Reciprocal Interspecific Crosses Research Articles

Genetic Cause of Hybrid Lethality Observed in Reciprocal Interspecific Crosses between Nicotiana simulans and N. tabacum.

Hybrid lethality, a type of postzygotic reproductive isolation, is an obstacle to wide hybridization breeding. Here, we report the hybrid lethality that was observed in crosses between the cultivated tobacco, Nicotiana tabacum (section Nicotiana), and the wild tobacco species, Nicotiana simulans (section Suaveolentes). Reciprocal hybrid seedlings were inviable at 28 °C, and the lethality was characterized by browning of the hypocotyl and roots, suggesting that hybrid lethality is due to the interaction of nuclear genomes derived from each parental species, and not to a cytoplasmic effect. Hybrid lethality was temperature-sensitive and suppressed at 36 °C. However, when hybrid seedlings cultured at 36 °C were transferred to 28 °C, all of them showed hybrid lethality. After crossing between an N. tabacum monosomic line missing one copy of the Q chromosome and N. simulans, hybrid seedlings with or without the Q chromosome were inviable and viable, respectively. These results indicated that gene(s) on the Q chromosome are responsible for hybrid lethality and also suggested that N. simulans has the same allele at the Hybrid Lethality A1 (HLA1) locus responsible for hybrid lethality as other species in the section Suaveolentes. Haplotype analysis around the HLA1 locus suggested that there are at least six and two haplotypes containing Hla1-1 and hla1-2 alleles, respectively, in the section Suaveolentes.

Reproductive systems and low outbreeding barriers between Jacaranda cuspidifolia and J. mimosifolia (Jacarandeae, Bignoniaceae)

The genus Jacaranda shows notable karyotype stability and a prevailing self‐sterile breeding system with evidence of late‐acting self‐incompatibility in several species. However, some studies have indicated self‐compatibility in J. mimosifolia, a species cultivated worldwide in tropical and subtropical areas. Jacaranda cuspidifolia is a closely related species with natural distribution broadly overlapping with that of J. mimosifolia, and manual heterospecific pollination studies have indicated that these species are interfertile, but there is no report on the breeding system of the former. In this study, we used hand‐pollination experiments, pistil longevity, epi‐fluorescence and histological analysis of post‐pollination events to determine the breeding system of J. cuspidifolia. We also employed intra and interspecific crosses and seed germination tests to reevaluate the breeding system of J. mimosifolia and the inter‐fertility between the two species. Some fruits were initiated from self‐pollinated pistils in J. mimosifolia, but none of them reached maturity. On the other hand, complete absence of fruit development by self‐pollination was verified in J. cuspidifolia, while in situ pollen tube growth and histological analysis of post‐pollination events in selfed pistils revealed the characteristic ovule penetration, fertilization and endosperm initiation also observed in other bignoniaceous species with late‐acting self‐incompatibility. Low outbreeding barriers seem to operate between these species because reciprocal interspecific crosses and hybrid seed germination tests indicated they are bilaterally interfertile. However, fruit/seed production and seed germinability were significantly lower when pistils of J. cuspidifolia were pollinated with pollen of J. mimosifolia, compared with crosses in the opposite direction, which indicates a partial unilateral incompatibility. This result is discussed in the context of the possible occurrence of self‐compatibility in J. mimosifolia. The low level of incongruity operating between the two species also points to their recent evolutionary divergence.

Hybrid vigour for thermal tolerance in hybrids between the allopatric kelps Laminaria digitata and L. pallida (Laminariales, Phaeophyceae) with contrasting thermal affinities

