Biparental inheritance of organelles in Pelargonium: evidence for intergenomic recombination of mitochondrial DNA

While uniparental transmission of mtDNA is widespread and dominating in eukaryotes leaving mutation as the major source of genotypic diversity, recently, biparental inheritance of mitochondrial genes has been demonstrated in reciprocal crosses of Pelargonium zonale and P. inquinans. The thereby arising heteroplasmy carries the potential for recombination between mtDNAs of different descent, i.e. between the parental mitochondrial genomes. We have analyzed these Pelargonium hybrids for mitochondrial intergenomic recombination events by examining differences in DNA blot hybridization patterns of the mitochondrial genes atp1 and cob. Further investigation of these genes and their flanking regions using nucleotide sequence polymorphisms and PCR revealed DNA segments in the progeny, which contained both P. zonale and P. inquinans sequences suggesting an intergenomic recombination in hybrids of Pelargonium. This turns Pelargonium into an interesting subject for studies of recombination and evolutionary dynamics of mitochondrial genomes.     

Paternal inheritance of mitochondria in Chlamydomonas

To analyze mitochondrial DNA (mtDNA) inheritance, differences in mtDNA between Chlamydomonas reinhardtii and Chlamydomonas smithii, respiration deficiency and antibiotic resistance were used to distinguish mtDNA origins. The analyses indicated paternal inheritance. However, these experiments raised questions regarding whether paternal inheritance occurred normally. Mitochondrial nucleoids were observed in living zygotes from mating until 3 days after mating and then until progeny formation. However, selective disappearance of nucleoids was not observed. Subsequently, experimental serial backcrosses between the two strains demonstrated strict paternal inheritance. The fate of mt+ and mt− mtDNA was followed using the differences in mtDNA between the two strains. The slow elimination of mt+ mtDNA through zygote maturation in darkness was observed, and later the disappearance of mt+ mtDNA was observed at the beginning of meiosis. To explain the different fates of mtDNA, methylation status was investigated; however, no methylation was detected. Variously constructed diploid cells showed biparental inheritance. Thus, when the mating process occurs normally, paternal inheritance occurs. Mutations disrupting mtDNA inheritance have not yet been isolated. Mutations that disrupt maternal inheritance of chloroplast DNA (cpDNA) do not disrupt inheritance of mtDNA. The genes responsible for mtDNA inheritance are different from those of chloroplasts.     

The mechanism of the mixed inheritance of chloroplast genes in Pelargonium

Summary The distributions are given of gene frequencies among embryos after G X W and W X G plastid crosses within and between eight Pelargonium cultivars and some of their inbred or hybrid derivatives.Two distinct segregation patterns are recognized. Homozygous type I female parents (Pr₁Pr₁) have a high frequency of progeny with only maternal alleles, are intermediate for biparental and low for paternal offspring. Heterozygous type II female plants (Pr₁Pr₂) have an equally high frequency of maternal and paternal offspring and a generally low biparental frequency. These correspond to L-shaped and U-shaped gene frequency distributions respectively in which the only modes are at 0 per cent (maternal embryos) and 100 per cent (paternal embryos), with no mode corresponding to the population mean and no sign of a Gaussian distribution.The extremely variable plastid gene frequencies are strongly influenced by the maternal nuclear genotype and by the plastid genotype in which the wild-type allele is always more successful than the mutant in strict comparisons.The relative frequencies of maternal and paternal zygotes, and the mean gene frequency among all the zygotes in a cross, are explicable in terms of the input frequencies of genes from the two parents, their degree of mixing, and by some form of selective replication of plastids. This selection is controlled by nuclear and plastid genotypes which may act in the same direction, to increase the frequency of either the maternal or the paternal alleles, or in opposition. But selection alone is inadequate to explain the shapes of the gene frequency distributions. Instead, a model is proposed in which the segregation or replication of plastids appears to have a strong random element, which results in random drift of gene frequencies within a heteroplasmic zygote or embryo.     

