叶绿体SSR标记：柑橘体细胞杂种胞质遗传分析的一种新方法

从水稻(Oryza sativa L．)、烟草(Nicotiana tabacum L．)和黑松(Pinus thunbergii Parl．)等植物的22对叶绿体SSR引物中筛选出5对能用于柑橘叶绿体SSR分析的引物,应用这5对引物对9个组合的柑橘体细胞杂种的叶绿体遗传进行了分析。结果表明：这些组合再生的杂种中叶绿体都呈现随机分离,该现象与以前报道的RFLP分析结果一致,而且其可靠性已被CAPS分析所证实。表明柑橘叶绿体SSR同RFLP及CAPS一样可靠,并且更简单高效、易于操作,特别适合对柑橘等植物体细胞杂种进行早期胞质遗传组成分析。     

Cytoplasmic inheritance in mammalian tissue culture cells.

A series of intraspecific, interspecific and interorder somatic cell cybrids and hybrids have been prepared by fusions in which one of the parents contained the cytoplasmically inherited marker for chloramphenicol (CAP) resistance. A clear relationship has been established between the expression of the CAP-resistant (CAP-R) determinants in the fusion products and the genetic homology of the parents. With increased genetic divergence, the acceptability of the CAP-R mitochondria decreased. Intraspecific cybrids and hybrids of the same strain were stable for the CAP-R marker, while those between strains were stable only in CAP. Intergeneric mouse-hamster cybrids occurred at a high frequency but were unstable in CAP, while CAP suppressed hybrid formation 100-fold. Interorder cybrids (CAP-R human X CAP-S mouse) occurred either at a moderate frequency and were stable at a low frequency and were unstable in CAP. Interorder hybrids could only be formed by challenging HAT-selected hybrids with CAP or by direct selection in ouabain and CAP. Reciprocal interorder crosses between CAP-R mouse and CAP-S human cells were unsuccessful. Interspecific cybrids contain only the chromosomes of the CAP-S parent. Interspecific hybrids selected directly in CAP segregated the chromosomes of the CAP-S parent, while hybrids selected in HAT and then CAP segregated those of the CAP-R parent. The mitochondrial DNA(mtDNA) of all mouse-human cybrids and most HAT and then CAP-selected hybrids contain only the mtDNA of the CAP-S mouse parent. However, preliminary evidence suggests that one of these hybrids contains both mouse and human mtDNA sequences.     

Growth and reproduction of two species of Akodon and their hybrids

Growth and reproduction of Akodon molinae (A.m.), A. dolores (A.d.) (Rodentia: Cricetidae), and their hybrids are described. A.d. showed less fertility under animal room conditions, fewer "successful" matings, similar litter sizes, and an altered sex proportion as compared to A.m.; A.d. ovulatory index is supposed to be higher than mean litter size. X-linked lethal gene(s) and chromosome polymorphisms are possible causes of these variations. Interspecific crosses showed a marked seasonal (summer) tendency in parturitions. Intraspecific matings also produced young during winter. F1 hybrids showed a reversed tendency, while F2 matings returned to the parental pattern. A.d. male X A.m. female and F1 crosses presented larger litter sizes than the other pairings. A.m. exhibited sexual differences in weight after 60 days of age. A.d. and F1 hybrids did not show significant differences in weight of both sexes at any age. A.m. males and females were heavier than A.d. individuals except 12 months after birth. F1 hybrids born to A.m. male were heavier at birth than the A.d. female s interspecific offspring, and may be interpreted as a maternal influence of the A.m. females. Hybrids were heavier than their parents at birth and some of them were also heavier at 21 days, and probably is due to pseudoheterosis. The comparison of body measurements between sexes of both species did not give conclusive results. A.m. females were larger than females of A.d. Males of both species showed similar total lengths; A.m. males had larger body lengths and shorter tails. Body measurements were not related to weight variations. However, differences in weight and measurements between sexes appeared simultaneously at 2-6 months. This probably reflected the attainment of maturity.     

