Cytoplasmic Genotypes Research Articles

Perspective and application of molecular markers linked to the cytoplasm types and male‐fertility restorer locus in onion (Allium cepa)

AbstractThe male sterility trait is an economically essential feature for the exploitation of hybrid vigour in onions; hence, scientists have looked into the molecular component of male sterility in onions. As a result, numerous molecular markers associated with male‐sterile (CMS‐S and CMS‐T) cytoplasms, male‐fertile normal (N) cytoplasm and nuclear‐male‐fertility restorers (Ms locus) were developed. The discovered markers have proven to be helpful in the effective selection and isolation of male‐sterile lines, their maintainer lines and restorer‐of‐fertility lines. These lines are required for breeding F1 hybrid to harness heterosis through marker‐assisted selection (MAS) of cytoplasmic genotypes with Ms or ms loci. Thus, the study came up with a compilation of molecular markers, by revealing their molecular perspective, practical utility and functional effectiveness in the MAS of male‐sterile cytoplasm types with nuclear‐fertility‐restorer locus, to speed up onion breeding, development of F1 hybrids and production of pure hybrid seeds.

Maternal karyogene and cytoplasmic genotype affect the induction efficiency of doubled haploid inducer in Brassica napus

BackgroundArtificial synthesis of octoploid rapeseed double haploid (DH) induction lines Y3380 and Y3560 was made possible by interspecific hybridization and genome doubling techniques. Production of pure lines by DH induction provides a new way to achieve homozygosity earlier in B.napus. Previously, the mechanism of induction, and whether the induction has obvious maternal genotypic differences or not, are not known so far.ResultsIn this study, different karyogene and cytoplasmic genotype of B.napus were pollinated with the previously reported DH inducers e.g. Y3380 and Y3560. Our study presents a fine comparison of different cytoplasmic genotypes hybridization to unravel the mechanism of DH induction. Ploidy identification, fertility and SSR marker analysis of induced F1 generation, revealed that ploidy and phenotype of the induced F1 plants were consistent with that type of maternal, rather than paternal parent. The SNP chip analysis revealed that induction efficiency of DH inducers were affected by the karyogene when the maternal cytoplasmic genotypes were the same. However, DH induction efficiency was also affected by cytoplasmic genotype when the karyogenes were same, and the offspring of the ogura cytoplasm showed high frequency inducer gene hybridization or low-frequency infiltration.ConclusionThe induction effect is influenced by the interaction between maternal karyogene and cytoplasmic genotype, and the results from the partial hybridization of progeny chromosomes indicate that the induction process may be attributed to the selective elimination of paternal chromosome. This study provides a basis for exploring the mechanism of DH inducer in B.napus, and provides new insights for utilization of inducers in molecular breeding.

The Application of Molecular Markers to Accelerate the Recovery of Cytoplasmic and Nuclear Male Sterility in South African Onion (Allium cepa L.) Hybrid Parental Lines

Male sterility is important to prevent self-pollination and loss of the onion hybrid genotype. Classic methods require 4-8 years of progeny testing before the cytoplasm type can be determined. An accurate and time-saving method was needed. Various types of markers were tested for application to South African onion parental lines of hybrid cultivars, and which could determine male sterility and maintainer genotypes accurately and easily with large numbers of samples. Five cytoplasmic (5’cob, orfA501, orf725, IGS and cob-type 2) and four nuclear markers (jnurf13, isotig34671_610, isotig30856_1351 and isotig29186_1830) were sourced. Genomic DNA was isolated from onion seedlings and young leaves growing from bulbs in the Agricultural Research Council (ARC) research field. PCR marker amplification products were separated by agarose or denaturing polyacrylamide gel electrophoresis (PAGE) gels. Real-time polymerase chain reaction (PCR) was performed with custom TaqMan® SNP genotyping assays containing primer/probe pairs designed to detect single nucleotide polymorphisms (SNPs) linked to the nuclear Ms locus. OrfA501 proved useful as a presence/absence marker for cytoplasmic male sterility, while TaqMan® SNP genotyping assays were superior to the jnurf13 nuclear marker in terms of rapid throughput. PCR molecular markers and custom TaqMan® SNP genotyping assays were efficient in screening the onion lines rapidly and accurately for their cytoplasmic and nuclear male sterility genotype. These methods reduced the time to identify the correct genotype of male sterile and maintainer lines, gave accurate genotypic information and proved to be useful on a larger scale. These molecular marker methods will facilitate the production of the correct seed for commercialization of onion lines worldwide.