ABSTRACTKelps are globally important bioengineering species with high ecological and economic value, but are increasingly threatened by climate-driven geographic range shifts. The inheritance of economically important traits from parents to offspring is poorly understood in kelps but it is of utmost interest to seaweed farmers wishing to select strains with superior performance and resilience to environmental change. For two allopatric kelp species (N-Atlantic Laminaria digitata and S-Atlantic L. pallida), we compared the speed of gametogenesis and reproductive success in parental gametophytes, and produced intraspecific and reciprocal interspecific crosses of female × male gametophyte parents isolated from the two species. We then compared the upper thermal resilience of microscopic and macroscopic sibling sporophytes in an exposure experiment over two weeks. The upper thermal limit of the sporophytes resulting from intraspecific crosses of the two species deviated by 1°C. In contrast, sporophytes from both interspecific hybrid crosses had a 2–3°C higher upper thermal tolerance than single species sporophytes, indicating heterosis for thermal tolerance. This phenotypic response appears partially sex-dependent in our study, with female parents being more important in determining the thermal-response phenotype than male parents. The presence of male gametophytes generally enhanced female reproductive success. Both gametogenesis rate and reproductive success differed among the types of reciprocal crosses. Although the interspecific crosses were artificial in an ecological sense, they may provide a tool for understanding the molecular basis of heterosis and thermal tolerance in kelps (e.g. by investigating species-specific gene expression), or for aquaculture breeding programmes against a background of rapid environmental change.

RNA-directed DNA methylation and seed development: an unexpected difference between Arabidopsis thaliana and Brassica rapa.

RNA-directed DNA methylation and seed development: an unexpected difference between Arabidopsis thaliana and Brassica rapa.

Gene effects of Cicer reticulatum on qualitative and quantitative traits in the cultivated chickpea

AbstractThe gene effects of Cicer reticulatum on both double‐podding as qualitative traits and yield criteria as quantitative traits in interspecific hybridization with cultivated chickpea (Cicer arietinum) have not yet been elucidated, despite the easy acquisition of hybrid progeny between two species. This study sought to answer three questions concerning qualitative and quantitative traits in reciprocal crosses between C. arietinum and C. reticulatum. (i) Is there a similarity in the gene effects of flower colour, pigmentation and double‐podded traits in reciprocal interspecific crosses? (ii) What are the expressivity and penetrance of the double‐podded trait in interspecific crosses? (iii) Which heterosis predicts the occurrence and the extent of transgressive variation? The materials for this study were F1, F2 and F3 progeny derived from a reciprocal cross between C. arietinum and C. reticulatum. As qualitative traits, purple flower colour, pigmentation and single‐podded traits in C. reticulatum were governed by a dominant single gene. Purple flower colour and pigmentation were detected to be linked traits as all progeny had the same phenotypes. As quantitative traits, yield criteria in progeny which were double‐podded had higher values than the single‐podded counterparts. Expressivity and penetrance of the double‐podding trait were superior in progeny derived from C. reticulatum × C. arietinum. The results showed that fruitful heterosis was more useful than residual heterosis in F3 as residual heterosis was mostly negative and fruitful heterosis was suggested in self‐pollinated species such as chickpea that lacks inbreeding depression. Interspecific transgression was significant with respect to chickpea improvement because it represented a potential source of novel genetic variation.

Transgressive segregations for yield criteria in reciprocal interspecific crosses between Cicer arietinum L. and C. reticulatum Ladiz.

Wild Cicer species are considered to be more resistant for biotic and abiotic stresses than that of the cultivated chickpea (Cicer arietinum L.). Alien genes conferring resistance for biotic and abiotic stresses can be transferred from wild Cicer species to the cultivated chickpea but success in interspecific hybridizations has already been achieved with only two wild Cicer species. The current study was undertaken to compare fruitful heterosis in F2 and F3 for yield and yield criteria and to identify transgressive segregation in F2 and F3 in reciprocal interspecific crosses between C. arietinum and C. reticulatum Ladiz. We define fruitful heterosis as a useful parameter that can be used instead of residual heterosis. Considerable fruitful heterosis in F2 and F3 was found for number of seeds, pods per plant, biological yield, and seed yield. Maximum values of most of the characteristics in F2 and F3 were higher than that of the best parent indicating that superior progeny could be selected for yield from transgressive segregation. Progeny selection should be based on number of seeds, pods per plant, and biological yield since these characteristics had the highest direct effect on seed yield. The narrow sense heritability was found to be the highest for 100-seed weight. It was suggested that the cultivated chickpea could be used as female parent in interspecific hybridization to increase yield and yield criteria since progeny in F2 and F3 had better performance when it was used as female. In conclusion, interspecific hybridization of wild and cultivated chickpea can be used to improve yield and yield components and resistance to biotic and abiotic stresses as well.

