The Distribution and Effects of Genes Causing F(1) Weakness in ORYZA BREVILIGULATA and O. GLABERRIMA

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.

Understanding and overcoming hybrid lethality in seed and seedling stages as barriers to hybridization and gene flow

Hybrid lethality is a type of reproductive isolation barrier observed in two developmental stages, hybrid embryos (hybrid seeds) and hybrid seedlings. Hybrid lethality has been reported in many plant species and limits distant hybridization breeding including interspecific and intergeneric hybridization, which increases genetic diversity and contributes to produce new germplasm for agricultural purposes. Recent studies have provided molecular and genetic evidence suggesting that underlying causes of hybrid lethality involve epistatic interaction of one or more loci, as hypothesized by the Bateson-Dobzhansky-Muller model, and effective ploidy or endosperm balance number. In this review, we focus on the similarities and differences between hybrid seed lethality and hybrid seedling lethality, as well as methods of recovering seed/seedling activity to circumvent hybrid lethality. Current knowledge summarized in our article will provides new insights into the mechanisms of hybrid lethality and effective methods for circumventing hybrid lethality.

Temperature-dependent sugar accumulation in interspecific Capsicum F1 plants showing hybrid weakness

In plants, F₁ hybrids showing hybrid weakness exhibit weaker growth than their parents. The phenotypes of hybrid weakness are often suppressed at certain temperatures. However, it is unclear whether hybrid weakness in Capsicum annuum × C. chinense is temperature-dependent or not. Our study showed that Capsicum hybrid weakness was suppressed at 30 and 35 °C and was induced at 15, 20, and 25 °C. Moreover, we investigated the time course of hybrid weakness in cell death, metabolite content, and gene expression in leaves of plants transferred to 20 °C after growing at 30 °C for 21 days. The expression of pathogen defense-related genes was upregulated at 1 day after transfer to 20 °C (DAT). Cell death was detected at 7 DAT, plant growth had almost stopped since 14 DAT, and sugars were accumulated at 42 DAT in hybrid plants. The study revealed that some sugar transporter genes, which had been upregulated since 7 DAT, were involved in sugar accumulation in Capsicum hybrid weakness. Thus, our results demonstrated that gene expression changes occur first, followed by physiological and morphological changes after induction of hybrid weakness. These responses observed in this study in Capsicum hybrid weakness are likely to be owed to plant defense responses-like reactions.

The rice pds1 locus genetically interacts with partner to cause panicle exsertion defects and ectopic tillers in spikelets

BACKGROUND

Rice (Oryza sativa L.) is a staple food crop worldwide. Its yield and quality are affected by its tillering pattern and spikelet development. Although many genes involved in the vegetative and reproductive development of rice have been characterized in previous studies, the genetic mechanisms that control axillary tillering, spikelet development, and panicle exsertion remain incompletely understood.

RESULTS

Here, we characterized a novel rice recombinant inbred line (RIL), panicle exsertion defect and aberrant spikelet (pds). It was derived from a cross between two indica varieties, S142 and 430. Intriguingly, no abnormal phenotypes were observed in the parents of pds. This RIL exhibited sheathed panicles at heading stage. Still, a small number of tillers in pds plants were fully exserted from the flag leaves. Elongated sterile lemmas and rudimentary glumes (occurred occasionally) were observed in the spikelets of the exserted panicles and were transformed into palea/lemma-like structures. Furthermore, more interestingly, tillers occasionally grew from the axils of the elongated rudimentary glumes. Via genetic linkage analysis, we found that the abnormal phenotype of pds manifesting as genetic incompatibility or hybrid weakness was caused by genetic interaction between a recessive locus, pds1, which was derived from S142 and mapped to chromosome 8, and a locus pds2, which not yet mapped from 430. We fine-mapped pds1 to an approximately 55-kb interval delimited by the markers pds-4 and 8 M3.51. Six RGAP-annotated ORFs were included in this genomic region. qPCR analysis revealed that Loc_Os080595 might be the target of pds1 locus, and G1 gene might be involved in the genetic mechanism underlying the pds phenotype.

