear organelles, either in meristematic cell divisions or those induced during the acquisition of totipotency. We have investigated organelle redistribution and inheritance mechanisms during cell division in cultured tobacco mesophyll protoplasts. Quantitative analysis of organelle repositioning observed by autofluorescence of chloroplasts or green fluorescent protein (GFP), targeted to mitochondria or endoplasmic reticulum (ER), demonstrated that these organelles redistribute in an ordered manner before division. Treating protoplasts with cytoskeleton-disrupting drugs showed that redistribution depended on actin filaments (AFs), but not on microtubules (MTs), and furthermore, that an intact actin cytoskeleton was required to achieve unbiased organelle inheritance. Labelling the actin cytoskeleton with a novel GFP-fusion protein revealed a highly dynamic actin network, with local reorganisation of this network itself, appearing to contribute substantially to repositioning of chloroplasts and mitochondria. Our observations show that each organelle exploits a different strategy of redistribution to ensure unbiased partitioning. We conclude that inheritance of chloroplasts, mitochondria and ER in totipotent plant cells is an ordered process, requiring complex interactions with the actin cytoskeleton.     

Endosymbiosis,cell evolution,and speciation

In 1905, the Russian biologist C. Mereschkowsky postulated that plastids (e.g., chloroplasts) are the evolutionary descendants of endosymbiotic cyanobacteria-like organisms. In 1927, I. Wallin explicitly postulated that mitochondria likewise evolved from once free-living bacteria. Here, we summarize the history of these endosymbiotic concepts to their modern-day derivative, the “serial endosymbiosis theory”, which collectively expound on the origin of eukaryotic cell organelles (plastids, mitochondria) and subsequent endosymbiotic events. Additionally, we review recent hypotheses about the origin of the nucleus. Model systems for the study of “endosymbiosis in action” are also described, and the hypothesis that symbiogenesis may contribute to the generation of new species is critically assessed with special reference to the secondary and tertiary endosymbiosis (macroevolution) of unicellular eukaryotic algae.     

Inheritance of mitochondrial and chloroplast genomes in the isogamous brown alga Scytosiphon lomentaria (Phaeophyceae)

Patterns of inheritance of chloroplasts and mitochondria were examined by fluorescence microscopy and haplotype genome markers in the isogamous brown alga Scytosiphon lomentaria (Lyngbye) Link. Germination of the zygote in this species was unilateral, the growing thallus developed entirely from the germ tube, and the original zygote cell did not develop except for the formation of a hair. Inheritance of chloroplasts was biparental, and partitioning of the two parental chloroplasts into the first sporophytic cells was accidental: either the maternal or the paternal chloroplast was migrated from the zygote into the germ tube cell, whereas the other chloroplast remained in the original cell. In contrast, the mitochondrial genome in all cells of the sporophyte came only from the female gamete (maternal inheritance). These inheritance patterns are similar to those of the isogamous brown alga Ectocarpus siliculosus (Dillwyn) Lyngbye. Maternal inheritance of mitochondria might be universal in brown algae.     

The bloom-forming ciliate Mesodinium rubrum harbours a single permanent endosymbiont

Whether the red tide Mesodinium rubrum contains a permanent cryptophyte symbiont or whether it only sequesters chloroplasts from cryptophyte prey was addressed using electron microscopy and the dynamics of photosynthesis, chloroplasts and nuclei. Mesodinium rubrum contains a branched cryptophyte symbiont consisting of many chloroplasts, mitochondria, nucleomorphs, an endoplasmic reticulum and one nucleus. The volume of the symbiont constitutes 36% of the consortium and it is separated from its host by a single-cell membrane. The chloroplasts of Mesodium are larger and morphologically different from two Teleaulax species that served as prey. The symbiont nucleus is also much larger than Teleaulax nuclei. Although M. rubrum is functionally a phototroph, sustained growth beyond two to four generations requires ingestion of prey, but less than one prey cell per generation suffices for maximum growth. This suggests that either the ciliate or its symbiont needs an essential growth factor for continuous growth.     