Cytonuclear Epistasis Controls the Density of Symbiont Wolbachia pipientis in Nongonadal Tissues of Mosquito Culex quinquefasciatus.

Wolbachia pipientis, a bacterial symbiont infecting arthropods and nematodes, is vertically transmitted through the female germline and manipulates its host’s reproduction to favor infected females. Wolbachia also infects somatic tissues where it can cause nonreproductive phenotypes in its host, including resistance to viral pathogens. Wolbachia-mediated phenotypes are strongly associated with the density of Wolbachia in host tissues. Little is known, however, about how Wolbachia density is regulated in native or heterologous hosts. Here, we measure the broad-sense heritability of Wolbachia density among families in field populations of the mosquito Culex pipiens, and show that densities in ovary and nongonadal tissues of females in the same family are not correlated, suggesting that Wolbachia density is determined by distinct mechanisms in the two tissues. Using introgression analysis between two different strains of the closely related species C. quinquefasciatus, we show that Wolbachia densities in ovary tissues are determined primarily by cytoplasmic genotype, while densities in nongonadal tissues are determined by both cytoplasmic and nuclear genotypes and their epistatic interactions. Quantitative-trait-locus mapping identified two major-effect quantitative-trait loci in the C. quinquefasciatus genome explaining a combined 23% of variance in Wolbachia density, specifically in nongonadal tissues. A better understanding of how Wolbachia density is regulated will provide insights into how Wolbachia density can vary spatiotemporally in insect populations, leading to changes in Wolbachia-mediated phenotypes such as viral pathogen resistance.

Hybrid Origins of Citrus Varieties Inferred from DNA Marker Analysis of Nuclear and Organelle Genomes.

Most indigenous citrus varieties are assumed to be natural hybrids, but their parentage has so far been determined in only a few cases because of their wide genetic diversity and the low transferability of DNA markers. Here we infer the parentage of indigenous citrus varieties using simple sequence repeat and indel markers developed from various citrus genome sequence resources. Parentage tests with 122 known hybrids using the selected DNA markers certify their transferability among those hybrids. Identity tests confirm that most variant strains are selected mutants, but we find four types of kunenbo (Citrus nobilis) and three types of tachibana (Citrus tachibana) for which we suggest different origins. Structure analysis with DNA markers that are in Hardy–Weinberg equilibrium deduce three basic taxa coinciding with the current understanding of citrus ancestors. Genotyping analysis of 101 indigenous citrus varieties with 123 selected DNA markers infers the parentages of 22 indigenous citrus varieties including Satsuma, Temple, and iyo, and single parents of 45 indigenous citrus varieties, including kunenbo, C. ichangensis, and Ichang lemon by allele-sharing and parentage tests. Genotyping analysis of chloroplast and mitochondrial genomes using 11 DNA markers classifies their cytoplasmic genotypes into 18 categories and deduces the combination of seed and pollen parents. Likelihood ratio analysis verifies the inferred parentages with significant scores. The reconstructed genealogy identifies 12 types of varieties consisting of Kishu, kunenbo, yuzu, koji, sour orange, dancy, kobeni mikan, sweet orange, tachibana, Cleopatra, willowleaf mandarin, and pummelo, which have played pivotal roles in the occurrence of these indigenous varieties. The inferred parentage of the indigenous varieties confirms their hybrid origins, as found by recent studies.