CONCLUSIONS

In this study, histological and genetic analyses revealed that the pyramided pds loci resulted in genetic incompatibility or hybrid weakness in rice might be caused by a genetic interaction between pds loci derived from different rice varieties. Further isolation of pds1 and its interactor pds2, would provide new insight into the molecular regulation of grass inflorescence development and exsertion, and the evolution history of the extant rice.

A novel discovery of a long terminal repeat retrotransposon-induced hybrid weakness in rice

Hybrid weakness is a post-zygotic hybridization barrier frequently observed in plants, including rice. In this study, we describe the genomic variation among three temperate japonica rice (Oryza sativa ssp. japonica) varieties 'Aranghyangchalbyeo' ('CH7'), 'Sanghaehyangheolua' ('CH8') and 'Shinseonchalbyeo' ('CH9'), carrying different hybrid weakness genes. The reciprocal progeny obtained from crossing any two varieties displayed characteristic hybrid weakness traits. We mapped and cloned a new locus, Hwc3 (hybrid weakness 3), on chromosome 4. Sequence analysis identified that a long terminal repeat (LTR) retrotransposon was inserted into the promoter region of the Hwc3 gene in 'CH7'. A 4-kb DNA fragment from 'CH7' containing the Hwc3 gene with the inserted LTR retrotransposon was able to induce hybrid weakness in hybrids with 'CH8' plants carrying the Hwc1 gene by genetic complementation. We investigated the differential gene expression profile of F1 plants exhibiting hybrid weakness and detected that the genes associated with energy metabolism were significantly down-regulated compared with the parents. Based on our results, we propose that LTR retrotransposons could be a potential cause of hybrid weakness in intrasubspecific hybrids in japonica rice. Understanding the molecular mechanisms underlying intrasubspecific hybrid weakness is important for increasing our knowledge on reproductive isolation and could have significant implications for rice improvement and hybrid breeding.

Towards development of new ornamental plants: status and progress in wide hybridization

The present review provides insights into the key findings of the hybridization process, crucial factors affecting the adaptation of new technologies within wide hybridization of ornamental plants and presents perspectives of further development of this strategy. Wide hybridization is one of the oldest breeding techniques that contributed enormously to the development of modern plant cultivars. Within ornamental breeding, it represents the main source of genetic variation. During the long history of wide hybridization, a number of methods were implemented allowing the evolution from a conventional breeding tool into a modern methodology. Nowadays, the research on model plants and crop species increases our understanding of reproductive isolation among distant species and partly explains the background of the traditional approaches previously used for overcoming hybridization barriers. Characterization of parental plants and hybrids is performed using molecular and cytological techniques that strongly facilitate breeding processes. Molecular markers and sequencing technologies are used for the assessment of genetic relationships among plants, as the genetic distance is typically depicted as one of the most important factors influencing cross-compatibility in hybridization processes. Furthermore, molecular marker systems are frequently applied for verification of hybrid state of the progeny. The flow cytometry and genomic in situ hybridization are used in the assessment of hybridization partners and characterization of hybrid progeny in relation to genome stabilization as well as genome recombination and introgression. In the future, new research and technologies are likely to provide more detailed information about genes and pathways responsible for interspecific reproductive isolation. Ultimately, this knowledge will enable development of strategies for obtaining compatible lines for hybrid production. Recent development in sequencing technologies and availability of sequence data will also facilitate creation of new molecular markers that will advance marker-assisted selection in hybridization process.

Interspecific hybridization among cultivars of hardy Hibiscus species section Muenchhusia

Rose mallows belong to the Muenchhusia section of the Hibiscus genus. They represent a small group of cold tolerant North American plants and are popular ornamentals mainly because of their abundant, large and colorful flowers. Due to their geographical origin they are well suited for garden use in temperate regions worldwide. The aim of the study was to investigate hybridization barriers in crosses among cultivars of Hibiscus species from the Muenchhusia section: H. coccineus, H. laevis and H. moscheutos. Crossing barriers were identified as both pre- and post-zygotic. The analysis of pollen tube growth revealed inhibition of pollen tubes and their abnormal growth. In specific crosses the fertilization success was low. The pre-fertilization barriers did not cause a complete reproductive isolation between the hybridization partners. In relation to post-fertilization barriers, the occurrence of hybrid incompatibilities such as unviability, chlorosis, necrosis, stunted growth and albinism were the main drawback in production of hybrids. The appearance of symptoms of hybrid incompatibilities was dependent upon specific parental plants. The obtained progeny had intermediate leaf morphology and flower morphology compared to parental plants. Hybridity state was verified by morphological analysis and RAPD markers. Based on the overall plant morphology, 472 hybrid progenies were obtained.