Maternal inheritance, sexual conflict and the maladapted male

Females differ from males in transmitting not only nuclear genes but also cytoplasmic genetic elements (CGEs), including DNA in mitochondria, chloroplasts and microorganisms that are present in the cell. Until recently, evolutionary research has adopted a nucleocentric approach in which organelles have been viewed as subservient energy suppliers. In this article, we propose that a more equitable view of nuclear genes and organelle genomes will lead to a better understanding of the dynamics of sexual selection and the constraints on male adaptation. Maternal inheritance of CGEs intensifies sexually-antagonistic coevolution and provides a parsimonious explanation for the relatively high frequency in males of such apparently maladaptive traits as infertility, homosexuality and baldness.     

Bacterial symbioses. Predation and mutually beneficial associations.

The endosymbiotic theory, which has proved to explain the origin of mitochondria and chloroplasts, also posits the origin of nucleus and other cellular organelles that could have derived from ancient relationships among bacteria. It seems that predation might have been a prerequisite to the establishment of symbiosis as a source of evolutionary novelty. This review describes current different examples of bacteria able not only to attack and degrade other bacteria, but also to establish stable symbiotic relationships with different eukaryotic organisms.     

Evolution of the chaperonin families (HSP60, HSP 10 and TCP-1) of proteins and the origin of eukaryotic cells

The members of the 10 kDa and 60 kDa heat-shock chaperonin proteins (Hsp10 and Hsp60 or Cpn10 and Cpn60), which form an operon in bacteria, are present in all eubacteria and eukaryotic ceil organelles such as mitochondria and chloroplasts. In archaebacteria and eukaryotic cell cytosol, no close homologues of Hsp10 or Hsp60 have been identified. However, these species (or ceil compartments) contain the Tcp-1 family of proteins (distant homologues of Hsp60). Phylogenetic analysis based on global alignments of Hsp60 and Hsp10 sequences presented here provide some evidence regarding the evolution of mitochondria from a member of the α-subdivision of Gram-negative bacteria and chloroplasts from cyanobacterial species, respectively. This inference is strengthened by the presence of sequence signatures that are uniquely shared between Hsp60 homologues from α-purple bacteria and mitochondria on one hand, and the chloroplasts and cyanobacterial hsp60s on the other. Within the α-purple subdivision, species such as Rickettsia and Ehrlichia, which live intracellularly within eukaryotic cells, are indicated to be the closest relatives of mitochondrial Homologues, In the Hsp60 evolutionary tree, rooted using the Tcp-1 homologue, the order of branching of the major groups was as follows: Gram-positive bacteria — cyanobacteria and chloroplasts — chlamydiae and spirochaetes —β and γ-Gram-negative purple bacteria —α-purple bacteria — mitochondria. A similar branching order was observed independently in the Hsp10 tree. Multiple Hsp60 homologues, when present in a group of species, were found to be clustered together in the trees, indicating that they evolved by independent gene-duplication events. This review also considers in detail the evolutionary relationship between Hsp50 and Tcp-1 families of proteins based on two different models (viz. archaebacterial and chimeric) for the origin of eukaryotic cell nucleus. Some predictions of the chimeric model are also discussed.     

Intensive nuclear selection in tryptophan limited cultures of Escherichia coli B/r/l, trp

Summary The paradoxical constancy of the rate of mutation to resistance to bacteriophage T5 was observed long ago by Novick and Szilard. Mutation rates are independent of growth rate in tryptophan limited chemostat cultures, even though both average cell volume and DNA content increase with generation time. To examine nuclear selection in these multinucleate cells, cultures of E. coli B/r/l, trp were exposed briefly to acridine orange/visible light, and cells repackaged to uninucleate forms by shifting growth rates up to 0.5 divisions per hr. The kinetics of accumulation of mutant nuclei indicates that only a single master nucleus replicates in each cell, one of its progeny becomes the master nucleus during the following nuclear generation in the same cell, and both progeny become master nuclei if the cell divides. Autoradiographs showed that master nuclei are located at the ends of cells and that uninucleate cells are produced at each division, except possibly for a small fraction of aberrant divisions. This intensive selection during replication and inheritance of master nuclei provides an explanation for the constancy of cellular mutation rates, which is due to the constancy of the rate of DNA replication in these cultures and the (almost) complete selection for mutant nuclei in mutated cells.Work supported by the U. S. Atomic Energy Commission.     