Creation of allo cytoplasmic wheat genotypes with high characteristics of gluten quality based on dna marker selection

Based on the analysis of allelic composition of genes related to bread baking quality grains, created through individual selection program in dry conditions of 2010, from the hybrid populations of allo cytoplasmic wheat (T. aestivum L.) genotypes with valuable cytoplasm Triticum timopheevii L. and Secale cereale L., combined good indicators of gluten quality and drought tolerance.According to the results of PCR analysis of high molecular weight glutenin screening of eleven allo cytoplasmic genotypes, three lines (genotypes) showed special value: two lines with a T. timopheevii cytoplasm and one line with a Secale cereale L. cytoplasm, whose distinguishing features were the presence of both subunits, 5+10 (for locus Glu-D1), as well as subunit 2* (for locus Glu-A1), considered to be of good bread baking properties, which is an advantage in the selection of wheat grain quality. In accordance with the regulations “flour quality” (sedimentation method), these three genotypes fall under the "strong" gluten category, of which nine genotypes were selected for this indicator.The results of the analysis of high molecular weight glutenin allelic state of the hybrid populations of allo cytoplasmic genotypes, by PCR method enabled significantly accelerate the creation of promising lines (11 lines) in the process of target selection on the quality of gluten. Created lines are valuable as donors of important qualitative properties of the grain and used as ingredients in the creation of new nuclear-cytoplasmic hybrid combinations in the target selection of wheat grain quality. The 11 lines are included in the competitive (station) for the test and variety integration as well as for a comprehensive study in comparison with standard varieties of spring wheat.

Seed size and chloroplast DNA of modern and ancient seeds explain the establishment of Japanese cultivated melon (Cucumis melo L.) by introduction and selection

Melon is a fruit/vegetable that has been grown in Japan for at least 2000 years. To obtain a better understanding of melon crop evolution in this island country, we measured the seed size and determined the cytoplasmic genotype of 135 modern melon accessions and 12 populations of ancient melon seed remains from archaeological sites for a 2000-year period in Japan. Based on differences in seed length, populations of melon seed remains at the Shikata site (Okayama Prefecture, Japan) consisted of seed types corresponding to those of modern East and South Asian melon. Although several types of melon seeds were found in and around the Shikata site, only Agrestis-type seeds, <6.1 mm in length, were found in melon populations from 1 CE. Intra-population length variation was higher in 1050 CE than in 1530–1680 CE. Ancient DNA from archaeological melon was analysed for SNPs in the chloroplast genome. These revealed that cytoplasm type was heterogeneous and consisted of Ia and Ib types in melon populations prior to ca. 1600 CE, and thereafter becoming homogenous by genetic erosion of Ib, which is absent in modern endemic Japanese melon accessions. The decrease in variation of both seed length and cytoplasm type together with historical records indicates that artificial selection in the Japanese melons for desired fruit traits intensified in the past 1000 years.

The effect of environmental factors and plant genotype on the severity of Claviceps purpurea in winter rye

Claviceps purpurea causes ergot, a serious disease of rye and grasses. Resistance genes to the pathogen have not been found in the genome of rye so far. Therefore we evaluated 90 rye genotypes in two seasons at three locations and analyzed the ergot incidence. In one location (Smolice) the same genotypes were also used to assess the influence of water spraying during rye flowering on ergot occurrence. Moreover, 29 cytoplasmic male-sterile genotypes of rye were used in Poznan, where within a 5 km distance there were no rye plants. In the first two experiments the source of the pathogen consisted of its sclerotia introduced into the soil, whereas in the third experiment a suspension of conidiospores was used to spray the plants from the moment of first flowering. Percentage of sclerotia in grain by weight in the first two experiments and sclerotia weight per ear in the third experiment were used as a measure of rye susceptibility to C. purpurea. Both rye genotype and location (weather conditions) showed significant variation for both traits. Low temperature and rainfall during rye flowering increased the occurrence of ergot. Water spraying of rye during flowering also boosted the occurrence of ergot. In another experiment, with sclerotia of C. purpurea used as inocula, ergot occurrence varied from 0 to 4.73 % in 2011/12 and from 0 to 5.22 % of sclerotia in grain yield in 2012/13. Cytoplasmic male-sterile rye inoculation with conidia of C. purpurea resulted in sclerotia presence which ranged from 0.10 to 0.26 g per ear.