A two-locus interaction causes interspecific hybrid weakness in rice

Reproductive barriers perform a vital role during speciation. Hybrid weakness, the poorer development of hybrids compared with their parents, hinders gene exchange between different species at the postzygotic stage. Here we show that two incompatible dominant loci (Hwi1 and Hwi2) involving three genes are likely to determine the high temperature-dependent expression of hybrid weakness in interspecific hybrids of rice. Hwi1 comprises two leucine-rich repeat receptor-like kinase (LRR-RLK) genes, 25L1 and 25L2, which are specific to wild rice (Oryza rufipogon) and induce hybrid weakness. Hwi2, a rare allele that is predominantly distributed in indica rice (Oryza sativa), encodes a secreted putative subtilisin-like protease. Functional analysis indicated that pyramiding of Hwi1 and Hwi2 activates the autoimmune response in the basal nodes of hybrids, interrupting root formation and then impairing shoot growth. These findings bring new insights into our understanding of reproductive isolation and may benefit rice breeding.

Genetic and physiological analysis of a novel type of interspecific hybrid weakness in rice

Hybrid weakness is an important reproductive barrier that hinders genetic exchange between different species at the post-zygotic stage. However, our understanding of the molecular mechanisms underlying hybrid weakness is limited. In this study, we report discovery of a novel interspecific hybrid weakness in a rice chromosome segment substitution line (CSSL) library derived from a cross between the indica variety Teqing (Oryza sativa) and common wild rice (O. rufipogon). The dominant Hybrid weakness i1 (Hwi1) gene from wild rice is genetically incompatible with Teqing and induced a set of weakness symptoms, including growth suppression, yield decrease, impaired nutrient absorption, and the retardation of crown root initiation. Phytohormone treatment showed that salicylic acid (SA) could restore the height of plants expressing hybrid weakness, while other phytohormones appear to have little effect. Fine mapping indicated that Hwi1 is located in a tandem leucine-rich repeat receptor-like kinase (LRR-RLK) gene cluster. Within the 13.2-kb candidate region on the short arm of chromosome 11, there are two annotated LRR-RLK genes, LOC_Os11g07230 and LOC_Os11g07240. The Teqing allele of LOC_Os11g07230 and the wild rice allele of LOC_Os11g07240 encode predicted functional proteins. Based on the genetic inheritance of hybrid weakness, LOC_Os11g07240 is implicated as the candidate gene for Hwi1. Functional analysis of Hwi1 will expand our knowledge of the regulation of hybrid weakness in rice.

A non-additive interaction in a single locus causes a very short root phenotype in wheat

Non-additive allelic interactions underlie over dominant and under dominant inheritance, which explain positive and negative heterosis. These heteroses are often observed in the aboveground traits, but rarely reported in root. We identified a very short root (VSR) phenotype in the F1 hybrid between the common wheat (Triticum aestivum L.) landrace Chinese Spring and synthetic wheat accession TA4152-71. When germinated in tap water, primary roots of the parental lines reached ~15 cm 10 days after germination, but those of the F1 hybrid were ~3 cm long. Selfing populations segregated at a 1 (long-root) to 1 (short-root) ratio, indicating that VSR is controlled by a non-additive interaction between two alleles in a single gene locus, designated as Vsr1. Genome mapping localized the Vsr1 locus in a 3.8-cM interval delimited by markers XWL954 and XWL2506 on chromosome arm 5DL. When planted in vermiculite with supplemental fertilizer, the F1 hybrid had normal root growth, virtually identical to the parental lines, but the advanced backcrossing populations segregated for VSR, indicating that the F1 VSR expression was suppressed by interactions between other genes in the parental background and the vermiculite conditions. Preliminary physiological analyses showed that the VSR suppression is independent of light status but related to potassium homeostasis. Phenotyping additional hybrids between common wheat and synthetics revealed a high VSR frequency and their segregation data suggested more Vsr loci involved. Because the VSR plants can be regularly maintained and readily phenotyped at the early developmental stage, it provides a model for studies of non-additive interactions in wheat.