Cytoplasmic inheritance in mammalian tissue culture cells

Summary A series of intraspecific, interspecific and interorder somatic cell cybrids and hybrids have been prepared by fusions in which one of the parents contained the cytoplasmically inherited marker for chloramphenicol (CAP) resistance. A clear relationship has been established between the expression of the CAP-resistant (CAP-R) determinants in the fusion products and the genetic homology of the parents. With increased genetic divergence, the acceptability of the CAP-R mitochondria decreased. Intraspecific cybrids and hybrids of the same strain were stable for the CAP-R marker, while those between strains were stable only in CAP. Intergeneric mouse-hamster cybrids occurred at a high frequency but were unstable in CAP, while CAP suppressed hybrid formation 100-fold. Interorder cybrids (CAP-R human × CAP-S mouse) occurred either at a moderate frequency and were stable or at a low frequency and were unstable in CAP. Interorder hybrids could only be formed by challenging HAT-selected hybrids with CAP or by direct selection in ouabain and CAP. Reciprocal interorder crosses between CAP-R mouse and CAP-S human cells were unsuccessful. Interspecific cybrids contain only the chromosomes of the CAP-S parent. Interspecific hybrids selected directly in CAP segregated the chromosomes of the CAP-S parent, while hybrids selected in HAT and then CAP segregated those of the CAP-R parent. The mitochondrial DNA(mtDNA) of all mouse-human cybrids and most HAT and then CAP-selected hybrids contain only the mtDNA of the CAP-S mouse parent. However, preliminary evidence suggests that one of these hybrids contains both mouse and human mtDNA sequences. Presented in the formal symposium on Somatic Cell Genetics at the 27th Annual Meeting of the Tissue Culture Association, Philadelphia, Pennsylvania, June 7–10, 1976. This work was supported by U.S.P.H.S. research grants GM-18186, GM-1948 and GM-21024 (to J. M. E.), and N.I.H. postdoctoral fellowship No. 1 F22 GM-02655 (to D. C. W.).     

The Photosynthetic Efficiency of Some Elite Rice Hybrids and Restorers

At the vegetative growth stage (40 d), the mean photosynthetic rate (PN) and canopy photosynthesis (PN × LAI) in F1 hybrids and their parents were similar, whereas the maintenance respiration rate (RM) was considerably higher and PN/RM lower in the F1 hybrids than in the parents. Yet at the flowering stage, the hybrids showed higher PN and PN × LAI values, while RM and PN/RM were similar in both. A specific F1 hybrid like IR 62829A×Vajram showed high readings in PN, PN/RM, and PN × LAI at the flowering stage, while IR 62829A×Swarna followed by IR 62829A×Vajram had high values 40 d after planting. The parents Swarna and Vajram, although moderate in PN, had the highest PN × LAI at the flowering stage due to a greater LAI.     

Clonal evolution in serially passaged Cryptococcus neoformans × deneoformans hybrids reveals a heterogenous landscape of genomic change

Cryptococcus neoformans × deneoformans hybrids (also known as serotype AD hybrids) are basidiomycete yeasts that are common in a clinical setting. Like many hybrids, the AD hybrids are largely locked at the F1 stage and are mostly unable to undergo normal meiotic reproduction. However, these F1 hybrids, which display a high (∼10%) sequence divergence are known to genetically diversify through mitotic recombination and aneuploidy, and this diversification may be adaptive. In this study, we evolved a single AD hybrid genotype in six diverse environments by serial passaging and then used genome resequencing of evolved clones to determine evolutionary mechanisms of adaptation. The evolved clones generally increased fitness after passaging, accompanied by an average of 3.3 point mutations, 2.9 loss of heterozygosity (LOH) events, and 0.7 trisomic chromosomes per clone. LOH occurred through nondisjunction of chromosomes, crossing over consistent with break-induced replication, and gene conversion, in that order of prevalence. The breakpoints of these recombination events were significantly associated with regions of the genome with lower sequence divergence between the parents and clustered in sub-telomeric regions, notably in regions that had undergone introgression between the two parental species. Parallel evolution was observed, particularly through repeated homozygosity via nondisjunction, yet there was little evidence of environment-specific parallel change for either LOH, aneuploidy, or mutations. These data show that AD hybrids have both a remarkable genomic plasticity and yet are challenged in the ability to recombine through sequence divergence and chromosomal rearrangements, a scenario likely limiting the precision of adaptive evolution to novel environments.     

Immune response of mice to the Thy-1.1 antigen: intra-H-2 mapping of the complementary Ir-Thy-1 loci.