Inheritance of chloroplast and mitochondrial DNA in alloplasmic forms of the genus Daucus

The inheritance of mitochondrial (mt) and chloroplast (ct) DNA in the progeny from interspecific crosses between the cultivated carrot (Daucus carota sativus) and wild forms of the genus Daucus was investigated by analysis of mt and ct RFLPs in single plants of the parental and filial generations. We observed a strict maternal inheritance of the organellar DNAs in all interspecific crosses examined. Previous studies on putative F₂ plants from a cross between Daucus muricatus x D. carota sativus suggested paternal inheritance of ctDNA. Our reinvestigation of this material revealed that the mtDNA of the putative F₂ plants differed from the mtDNA of both putative parents. Therefore, our data suggest that the investigated material originated from other, not yet identified, parents. Consequently, the analysis of this material cannot provide evidence for a paternal inheritance of ctDNA.     

Paternal inheritance of plastids in Medicago sativa

Summary Plastids are plant cellular organelles that are generally inherited from the maternal parent in the angiosperms. Many species exhibit biparental inheritance of plastids, but usually with a predominantly maternal influence. In contrast to this, we report strong paternal inheritance of plastids in reciprocal crosses of alfalfa, Medicago sativa, by following restriction fragment length polymorphisms for plastid DNA in two normal green plastids. Mitochondrial inheritance remained exclusively maternal.     

Chloroplast DNA inheritance in theStellaria longipes complex (Caryophyllaceae)

Inheritance of chloroplast DNA (cpDNA) was examined in F₁ progenies derived from three crosses and three corresponding reciprocal crosses betweenStellaria porsildii andS. longifolia. Chloroplast DNA restriction fragments were analyzed using methods of nonradioactive digoxigenin-11-dUTP labeling and chemiluminescent detection with Lumi-Phos 530. Distinct interspecific restriction fragment polymorphisms were identified and used to demonstrate the mode of cpDNA inheritance. Mode of cpDNA inheritance differed among crosses. Two crosses in whichS. porsildii, SP2920-21, was the maternal parent exhibited three different types of plastids, maternal, paternal and biparental, among the F₁ hybrids, suggesting a biparental cpDNA inheritance and plastid sorting-out inStellaria.     

Variations of chloroplast DNAs in the genus Pelargonium and their biparental inheritance

Summary The comparison of EcoRI patterns of chloroplast DNAs (ctDNAs) from five species of the genus Pelargonium and from 16 cultivars and varieties of Pelargonium zonale hort. demonstrates a remarkable inter- and intraspecific ctDNA (plastome) variation. The plastome of the P. zonale varieties could be differentiated into groups I, II and III. Reasons for this variation seem to be: occurrence of numerous spontaneous plastome mutations, intense hybridisation by gardeners and breeders, and biparental plastid inheritance.Crosses of P. zonale varieties with different ctDNA types lead to the direct evidence on the molecular level of biparental plastid inheritance and plastid sorting-out in F₁-hybrids.     

Paternal leakage of mitochondrial DNA in experimental crosses of populations of the potato cyst nematode Globodera pallida

Animal mtDNA is typically assumed to be maternally inherited. Paternal mtDNA has been shown to be excluded from entering the egg or eliminated post-fertilization in several animals. However, in the contact zones of hybridizing species and populations, the reproductive barriers between hybridizing organisms may not be as efficient at preventing paternal mtDNA inheritance, resulting in paternal leakage. We assessed paternal mtDNA leakage in experimental crosses of populations of a cyst-forming nematode, Globodera pallida. A UK population, Lindley, was crossed with two South American populations, P5A and P4A. Hybridization of these populations was supported by evidence of nuclear DNA from both the maternal and paternal populations in the progeny. To assess paternal mtDNA leakage, a ~3.4?kb non-coding mtDNA region was analyzed in the parental populations and in the progeny. Paternal mtDNA was evident in the progeny of both crosses involving populations P5A and P4A. Further, paternal mtDNA replaced the maternal mtDNA in 22 and 40?% of the hybrid cysts from these crosses, respectively. These results indicate that under appropriate conditions, paternal leakage occurs in the mtDNA of parasitic nematodes, and supports the hypothesis that hybrid zones facilitate paternal leakage. Thus, assumptions of strictly maternal mtDNA inheritance may be frequently violated, particularly when divergent populations interbreed.     