Analysis of genes controlling f(1) sterility in rice by the use of isogenic lines

In order to look into the genetic basis of intervarietal F(1) sterility in rice (Oryza sativa L.), a series of backcrosses (up to B(13)) was carried out using Taichung 65 (Japonica type) as the recurrent parent and several Indica varieties as donor parents. A number of "isogenic F(1)-sterile lines" were isolated by test-crossing fertile F(2) plants obtained from the selfing of partly pollensterile backcross segregants. Crossing experiments with the isogenic lines confirmed the author's previous hypothesis that there are sets of duplicate gametic lethals (s genes) and that gametes carrying a double recessive combination (s(1)s(2)) of these deteriorate during development, though in the present hypothesis the genes are considered to affect the development of microspores only. Assuming that Taichung 65 has the genotype s(1)/s(1) + (2)/+(2) and a donor parent (like an isogenic F(1)-sterile line derived from it) has +(1)/+(1)s(2)/s(2), pollen grains with +(1)s(2) have shown a higher fertilizing capacity in the genetic background of Taichung 65 than those with s(1) +(2), while those with +(1) +(2) have a lower fertilizing capacity. This certational advantage of alien genes was considered to be an internal mechanism that helped the development of F(1) sterility relationships among rice varieties. The isogenic F(1)-sterile lines derived from different donor parents each had a set of s genes at different loci. Linkage relations were detected between the s loci and three gene markers.

Interaction of novel Dobzhansky-Muller type genes for the induction of hybrid lethality between Gossypium hirsutum and G. barbadense cv. Coastland R4-4

Hybrid lethality was identified in interspecific hybrids between two cotton species, Gossypium hirsutum and G. barbadense cv. Coastland R4-4 (R4-4). Genetic analysis indicated that the lethal symptom was controlled by two dominant complementary genes, one from G. hirsutum and another from R4-4. Microsatellite mapping identified the location of the causal gene in G. hirsutum as chromosome D8, while the R4-4 gene was placed on chromosome D11. Our data indicate that these genes conform to the Dobzhansky-Muller model, and are novel for the induction of hybrid lethality in Gossypium. Following the genetic nomenclature, we propose that the two novel Dobzhansky-Muller genes from G. hirsutum and from R4-4 be named Le ( 3 ) and Le ( 4 ), respectively. Given what we know about their inheritance patterns, their genotypes should be Le ( 3 ) Le ( 3 ) le ( 4 ) le ( 4 ) in G. hirsutum, and le ( 3 ) le ( 3 ) Le ( 4 ) Le ( 4 ) in R4-4. Data from this study supported previous information in that expression of the lethal symptom might be affected by the dosage of causal alleles and the environment in which plants are growing.

The evolution of sex-independent transmission ratio distortion involving multiple allelic interactions at a single locus in rice

Transmission ratio distortion (TRD) is frequently observed in inter- and intraspecific hybrids of plants, leading to a violation of Mendelian inheritance. Sex-independent TRD (siTRD) was detected in a hybrid between Asian cultivated rice and its wild ancestor. Here we examined how siTRD caused by an allelic interaction at a specific locus arose in Asian rice species. The siTRD is controlled by the S6 locus via a mechanism in which the S6 allele acts as a gamete eliminator, and both the male and female gametes possessing the opposite allele (S6a) are aborted only in heterozygotes (S6/S6a). Fine mapping revealed that the S6 locus is located near the centromere of chromosome 6. Testcross experiments using near-isogenic lines (NILs) carrying either the S6 or S6a alleles revealed that Asian rice strains frequently harbor an additional allele (S6n) the presence of which, in heterozygotic states (S6/S6n and S6a/S6n), does not result in siTRD. A prominent reduction in the nucleotide diversity of S6 or S6a carriers relative to that of S6n carriers was detected in the chromosomal region. These results suggest that the two incompatible alleles (S6 and S6a) arose independently from S6n and established genetically discontinuous relationships between limited constituents of the Asian rice population.