The primary immune response to the Thy-1.1 antigen was measured by a plaque assay that detected cells producing antibodies lytic for AKR thymocytes. F1 hybrids which were b/d heterozygous at the entire H-2 complex or at only some of its regions were studied. All F1 hybrids which were b/d heterozygotes at the K, IA and possibly IB regions displayed a complementary effect of the alleles at the Ir-Thy-1A and Ir-Thy-1B loci. The complementary effect was indicated by the significantly higher responses of the F1 hybrids than the responses of their parental strains. No complementary effect could be demonstrated in F1 hybrids which were b/d heterozygous at the IB, IC, S and D regions, but not at the K and IA regions. The results excluded participation of the IC, S and D regions in the phenomenon of the complementation. Furthermore, one of the Ir-Thy-1 loci could be mapped to the left of the IB subregion, most likely in the IA subregion of the H-2 complex. The second Ir-Thy-1 locus could be mapped to the left of the IC subregion in either the IA or IB subregion.     

Design,Synthesis, and Biological Evaluation of Novel Amyl Ester Tethered Dihydroartemisinin-Isatin Hybrids as Potent Anti-Breast Cancer Agents

A series of novel amyl ester tethered dihydroartemisinin-isatin hybrids 4a–d and 5a–h were designed, synthesized, and evaluated as anti-breast cancer agents. The synthesized hybrids were preliminarily screened against estrogen receptor-positive (MCF-7 and MCF-7/ADR) and triple-negative (MDA-MB-231 and) breast cancer cell lines. Three hybrids 4a , d and 5e not only were more potent than artemisinin and adriamycin against drug-resistant MCF-7/ADR and MDA-MB-231/ADR breast cancer cell lines, but also displayed non-cytotoxicity towards normal MCF-10 A breast cells, and the SI values were >4.15, indicating their excellent selectivity and safety profiles. Thus, hybrids 4a , d and 5e could act as potential anti-breast cancer candidates and were worthy of further preclinical evaluations. Moreover, the structure–activity relationships which may facilitate further rational design of more effective candidates were also enriched.     

The Application of Fluorescence In Situ Hybridization in Different Ploidy Levels Cross-Breeding of Lily

21 crossing were conducted between Asiatic Lily with different ploidy levels, the results showed that the interploidy hybridization between diploid and tetraploid lilies was not as successful as intraploidy hybridization. Regardless of male sterility, triploid lilies could be used as female parents in the hybridization which the progenies were aneuploidy. 3x×4x crosses could be cultured more successfully than 3x×2x crosses. 45S rDNA was mapped on the chromosomes of seven Lilium species and their progenies using fluorescence in situ hybridization (FISH). FISH revealed six to sixteen 45S rDNA gene loci, and normally the sites were not in pairs. The asymmetry indexes of LA (Longiflorum hybrids × Asiatic hybrids) hybrids was higher than Asiatic hybrids, the evolution degree was LA hybrids > Asiatic hybrids. 45S rDNA distributed variably on chromosome 1-10 and 12 among Asiatic hybrids. Chromosome 1 had invariable sites of 45S rDNA in all Asiatic hybrids, which could be considered as the characteristic of Asiatic hybrids. LA hybrid ‘Freya’ had two sites of 45S rDNA on one homologous chromosome 5, and also it could be found in the progenies. The karyotype and fluorescence in situ hybridization with 45S rDNA as probe were applied to identify the different genotypes of 9 hybrids. Typical chromosomes with parental signal sites could be observed in all the genotypes of hybrids, it was confirmed that all the hybrids were true.     

Effects of nitrogen deficiency on photosynthetic traits of maize hybrids released in different years

BACKGROUND AND AIMS: New maize (Zea mays) hybrids outperformed old ones even at reduced N rates. Understanding the mechanisms of the differences in performance between newer and older hybrids under N deficiency could provide avenues for breeding maize cultivars with large yield under N deficiency, and reduce environmental pollution caused by N fertilizers. METHODS: N deficiency effects on grain weight, plant weight, harvest index, leaf area and photosynthetic traits were studied in the field for six maize hybrids released during the past 50 years to compare their tolerance and to explore their physiological mechanisms. KEY RESULTS: N deficiency decreased grain yield and plant weight in all hybrids, especially in the older hybrids. However, there was no significant difference in harvest index, rate of light-saturated photosynthesis (Psat) 20 d before flowering, leaf area or plant weight at flowering between the N-deficient and control plants of all hybrids. Dry matter production after flowering of the N-deficient plants was significantly lower than that of the control plants in all hybrids, especially in the older hybrids, and was mostly due to differences in the rate of decrease in photosynthetic capacity during this stage. The lower Psat of the older hybrids was not due to stomatal limitation, as there was no significant difference in stomatal conductance (gs) and intercellular CO2 concentration (Ci) between the hybrids. N deficiency accelerated senescence, i.e. decreased chlorophyll and soluble protein contents, after anthesis more for the earlier released hybrids than for the later ones. N deficiency decreased phosphoenolpyruvate carboxylase (PEPCase) activity significantly more in older hybrids than newer hybrids, and affected the maximal efficiency of PSII photochemistry (Fv/Fm) only in the old hybrids and at the late stage. CONCLUSIONS: Compared with older (earlier released) hybrids, newer (later released) hybrids maintained greater plant and grain weight under N deficiency because their photosynthetic capacity decreased more slowly after anthesis, associated with smaller non-stomatal limitations due to maintenance of PEPCase activity, and chlorophyll and soluble protein content.     

