Hybrid Sterility Loci Research Articles

Identification of a novel hybrid sterility locus S67 between temperate japonica subgroup and basmati subgroup in Oryza sativa L

Asian cultivated rice (Oryza sativa L.) is the most important cultivated species in the AA genome species of the genus Oryza. basmati is a special and famous subgroup in Asian cultivated rice, and temperate japonica is one of the most important cultivated subgroup, too. However, hybrid sterility hinders the introgression of favorable traits and the utilization of hybrid vigour between the two subgroups. The genetic basis of intraspecific hybrid sterility between temperate japonica and basmati remained elusive. In this study, a novel hybrid sterility locus S67 was identified, which caused hybrid male sterility in hybrids between the temperate japonica rice variety Dianjingyou 1 (DJY1) and the basmati rice variety Dom-sufid. Initial mapping with BC1F1, BC4F1, BC4F2 populations and DNA markers located S67 between RM5362 and K1-40.6 on the long arm of chromosome 1. Genetic analysis confirmed that S67 caused a transmission advantage for the temperate japonica rice S67-te allele in the hybrid offsprings. This result not only fills the gap in the research on hybrid sterility between basmati and temperate japonica, but also lays a good foundation for the systematic study of the genetic nature of hybrid sterility between basmati and other subgroups, as well as the full exploration and utilization of this subgroup through the creation of wide or specific compatibility lines to overcome hybrid sterility. In addition, this result can also help us broaden our understanding of genetic differentiation within Asian cultivated rice and hybrid sterility between inter-subgroups.

A Minimal Hybrid Sterility Genome Assembled by Chromosome Swapping Between Mouse Subspecies (Mus musculus).

Hybrid sterility is a reproductive isolation barrier between diverging taxa securing the early steps of speciation. Hybrid sterility is ubiquitous in the animal and plant kingdoms, but its genetic control is poorly understood. In our previous studies, we have uncovered the sterility of hybrids between musculus and domesticus subspecies of the house mouse, which is controlled by the Prdm9 gene, the X-linked Hstx2 locus, and subspecific heterozygosity for genetic background. To further investigate this form of genic-driven chromosomal sterility, we constructed a simplified hybrid sterility model within the genome of the domesticus subspecies by swapping domesticus autosomes with their homologous partners from the musculus subspecies. We show that the "sterility" allelic combination of Prdm9 and Hstx2 can be activated by a musculus/domesticus heterozygosity of as few as two autosomes, Chromosome 17 (Chr 17) and Chr 18 and is further enhanced when another heterosubspecific autosomal pair is present, whereas it has no effect on meiotic progression in the pure domesticus genome. In addition, we identify a new X-linked hybrid sterility locus, Hstx3, at the centromeric end of Chr X, which modulates the incompatibility between Prdm9 and Hstx2. These results further support our concept of chromosomal hybrid sterility based on evolutionarily accumulated divergence between homologous sequences. Based on these and previous results, we believe that future studies should include more information on the mutual recognition of homologous chromosomes at or before the first meiotic prophase in interspecific hybrids, as this may serve as a general reproductive isolation checkpoint in mice and other species.

Tetraploid interspecific hybrids between Asian and African rice species restore fertility depending on killer-protector loci for hybrid sterility.

Interspecific F1 hybrids between Asian (Oryza sativa) and African rice (Oryza glaberrima) exhibit severe sterility caused by the accumulation of hybrid sterility genes/loci at 15 or more loci. The mechanisms underlying the hybrid sterility genes are largely unknown; however, a few genes associated with the killer-protector system, which is the system most frequently associated with hybrid sterility genes, have been identified. We previously produced fertile plants as tetraploids derived from diploid interspecific F1 hybrids through anther culture; therefore, it was suggested that hybrid sterility could be overcome following tetraploidization. We investigated whether tetraploid interspecific plants produced by crossing are fertile and tested the involvement of hybrid sterility genes in the process. Fertile tetraploid interspecific F1 hybrid plants were obtained by crossing 2 tetraploids of O. sativa and O. glaberrima. To elucidate the relationships between pollen fertility and the hybrid sterility loci in the tetraploid F1 microspores, we performed genetic analyses of the tetraploid F2 hybrids and diploid plants obtained from the microspores of tetraploid interspecific hybrids by anther culture. The result suggested that the tetraploid interspecific hybrids overcame pollen and seed infertility based on the proportion of loci with the killer-protector system present in the tetraploids. The heterozygous hybrid sterility loci with the killer-protector system in the tetraploid segregate the homozygous killed allele (16.7-21.4%), with more than three-quarters of the gametes surviving. We theoretically and experimentally demonstrated that fertile rice progenies can be grown from tetraploid interspecific hybrids.

