Mitochondrial genome organization and cytoplasmic male sterility in plants

Plant mitochondrial genomes are much larger and more complex than those of other eukaryotic organisms. They contain a very active recombination system and have a multipartite genome organization with a master circle resolving into two or more subgenomic circles by recombination through repeated sequences. Their protein coding capacity is very low and is comparable to that of animal and fungal systems. Several subunits of mitochondrial functional complexes, a complete set of tRNAs and 26S, 18S and 5S rRNAs are coded by the plant mitochondrial genome. The protein coding genes contain group II introns. The organelle genome contains stretches of DNA sequences homologous to chloroplast DNA. It also contains actively transcribed DNA sequences having open reading frames. Plasmid like DNA molecules are found in mitochondria of some plants Cytoplasmic male sterility in plants, characterized by failure to produce functional pollen grains, is a maternally inherited trait. This phenomenon has been found in many species of plants and is conveniently used for hybrid plant production. The genetic determinants for cytoplasmic male sterility reside in the mitochondrial genome. Some species of plants exhibit more than one type of cytoplasmic male sterility. Several nuclear genes are known to control expression of cytoplasmic male sterility. Different cytoplasmic male sterility types are distinguished by their specific nuclear genes(rfs) which restore pollen fertility. Cytoplasmic male sterility types are also characterized by mitochondrial DNA restriction fragment length polymorphism patterns, variations in mitochondrial RNAs, differences in protein synthetic profiles, differences in sensitivity to fungal toxins and insecticides, presence of plasmid DNAs or RNAs and also presence of certain unique sequences in the genome. Recently nuclear male sterility systems based on (i) over expression of agrobacterialrol C gene and (ii) anther specific expression of an RNase gene have been developed in tobacco andBrassica by genetic engineering methods.     

RAPD, ISSR and RFLP fingerprints as useful markers to evaluate genetic integrity of micropropagated plants of three diploid and triploid elite tea clones representing Camellia sinensis (China type) and C. assamica ssp. assamica (Assam-India type)

An efficient in vitro propagation method using enhanced axillary branching cultures produced plants from nodal explants of three mature, elite tea clones: diploid UPASI 26 and UPASI 27 (2n=2x=30) representing Camellia sinensis (China type) and triploid UPASI 3 (2n=3x=45) representing C. assamica ssp. assamica (Assam-India type). The genetic fidelity of the micropropagated plants of these three tea clones was assessed by analysing their nuclear, mitochondrial (mt), and chloroplast (cp) genomes using multiple molecular DNA markers. A total of 465, 446 and 462 genetic loci were produced with RFLP, RAPD and ISSR fingerprinting in the micropropagated plants and the corresponding mother plant of C. sinensis clone U (UPASI) 26, and C. assamica ssp. assamica clones U3 and U27, respectively. RFLP fingerprinting was performed using six restriction endonuclease digests and 14 mt and cp gene probes in 84 enzyme-probe combinations. For PCR fingerprinting, 50 RAPD and SSR primers were used for amplifications. The micropropagated plants of both the U3 and U27 clones revealed complete stability in the 462 and 446 genetic loci analysed. In comparison, 36 (7.7%) of the 465 loci were polymorphic among micropropagated plants of the U26 clone. The observed polymorphic loci were not restricted to a particular genome (nuclear or organellar), although a relatively low (7.43%) level of polymorphism was observed in the nuclear as compared to the mt genome (16.3%). ISSR fingerprinting (12.8%) detected more polymorphic loci than RAPD fingerprinting (4.28%). No polymorphism was observed in the cp genome of the micropropagated plants of the three tea clones. The rigorous screening of nuclear and two organellar genomes has demonstrated, for the first time, subtle genetic variation at the DNA sequence level in organized meristem-derived micropropagated plants of tea. Clearly, this is another example demonstrating that organized meristem cultures are not always genetically true-to-type. The genomic changes in tea clones are genotype dependent rather than culture condition dependent.     