How do mitochondrial genes get into the nucleus?

It is well known that genes from chloroplasts and mitochondria were transferred to the nucleus many times during plant evolution. But in what form do the transferred genes physically make that intracellular journey--as RNA, as cDNA, as pieces of organelle DNA, or as whole organelle chromosomes? Current views focus upon cDNA as the vehicle, based upon some examples from plants. But other mechanisms, involving direct transfer of DNA from organelle chromosomes, could also account for the available data. Direct DNA transfer, rather than cDNA-mediated transfer, does occur today, and it probably prevailed during the early phases of organelle evolution.     

Bacteria and Archaea Group II introns: additional mobile genetic elements in the environment

Self-splicing group II introns are present in the organelles of lower eukaryotes, plants and Bacteria and have been found recently in Archaea. It is generally accepted that group II introns originated in bacteria before spreading to mitochondria and chloroplasts. These introns are thought to be related to the progenitors of spliceosomal introns. Group II introns are also mobile genetic elements. In bacteria, they appear to spread using either other mobile genetic elements or low-expression regions as target sites. Bacteria and Archaea genome sequence annotations have revealed the diversity of group II intron classes and that they are involved in vertical and horizontal inheritance.     

Maternal transmission of cytoplasmic DNA in interspecific hybrids of peat mosses, Sphagnum (Bryophyta)

The progeny of spontaneous interspecific hybrid sporophytes of Sphagnum were used to analyse the inheritance of cytoplasmic DNA. The analysis showed that only the female parent donated chloroplasts and mitochondria in Sphagnum hybrids. Thus, this is the first study demonstrating maternal cytoplasmic inheritance in a nonvascular land plant. This finding has important implications for phylogenetic reconstructions utilizing chloroplast and mitochondrial DNA sequences as well as for the evolution of cytoplasmic inheritance in relation to the life cycle of land plants.     

Cytoplasmic inheritance of mitochondria and chloroplasts in the anisogamous brown alga Mutimo cylindricus (Phaeophyceae)

Based on the morphology of gametes, sexual reproduction in brown algae is usually classified into three types: isogamy, anisogamy, and oogamy. In isogamy, chloroplasts and chloroplast DNA (chlDNA) in the sporophyte cells are inherited biparentally, while mitochondria (or mitochondrial DNA, mtDNA) is inherited maternally. In oogamy, chloroplasts and mitochondria are inherited maternally. However, the patterns of mitochondrial and chloroplast inheritance in anisogamy have not been clarified. Here, we examined derivation of mtDNA and chlDNA in the zygotes through strain-specific PCR analysis using primers based on single nucleotide polymorphism in the anisogamous brown alga Mutimo cylindricus. In 20-day-old sporophytes after fertilization, mtDNA and chlDNA derived from female gametes were detected, thus confirming the maternal inheritance of both organelles. Additionally, the behavior of mitochondria and chloroplasts in the zygotes was analyzed by examining the consecutive serial sections using transmission electron microscopy. Male mitochondria were isolated or compartmentalized by a double-membrane and then completely digested into a multivesicular structure 2 h after fertilization. Meanwhile, male chloroplasts with eyespots were observed even in 4-day-old, seven-celled sporophytes. The final fate of male chloroplasts could not be traced. Organelle DNA copy number was also examined in female and male gametes. The DNA copy number per chloroplast and mitochondria in male gametes was lower compared with female organelles. The degree of difference is bigger in mtDNA. Thus, changes in different morphology and DNA amount indicate that maternal inheritance of mitochondria and chloroplasts in this species may be based on different processes and timing after fertilization.