Natural variation in the zinc-finger-encoding exon of Prdm9 affects hybrid sterility phenotypes in mice.

PRDM9-mediated reproductive isolation was first described in the progeny of Mus musculus musculus (MUS) PWD/Ph and Mus musculus domesticus (DOM) C57BL/6J inbred strains. These male F1 hybrids fail to complete chromosome synapsis and arrest meiosis at prophase I, due to incompatibilities between the Prdm9 gene and hybrid sterility locus Hstx2. We identified 14 alleles of Prdm9 in exon 12, encoding the DNA-binding domain of the PRDM9 protein in outcrossed wild mouse populations from Europe, Asia, and the Middle East, 8 of which are novel. The same allele was found in all mice bearing introgressed t-haplotypes encompassing Prdm9. We asked whether 7 novel Prdm9 alleles in MUS populations and the t-haplotype allele in 1 MUS and 3 DOM populations induce Prdm9-mediated reproductive isolation. The results show that only combinations of the dom2 allele of DOM origin and the MUS msc1 allele ensure complete infertility of intersubspecific hybrids in outcrossed wild populations and inbred mouse strains examined so far. The results further indicate that MUS mice may share the erasure of PRDM9msc1 binding motifs in populations with different Prdm9 alleles, which implies that erased PRDM9 binding motifs may be uncoupled from their corresponding Prdm9 alleles at the population level. Our data corroborate the model of Prdm9-mediated hybrid sterility beyond inbred strains of mice and suggest that sterility alleles of Prdm9 may be rare.

A minimal genome design to maximally guarantee fertile inter-subspecific hybrid rice

Hybrid rice has made considerable contributions to achieve the ambitious goal of food security for the world’s population. Hybrid rice from indica/xian and japonica/geng subspecies shows much higher heterosis and is thereby an important innovation in promoting rice production in the next decade. However, such inter-subspecific hybrid rice has long suffered from serious hybrid sterility, which is a major challenge that needs to be addressed. In this study, we performed a genome design strategy to produce fertile inter-subspecific hybrid by creation of wide compatibility varieties that are able to overcome hybrid sterility. Based on combined genetic analyses in two indica-japonica crosses, we determined that four hybrid sterility loci, S5, f5, pf12 and Sc, are the major QTLs controlling inter-subspecific hybrid sterility and thus the minimal targets that can be manipulated for breeding sub-specific hybrid rice. We then cloned the pf12 locus, one of the most effective loci for hybrid male sterility, by map-based cloning, and showed that artificial disruption of pf12A gene at this locus could successfully rescue hybrid fertility. We further dissected the genetic basis of wide compatibility using three pairwise crosses from a wide-compatibility variety Dular and representative indica and japonica varieties. On this basis, we constructed and assembled different combinations of naturally compatible alleles of four loci, S5, Sc, pf12, and f5, and found that the improved lines could fully recover pollen and embryo sac fertility in test-crossed F1s, thereby completely fulfilling the demands of inter-subspecific hybrid spikelet fertility in agricultural production. This breeding scheme would facilitate redesign of future inter-subspecific hybrid rice with a higher yield potential.

Understanding the Nature of Hybrid Sterility and Divergence of Asian Cultivated Rice.

Intraspecific hybrid sterility is a common form of postzygotic reproductive isolation in Asian cultivated rice, which is also the major obstacle to utilize the strong heterosis in the rice breeding program. Here, we review recent progress in classification and hybrid sterility in Asian cultivated rice. A genome-wide analysis of numerous wild relatives of rice and Asian cultivated rice has provided insights into the origin and differentiation of Asian cultivated rice, and divided Asian cultivated rice into five subgroups. More than 40 conserved and specific loci were identified to be responsible for the hybrid sterility between subgroup crosses by genetic mapping, which also contributed to the divergence of Asian cultivated rice. Most of the studies are focused on the sterile barriers between indica and japonica crosses, ignoring hybrid sterility among other subgroups, leading to neither a systematical understanding of the nature of hybrid sterility and subgroup divergence, nor effectively utilizing strong heterosis between the subgroups in Asian cultivated rice. Future studies will aim at identifying and characterizing genes for hybrid sterility and segregation distortion, comparing and understanding the molecular mechanism of hybrid sterility, and drawing a blueprint for intraspecific hybrid sterility loci derived from cross combinations among the five subgroups. These studies would provide scientific and accurate guidelines to overcome the intraspecific hybrid sterility according to the parent subgroup type identification, allowing the utilization of heterosis among subgroups, also helping us unlock the mysterious relationship between hybrid sterility and Asian cultivated rice divergence.