Somatic hybrids with substitution type genomic configuration TCBB for the transfer of nuclear and organelle genes from Brassica tournefortii TT to allotetraploid oilseed crop B. carinata BBCC

Oilseed crop Brassica carinata BBCC is a natural allotetraploid of diploid species B. nigra BB and B. oleracea CC. To transfer the nuclear and organelle genes in a concerted manner from an alien species, B. tournefortii TT, to B. carinata, we produced somatic hybrids with genomic configuration TCBB using B. nigra and B. oleracea stocks that carried selectable marker genes. B. tournefortii TT was sexually crossed with hygromycin-resistant B. oleracea CC. Protoplasts isolated from shoot cultures of hygromycin-resistant F₁ hybrids of B. tournefortiixB. oleracea TC were fused with protoplasts of kanamycin-resistant B. nigra BB. In two different fusion experiments 80 colonies were obtained through selection on media containing both hygromycin and kanamycin. Of these, 39 colonies regenerated into plants. Analysis of 15 regenerants by random amplified polymorphic DNA (RAPD) markers showed the presence of all three genomes, thereby confirming these to be true hybrids. Restriction fragment length polymorphism (RFLP) analysis of organelle genomes with heterologous chloroplast (cp)and mitochondrial (mt) DNA probes showed that the chloroplast genome was inherited from either of the two parents while mitochondrial genomes predominantly showed novel configurations due to either rearrangements or intergenomic recombinations. We anticipate that the TCBB genomic configuration will provide a more conducive situation for recombination between the T and C genomes during meiosis than the TTCCBB or TCCBB type configurations that are usually produced for alien gene transfer. The agronomic aim of producing TCBB hybrids is to transfer mitochondrial genes conferring cytoplasmic male sterility and nuclear genes for fertility restoration from B. tournefortii to B. carinata.     

An update on chloroplast genomes

Plant cells possess two more genomes besides the central nuclear genome: the mitochondrial genome and the chloroplast genome (or plastome). Compared to the gigantic nuclear genome, these organelle genomes are tiny and are present in high copy number. These genomes are less prone to recombination and, therefore, retain signatures of their age to a much better extent than their nuclear counterparts. Thus, they are valuable phylogenetic tools, giving useful information about the relative age and relatedness of the organisms possessing them. Unlike animal cells, mitochondrial genomes of plant cells are characterized by large size, extensive intramolecular recombination and low nucleotide substitution rates and are of limited phylogenetic utility. Chloroplast genomes, on the other hand, show resemblance to animal mitochondrial genomes in terms of phylogenetic utility and are more relevant and useful in case of plants. Conservation in gene order, content and lack of recombination make the plastome an attractive tool for plant phylogenetic studies. Their importance is reflected in the rapid increase in the availability of complete chloroplast genomes in the public databases. This review aims to summarize the progress in chloroplast genome research since its inception and tries to encompass all related aspects. Starting with a brief historical account, it gives a detailed account of the current status of chloroplast genome sequencing and touches upon RNA editing, ycfs, molecular phylogeny, DNA barcoding as well as gene transfer to the nucleus.     

Characterization of genetic diversity of nuclear and mitochondrial genomes in Daucus varieties by RAPD and AFLP

The genetic diversity of nuclear genomes of five Daucus species and seven Daucus carota L. subspecies involving 26 accessions was characterized with random amplified polymorphic DNA (RAPD) and amplified fragment length polymorphism (AFLP). AFLP produced more than four times as many discrete bands per reaction compared with RAPD analysis, while both AFLP and RAPD basically led to similar conclusions. The dendrograms constructed with both RAPD and AFLP revealed that all accessions of D. carota were grouped into a major cluster delimited from other Daucus species, in good agreement with the classification by morphological char-acteristics. All accessions of cultivated carrots [(D. carota ssp. sativus (Hoffm.) Arcang.] were clustered in the same group while the variation within D. carota was relatively extensive. Genetic diversity of mitochondrial genomes was also documented with RAPD for the same accessions. The mitochondrial dendrogram differed from that of the nuclear genome, suggesting that nuclear and mitochondrial genomes of some accessions had separate evolutionary histories. Received: 20 September 1997 / Revision received: 19 January 1998 / Accepted: 28 March 1998     

Cucumber: a model angiosperm for mitochondrial transformation?