ISSR-PCR technique: a useful method for characterizing new allotetraploid somatic hybrids of mandarin

Somatic hybrids between Redblush and Duncan grapefruit (Citrus paradisi Macfadyen) with Avana, Tardivo di Ciaculli mandarin (C. deliciosa Tenore) and Fortune mandarin (C. reticulata Blanco) were obtained by symmetric protoplast fusion. Isozyme banding pattern analysis and flow cytometry were used for early screening. DNA of the hybrids was then extracted and subjected to inter-simple sequence repeat polymerase chain reaction (ISSR-PCR) using different primers anchored at the 5' and 3' ends. PCR products were resolved on 1.5% agarose gels and stained with ethidium bromide. All the bands from the parental cultivars were present in the somatic hybrids. The usefulness and efficiency of the ISSR-PCR method as a screening technique for Citrus somatic hybrids are discussed.     

Interspecies Crosses in Paramecium aurelia (Syngen 4 by Syngen 8)*

Two syngens (biologic species) of Paramecium aurelia that appear to be closely related were crossed. Parental stocks carrying different homozygous recessive marker genes were utilized to identify true hybrids (those that had undergone cross-fertilization followed by normal nuclear processes). From these crosses of syngens 4 and 8, 32% of the conjugants survived but only 9% (27% of the survivors) were true hybrids. All 19 viable hybrids recovered were cytoplasmically descended from the syngen 8 parent; but the viable nonhybrids were cytoplasmically descended from either of the 2 parents and with equal frequency from each. Surprisingly, the hybrids could not be infected with the symbiont kappa, even though they should have carried a K gene donated by their syngen 4 parent which is necessary and sufficient to allow infection of the syngen 4 parent stocks. The hybrids required special care to cultivate and were sterile, thus indicating that they are at an evolutionary dead end. However, one type of nonhybrid clone produced by the intersyngenic conjugants was able to produce viable progeny. It is speculated that genetic elements of the cortex and in the cytoplasm (e.g. mitochondria) could be transferred intersyngenically via this type of nonhybrid clone.     

Assessing patterns of hybridization between North Atlantic eels using diagnostic single-nucleotide polymorphisms

The two North Atlantic eel species, the European eel (Anguilla anguilla) and the American eel (Anguilla rostrata), spawn in partial sympatry in the Sargasso Sea, providing ample opportunity to interbreed. In this study, we used a RAD (Restriction site Associated DNA) sequencing approach to identify species-specific diagnostic single-nucleotide polymorphisms (SNPs) and design a low-density array that combined with screening of a diagnostic mitochondrial DNA marker. Eels from Iceland (N=159) and from the neighboring Faroe Islands (N=29) were genotyped, along with 94 larvae (49 European and 45 American eel) collected in the Sargasso Sea. Our SNP survey showed that the majority of Icelandic eels are pure European eels but there is also an important contribution of individuals of admixed ancestry (10.7%). Although most of the hybrids were identified as F1 hybrids from European eel female × American eel male crosses, backcrosses were also detected, including a first-generation backcross (F1 hybrid × pure European eel) and three individuals identified as second-generation backcrosses originating from American eel × F1 hybrid backcrosses interbreeding with pure European eels. In comparison, no hybrids were observed in the Faroe Islands, the closest bodies of land to Iceland. It is possible that hybrids show an intermediate migratory behaviour between the two parental species that ultimately brings hybrid larvae to the shores of Iceland, situated roughly halfway between the Sargasso Sea and Europe. Only two hybrids were observed among Sargasso Sea larvae, both backcrosses, but no F1 hybrids, that points to temporal variation in the occurrence of hybridization.     