Biparental inheritance of chloroplast DNA and the existence of heteroplasmic cells in alfalfa

Summary Mapping of chloroplast DNA (ctDNA) restriction fragment patterns from a chlorophyll deficient mutant and two phenotypically normal alfalfa genotypes (Medicago sativa L.) has demonstrated the existence of a distinct ctDNA genotype from each source. These unique restriction fragment patterns were utilized to identify maternal or paternal origin of ctDNA in hybrid plants from crosses involving the normal alfalfa genotypes as females and the yellow-green chlorophyll deficient sectors as males. Progeny from these crosses expressing the yellow-green sectored phenotypes contained paternal ctDNA in the chlorophyll deficient sectors and maternal ctDNA in the normal sectors, confirming biparental plastid inheritance. The existence of mixed cells containing both mutant and normal plastids at various stages of sorting-out was observed by transmission electron microscopy of mesophyll cells in mosaic tissue from hybrid plants. This observation verified the biparental transmission of plastids in alfalfa.     

Paternal, maternal, and biparental inheritance of the chloroplast genome in Passiflora (Passifloraceae): implications for phylogenetic studies

Patterns of inheritance of the chloroplast genome in Passiflora were analyzed by examining the progeny from both interspecific and intraspecific crosses. Artificial crosses of field-collected material were performed in greenhouses at The University of Texas at Austin. DNA from fresh leaf material was analyzed by Southern blot techniques to identify the donor of the chloroplast genome. Initially, single progeny were analyzed for 11 crosses; two intraspecific crosses demonstrated maternal inheritance, whereas the nine interspecific crosses had paternal inheritance. Subsequently, the donor of the chloroplast genome was determined for multiple progeny in seven crosses. Passiflora oerstedii × P. retipetala showed strict paternal inheritance in all of 17 progeny. A series of five crosses and backcrosses between P. oerstedii and P. menispermifolia demonstrated strictly paternal inheritance. Finally, when 15 progeny were analyzed for the P. costaricensis × P. costaricensis cross, 12 of the 15 showed maternal inheritance, whereas the remaining three were biparental. Interestingly, all interspecific crosses had primarily paternal inheritance, whereas all intraspecific crosses had primarily maternal inheritance. The implications of heteroplasmy on phylogenetic analyses of chloroplast DNA are discussed.     

Segregation and Recombination of Non-Mendelian Genes in Chlamydomonas

Non-Mendelian genes in Chamydomonas reinhardtii are inherited in a uniparental (UP) fashion. Most zygotes and their progeny receive UP genes only from the mt(+) or maternal parent. However, a few exceptional zygotes are also found in which the mt(-) or paternal UP genome is transmitted. Most of the exceptional zygotes are biparental in that their progeny segregate UP genes transmitted by both parents. As a result, biparental zygotes have been extensively used to study the rules governing UP inheritance.The frequency of biparental zygotes can be greatly increased if the maternal parent is irradiated with ultraviolet light prior to mating. Based principally on studies with ultraviolet-induced biparental zygotes, Sager has argued that a vegetative cell contains two copies of the UP genome and that the progeny of a biparental zygote receive a copy derived from each parent. Results reported in this paper with spontaneous and ultraviolet-induced biparental zygotes do not support the two copy model, but argue for a mulitple copy model with most of the copies normally being transmitted by the maternal parent. A multiple copy model which accounts for both Sager's results and ours is presented.     