Uncovering symbiont‐driven genetic diversity across North American pea aphids

Heritable genetic variation is required for evolution, and while typically encoded within nuclear and organellar genomes, several groups of invertebrates harbour heritable microbes serving as additional sources of genetic variation. Hailing from the symbiont-rich insect order Hemiptera, pea aphids (Acyrthosiphon pisum) possess several heritable symbionts with roles in host plant utilization, thermotolerance and protection against natural enemies. As pea aphids vary in the numbers and types of harboured symbionts, these bacteria provide heritable and functionally important variation within field populations. In this study, we quantified the cytoplasmically inherited genetic variation contributed by symbionts within North American pea aphids. Through the use of Denaturing Gradient Gel Electrophoresis (DGGE) and 454 amplicon pyrosequencing of 16S rRNA genes, we explored the diversity of bacteria harboured by pea aphids from five populations, spanning three locations and three host plants. We also characterized strain variation by analysing 16S rRNA, housekeeping and symbiont-associated bacteriophage genes. Our results identified eight species of facultative symbionts, which often varied in frequency between locations and host plants. We detected 28 cytoplasmic genotypes across 318 surveyed aphids, considering only the various combinations of secondary symbiont species infecting single hosts. Yet the detection of multiple Regiella insecticola, Hamiltonella defensa and Rickettsia strains, and diverse bacteriophage genotypes from H. defensa, suggest even greater diversity. Combined, these findings reveal that heritable bacteria contribute substantially to genetic variation in A. pisum. Given the costs and benefits of these symbionts, it is likely that fluctuating selective forces play a role in the maintenance of this diversity.

Suppressed expression of RETROGRADE-REGULATED MALE STERILITY restores pollen fertility in cytoplasmic male sterile rice plants

Conflict/reconciliation between mitochondria and nuclei in plants is manifested by the fate of pollen (viable or nonviable) in the cytoplasmic male sterility (CMS)/fertility restoration (Rf) system. Through positional cloning, we identified a nuclear candidate gene, RETROGRADE-REGULATED MALE STERILITY (RMS) for Rf17, a fertility restorer gene for Chinese wild rice (CW)-type CMS in rice (Oryza sativa L.). RNA interference-mediated gene silencing of RMS restored fertility to a CMS plant, whereas its overexpression in the fertility restorer line induced pollen abortion. The mRNA expression level of RMS in mature anthers depended on cytoplasmic genotype, suggesting that RMS is a candidate gene to be regulated via retrograde signaling. We found that a reduced-expression allele of the RMS gene restored fertility in haploid pollen, whereas a normal-expression allele caused pollen to die in the CW-type CMS. RMS encodes a mitochondrial protein, 178 aa in length, of unknown function, unlike the majority of other Rf genes cloned thus far, which encode pentatricopeptide repeat proteins. The unique features of RMS provide novel insights into retrograde signaling and CMS.

Analysis of embryo, cytoplasm and maternal effects on fatty acid components in soybean (Glycine max Merill.)

The quality of oil determined by the constituents and proportion of fatty acid components, and the understanding of heredity of fatty acid components are of importance to breeding good quality soybean varieties. Embryo, cytoplasmic and maternal effects and genotype × environment interaction effects for quality traits of soybean [Glycine max (L.) Merrill.] seeds were analyzed using a general genetic model for quantitative traits of seeds with parents, F1 and F2, of 20 crosses from a diallel mating design of five parents planted in the field in 2003 and 2004 in Harbin, China. The interaction effects of palmitic, stearic, and linoleic acid contents were larger than the genetic main effects, while the genetic main effects were equal to interaction effects for linolenic and oleic acid content. Among all kinds of genetic main effects, the embryo effects were the largest for palmitic, stearic, and linoleic acids, while the cytoplasm effects were the largest for oleic and linolenic acids. Among all kinds of interaction effects, the embryo interaction effects were the largest for fatty acids. The sum of additive and additive × environment effects were larger than that of dominance and dominance × environment effects for the linolenic acid content, but not for other quality traits. The general heritabilities were the main parts of heritabilities for palmitic and oleic acid contents, but the interaction was more important for stearic, linoleic, and linolenic acid contents. For the general heritability, maternal and cytoplasm heritabilities were the main components for palmitic, oleic, and linolenic acid contents. It was shown for the interaction heritabilities that the embryo interaction heritabilities were more important for oleic and linolenic acid contents, while the maternal interaction heritabilities were more important for linoleic acid content. Among selection response components, the maternal and cytoplasm general responses and/or interaction responses were more important for palmitic, stearic, oleic, and linoleic acid contents. The main selection response components were from the embryo general response and/or interaction response for linolenic acid content. It suggested that the selection of palmitic, stearic, oleic, and linoleic acid contents in offspring should be in maternal plants, while linolenic acid content should be improved by screening or selecting the single seed in higher generations.