Expression profiling of reciprocal maize hybrids divergent for cold germination and desiccation tolerance

Recombinant inbred lines (RILs) derived from B73 x M017 were screened for cold germination (CG) and desiccation tolerance (DT) phenotypes. Reciprocal F(1) hybrids were made between divergent RILs, and hybrids that showed differential phenotypes (parent-of-origin effect) for CG or DT were selected for profiling mRNA and protein expression. mRNA and proteins were extracted from embryo axes of seed germinated for 11 d at 12.5 degrees C in the dark and developing embryos at 40% seed moisture (R5 stage) for CG and DT, respectively. GeneCalling analysis, an open-ended mRNA profiling method, identified 336 of 32,496 and 656 of 32,940 cDNA fragments that showed >or=1.5-fold change in expression between the reciprocal F(1) hybrids for CG and DT, respectively. Protein expression map (PEM) analysis, an open-ended two-dimensional polyacrylamide gel electrophoresis, identified 117 of 2,641 and 205 of 1,876 detected proteins to be differentially expressed with >or=1.5-fold change between the reciprocal F(1) hybrids in CG and DT samples, respectively. A subset of these proteins was identified by tandem mass spectrometry followed by database query of the spectra. The differentially expressed genes/proteins were classified into various functional groups including carbohydrate and amino acid metabolism, ion transporters, stress and defense response, polyamine metabolism, chaperonins, cytoskeleton associated, etc. Phenotypic analysis of seed from self-pollinated ears of the reciprocal F(1) hybrids displayed small differences compared with the reciprocal hybrids themselves, suggesting a negligible effect of cytoplasmic factors on CG and DT traits. The results provide leads to improving our understanding of the genes involved in stress response during seed maturation and germination.     

Fermentation patterns of poorly fermenting cabbage hybrids

Thirteen varieties of cabbage, including five commonly used commercial varieties and eight newly developed hybrids, were analyzed for their ability to undergo lactic acid fermentations. The rates of fermentation, total quantities of acids produced, and chemical analyses indicate that 11 of the 13 selections could support adequate fermentation at any of the three stages of maturation. However, two varieties G27 × G51 (105) and G60 × W-1 (106), harvested during the earlier phases of the 1967 and 1968 growing seasons, failed to undergo proper fermentations and consistently resulted in producing putrid products. These hybrids when harvested at the late stage of maturation were capable of producing total titratable acidities comparable to the acceptable varieties. However the resulting sauerkrauts were judged to be of poor quality. The inability of these varieties to support adequate fermentations may be due to growth inhibitory substances or the lack of inherent maturation-dependent growth factor(s) essential for the growth of lactic acid bacteria.     

Comparison of Triazine-Resistant and -Susceptible Biotypes of Senecio vulgaris and Their F(1) Hybrids

The relationship of triazine resistance to decreased plant productivity was investigated in Senecio vulgaris L. F₁ reciprocal hybrids were developed from pure-breeding susceptible (S) and resistant (R) lines. The four biotypes (S, S × R, R, R × S) were compared in terms of atrazine response, electron transport, carbon fixation, and biomass production. Atrazine response, carbon fixation rate, and PSII and whole-chain electron transport rates of hybrids were nearly identical to those of their respective maternal parents. Significant differences occurred between the two susceptible (S, S × R) and two resistant (R, R × S) biotypes in atrazine response (I₅₀), carbon fixation rate, and PSII and whole-chain electron transport rates; PSI rates were identical in all four biotypes. Coupled and uncoupled, whole-chain electron transport rates of thylakoids of the two susceptible biotypes were approximately 50% greater than those of the two resistant biotypes at photon flux densities greater than 215 micromoles per square meter per second. Carbon exchange rates of the two susceptible biotypes were 23% greater than those of the two resistant biotypes. Hybrid biotypes (S × R, R × S) were not identical to their maternal parents in biomass production. The S, S × R, and R × S plants all achieved greater biomass than R plants. These results suggest that while the resistance mutation influences thylakoid performance, reduced productivity of triazine-resistant plants cannot be ascribed solely to decreases in electron transport or carbon assimilation rates brought about by the altered binding protein. Since the F₁ hybrids differed from their maternal parents only in nuclear genes, it appears that the detrimental effects of the triazine resistance mutation on plant growth may be attenuated by interactions of the plastid and nuclear genomes.     

The fusion of oocytes of the starfish Aphelasterias japonica. III. Reconstruction of oocytes from cells and cell fragments (cytoplasts)

The oocytes of the starfish Aphelasterias japonica were divided into anuclear (cytoplasts) and nuclear (germinal vesicles) fragments by centrifugation in a Ficoll gradient with cytochalasin B. Cell hybrids between the oocytes and the cytoplasts were then prepared with the aid of polyethylene glycol treatment. These cell hybrids were capable of responding to 1-methyladenine by maturation.     