English

Weeds are the most widespread biotic production constraint of rice in Africa and one of the major factors limiting grain yield. An efficient breeding strategy could be particularly important for improving weed management in sub-Saharan Africa (SSA) because most smallholder rice farmers use few external inputs. To understand rice weed competitiveness, experiments on reciprocal interspecific crosses derived from FKR19 (Oryza sativa) and CG20 (Oryza glaberrima) were carried out to estimate gene effects and heritability of traits: plant height at five leaves, plant height 30 days after transplanting, plant height at maturity, number of tillers at 30 and 60 DAT, number of fertile tillers, width of leaves at 80 DAT and at maturity, and length of leaves at 80 DAT and at maturity for rice–weed competitiveness. Six generations – P1, P2, F1, F2, BC1F1 and BC2F1 – were raised and subjected to generation mean analysis. The lowest heterosis of F1 was obtained in both crosses (CG20/FKR19 and FKR19/CG20), except for plant height at 30 days after transplanting and leaf width at maturity in the CG20/FKR19 cross. The majority of traits displayed higher dominance gene effects (H5_L, H30 and L_80 for CG20/FKR19; W_mat and L_mat for FKR19/CG20) than additive gene effects; the latter were slight and non-significant for the majority of traits. Duplicate epistasis was observed for the number of tillers 30 days after transplanting and leaf length at maturity and plant height at maturity. Additive genetic variance values were higher in CG20/FKR19, revealing that the CG20 variety can be used as a donor parent. Plant height at maturity, length of leaves at 80 DAT and at maturity showed high narrow-sense heritability (hn2>0.70), influencing weed competitiveness. Key words: Additive, dominance, heritability, rice, variance components, weed competitiveness.

Interspecific Hybridization between Coreopsis leavenworthii and Coreopsis tinctoria Differently Affected Growth, Development, and Reproduction of Their Progeny

The genus Coreopsis L. is Florida’s state wildflower; there is a strong interest in commercial production and large-scale planting of Coreopsis seed in Florida, especially the seed of Coreopsis leavenworthi Torr. & A. Gray (COLE) and Coreopsis tinctoria Nutt. (COTI). Both species belong to the same section [Calliopsis (Reichenb.) Nutt.] within Coreopsis and were known to be cross-compatible and produce interspecific hybrids when hand-pollinated or grown in close proximity. Little was known about the effects of such hybridization on progeny growth, development, and reproduction, which are very important to seed production and planting. F1 and F2 interspecific populations between COLE and COTI were created in the greenhouse and then evaluated in replicated field studies in two growing seasons. Results showed that interspecific hybridization between COLE (as the maternal parent) and COTI (as the paternal parent) significantly increased the plant height (by 11.4% to 18.7%), plant dry weight (by 38.6% to 63.9%), and time to flower (by 3.7 to 9.8 days) of the F1 and F2 progeny of COLE × COTI crosses. By contrast, interspecific hybridization between COTI (as the maternal parent) and COLE (as the paternal parent) did not cause significant changes in these characteristics of the F1 and F2 progeny of COTI × COLE crosses. The differences between the two species in responding to interspecific hybridization suggest that COTI played a more dominant role in controlling plant height, dry weight, and time to flower in its hybrids with COLE. Results pooled from all F1 or F2 progeny of reciprocal interspecific crosses showed that interspecific hybridization did not seem to affect the plant height and seedling emergence of F1 and F2 progeny but affected the dry weight, time to flower, pollen stainability, and seed production (per seed head) of these progeny. Heterosis was observed in the time to flower of F1 progeny in 2009. Heterosis was also evident in F1 progeny’s dry weight but followed with slight hybrid breakdown in F2 progeny. Pollen stainability and seed production both showed significant breakdown in F1 and F2 progeny: 53.3% to 81.1% reduction in pollen stainability and 12.6% to 38.2% reduction in seed production, respectively. Chromosome mispairing resulting from reported reciprocal translocations between the chromosomes of COLE and COTI might be the primary cause of low pollen stainability and seed production in F1 and F2 progeny. Maternal effects were detected in plant height and dry weight of F1 and F2 progeny. These results showed that interspecific hybridization between COLE and COTI would result in deleterious effects to both species; thus, it is very important to prevent cross-pollination and hybridization between them in commercial seed production and native plantings.