Development of Wide-Compatible Indica Lines by Pyramiding Multiple Neutral Alleles of Indica-Japonica Hybrid Sterility Loci.

Since the development of indica hybrid rice in the 1970s, great success has been achieved in hybrid rice production in China and around the world. The utilization of inter-subspecific indica–japonica hybrid rice has always been considered due to its stronger heterosis characteristics. However, indica–japonica hybrids face a serious problem of sterility, which hinders the exploitation of their heterosis. In the past decades, the genetic basis of indica–japonica hybrid sterility has been well studied. It was found that in sterile indica–japonica hybrids, female sterility was mainly controlled by the S5 locus and male sterility by the Sa, Sb, Sc, Sd, and Se loci. In this study, we developed wide-compatible indica lines (WCILs) by pyramiding multiple neutral (n) alleles of the hybrid sterility loci. First, we identified Sn alleles of the loci in single-segment substitution lines (SSSLs) in the genetic background of indica Huajingxian 74 (HJX74). Then, the Sn alleles of S5, Sb, Sc, Sd, and Se loci in SSSLs were pyramided in the HJX74 genetic background. The WCILs carrying Sn alleles at the S5, Sb, Sc, Sd, and Se loci showed wide compatibility with indica and japonica rice varieties. Therefore, the WCILs will be used to develop inter-subspecific indica–japonica hybrid rice with normal fertility.

Rapid Macrosatellite Evolution Promotes X-Linked Hybrid Male Sterility in a Feline Interspecies Cross.

The sterility or inviability of hybrid offspring produced from an interspecific mating result from incompatibilities between parental genotypes that are thought to result from divergence of loci involved in epistatic interactions. However, attributes contributing to the rapid evolution of these regions also complicates their assembly, thus discovery of candidate hybrid sterility loci is difficult and has been restricted to a small number of model systems. Here we reported rapid interspecific divergence at the DXZ4 macrosatellite locus in an interspecific cross between two closely related mammalian species: the domestic cat (Felis silvestris catus) and the Jungle cat (Felis chaus). DXZ4 is an interesting candidate due to its structural complexity, copy number variability, and described role in the critical yet complex biological process of X-chromosome inactivation. However, the full structure of DXZ4 was absent or incomplete in nearly every available mammalian genome assembly given its repetitive complexity. We compared highly continuous genomes for three cat species, each containing a complete DXZ4 locus, and discovered that the felid DXZ4 locus differs substantially from the human ortholog, and that it varies in copy number between cat species. Additionally, we reported expression, methylation, and structural conformation profiles of DXZ4 and the X chromosome during stages of spermatogenesis that have been previously associated with hybrid male sterility. Collectively, these findings suggest a new role for DXZ4 in male meiosis and a mechanism for feline interspecific incompatibility through rapid satellite divergence.

Genetic Mapping of the Gamete Eliminator Locus, S2, Causing Hybrid Sterility and Transmission Ratio Distortion Found between Oryza sativa and Oryza glaberrima Cross Combination

Hybrid sterility is a reproductive barrier that prevents gene flow between species. In Oryza species, some hybrid sterility loci, which are classified as gamete eliminators, cause pollen and seed sterility and sex-independent transmission ratio distortion (siTRD) in hybrids. However, the molecular basis of siTRD has not been fully characterized because of lacking information on causative genes. Here, we analyze one of the hybrid sterility loci, S2, which was reported more than forty years ago but has not been located on rice chromosomes. Hybrids between African rice (Oryza glaberrima) and a near-isogenic line that possesses introgressed chromosomal segments from Asian rice (Oryza sativa) showed sterility and siTRD, which confirms the presence of the S2 locus. Genome-wide SNP marker survey revealed that the near-isogenic line has an introgression on chromosome 4. Further substitution mapping located the S2 locus between 22.60 Mb and 23.54 Mb on this chromosome. Significant TRD in this chromosomal region was also observed in a calli population derived from cultured anther in hybrids of another cross combination of African and Asian rice species. This indicates that the pollen abortion caused by the S2 locus occurs before callus induction in anther culture. It also suggests the wide existence of the S2-mediated siTRD in this interspecific cross combination. Chromosomal location of the S2 locus will be valuable for identifying causative genes and for understanding of the molecular basis of siTRD.