Autoimmune response as a mechanism for a Dobzhansky-Muller-type incompatibility syndrome in plants

Epistatic interactions between genes are a major factor in evolution. Hybrid necrosis is an example of a deleterious phenotype caused by epistatic interactions that is observed in many intra- and interspecific plant hybrids. A large number of hybrid necrosis cases share phenotypic similarities, suggesting a common underlying mechanism across a wide range of plant species. Here, we report that approximately 2% of intraspecific crosses in Arabidopsis thaliana yield F1 progeny that express necrosis when grown under conditions typical of their natural habitats. We show that several independent cases result from epistatic interactions that trigger autoimmune-like responses. In at least one case, an allele of an NB-LRR disease resistance gene homolog is both necessary and sufficient for the induction of hybrid necrosis, when combined with a specific allele at a second locus. The A. thaliana cases provide insights into the molecular causes of hybrid necrosis, and serve as a model for further investigation of intra- and interspecific incompatibilities caused by a simple epistatic interaction. Moreover, our finding that plant immune-system genes are involved in hybrid necrosis suggests that selective pressures related to host-pathogen conflict might cause the evolution of gene flow barriers in plants.

Fine mapping and allelic dosage effect of Hwc1, a complementary hybrid weakness gene in rice

Hybrid weakness is a reproductive barrier that is found in many plant species. In rice, the hybrid weakness caused by two complementary genes, Hwc1 and Hwc2, has been surveyed intensively. However, their gene products and the molecular mechanism that causes hybrid weakness have remained unknown. We performed linkage analyses of Hwc1, narrowed down the area of interest to 60 kb, and identified eight candidate genes. In the F(2) population, in which both Hwc1 and Hwc2 genes were segregated, plants were separable into four classes according to their respective phenotypes: severe type, semi-severe type, F(1) type, and normal type. Severe type plants show such severe symptoms that they could produce only tiny shoot-like structures; they were unable to generate roots. Genetic analyses using closely linked DNA markers of the two genes showed that the symptoms of the F(2) plants were explainable by the genotypes of Hwc1 and Hwc2. Weakness was observed in plants that have both Hwc1 and Hwc2. In Hwc1 homozygote, the symptoms worsened and severe type or semi-severe type plants appeared. Consequently, Hwc1 should have a gene dosage effect and be a semi-dominant gene. The dosage effect of Hwc2 was recognizable, but it was not so severe as that in Hwc1. These results are useful to elucidate the mechanism that causes the hybrid weakness phenomenon and the role of each causal gene in hybrid weakness.

Hybrid necrosis: autoimmunity as a potential gene-flow barrier in plant species

Ecological factors, hybrid sterility and differences in ploidy levels are well known for contributing to gene-flow barriers in plants. Another common postzygotic incompatibility, hybrid necrosis, has received comparatively little attention in the evolutionary genetics literature. Hybrid necrosis is associated with a suite of phenotypic characteristics that are similar to those elicited in response to various environmental stresses, including pathogen attack. The genetic architecture is generally simple, and complies with the Bateson-Dobzhansky-Muller model for hybrid incompatibility between species. We survey the extensive literature on this topic and present the hypothesis that hybrid necrosis can result from autoimmunity, perhaps as a pleiotropic effect of evolution of genes that are involved in pathogen response.

Transgene escape: what potential for crop-wild hybridization?

To date, regional surveys assessing the risk of transgene escape from GM crops have focused on records of spontaneous hybridization to infer the likelihood of crop transgene escape. However, reliable observations of spontaneous hybridization are lacking for most floras, particularly outside Europe. Here, we argue that evidence of interspecific reproductive compatibility derived from experimental crosses is an important component of risk assessment, and a useful first step especially where data from field observations are unavailable. We used this approach to assess the potential for transgene escape via hybridization for 123 widely grown temperate crops and their indigenous and naturalized relatives present in the New Zealand flora. We found that 66 crops (54%) are reproductively compatible with at least one other indigenous or naturalized species in the flora. Limited reproductive compatibility with wild relatives was evident for a further 12 crops (10%). Twenty-five crops (20%) were found to be reproductively isolated from all their wild relatives in New Zealand. For the remaining 20 crops (16%), insufficient information was available to determine levels of reproductive compatibility with wild relatives. Our approach may be useful in other regions where spontaneous crop-wild hybridization has yet to be well documented.