Plants possess three major genomes, carried in the chloroplast, mitochondrion, and nucleus. The chloroplast genomes of higher plants tend to be of similar sizes and structure. In contrast both the nuclear and mitochondrial genomes show great size differences, even among closely related species. The largest plant mitochondrial genomes exist in the genus Cucumis at 1500 to 2300 kilobases, over 100 times the sizes of the yeast or human mitochondrial genomes. Biochemical and molecular analyses have established that the huge Cucumis mitochondrial genomes are due to extensive duplication of short repetitive DNA motifs. The organellar genomes of almost all organisms are maternally transmitted and few methods exist to manipulate these important genomes. Although chloroplast transformation has been achieved, no routine method exists to transform the mitochondrial genome of higher plants. A mitochondrial-transformation system for a higher plant would allow geneticists to use reverse genetics to study mitochondrial gene expression and to establish the efficacy of engineered mitochondrial genes for the genetic improvement of the mitochondrial genome. Cucumber possesses three unique attributes that make it a potential model system for mitochondrial transformation of a higher plant. Firstly, its mitochondria show paternal transmission. Secondly, microspores possess relatively few, huge mitochondria. Finally, there exists in cucumber unique mitochondrial mutations conditioning strongly mosaic (msc) phenotypes. The msc phenotypes appear after regeneration of plants from cell culture and sort with specific rearranged and deleted regions in the mitochondrial genome. These mitochondrial deletions may be a useful genetic tool to develop selectable markers for mitochondrial transformation of higher plants.     

The complete chloroplast genome of Gracilariopsis lemaneiformis (Rhodophyta) gives new insight into the evolution of family Gracilariaceae

The complete chloroplast genome of Gracilariopsis lemaneiformis was recovered from a Next Generation Sequencing data set. Without quadripartite structure, this chloroplast genome (183,013 bp, 27.40% GC content) contains 202 protein‐coding genes, 34 tRNA genes, 3 rRNA genes, and 1 tmRNA gene. Synteny analysis showed plasmid incorporation regions in chloroplast genomes of three species of family Gracilariaceae and in Grateloupia taiwanensis of family Halymeniaceae. Combined with reported red algal plasmid sequences in nuclear and mitochondrial genomes, we postulated that red algal plasmids may have played an important role in ancient horizontal gene transfer among nuclear, chloroplast, and mitochondrial genomes. Substitution rate analysis showed that purifying selective forces maintaining stability of protein‐coding genes of nine red algal chloroplast genomes over long periods must be strong and that the forces acting on gene groups and single genes of nine red algal chloroplast genomes were similar and consistent. The divergence of Gp. lemaneiformis occurred ~447.98 million years ago (Mya), close to the divergence time of genus Pyropia and Porphyra (443.62 Mya).     

Development of synthetic Brassica amphidiploids by reciprocal hybridization and comparison to natural amphidiploids