     

Maternal inheritance of plastids and mitochondria in Cycas L. (Cycadaceae)

Cycas is often considered a living fossil, thereby providing a unique model for revealing the evolution of spermatophytes. To date, the genetic inheritance of these archaic plants is not fully understood. The present study seeks to document the process of organelle inheritance in an interspecific cross of Cycas species. Extranuclear organelle DNA from chloroplasts and mitochondria was analyzed using both polymerase chain reaction-restriction fragment length polymorphism analysis and microscopy. Here, we show that the chloroplasts and mitochondria in the progeny of interspecific crosses between Cycas taitungensis and Cycas ferruginea were exclusively inherited from the female parent. Epifluorescence microscopic analyses of the pollen cells from Cycas elongata indicated that there was a significant degradation of organelle DNA in male reproductive cells following maturation; the DNA fluorescent signals were only seen after pollen mitosis two, but not detectable at mature stage. Lack of organelle DNA fluorescent signal in prothallial cells was confirmed by the absence of plastids and mitochondria in electronic microscopic images. In conclusion, these data suggest that the maternal plastid and mitochondrial inheritance in Cycas, native to the old world, are the same as seen in seed plants.     

Organelle division: Self-assembling GTPase caught in the middle

Recent findings indicate that the dynamin GTPase helps to divide animal and fungal mitochondria, and that the tubulin-like FtsZ GTPase is involved in division of, not only most bacteria, but also chloroplasts and probably mitochondria of unicellular eukaryotes.     

Horizontal gene transfer between bacteria and animals

Horizontal gene transfer is increasingly described between bacteria and animals. Such transfers that are vertically inherited have the potential to influence the evolution of animals. One classic example is the transfer of DNA from mitochondria and chloroplasts to the nucleus after the acquisition of these organelles by eukaryotes. Even today, many of the described instances of bacteria-to-animal transfer occur as part of intimate relationships such as those of endosymbionts and their invertebrate hosts, particularly insects and nematodes, while numerous transfers are also found in asexual animals. Both of these observations are consistent with modern evolutionary theory, in particular the serial endosymbiotic theory and Muller's ratchet. Although it is tempting to suggest that these particular lifestyles promote horizontal gene transfer, it is difficult to ascertain given the nonrandom sampling of animal genome sequencing projects and the lack of a systematic analysis of animal genomes for such transfers.     

Biogenesis of beta-barrel membrane proteins of mitochondria

beta-Barrel membrane proteins have several important functions in outer membranes of Gram-negative bacteria and in the organelles of endosymbiotic origin, mitochondria and chloroplasts. The biogenesis of beta-barrel membrane proteins was, until recently, an unresolved process. A breakthrough was achieved when a specific pathway for the insertion of beta-barrel outer-membrane proteins was identified in both mitochondria and Gram-negative bacteria. The key component of this pathway is Tob55 (also known as Sam50) in mitochondria and Omp85 in bacteria, both beta-barrel membrane proteins themselves. Tob55 is part of the hetero-oligomeric TOB (topogenesis of mitochondrial outer-membrane beta-barrel proteins) or SAM (sorting and assembly of mitochondria) complex, which is present in the mitochondrial outer membrane. Tob55 belongs to an evolutionarily conserved protein family, the members of which are present in almost all eukaryotes and in Gram-negative bacteria and chloroplasts. Thus, is it emphasized that the insertion pathway of mitochondrial beta-barrel membrane proteins was conserved during evolution of mitochondria from endosymbiotic bacterial ancestors.     

Effects of marked chromosome sections on milk performance in cattle

Summary The investigations of paternal half sibs start with the assumption that the transfer of an allele from a father to an offspring also indicates the inheritance of a distinct section of two homologous chromosomes from the father concerned. With the help of 20 gene systems, the transfers of single chromosome sections were marked and tested with regard to influences on milk performance traits. 1,457 German Friesian cattle, registered as daughters of three sires, were used. Some of the chromosome sections showed significant effects on the traits considered. Since especially those chromosomes which bear genes for milk proteins were involved, it was assumed that groups of linked loci influence the genetic variance of milk production. Possibilities for applying the results to the practical breeding situation and their significance are discussed.     