Assessment of purity of rice CMS lines using cpDNA marker

AFLP technique was used to analyse the polymorphism between rice cytoplasmic male sterility (CMS) line Jin2A and its maintainer Jin2B. A stable differential band was discovered, and sequence analysis showed that Jin2A contained a more tandem repeat of 6 base pairs (AGAAAA) than Jin2B. Further studies confirmed that the diversity came from cpDNA and occurred at three kinds of abortive cytoplasmic genotypes. Accordingly, specific primers were designed and utilized to assess the purity of rice CMS lines during multiplication with pollen fertility and seed setting rates of bagged panicles as control. The result indicated that this cpDNA locus could be utilized to precisely distinguish maintainer plants from rice CMS lines. PCR analysis was consistent with that from Grow-out test in CMS line seed purity assessment during multiplication, despite it was helpless in distinguishing F1 hybrids from CMS lines due to similar cytoplasms. Because of fewer hybrid and more maintainer off-plants, this cpDNA locus was still appropriate for seed purity assessment of rice CMS line during multiplication. This is first report that a marker on cpDNA could be utilized to assess the genetic purity of rice CMS lines with three abortive cytoplasmic genotypes.

Cytonuclear disequilibria in a spatially structured hybrid zone

A hybrid zone model was developed that uses a stepping-stone model of population structure along with both nuclear and cytoplasmic genotypes to evaluate the effect that migration has on random mating organisms when no selection is present. Numerical simulations indicate that a number of different allele frequency and cytonuclear disequilibrium patterns can be found across the hybrid zone when it has reached equilibrium, and these results are discussed in the context of relative migration rates of the two sexes and the two source populations. The importance of numbers of subpopulations sampled, census time, and the persistence of pure species individuals into the hybrid zone are also discussed. Although this model does not consider selection or assortative mating in the hybrid zone, it should provide a useful baseline for evaluating joint cytonuclear genetic data from a structurally complex hybrid zone.

Different maternal origins of Japanese lowland and upland rice populations.

Plastid subtype ID (PS-ID) sequences were determined from sequence data based on CA repeats between genes rpl16 and rpl14 in Japanese lowland and upland cultivars. The PS-ID sequences of Japanese rice cultivars showed that there are different maternal origins between lowland and upland cultivars. One subtype, 6C7A, of PS-ID sequences was predominant in all but one Japanese lowland cultivar and carried a combination of the indica-specific subtype 8C8A and japonica-specific nuclear markers for the isozyme genotype. It is probably a nuclear-cytoplasmic recombinant resulting from natural out-crossing and succeeding self-pollination. The origin of the plastid was re-confirmed by the existence of an indica-specific deletion in the plastid genome. In contrast, the Japanese upland cultivars showed two subtypes, 7C6A and 6C7A, of PS-ID sequences. An upland-specific isozyme allele as a nuclear marker was equally predominant in cultivars carrying each subtype. The existence of these particular upland-specific nuclear and cytoplasmic genotypes suggests that the origin of Japanese upland cultivars is different from that of Japanese lowland cultivars. Cultivars carrying the upland-specific nuclear genotype are common in Southeast Asia, but the combination of the upland-specific nuclear and cytoplasmic genotypes which is the same as the Japanese upland predominant type was found in cultivars only in Taiwan and Indonesia. Japanese upland cultivars are closely related to those cultivars.