Diploid Male Gametes Circumvent Hybrid Sterility Between Asian and African Rice Species.

In F1 hybrids of Oryza sativa (Asian rice) and Oryza glaberrima (African rice), heterozygosity leads to a complete gamete abortion because of allelic conflict at each of the 13 hybrid sterility (HS) loci. We systematically produced 19 plants from the F1 hybrids of both the rice species by the anther culture (AC) method. Five of the 19 interspecific hybrid plants were partially fertile and able to produce seeds. Unlike ordinal doubled haploid plants resulting from AC, these regenerated plants showed various ploidy levels (diploid to pentaploid) and different zygosities (completely homozygous, completely heterozygous, and a combination). These properties were attributable to meiotic anomalies in the interspecific hybrid F1 plants. Examination of the genetic structures of the regenerated plants suggested meiotic non-reduction took place in the interspecific hybrid F1 plants. The centromeric regions in the regenerated plants revealed that the abnormal first and/or second divisions of meiosis, namely the first division restitution (FDR) and/or second division restitution (SDR), had occurred in the interspecific hybrid. Immunohistochemical observations also verified these phenomena. FDR and SDR occurrences at meiosis might strongly lead to the formation of diploid microspores. The results demonstrated that meiotic anomalies functioned as a reproductive barrier occurred before the HS genes acted in gamete of the interspecific hybrid. Although such meiotic anomalies are detrimental to pollen development, the early rescue of microspores carrying the diploid gamete resulted in the fertile regenerated plants. The five partially fertile plants carrying tetraploid genomes with heterozygous alleles of the HS loci produced fertile diploid pollens, implying that the diploid gametes circumvented the allelic conflicts at the HS loci. We also proposed how diploid male gametes avoid HS with the killer–protector model.

New Insights Into the Nature of Interspecific Hybrid Sterility in Rice.

Interspecific and intraspecific hybrid sterility is a typical and common phenomenon of postzygotic reproductive barrier in rice. This is an indicator of speciation involved in the formation of new species or subspecies, and it significantly hampers the utilization of favorable genes from distant parents for rice improvement. The Oryza genus includes eight species with the same AA genome and is a model plant for studying the nature of hybrid sterility and its relationship with speciation. Hybrid sterility in rice is mostly controlled by nuclear genes, with more than 50 sterility loci genetically identified to date, of which 10 hybrid sterility loci or pairs were cloned and characterized at the molecular level. Comparing the mapping results for all sterility loci reported indicated that some of these loci from different species should be allelic to each other. Further research revealed that interactions between the multiple alleles at the hybrid sterility locus caused various genetic effect. One hypothesis for this important phenomenon is that the hybrid sterility loci are orthologous loci, which existed in ancient ancestors of rice. When one or more ancestors drifted to different continents, genetic divergence occurred because of adaptation, selection, and isolation among them such that various alleles from orthologous loci emerged over evolutionary time; hence, interspecific hybrid sterility would be mainly controlled by a few orthologous loci with different alleles. This hypothesis was tested and supported by the molecular characterization of hybrid sterility loci from S1, S5, Sa, qHMS7, and S27. From this, we may further deduce that both allelic and non-allelic interactions among different loci are the major genetic basis for the interspecific hybrid sterility between O. sativa and its AA genome relatives, and the same is true for intraspecific hybrid sterility in O. sativa. Therefore, it is necessary to raise the near-isogenic lines with various alleles/haplotypes and pyramided different alleles/haplotypes from sterile loci in the same genetic background aiming to study allelic and non-allelic interaction among different hybrid sterility loci in the AA genome species. Furthermore, the pyramiding lines ought to be used as bridge parents to overcome hybrid sterility for rice breeding purposes.