Paternally biased cpDNA inheritance in Turnera ulmifolia (Turneraceae)

We end-labeled Hin fI restriction digests of a PCR-amplified plastid encoded gene, the large subunit of ribulose bisphosphate carboxylase, to investigate patterns of cpDNA inheritance in Turnera ulmifolia. A total of 70 progeny from crosses among plants taken from ten populations revealed varying patterns of inheritance. A majority of progeny inherited the paternal cpDNA (64%), while 19% exhibited maternal and 17% biparental inheritance. Eight variegated progeny showed biparental inheritance and were analyzed in greater detail. We extracted and analyzed the cpDNA content of light- vs. dark- green leaf sectors from these plants. The results showed that vegetative segregation of cpDNA had occurred for seven of the eight plants.     

GENETIC EVIDENCE FOR TRACE PATERNAL TRANSMISSION OF PLASTIDS IN LIRIODENDRON AND MAGNOLIA (MAGNOLIACEAE)

Transmission of plastids in Liriodendron and Magnolia was studied by examining inheritance of plastid DNA markers in interspecific crosses. In congeneric hybrids of Liriodendron and Magnolia, 2.9% and 11.1% of progeny, respectively, exhibited uniparental paternal transmission of plastids. This departure from strict uniparental maternal transmission of plastids indicates that mechanisms for the elimination of paternal plastids are not 100% efficient and that any proposed model for plastid transmission must account for a continuum of variation. Our results, and other results reviewed here, show a discrepancy with data obtained by cytological observations and point to the need of confirming the mode of plastid inheritance by genetic analysis. In addition, if paternal plastids are occasionally inherited, intensive sampling of plastid DNA in a putative hybrid population could identify both parents of hybrids and thereby aid in the phylogenetic reconstruction of hybrid zones.     

Complementary genes control biparental plastid inheritance in Pelargonium

Summary Zonal pelargoniums exhibit biparental plastid inheritance. After G x W plastid crosses the progeny are a mixture of green, variegated and white embryos corresponding to a maternal, biparental or paternal inheritance of plastids, respectively. There are two patterns of segregation: type-I females have families in which the majority of embryos are green, variegated are of intermediate frequency and white are the least frequent. Type-II females have families in which green and white embryos are present at about the same frequency and variegated are the least common. The results of many selfs and crosses made within and between 8 type-I and 8 type-II plants led us to conclude that the type of female was determined by its genotype with respect to a pair of complementary genes. Plants giving rise to the type-II pattern contained one or two copies of the dominant alleles of both genes, whereas in the absence of either one or both dominant alleles the plants were type I. The genes were called Pr1/pr1 and Pr2/pr2, an adaptation of symbolism used previously. All 8 type IIs were double heterozygotes Pr1pr1, Pr2pr2, whereas we found 3 genotypes among the type Is, Pr1Pr1, pr2pr2; pr1pr1, Pr2Pr2 and pr1pr1, Pr2pr2. In unrelated experiments we found type IIs of which some were again double heterozygotes and others single heterozygotes Pr1pr1, Pr2Pr2 or Pr1Pr1, Pr2pr2. The model displaces an earlier model based on the proposed operation of a gametophytic lethal or incompatibility system.     

Disappearance of the heteroplasmic state for chloroplast markers in zygospores of Chlamydomonas reinhardtii

In the investigations reported here, the length of zygospore incubation or “maturation” prior to the induction of meiosis was found to affect the inheritance pattern of chloroplast genes. The frequency of zygospores transmitting chloroplast alleles from both parents drops with increasing zygospore age following mating, while the frequencies of zygospores homoplasmic for maternal or paternal chloroplast alleles increase correspondingly. Since there is a negligible reduction in viability, zygospores which are initially biparental appear to become pure for the chloroplast genes from one or the other parent prior to the occurrence of cell division. These results are amplified in crosses of mt⁺ cells which have been irradiated with ultraviolet (uv) light or grown in the presence of the base analog, 5-fluorodeoxyuridine, which also perturbs maternal inheritance. Low doses of uv irradiation, applied to zygospores derived from crosses in which the maternal parent was also irradiated prior to mating, increase the biparental zygospore frequency while reducing the proportion of maternal zygospores. This indicates that at least some maternal zygospore clones are actually derived from zygospores which still contain both parental chloroplast genomes prior to the induction of germination. Thus, a subclass of zygospores must contain paternal chloroplast genomes which are either eliminated upon germination or are not expressed in the resulting zygospore clone. Tetrad analysis of biparental zygospores derived from uv-irradiated mt⁺ gametes demonstrates that the frequency of maternal chloroplast alleles in biparental zygospores decreases as they age. One result is an increase in the proportion of meiotic products homoplasmic for all paternal markers. The increased segregation of homoplasmic daughter cells during the meiotic divisions may result from a reduction in chloroplast ploidy by elimination of maternal genomes. Alternatively, it may reflect an altered ratio of maternal:paternal genomes due to continuous rounds of pairing and gene conversion between heterologous chloroplast DNAs leading to genetic drift within the DNA population of the organelle.     