シデコブシとタムシバの正逆種間交配間における種子形成と発芽率の差異

シデコブシとタムシバの雌雄を入れ替えた種間交配（正逆種間交配）を行い，種間交配の方向によって果実と種子の形成程度および形成された種子の発芽率に差異が生じるかを調べた。授粉から約60～90日後において，タムシバを母樹とする種間交配よりシデコブシを母樹とする種間交配で果実の脱落が多かった。また，結果率（成熟果実数／初期生残果実数）と結実率（成熟果実の種子数／成熟果実の胚珠数），雌性繁殖成功度（結果率×結実率）は，いずれもシデコブシを母樹とする種間交配の方が低い値を示した。種子の発芽率は，タムシバを母樹とする種間交配よりシデコブシを母樹とする種間交配で低かったが，統計的な差は認められなかった。したがって，自然条件下においてタムシバが母樹，シデコブシが父樹となる一方向性の種間交雑が生じるのは，主に，シデコブシを母樹とする種間交配において果実の成熟と種子の形成に障害が生じるためであると考えられる。

Hybrid lethality with programmed cell death results from reciprocal interspecific crosses between Nicotiana nudicaulis Watson and N. tabacum L.

Hybrid lethality with programmed cell death results from reciprocal interspecific crosses between Nicotiana nudicaulis Watson and N. tabacum L.

Effect of cross direction and cultivars on crossability of interspecific hybridization between Vaccinium corymbosum and Vaccinium virgatum

The purpose of this study was to evaluate the effect of cross direction and cultivars on interspecific hybridization between Vaccinium corymbosum (4x) and Vaccinium virgatum (6x). Totally 99 reciprocal interspecific crosses were made by using ten cultivars of V. corymbosum and five cultivars of V. virgatum. Percent fruit set, number of developed seeds per berry, germination rate and number of seedlings per flower were evaluated. The progeny seedlings were obtained from 65% of all cross combinations, and more than 99% of the seedlings were revealed to be pentaploid hybrids by flow cytometric analysis. In the comparison of crossability between interspecific reciprocal crossing, 4x×6x crosses showed 2.9 times higher rate of mean fruit set (67%) and 3.3 times higher rate of number of seeds per berry (6.2) than the opposite crosses (6x×4x). However, the average germination rate (38%) of the 6x×4x crosses was 1.7 times higher than that of 4x×6x crosses. Consequently, average number of seedlings per flower (0.96) in 4x×6x was 3.8 times higher than that in the opposite crosses. According to the number of seedlings per flower significant difference in the crossability was found among the cultivars in V. corymbosum in the reciprocal crosses with V. virgatum, whereas no such significant difference was observed in V. virgatum.

Patterns of Inheritance of Mating Signals in Sailfin and Shortfin Mollies (Poeciliidae: Poecilia: Mollienesia)

AbstractDivergence in mating signals is a primary factor leading to reproductive isolation, and thus, speciation. However, the genetic changes underlying such divergence are poorly understood, especially in vertebrates. We used two species of poeciliid fishes, Poecilia velifera and P. mexicana, to explore the link between genes and mating behaviors that has resulted in pre‐mating reproductive isolation between these species. Using backcross hybrids created from the F1 male offspring of reciprocal interspecific crosses between a sailfin molly (P. velifera) and a shortfin molly (P. mexicana), we examined the effects of Y‐linkage and autosomal contributions on the expression of two male mating behaviors: courtship displays and gonopodial thrusts. The F1 hybrid males displayed a strong influence of sire on courtship display rates, with F1 males sired by the sailfin species showing courtship display rates that were up to three times higher than the rates of displays performed by F1 males sired by the shortfin species. These results suggest a Y‐linked genetic effect on the expression of courtship display behavior. Comparisons between backcross hybrid males with sailfin Y‐chromosomes or shortfin Y‐chromosomes suggested that the interaction of autosomal genes also influences the inheritance of courtship display rates. Sailfin autosomal genes significantly increased the probability of performing courtship displays for hybrid males, and increased display rate for males from the sailfin Y‐chromosome line. Autosomal genes had less of an impact on gonopodial thrusting behavior, however, thrust rates did significantly decrease with an increasing proportion of sailfin autosomes in males from the shortfin Y‐chromosome line. These results suggest that the inheritance of species differences in mating signals between shortfin and sailfin mollies involves both genes found on the Y‐chromosome and autosomal gene influences on their expression.