A hybrid sterile locus leads to the linkage drag of interspecific hybrid progenies

Interspecific hybridization plays an important role in rice breeding by broadening access to desirable traits such as disease resistance and improving yields. However, interspecific hybridization is often hindered by hybrid sterility, linkage drag, and distorted segregation. To mine for favorable genes from Oryza glaberrima, we cultivated a series of BC4 introgression lines (ILs) of O. glaberrima in the japonica rice variety background (Dianjingyou 1) in which the IL-2769 (BC4F10) showed longer sterile lemmas, wider grains and spreading panicles compared with its receptor parent, suggesting that linkage drag may have occurred. Based on the BC5F2 population, a hybrid sterility locus, S20, a long sterile lemma locus, G1-g, and a new grain width quantitative trait locus (QTL), qGW7, were mapped in the linkage region about 15 centimorgan (cM) from the end of the short arm of chromosome 7. The hybrid sterility locus S20 from O. glaberrima eliminated male gametes of Oryza sativa, and male gametes carrying the alleles of O. sativa in the heterozygotes were aborted completely. In addition, the homozygotes presented a genotype of O. glaberrima, and homozygous O. sativa were not produced. Surprisingly, the linked traits G1-g and qGW7 showed similar segregation distortion. These results indicate that S20 was responsible for the linkage drag. As a large number of detected hybrid sterility loci are widely distributed on rice chromosomes, we suggest that hybrid sterility loci are the critical factors for the linkage drag in interspecific and subspecific hybridization of rice.

Molecular mechanisms of hybrid sterility in rice.

Hybrid sterility presents a major bottleneck in hybrid crop breeding and causes postzygotic reproductive isolation in speciation. Here, we summarize the current understanding of the genetics of rice hybrid sterility and highlight new advances in deciphering the molecular basis of the major genetic loci for hybrid sterility in rice. We also discuss practical strategies for overcoming reproductive barriers to utilize hybrid vigor in inter-specific and inter-subspecific hybrid rice breeding.

Identification and Mapping of a New Soybean Male-Sterile Gene, mst-M.

The use of sterility is common in plants and multiple loci for hybrid sterility have been identified in crops such as rice. In soybean, fine-mapping and research on the molecular mechanism of male sterility is limited. Here, we identified a male-sterile soybean line, which produces larger, abnormal pollen grains that stain poorly with I2-KI. In an inheritance test, all F1 plants were fertile and the F2 and F2:3 populations conformed with the expected segregation ratio of 3:1 (fertility:sterility) (p = 0.82) and showed a 1:2:0 ratio of homozygous fertile: heterozygous fertile: homozygous sterile genotypes (p = 0.73), suggesting that the sterility was controlled by a single recessive gene (designated “mst-M”). Bulked segregant analysis showed that almost all single-nucleotide polymorphisms (SNPs; 95.92%) were distributed on chromosome 13 and 868 SNPs (95.81%) were distributed in the physical region of Chromosome 13.21877872 to Chromosome 13.22862641. Genetic mapping revealed that mst-M was flanked by W1 and dCAPS-1 with genetic distances of 0.6 and 1.8 cM, respectively. The order of the consensus markers and known sterility genes was: Satt146 – (5.0 cM) – st5 – (2.5 cM) – Satt030 – (15.3 cM) – ms6 – (5.0 cM) – Satt149 – (39.5 cM) – W1 – (0.6 cM) – mst-M – (14.1 cM) – Satt516 (7.5 cM) – ms1 – (16.3 cM) – Satt595. These results suggest that mst-M is a newly identified male-sterility gene, which represents an alternative genetic resource for developing a hybrid seed production system for soybean.

Anther culture in rice proportionally rescues microspores according to gametophytic gene effect and enhances genetic study of hybrid sterility

BackgroundTo investigate plant hybrid sterility, we studied interspecific hybrids of two cultivated rice species, Asian rice (Oryza sativa) and African rice (O. glaberrima). Male gametes of these hybrids display complete sterility owing to a dozen of hybrid sterility loci, termed HS loci, but this complicated genetic system remains poorly understood.ResultsMicrospores from these interspecific hybrids form sterile pollen but are viable at the immature stage. Application of the anther culture (AC) method caused these immature microspores to induce callus. The segregation distortion of 11 among 13 known HS loci was assessed in the callus population. Using many individual calli, fine mapping of the HS loci was attempted based on heterozygotes produced from chromosome segment substitution lines (CSSLs). Transmission ratio distortion (TRD) from microspores was detected at 6 of 11 HS loci in the callus population. The fine mapping of S1 and S19 loci using CSSLs revealed precise distances of markers from the positions of HS loci exhibiting excessive TRD.ConclusionsWe demonstrated that AC to generate callus populations derived from immature microspores is a useful methodology for genetic study. The callus population facilitated detection of TRD at multiple HS loci and dramatically shortened the process for mapping hybrid sterility genes.