In a previous study we proposed that cytoplasmic genomes have played an important role in the evolution of Brassica amphidiploid species. Based on this and other studies, we hypothesized that interactions between the maternal cytoplasmic genomes and the paternal nuclear genome may cause alterations in genome structure and/or gene expression of a newly synthesized amphidiploid, which may play an important role in the evolution of natural amphidiploid species. To test this hypothesis, a series of synthetic amphidiploids, including all three analogs of the natural amphidiploids B. napus, B. juncea, and B. Carinata and their reciprocal forms, were developed. These synthetic amphidiploids were characterized for morphological traits, chromosome number, and RFLPs revealed by chloroplast, mitochondrial, and nuclear DNA clones. The maternal transmission of chloroplast and mitochondrial genomes was observed in all of the F₁ hybrids examined except one hybrid plant derived from the B. rapa x B. oleracea combination, which showed a biparental transmission of organelles. However, the paternal chloroplast and mitochondrial genomes were not observed in the F₂ progeny. Nuclear genomes of synthetic amphidiploids had combined RFLP patterns of their parental species for all of the nuclear DNA clones examined. A variation in fertility was observed among self-pollinated progenies of single amphidiploids that had completely homozygous genome constitutions. Comparisons between natural and synthetic amphidiploids based on restriction fragment length polymorphism (RFLP) patterns indicated that natural amphidiploids are considerably more distant from the progenitor diploid species than the synthetic amphidiploids. The utility of these synthetic amphidiploids for investigating the evolution of amphidiploidy is discussed.     

叶绿体基因组研究进展

作为植物细胞器的重要组成部分和光合作用的器官,叶绿体在生物进化的漫长历史中发挥了重要作用.伴随着生物技术的深入发展,人们发现叶绿体基因组结构和序列的信息在揭示物种起源、进化演变及其不同物种之间的亲缘关系等方面具有重要价值.与此同时,比核转化具有明显优势的叶绿体转化技术在遗传改良、生物制剂的生产等方面显示出巨大潜力,而叶绿体基因组结构和序列分析则是叶绿体转化的基石.基于叶绿体的这些重要作用,收集整理了有关的资料,从几个方面归纳了本领域最近的研究进展,希望能使读者对迅速发展的叶绿体基因组研究有更全面的了解,以及对叶绿体基因组在物种的进化、遗传、系统发育关系等方面的作用有更深刻的认识,同时也希望对叶绿体转化技术的研究和广泛应用产生积极作用.     

Construction and characterization of a BAC library from the Coffea arabica genotype Timor Hybrid CIFC 832/2

Most of the world’s coffee production originates from Coffea arabica, an allotetraploid species with low genetic diversity and for which few genomic resources are available. Genomic libraries with large DNA fragment inserts are useful tools for the study of plant genomes, including the production of physical maps, integration studies of physical and genetic maps, genome structure analysis and gene isolation by positional cloning. Here, we report the construction and characterization of a Bacterial Artificial Chromosome (BAC) library from C. arabica Timor Hybrid CIFC 832/2, a parental genotype for several modern coffee cultivars. The BAC library consists of 56,832 clones with an average insert size of 118 kb, which represents a dihaploid genome coverage of five to sixfold. The content of organellar DNA was estimated at 1.04 and 0.5 % for chloroplast and mitochondrial DNA, respectively. The BAC library was screened for the NADPH-dependent mannose-6-phosphate reductase gene (CaM6PR) with markers positioned on four linkage groups of a partial C. arabica genetic map. A mixed approach using PCR and membrane hybridization of BAC pools allowed for the discovery of nine BAC clones with the CaM6PR gene and 53 BAC clones that were anchored to the genetic map with simple sequence repeat markers. This library will be a useful tool for future studies on comparative genomics and the identification of genes and regulatory elements controlling major traits in this economically important crop species.     

Taxonomic individuality of Leonurus cardiaca and Leonurus quinquelobatus in view of morphological and molecular studies

The main goal of this study was to determine the number and taxonomic rank of taxa belonging to the complex Leonurus cardiaca agg. in Poland. Based on statistical analysis of selected features, two morphological forms of this plant were distinguished. In order to determine their genetic polymorphism and the relationships between them, the nuclear, mitochondrial and chloroplast genomes were analysed with the use of RAPD and PCR–RFLP markers. 39 RAPD primers produced a total of 234 nuclear DNA fragments, of which 128 were polymorphic and distributed almost equally between two forms. It was found that 87 % of the compared pairs of RAPD profiles differ from each other. Five chloroplast and two mitochondrial primer pairs were used to amplify non-coding regions of organelle genomes. Restriction analysis revealed uniformity of mtDNA and occurrence of two cpDNA haplotypes, corresponding to naked and hairy forms of L. cardiaca agg. The obtained results justifies the recognition of these forms as separate species L. cardiaca s. s. L. and L. quinquelobatus Gilib., respectively. The distribution of both species in Poland is given in the paper.     