Crossability, Cytogenetics, and Reproductive Pathways in Rudbeckia Subgenus Rudbeckia

Rudbeckia L. are valuable nursery crops that offer broad adaptability and exceptional ornamental merit. However, there is little information on interspecific and interploid crossability and ploidy levels of specific cultivars. The objectives of this study were to determine the ploidy levels and relative DNA contents (genome sizes) of selected species and cultivars, to evaluate self-compatibility and crossability among species and ploidy levels, and to explore reproductive pathways in triploid R. hirta L. with the goal of facilitating future breeding endeavors and development of new hybrids. Reciprocal interspecific crosses were performed between R. hirta cultivars and R. fulgida Ait., R. missouriensis Engelm. ex C.L. Boynton & Beadle, and R. subtomentosa Pursh. as well as reciprocal interploid crosses among four R. hirta cultivars. A combination of relative DNA content analysis and chromosome counts was used to test for hybridity and to determine ploidy levels for selected species, cultivars, and interploid R. hirta F1 hybrids. Of the specific clones tested, R. subtomentosa and R. missouriensis were diploid, R. fuligida varieties were tetraploid, and R. hirta include both diploid and tetraploid cultivars. Mean 1Cx DNA content varied over 320% among species. The interploid R. hirta crosses produced triploids as well as pentaploids and hexaploids. Seedlings from open-pollinated triploid R. hirta appeared, based on diverse phenotypes and DNA contents, to be aneuploids resulting from sexual fertilization, not apomixis. Of the 844 seedlings from interspecific F1 crosses, only one individual, R. subtomentosa ×R. hirta, had a DNA content intermediate between its parents and was confirmed as the only interspecific hybrid. Although most taxa had low self-fertility, seedlings (with genomic sizes similar to their maternal parent) resulted after interspecific crosspollination, indicating that pseudogamy is one reproductive pathway in Rudbeckia species.

Cytological mechanisms of 2n pollen formation in the wild potato Solanum okadae and pollen-pistil relations with the cultivated potato, Solanum tuberosum

Solanum okadae Hawkes et Hjert is a wild diploid potato species endemic to Argentina and Bolivia, of potential breeding value. However, no genetic studies have been carried out with this species and its crossability relations with the common tetraploid potato, S. tuberosum L. ssp. tuberosum, are unknown. Furthermore, accessions from both countries differed in their morphological phenotypes. To ascertain the feasibility of incorporating this wild germplasm into cultivated potato, 2n pollen screening was carried out in 10 accessions and families derived from crosses between accessions; also, pollen-pistil compatibility relations were studied in reciprocal interspecific crosses. Plants of four of the 10 accessions produced 2n pollen (0.1 to 5.0%) and 4n pollen (0.0 to 3.0%). Parallel and tripolar spindles at Anaphase II were the cytological mechanisms involved in 2n pollen formation; lack of chromosome migration in both meiotic divisions originated the 4n pollen. Both full compatibility and incompatibility at various sites along the pistil were observed in the S. tuberosum × S. okadae combinations; most reciprocal combinations were incompatible. Compatible genotypes produced 2n pollen. However, only a few seeds were obtained and chromosome counts could not be carried out in the hybrid progeny because seedlings died at an early stage. The pollen-pistil barriers are incomplete and can be circumvented by the appropiate choice of parents. The identification of the post-zygotic barriers will be the focus of further studies.