CPDNA INHERITANCE IN INTERSPECIFIC CROSSES AND EVOLUTIONARY INFERENCE IN LOUISIANA IRISES

Inheritance of chloroplast DNA haplotypes was determined for progeny from interspecific crosses involving Iris fulva and Iris hexagona. Polymerase chain reaction amplification of chloroplast DNA followed by restriction fragment length analysis of the amplification products was used to identify the haplotypes of 213 experimental hybrids. This analysis allowed a test for maternal, paternal, and biparental inheritance in the hybrid offspring. Two of the hybrid progeny possessed haplotypes that were combinations of those present in the pollen and seed parents. One of the offspring possessed only the paternal haplotype. The remaining 210 plants had the haplotypes characteristic of the maternal plant. Chloroplast DNA variation in iris populations has previously been used to infer not only introgressive hybridization between I. fulva and I. hexagona, but also the greater role of direct pollen transfer relative to seed dispersal as the avenue for interspecific gene flow. We reexamined the previous conclusions concerning the mode of introgressive hybridization between I. fulva and I. hexagona in light of the results from the chloroplast DNA inheritance analysis. The low level of paternal and biparental inheritance detected in this analysis suggests that previous analyses using the chloroplast DNA as a seed-specific marker were robust. Furthermore, data concerning barriers to hybridization between I. fulva and I. hexagona suggest that the probability of chloroplast DNA introgression via pollen is low.     

Sex-specific effects of hybridization on reproductive fitness in Mytilus

Blue mussels of the genus Mytilus form extensive hybrid zones in the North Atlantic and elsewhere where the distributions of different species overlap. Mytilus species transmit both maternal and paternal mtDNA through egg and sperm, respectively, a process known as doubly uniparental inheritance (DUI), and some females produce offspring with extremely biased sex ratios. These two traits have been shown to be linked and maternally controlled, with sex determination involving nuclear–cytoplasmic interactions. Hybridization has been shown to disrupt DUI mitochondrial inheritance and sex ratio bias; however, the effect of hybridization on reproductive fitness has not previously been examined. We investigated this effect in M. edulis × M. trossulus crosses through histological examination of mature F1 progeny, and spawning of F1 hybrids to monitor survival of their progeny through to the D stage of larval development. For progeny produced from mothers with a strong bias toward female offspring (often 100%) in pure-bred crosses, there was a clear breakdown in female dominance of progeny and significantly more hermaphrodites in the hybrid crosses produced from sperm with the M-tr1 mitotype. We also found significant sex-specific differences among hybrid progeny, with females producing normal eggs while males and hermaphrodites evidenced impaired gonadal development with significantly greater numbers of Sertoli cells, phagocytic hemocytes, and degenerating germ cells, all associated with gonad resorption. Males from crosses where DUI was disrupted and where male progeny were homoplasmic for the female mtDNA were the most severely compromised. Allelic incongruity between maternal and paternal mitotypes in hybrid crosses was associated with significant disruption of male gonadal development.     