Comprehensive molecular characterization of tissue-culture-derived Hordeum marinum plants

Summary Scuttelar calli of Hordeum marinum readily and efficiently regenerate functional plants. In order to assess genetic variability among the regenerants we employed multiple analytic tools, which included molecular and biochemical assays. Total DNA extract from regenerated plants was digested with at least two restriction enzymes and hybridized to four nuclear and six mitochondrial coding sequences, in addition to one nuclear and three mitochondrial noncoding probes. SDS-PAGE analyses of hordein extracted from seeds of regenerated plants and activity assays of -amylase were also performed. The nuclear and mitochondrial genomes of 50 regenerated plants demonstrated relative stability when assessed with coding sequences and by biochemical analyses. However, the mitochondrial noncoding probes revealed one qualitative somaclonal variant characterized by a loss of a hybridizing fragment. Moreover, changes in the methylation patterns of the rRNA genes and the nontranscribed spacer were revealed in another regenerated plant. The albino plant regenerated was characterized by a loss of three chloroplast DNA BamHI fragments.     

The Mitochondrial Genome of Soybean Reveals Complex Genome Structures and Gene Evolution at Intercellular and Phylogenetic Levels

Determining mitochondrial genomes is important for elucidating vital activities of seed plants. Mitochondrial genomes are specific to each plant species because of their variable size, complex structures and patterns of gene losses and gains during evolution. This complexity has made research on the soybean mitochondrial genome difficult compared with its nuclear and chloroplast genomes. The present study helps to solve a 30-year mystery regarding the most complex mitochondrial genome structure, showing that pairwise rearrangements among the many large repeats may produce an enriched molecular pool of 760 circles in seed plants. The soybean mitochondrial genome harbors 58 genes of known function in addition to 52 predicted open reading frames of unknown function. The genome contains sequences of multiple identifiable origins, including 6.8 kb and 7.1 kb DNA fragments that have been transferred from the nuclear and chloroplast genomes, respectively, and some horizontal DNA transfers. The soybean mitochondrial genome has lost 16 genes, including nine protein-coding genes and seven tRNA genes; however, it has acquired five chloroplast-derived genes during evolution. Four tRNA genes, common among the three genomes, are derived from the chloroplast. Sizeable DNA transfers to the nucleus, with pericentromeric regions as hotspots, are observed, including DNA transfers of 125.0 kb and 151.6 kb identified unambiguously from the soybean mitochondrial and chloroplast genomes, respectively. The soybean nuclear genome has acquired five genes from its mitochondrial genome. These results provide biological insights into the mitochondrial genome of seed plants, and are especially helpful for deciphering vital activities in soybean.     

Chloroplast and mitochondrial DNA diversity in Theobroma cacao

The variability of cocoa (Theobroma cacao) cytoplasmic genomes has been investigated. A total of 177 cocoa clones was surveyed for restriction fragment length polymorphism (RFLP) in chloroplast DNA and in mitochondrial DNA using two restriction endonucleases and various heterologous cytoplasmic probes. A high level of polymorphism was found for the mitochondrial genome. This study points up a structuring of the species that fits with the distinction between the Criollo and Forastero populations. In contrast to all previous analyses, a higher level of polymorphism is found among the Criollo clones while the Forastero clones form quite a homogeneous group.     