Genetic diversity among wild sugarcane germplasm from Laos revealed with markers

A group of 35 wild Saccharum complex clones, collected in Laos(ECL), was studied for its nuclear genetic diversity by RFLP revealed by 10dispersed low copy probes. Representatives of the main germplasmdiversity, regularly used in introgression, were also included. A preliminarycharacterisation of the new clones was performed by Erianthus specificprimers. A set of the 11 ECL clones most outstanding for their diseaseresistance and vigor and basic germplasm members were also characterisedby 9 cytoplasmic probes. Nuclear diversity evidenced that ECL clonesrepresented an independent genetic pool consisting at least of 3 differentgroups. A group of bands was exclusively exhibited by ECL clones. Theoccurrence of genetic recombination among them was suggested by thepresence of the new bands in 2 or 3 clonal groups. Erianthus specificamplification was positive for 7 ECL clones from different nuclear groups.Dendrogram analysis based on cytoplasmic diversity divided genotypes into2 major cytoplasmic types comprising 5 groups. The ECL clones separatedin 2 distinct groups: one located in the same cytoplasmic type as Erianthus clones; the other one, in an intermediate position, closer to Saccharum species. Basic clones groups were consistent with theirtaxonomic classification. The diversity revealed by nuclear singlecopy/nuclear repetitive/cytoplasmic patterns of the ECL germplasmcontrasts with what is usually observed and points to an active populationevolution. Further studies needed to confirmed this hypothesis arediscussed.

ANCA Pattern and LTA haplotype relationship to clinical responses to anti-TNF antibody treatment in crohn's disease

In the clinical trial, a lower response to infliximab was observed in some patients after multiple infusions, suggesting that clinical subgroups of Crohn's disease (CD) exist based on response to anti-tumor necrosis factor (anti-TNF). The aim of this study was to characterize these subgroups further by antineutrophil cytoplasmic antibody (ANCA) pattern and TNF genotype. Crohn's Disease Activity Index (CDAI) data from the North American patients in the clinical trial (n = 59) were evaluated as the response parameter. Speckled ANCA (sANCA) subjects were ANCA positive by ELISA with a speckling over the entire neutrophil on indirect immunofluorescence. Genotypes were determined for polymorphisms in the TNF/lymphotoxin alpha (LTA) region. Response to infliximab as median change in CDAI was placebo (least response) < perinuclear ANCA (pANCA) < not pANCA or sANCA < sANCA (greatest response) (P(overall) = 0.003; 4 weeks). The response of subjects with sANCA was significantly different from that of placebo at all time points; that of pANCA subjects was not. Homozygotes for the LTA NcoI-TNFc-aa13L-aa26 haplotype 1-1-1-1 did not respond (P(overall) = 0.007). These observations suggest that sANCA may identify a CD subgroup with a better response to infliximab and that pANCA and homozygosity for the LTA 1-1-1-1 haplotype may identify subgroups with a poorer response.

The spatial structure of sexual and cytonuclear polymorphism in the gynodioecious Beta vulgaris ssp. maritima: I/ at a local scale.

We have analyzed the spatial distribution of the sex phenotypes and of mitochondrial, chloroplast, and nuclear markers within two gynodioecious populations of Beta vulgaris ssp. maritima. Within both populations, sexual phenotype variation is controlled mainly by the cytoplasmic genotype, although in one study population a joint polymorphism of cytonuclear factors is clearly involved. In spite of contrasts in the ecology (mainly due to different habitats), a clear common feature in both populations is the highly patchy distribution of cytoplasmic haplotypes, contrasting with the wide distribution of nuclear diversity. This high contrast between cytoplasmic vs. nuclear spatial structure may have important consequences for the maintenance of gynodioecy. It provides opportunities for differential selection since nuclear restorer alleles are expected to be selected for in the presence of their specific cytoplasmic male sterile (CMS) type, but to be neutral (or selected against if there is a cost of restoration) in the absence of their CMS type. Selective processes in such a cytonuclear landscape may explain the polymorphism we observed at restorer loci for two CMS types.