A novel in vitro system for simultaneous import of precursor proteins into mitochondria and chloroplasts

Most chloroplast and mitochondrial precursor proteins are targeted specifically to either chloroplasts or mitochondria. However, there is a group of proteins that are dual targeted to both organelles. We have developed a novel in vitro system for simultaneous import of precursor proteins into mitochondria and chloroplasts (dual import system). The mitochondrial precursor of alternative oxidase, AOX was specifically targeted only to mitochondria. The chloroplastic precursor of small subunit of pea ribulose bisphosphate carboxylase/oxygenase, Rubisco, was mistargeted to pea mitochondria in a single import system, but was imported only into chloroplasts in the dual import system. The dual targeted glutathione reductase GR precursor was targeted to both mitochondria and chloroplasts in both systems. The GR pre-sequence could support import of the mature Rubisco protein into mitochondria and chloroplasts in the single import system but only into chloroplasts in the dual import system. Although the GR pre-sequence could support import of the mature portion of the mitochondrial FAd subunit of the ATP synthase into mitochondria and chloroplasts, mature AOX protein was only imported into mitochondria under the control of the GR pre-sequence in both systems. These results show that the novel dual import system is superior to the single import system as it abolishes mistargeting of chloroplast precursors into pea mitochondria observed in a single organelle import system. The results clearly show that although the GR pre-sequence has dual targeting ability, this ability is dependent on the nature of the mature protein.     

Selection for mitonuclear co-adaptation could favour the evolution of two sexes

Mitochondria are descended from free-living bacteria that were engulfed by another cell between one and a half to two billion years ago. A redistribution of DNA led to most genetic information being lost or transferred to a large central genome in the nucleus, leaving a residual genome in each mitochondrion. Oxidative phosphorylation, the most critical function of mitochondria, depends on the functional compatibility of proteins encoded by both the nucleus and mitochondria. We investigate whether selection for adaptation between the nuclear and mitochondrial genomes (mitonuclear co-adaptation) could, in principle, have promoted uniparental inheritance of mitochondria and thereby the evolution of two mating types or sexes. Using a mathematical model, we explore the importance of the radical differences in ploidy levels, sexual and asexual modes of inheritance, and mutation rates of the nucleus and mitochondria. We show that the major features of mitochondrial inheritance, notably uniparental inheritance and bottlenecking, enhance the co-adaptation of mitochondrial and nuclear genes and therefore improve fitness. We conclude that, under a wide range of conditions, selection for mitonuclear co-adaptation favours the evolution of two distinct mating types or sexes in sexual species.     

Interspecific evolution: microbial symbiosis,endosymbiosis and gene transfer

Microbial symbioses are interesting in their own right and also serve as exemplary models to help biologists to understand two important symbioses in the evolutionary past of eukaryotic cells: the origins of chloroplasts and mitochondria. Most, if not all, microbial symbioses have a chemical basis: compounds produced by one partner are useful for the other. But symbioses can also entail the transfer of genes from one partner to the other, which in some cases cements two cells into a bipartite, co-evolving unit. Here, we discuss some microbial symbioses in which progress is being made in uncovering the nature of symbiotic interactions: anaerobic methane-oxidizing consortia, marine worms that possess endosymbionts instead of a digestive tract, amino acid-producing endosymbionts of aphids, prokaryotic endosymbionts living within a prokaryotic host within mealybugs, endosymbionts of an insect vector of human disease and a photosynthetic sea slug that steals chloroplasts from algae. In the case of chloroplasts and mitochondria, examples of recent and ancient gene transfer to the chromosomes of their host cell illustrate the process of genetic merger in the wake of organelle origins.