Regeneration of somatic hybrid plants formed between Lycopersicon esculentum and Solatium rickii

Summary A single somatic hybrid callus clone was identified following the fusion of Lycopersicon esculentum protoplasts and Solanum rickii suspension culture protoplasts. The hybrid nature of the callus and the plants regenerating from it was determined by assaying phosphoglucomutase-2 isozyme expression. The chloroplast genome present in four somatic hybrid plants was characterized by probing digests of total DNA with nick translated L. esculentum chloroplast DNA(cpDNA). All four somatic hybrid plants had inherited S. rickii cpDNA. Two clones of plant mitochondrial DNA (mtDNA), soybean 18S and 5S rDNA and maize cytochrome oxidase subunit II were used to characterize the mtDNA present in total DNA digests of four somatic hybrid plants. In both cases, the somatic hybrid plants had inherited most but not all of the S. rickii specific fragments, but none of the L. esculentum specific fragments.     

GISH,AFLP and PCR-RFLP analysis of an intergeneric somatic hybrid combining Goutou sour orange and <Emphasis Type="Italic">Poncirus trifoliata</Emphasis>

Intergeneric somatic hybrids combining Goutou sour orange (Citrus aurantium L.) with trifoliate orange [Poncirus trifoliata (L.) Raf] were produced by electrofusion and their genetic inheritance analyzed by amplified fragment length polymorphism (AFLP), genomic in situ hybridization (GISH), and PCR-restriction fragment length polymorphism (PCR-RFLP). Sixteen mini-calluses were obtained after 20 days of culture; they all developed into embryoids on EME500 medium. Following several subcultures on shoot induction medium for a total culture period of 6 months, shoots regenerated. The plants grew vigorously with a well-developed root system and exhibited the trifoliate leaf character of P. trifoliata. Ploidy analysis verified that all of the regenerates were tetraploids (2n=4x=36) as expected. GISH analysis confirmed that 18 chromosomes came from trifoliate orange and the remaining 18 from Goutou sour orange, as with most symmetric somatic hybrid plants; moreover, chromosome translocations were also observed in one plant. AFLP analysis of 16 regenerates and their fusion parents indicated that all of the somatic hybrids except one were genetically uniform. Analysis of the somatic hybrid cytoplasmic genomes with universal primers revealed that their chloroplast DNA (cpDNA) banding patterns were identical to those of the mesophyll parent trifoliate orange, while their mitochondria (mt) genomes were of the callus parent sour orange. The potential of GISH in Citrus somatic hybrid analysis is discussed.The first two authors contributed equally to this paper.     

Complete mitochondrial genomes of prasinophyte algae Pyramimonas parkeae and Cymbomonas tetramitiformis

Mitochondria are archetypal eukaryotic organelles that were acquired by endosymbiosis of an ancient species of alpha‐proteobacteria by the last eukaryotic common ancestor. The genetic information contained within the mitochondrial genome has been an important source of information for resolving relationships among eukaryotic taxa. In this study, we utilized mitochondrial and chloroplast genomes to explore relationships among prasinophytes. Prasinophytes are represented by diverse early‐diverging green algae whose physical structures and genomes have the potential to elucidate the traits of the last common ancestor of the Viridiplantae (or Chloroplastida). We constructed de novo mitochondrial genomes for two prasinophyte algal species, Pyramimonas parkeae and Cymbomonas tetramitiformis, representing the prasinophyte clade. Comparisons of genome structure and gene order between these species and to those of other prasinophytes revealed that the mitochondrial genomes of P. parkeae and C. tetramitiformis are more similar to each other than to other prasinophytes, consistent with other molecular inferences of the close relationship between these two species. Phylogenetic analyses using the inferred amino acid sequences of mitochondrial and chloroplast protein‐coding genes resolved a clade consisting of P. parkeae and C. tetramitiformis; and this group (representing the prasinophyte clade I) branched with the clade II, consistent with previous studies based on the use of nuclear gene markers.     