The foundation of extranuclear inheritance: plastid and mitochondrial genetics

In 1909 two papers by Correns and by Baur published in volume 1 of Zeitschrift für induktive Abstammungs- und Vererbungslehre (now Molecular Genetics and Genomics) reported on the non-Mendelian inheritance of chlorophyll deficiencies. These papers, reporting the very first cases of extranuclear inheritance, laid the foundation for a new field: non-Mendelian or extranuclear genetics. Correns observed a purely maternal inheritance (in Mirabilis), whereas Baur found a biparental inheritance (in Pelargonium). Correns suspected the non-Mendelian factors in the cytoplasm, while Baur believed that the plastids carry these extranuclear factors. In the following years, Baur’s hypothesis was proved to be correct. Baur subsequently developed the theory of plastid inheritance. In many genera the plastids are transmitted only uniparentally by the mother, while in a few genera there is a biparental plastid inheritance. Commonly there is random sorting of plastids during ontogenetic development. Renner and Schwemmle as well as geneticists in other countries added additional details to this theory. Pioneering studies on mitochondrial inheritance in yeast started in 1949 in the group of Ephrussi and Slonimski; respiration-deficient cells (petites in yeast, poky in Neurospora) were demonstrated to be due to mitochondrial mutations. Electron microscopical and biochemical studies (1962–1964) showed that plastids and mitochondria contain organelle-specific DNA molecules. These findings laid the molecular basis for the two branches of extranuclear inheritance: plastid and mitochondrial genetics.     

The zygote and proembryo of alfalfa: quantitative,three-dimensional analysis and implications for biparental plastid inheritance

Summary Genetic studies have demonstrated biparental inheritance of plastids in alfalfa. The ratio of paternal to maternal plastids in the progeny varies according to the genotypes of the parents, which can be classified as strong or weak transmitters of plastids. Previous cytological investigations of generative cells and male gametes have provided no consistent explanation for plastid inheritance patterns among genotypes. However, plastids in the mature egg cells of a strong female genotype (6–4) were found to be more numerous and larger than in mature eggs of a weak female genotype (CUF-B), and the plastids in 6–4 eggs are positioned equally around the nucleus. In CUF-B, the majority of plastids are positioned below (toward the micropyle) the mid level of the nucleus, which is the future division plane of the zygote. Since only the apical portion of the zygote produces the embryo proper, plastids in the basal portion were predicted to become included in the suspensor cells and not be inherited. In the present study, we examined zygotes and a two-celled proembryo from a cross between CUF-B and a strong male genotype (301), a cross that results in over 90% of the progeny possessing paternal plastids only. Our results indicate that the distribution of plastids observed in the CUF-B egg cell is maintained through the first division of the zygote. Further, paternal plastids are similarly distributed; however, within the apical portion of the zygote and in the apical cell of the two-celled proembryo, the number of paternal plastids is typically much greater than the number of maternal plastids. These findings suggest that maternal and paternal plastid distribution within the zygote is a significant factor determining the inheritance of maternal and paternal plastids in alfalfa.     

Biparental mitochondrial DNA inheritance in the parasitic trematode Schistosoma mansoni

The maternal inheritance of mitochondrial DNA (mtDNA) in eukaryotic organisms occurs because of the selective destruction of paternal mtDNA molecules that may be present in the zygote. The elimination of sperm mtDNA is less efficient in interspecific crosses, and biparental inheritance of mtDNA has been observed in a variety of species. Because interspecific crosses are likely to be extremely rare in nature, parental inheritance of mtDNA has been deemed of little relevance to population genetics. The mtDNA of the parasitic trematode Schistosoma mansoni was examined for its utility in addressing epidemiological questions related to the transmission and spread of schistosomiasis. Prior to embarking on such experiments, we sought to confirm the mode of inheritance of this molecule using the highly polymorphic mtDNA minisatellite as a marker. In 3 separate crosses, mtDNA apparently identical to paternal DNA was observed in some individuals of the F2 and F3 generations. These observations thus suggest the intraspecific paternal inheritance of mtDNA across multiple generations in Schistosoma mansoni.