Allelic interaction of F1 pollen sterility loci and abnormal chromosome behaviour caused pollen sterility in intersubspecific autotetraploid rice hybrids

The intersubspecific hybrids of autotetraploid rice has many features that increase rice yield, but lower seed set is a major hindrance in its utilization. Pollen sterility is one of the most important factors which cause intersubspecific hybrid sterility. The hybrids with greater variation in seed set were used to study how the F(1) pollen sterile loci (S-a, S-b, and S-c) interact with each other and how abnormal chromosome behaviour and allelic interaction of F(1) sterility loci affect pollen fertility and seed set of intersubspecific autotetraploid rice hybrids. The results showed that interaction between pollen sterility loci have significant effects on the pollen fertility of autotetraploid hybrids, and pollen fertility further decreased with an increase in the allelic interaction of F(1) pollen sterility loci. Abnormal ultra-structure and microtubule distribution patterns during pollen mother cell (PMC) meiosis were found in the hybrids with low pollen fertility in interphase and leptotene, suggesting that the effect-time of pollen sterility loci interaction was very early. There were highly significant differences in the number of quadrivalents and bivalents, and in chromosome configuration among all the hybrids, and quadrivalents decreased with an increase in the seed set of autotetraploid hybrids. Many different kinds of chromosomal abnormalities, such as chromosome straggling, chromosome lagging, asynchrony of chromosome disjunction, and tri-fission were found during the various developmental stages of PMC meiosis. All these abnormalities were significantly higher in sterile hybrids than in fertile hybrids, suggesting that pollen sterility gene interactions tend to increase the chromosomal abnormalities which cause the partial abortion of male gametes and leads to the decline in the seed set of the autotetraploid rice hybrids.     

Maize Leaf Rolling Initiation

Maize plants of CPB2 and CPB8 hybrids were kept under water deficit for 22 d. In the CPB8 hybrid, leaf rolling initiated at the 9^th d of water deficit period, while in CPB2 hybrid it was at the 15^th d. Both hybrids showed leaf rolling initiation at the same leaf water potential, Ψ_W of -0.480±0.095 MPa. At leaf rolling initiation, the leaf osmotic potential, Ψ_S was -0.730±0.085 MPa in CPB8 and 0.630±0.110 MPa in CPB2. The leaf temperature and stomatal conductance were higher in CPB8 than in CPB2. Values of leaf Ψ_W, ribulose-1,5-bisphosphate carboxylase activity, chlorophyll content, and specific leaf area were similar in both hybrids. Phosphoenolpyruvate carboxylase activity and protein content were lower in the CPB2 hybrid than in CPB8. In both hybrids leaf rolling initiation was associated with: (1) higher leaf temperature, with leaf rolling effect related to leaf temperature reduction, and (2) lower leaf Ψ_S, related to osmotic adjustment as an additional component of drought-tolerance strategy. This revised version was published online in June 2006 with corrections to the Cover Date.     

Receptors for the complement C3d component and the Epstein-Barr virus are quantitatively coexpressed on a series of B-cell lines and their derived somatic cell hybrids

Various human Burkitt lymphoma and LCL lines established in vitro and their derived somatic cell hybrids were tested for their comparative EBV receptor levels in a virus binding assay. Their graded C3b and C3d complement receptor expression was estimated simultaneously by means of isotope labeled rosette marker cells. The receptor concentration of each cell line was related to Raji as the standard of comparison, K 562, P3HR-1, and YACUT were used as negative controls. In general, the charging curves for EBV and C3d receptors parallelled each other (r = 0.97) while C3b receptor charging showed no correlation (r < 0.60). In the Raji hybrids between the C3b receptor positive Raji parent and various patents that were negative for this receptor, C3b receptor expression was low or negative. In contrast, the C3d negative P3HR-1 line gave rise to hybrids, after fusion with receptor-positive cells, that were intermediate with regard to their C3d receptor expression. The host range restriction of the Epstein-Barr virus is determined at the receptor level. The close relationship between the EBV receptor and the C3d receptor, a B-lymphocyte-specific moiety, suggests that the moderate interaction with EBV with the B lymphocytes may have had a selective advantage, favoring the presence of EBV. Since EBV causes lytic infections after artificial introduction into nonnatural host cells, it may represent a B-lymphocyte-specific host range mutant, derived from an originally lytic herpesvirus with a much broader target cell range.