Molecular characterisation and origin of the Coffea arabica L. genome

Restriction fragment length polymorphism (RFLP) markers were used in combination with genomic in situ hybridisation (GISH) to investigate the origin of the allotetraploid species Coffea arabica (2n = 44). By comparing the RFLP patterns of potential diploid progenitor species with those of C. arabica, the sources of the two sets of chromosomes, or genomes, combined in C. arabica were identified. The genome organisation of C. arabica was confirmed by GISH using simultaneously labelled total genomic DNA from the two putative genome donor species as probes. These results clearly suggest that C. arabica is an amphidiploid formed by hybridisation between C. eugenioides and C. canephora, or ecotypes related to these diploid species. Our results also indicate low divergence between the two constituent genomes of C. arabica and those of its progenitor species, suggesting that the speciation of C. arabica took place relatively recently. Precise localisation in Central Africa of the site of the speciation of C. arabica, based on the present distribution of the coffee species, appears difficult, since the constitution and extent of tropical forest has varied considerably during the late Quaternary period. Received: 6 June 1998 / Accepted: 10 November 1998     

Identification of the Mitochondrial Genome in the Chrysophyte Alga Ochromonas danica

ABSTRACT. Analysis of total DNA isolated from the Chrysophyte alga Ochromonas danica revealed, in addition to nuclear DNA, two genomes present as numerous copies per cell. The larger genome (?120 kilobase pairs or kbp) is the plastid DNA, which is identified by its hybridization to plasmids containing sequences for the photosynthesis genes rbcL, psbA, and psbC. The smaller genome (40 kbp) is the mitochondrial genome as identified by its hybridization with plasmids containing gene sequences of plant cytochrome oxidase subunits I and II. Both the 120- and 40-kbp genomes contain genes for the small and large subunits of rDNA. The mitochondrial genome is linear with terminal inverted repeats of about 1.6 kbp. Two other morphologically similar species were examined, Ochromonas minuta and Poteriochromonas malhamensis. All three species have linear mitochondrial DNA of 40 kbp. Comparisons of endonuclease restriction-fragment patterns of the mitochondrial and chloroplast DNAs as well as those of their nuclear rDNA repeats failed to reveal any fragment shared by any two of the species. Likewise, no common fragment size was detected by hybridization with plasmids containing heterologous DNA or with total mitochondrial DNA of O. danica; these observations support the taxonomic assignment of these three organisms to different species. The Ochromonas mitochondrial genomes are the first identified in the chlorophyll a/c group of algae. Combining these results with electron microscopic observations of putative mitochondrial genomes reported for other chromophytes and published molecular studies of other algal groups suggests that all classes of eukaryote algae may have mitochondrial genomes < 100 kbp in size, more like other protistans than land plants.     

Differential fate of plastid and mitochondrial genomes in Petunia somatic hybrids

Summary The chloroplast (cp) and mitochondrial (mt) DNAs of Petunia somatic hybrid plants, which were derived from the fusion of wild-type P. parodii protoplasts with albino P. inflata protoplasts, were analyzed by endonuclease restriction and Southern blot hybridization. Using ³²P-labelled probes that distinguished the two parental cpDNAs at a BamH1 site and at a HpaII site, only the P. parodii chloroplast genome was detected in the 10 somatic hybrid plants analyzed. To examine whether cytoplasmic mixing had resulted in rearrangement of the mitochondrial genome in the somatic hybrids, restriction patterns of purified somatic hybrid and parental mtDNAs were analyzed. Approximately 87% of those restriction fragments which distinguish the two parental genomes are P. inflata-specific. Restriction patterns of the somatic hybrid mtDNAs differ both from the parental patterns and from each other, suggesting that an interaction occurred between the parental mitochondrial genomes in the somatic fusion products which resulted in generation of the novel mtDNA patterns. Southern blot hybridization substantiates this conclusion. In addition, somatic hybrid lines derived from the same fusion product were observed to differ in mtDNA restriction pattern, reflecting a differential sorting-out of mitochondrial genomes at the time the plants were regenerated.