Time course analysis of apoptotic cell death during expression of hybrid lethality in hybrid tobacco cells (Nicotiana suaveolens x N. tabacum)

Hybrid cells from the cross Nicotiana suaveolens x N. tabacum expressed hybrid lethality at 28 degrees C in a thin layer cell culture system. Features characteristic of apoptosis, such as DNA fragmentation, chromatin condensation and nuclear fragmentation, were detected during expression of hybrid lethality. Actinomycin D (ActD) or cycloheximide (CHX) added to the medium suppressed apoptotic cell death during hybrid lethality. This indicates that hybrid lethality requires de novo transcription and translation, and is thus under genetic control. To estimate the time course of apoptotic cell death during the expression of hybrid lethality, we determined when factors controlling hybrid lethality were expressed by observing the point of no return. When cells were exposed to 28 degrees C for 2 h or less in inhibitor-free medium before addition of ActD or CHX, the percentage of dead cells did not increase. However, when cells were exposed to 28 degrees C for 4 h before the addition of inhibitor, the percentage of dead cells increased. When cells were exposed to 28 degrees C for 3 h before the addition of inhibitor, the percentage of dead cells varied from experiment to experiment. These data indicate that the factors controlling hybrid lethality are expressed 3 h after induction of hybrid lethality. In addition, we found a time difference between the expression of cell death and nuclear fragmentation. This suggests that the factor controlling cell death is different from the one controlling nuclear fragmentation.

Facile induction of apoptosis into plant cells associated with temperature-sensitive lethality shown on interspecific hybrid from the cross Nicotiana suaveolens x N. tabacum

Two lines of suspension culture cells were obtained from a hybrid seedling of Nicotiana suaveolens Lehm. x N. tabacum L. cv. Hicks-2 expressing temperature-sensitive lethality. One of them (LH line) was inducible cell death in accordance with the lethality at 28 degrees C but not under high-temperature conditions (36 degrees C). Another one (SH line) lost the lethality and survived at 28 degrees C. The cells of LH line showed apoptotic changes when they were cultured at 28 degrees C. Fragmentation of nuclei was correlated with the lethality in the cells, as confirmed by fluorimetry of the nuclear DNA using laser scanning cytometry. Agarose gel analysis of DNA extracted from the cells expressing the lethality revealed a specific ladder pattern suggesting nucleosomal fragmentation that is one of the biochemical characteristics of apoptosis. From these facts, we confirmed that the process of cell death leading to hybrid lethality in the cells is certainly apoptosis. Hybrid cells were used in the experiments to estimate the point of no return in temperature-sensitive lethality and to examine the influence of cation in DNA fragmentation during apoptosis. The utility of hybrid cells as an experimental system for studies of hybrid lethality and apoptosis in plants was confirmed.

REPRODUCTIVE BARRIERS DISTRIBUTED INMELILOTUSSPECIES AND THEIR GENETIC BASES

In order to look into species relationships, crosses were made in all possible combinations between 9 species belonging to the subgenus Eumelilotus; 504 crosses were made among 53 strains in total. The reproductive barriers found among the species were a crossing barrier (death of young F1zygotes), F1inviability and weakness due to chlorosis, F2breakdown due to the occurrence of chlorotic segregants (classified into lethal and weak types), partial F1sterility, and a segmental interchange. All the hybrids within species were normal. In crosses between M. alba and M. suaveolens, the hybrids of annual strains showed more pronounced F1and F2chlorosis than those of biennial strains. The hybrids of self-fertile species or forms tended to have a low reproductive potential. The F1chlorosis between the above two species appeared to be controlled by a set of complementary dominant lethals, or by alleles at one locus which express a disharmonious interaction in heterozygote. The F2chlorosis in the same species hybrid was controlled by two sets of duplicate recessive genes, chl1and chl2for the lethal type and chw1and chw2for the weak type. The chl2and chw2loci were linked with a recombination value of about 13%.