Asymmetric Postmating Isolation: Darwin's Corollary to Haldane's Rule

Asymmetric postmating isolation, where reciprocal interspecific crosses produce different levels of fertilization success or hybrid sterility/inviability, is very common. Darwin emphasized its pervasiveness in plants, but it occurs in all taxa assayed. This asymmetry often results from Dobzhansky-Muller incompatibilities (DMIs) involving uniparentally inherited genetic factors (e.g., gametophyte-sporophyte interactions in plants or cytoplasmic-nuclear interactions). Typically, unidirectional (U) DMIs act simultaneously with bidirectional (B) DMIs between autosomal loci that affect reciprocal crosses equally. We model both classes of two-locus DMIs to make quantitative and qualitative predictions concerning patterns of isolation asymmetry in parental species crosses and in the hybrid F(1) generation. First, we find conditions that produce expected differences. Second, we present a stochastic analysis of DMI accumulation to predict probable levels of asymmetry as divergence time increases. We find that systematic interspecific differences in relative rates of evolution for autosomal vs. nonautosomal loci can lead to different expected F(1) fitnesses from reciprocal crosses, but asymmetries are more simply explained by stochastic differences in the accumulation of U DMIs. The magnitude of asymmetry depends primarily on the cumulative effects of U vs. B DMIs (which depend on heterozygous effects of DMIs), the average number of DMIs required to produce complete reproductive isolation (more asymmetry occurs when fewer DMIs are required), and the shape of the function describing how fitness declines as DMIs accumulate. Comparing our predictions to data from diverse taxa indicates that unidirectional DMIs, specifically involving sex chromosomes, cytoplasmic elements, and maternal effects, are likely to play an important role in postmating isolation.

The Mutated S1-Haplotype in Sour Cherry Has an Altered S-Haplotype–Specific F-Box Protein Gene

Gametophytic self-incompatibility (GSI) is an outcrossing mechanism in flowering plants that is genetically controlled by 2 separate genes located at the highly polymorphic S-locus, termed S-haplotype. This study characterizes a pollen part mutant of the S(1)-haplotype present in sour cherry (Rosaceae, Prunus cerasus L.) that contributes to the loss of GSI. Inheritance of S-haplotypes from reciprocal interspecific crosses between the self-compatible sour cherry cultivar Ujfehértói Fürtös carrying the mutated S(1)-haplotype (S(1)'S(4)S(d)S(null)) and the self-incompatible sweet cherry (Prunus avium L.) cultivars carrying the wild-type S(1)-haplotype revealed that the mutated S(1)-haplotype confers unilateral incompatibility with a functional pistil component and a nonfunctional pollen component. The altered sour cherry S(1)-haplotype pollen part mutant, termed S(1)', contains a 615-bp Ds-like element within the S(1)-haplotype-specific F-box protein gene (SFB(1)'). This insertion generates a premature in-frame stop codon that would result in a putative truncated SFB(1) containing only 75 of the 375 amino acids present in the wild-type SFB(1). S(1)' along with 2 other previously characterized Prunus S-haplotype mutants, S(f) and S(6m), illustrate that mobile element insertion is an evolutionary force contributing to the breakdown of GSI.

Interspecific hybrid production by reciprocal crosses between Dianthus caryophyllus L. and Dianthus × isensis Hirahata et Kitamura

SummaryTo introduce the characters of extremely early flowering and unique flower shape of Dianthus isensis Hirahata et Kitamura into Dianthus caryophyllus L., reciprocal interspecific crosses were carried out between seven lines of D. caryophyllus and one line of D. isensis. When D. caryophyllus was used as the maternal parent, 79 progeny were obtained from only four lines of D. caryophyllus. Flow cytometric analysis revealed that they consisted of 30 diploid hybrid plants and 49 diploid maternal-type progeny. When D. isensis was used as the maternal parent, five progeny including three diploid hybrids, one triploid hybrid and one maternal-type were obtained by pollination with only two lines of D. caryophyllus. The ploidy level of these hybrids was confirmed by counting chromosome numbers in root-tip cells. Flow cytometric analysis also revealed that the triploid hybrid had two genomes of D. isensis and one genome of D. caryophyllus, suggesting that D. isensis had produced an unreduced female gamete. Since the diploid interspecific hybrids were fertile and have some of the profitable characters of D. isensis such as early flowering and a unique flower shape, they are expected to be used for future breeding in carnation.