Genetic and genotype × environment interaction effects from embryo, endosperm, cytoplasm and maternal plant for rice grain shape traits of indica rice

A seed genetic model with embryo, endosperm, cytoplasmic and maternal effects and genotype×environment (GE) interaction effects was used for analyzing ratio of length-to-width (RLW) and ratio of length-to-thickness (RLT) of indica rice ( Oryza sativa L.). A factorial mating design in two environments was conducted by including nine cytoplasmic male sterile lines (A) and five restorer lines (R) along with their F 1 (A×R), F 2, reciprocal RF 2 and BC 1 (A×F 1). RLW and RLT were found to be controlled mainly by genetic main effects, but also by GE interaction effects. Variance of maternal effects was larger than other variance components. Variance due to embryo effects and embryo×environment interaction effects could also significantly affect the rice grain shape traits. The total narrow-sense heritability for RLW and RLT were about 85.6 and 69.9%, respectively, with the general heritabilities being 62.5 and 61.9% and the interaction heritabilities being 23.1 and 7.9%. It was suggested by the predicted genetic effects that Zhexie 2, Xieqingzao and Cezao 2-2 were better than other parents for improving rice grain shape traits.

Mitochondrial genotype segregation and effects during mammalian development: Applications to biotechnology

Mitochondria are endosymbiotic organelles responsible for energy production in practically every eukaryotic cell. Their uniparental fashion of inheritance, maternally inherited in mammals, and the homogeneity of mitochondrial DNA (mtDNA) within individuals and matrilineages, are biological phenomena that remain unexplained. This paper reviews some of the recent findings on mitochondrial influences on the manner in which embryos develop and how their genotypes are inherited in mammals, with particular emphasis on the genetic “bottleneck” effect. Animal models carrying a mix of mtDNAs (heteroplasmic) have been produced by karyoplast and cytoplast transplantation to analyze the segregation patterns at different stages during embryogenesis, in fetuses and offspring. Comparisons performed between murine and bovine reveal interesting changes in segregation and replication of transplanted mtDNAs. We have recently obtained Bos indicus and Bos taurus fetuses and calves from embryos reconstructed using enucleated polymorphic oocytes of Bos taurus origin. These and other findings on mitochondrial biology will have important implications in determining the cytoplasmic genotype of clones and in the preservation of endangered breeds and species.

Empirical evaluation of cytonuclear models incorporating genetic drift and tests for neutrality of mtDNA variants: data from experimental Gambusia hybrid zones

Statistical tests of genetic drift and of the neutrality of mtDNA are presented using empirical time‐series data on multi‐generational changes in cytonuclear disequilibria within replicated experimental hybrid populations of two species of live‐bearing Poeciliid fishes (Gambusia holbrooki and G.affinis) which were monitored over a period of two years (three generations). Cytonuclear disequilibria D and D (which measure departures from random associations of cytoplasmic and nuclear genotypes) over the three generations of the experiment were non‐zero for all replicate populations. For each of five nuclear loci, the observed measures of D and D were highly concordant between replicates during each generation. Significant departures from expectations were observed after one and two generations. A statistical measure of goodness of fit of observed changes in cytonuclear disequilibria (and implicitly of the neutrality of the mtDNA markers) was calculated for each nuclear locus. When the results for the replicates were combined into an overall test of neutrality, the fit to the random union of zygotes (RUZ) model was rejected for four of the five nuclear loci (P < 0.05). A simple genetic drift model does not explain the temporal changes in composite cytonuclear genotypic frequencies. Frequencies of parental G. holbrooki mitochondrial alleles and nuclear genotypes exceeded expected values during most time periods, implying some selective advantage of offspring produced by G. holbrooki females. Expansion of cytonuclear models to explicitly address questions of genetic drift and neutrality have general relevance to studies of natural populations.