Unilateral compatibility and genotypic difference in crossability in interspecific hybridization between Dianthus caryophyllus L. and Dianthus japonicus Thunb.

Reciprocal interspecific crosses were carried out between six lines of Dianthus caryophyllus L. and one line of Dianthus japonicus Thunb. Although no seed was set when D. japonicus was used as the seed parent, six seedlings were successfully obtained from 2,380 immature ovules by applying the embryo-rescue technique. However, they showed seed parent-like morphology and no evidence for the hybridity by flow cytometry and RAPD analyses. When six lines of D. caryophyllus were used as seed parents, a total of 192 seedlings were successfully obtained without using the embryo-rescue technique. Among these seedlings, 12 out of 25 progenies obtained from the carnation line '98sp1651' were confirmed to be the hybrids. The remaining 13 progenies of this line, and the total 167 progenies obtained from the other carnation lines, had carnation-like morphology without any evidence of hybridity by flow cytometry and RAPD analyses. The progenies confirmed as hybrids had intermediate characters of the parents with respect to leaf width and flower size, but they had a uniform flower color, reddish purple, which was different from that of either parent. Since the hybrids obtained in the present study have some profitable characters such as vigorous growth in summer time, upright robust stem, broad leaves and early flowering, they are expected to be used for the breeding of carnation which is suitable for growing under the Japanese climate.

The production of yellow-seeded Brassica napus (AACC) through crossing interspecific hybrids of B. campestris (AA) and B. carinata (BBCC) with B. napus

To transfer the genes for yellow seed coat from both genomes A and C to B. napus (AACC), the hexaploid of Brassica (AABBCC) was synthesised from reciprocal interspecific crosses between yellow-seeded B.campestris (AA) and B.carinata (BBCC). The hexaploid with 27 pairs of chromosomes was red-seeded which showed that genic interaction existed in the trigenomic plants for the colour of the seed coat. Hundreds of hybrid seeds were obtained from crosses between the red-seeded hexaploid and partial yellow or brown-seeded varieties of B. napus as pollen donor. The majority of the hybrid plants (AABCC) were self fertile with brown seeds. It appeared that the chromosomes of the B genome were excluded during the meiosis of the pentaploid and a high proportion of the genetically balanced AC gametes could be produced. The fertility of the F2 population was increased and even reached normal levels for some plants. Seventy-three plants with the yellow-seeded character were isolated from 2590 open-pollinated F2 plants, most with increased fertility. After two successive self-pollinations, 18 lines produced yellow seeds and no brown seeds segregated from these populations. The morphology of the novel yellow-seeded plants was basically towards B. napus. Esterase isoenzyme electrophoresis showed that the plants contained some of the genetic background of B. campestris, B. carinata and B. napus. Cytological analysis has shown that at least some yellow-seeded lines have the B.napus AACC genome composition with 38 chromosomes and normal meiotic pairing.

Developmental factors responsible for heterosis in aspen hybrids (Populus tremuloides x P. tremula.

Juvenile growth and bud set phenology were analyzed to study the biological basis of heterosis (hybrid vigor) in interspecific hybrids of Populus tremuloides Michx. (T) and P. tremula L. (Ta). Growth, measured as seedling volume index, was significantly higher for each of the two reciprocal interspecific crosses, T x Ta and Ta x T, than for the T x T intraspecific cross. Broad-sense heritabilities were 2-6 times larger than narrow-sense heritabilities for growth and shoot components in the T x T intraspecific cross, suggesting an important role for dominance or overdominance in aspen growth. Previous genetic analyses have indicated that hybrid vigor may be the result of overdominance at several key loci each with an allele inherited from each of parental species. Internode length and leaf number contributed substantially to the heterosis of stem volume, but their effects on heterosis differed between the T x Ta and Ta x T hybrids. In T x Ta seedlings, heterosis of stem volume was attributed to a high diameter growth rate, whereas in Ta x T seedlings heterosis of stem volume was probably the result of delayed bud set resulting in a longer duration of height growth. In addition to internode number and length and leaf number, other morphological or physiological components might affect heterosis, for example, extended leaf retention.