Neutral alleles at hybrid sterility loci of Oryza glaberrima from AA genome relatives in Genus Oryza.

Hybrid sterility between Oryza sativa and O. glaberrima is a main reproduction barrier when transferring the favorable alleles from O. glaberrima to O. sativa and it happens due to allelic interaction at sterility loci. Neutral alleles at each locus have the potential to overcome the sterility between the two cultivated rice species. In this study, an O. sativa cultivar Dianjingyou 1 (DJY1) and its near-isogenic lines (NILs) harboring the single sterility allele S1-glab, S19-glab, S20-glab, S37-glab, S38-glab and S39-glab as the tested lines were crossed with O. glaberrima, O. rufipogon, O. nivara, O. glumaepatula, O. barthii, O. meridionalis and O. sativa so as to detect the neutral alleles of these loci. Pollen fertility was investigated in the paired F1s based on two seasons’ result and genotypic segregation was also analyzed in some F2 populations to confirm the results of pollen fertility investigation. The neutral alleles of S38-n and S39-n were identified based upon the pollen fertility and genotypic segregation analysis for the first time. The neutral alleles of sterility loci detected from present report have the potential to know of the nature of interspecific hybrid sterility, and to overcome the interspecific hybrid sterility between O. sativa and O. glaberrima.

Mechanisms of Transmission Ratio Distortion at Hybrid Sterility Loci Within and Between Mimulus Species.

Hybrid incompatibilities are a common correlate of genomic divergence and a potentially important contributor to reproductive isolation. However, we do not yet have a detailed understanding of how hybrid incompatibility loci function and evolve within their native species, or why they are dysfunctional in hybrids. Here, we explore these issues for a well-studied, two-locus hybrid incompatibility between hybrid male sterility 1 (hms1) and hybrid male sterility 2 (hms2) in the closely related yellow monkeyflower species Mimulus guttatus and M. nasutus. By performing reciprocal backcrosses with introgression lines (ILs), we find evidence for gametic expression of the hms1-hms2 incompatibility. Surprisingly, however, hybrid transmission ratios at hms1 do not reflect this incompatibility, suggesting that additional mechanisms counteract the effects of gametic sterility. Indeed, our backcross experiment shows hybrid transmission bias toward M. guttatus through both pollen and ovules, an effect that is particularly strong when hms2 is homozygous for M. nasutus alleles. In contrast, we find little evidence for hms1 transmission bias in crosses within M. guttatus, providing no indication of selfish evolution at this locus. Although we do not yet have sufficient genetic resolution to determine if hybrid sterility and transmission ratio distortion (TRD) map to the same loci, our preliminary fine-mapping uncovers a genetically independent hybrid lethality system involving at least two loci linked to hms1. This fine-scale dissection of TRD at hms1 and hms2 provides insight into genomic differentiation between closely related Mimulus species and reveals multiple mechanisms of hybrid dysfunction.

Mapping QTLs for hybrid sterility in three AA genome wild species of Oryza.

In order to know the genetic nature of hybrid sterility further, three populations, a BC4F2 population derived from Oryza nivara crossed with Yundao 1, a BC4F2 population derived from O. rufipogon crossed with Yundao 1, and a BC10F1 population derived from a cross between O. barthii and Dianjingyou 1 were developed, respectively. Three hybrid sterility QTLs, qHS-6a, qHS-6b, and qHS-6c, detected from those three populations, were mapped into the region between RM190 and RM510, RM190 and RM3414, RM190 and RM587 on chromosome 6, respectively. These QTLs showed collinearity, and explained 88.24%, 61.52%, 44.46% of the phenotypic variance in pollen fertility and 80.60%, 35.20%, 29.01% of the phenotypic variance in spikelet fertility, respectively. In all three crosses, the gametes carrying Yundao 1 or Dianjingyou 1 alleles were eliminated by gametes carrying the wild species alleles. Comparison of the location and the mode of gene action of three QTLs correspond to the S1 locus indicates a common and conserved hybrid sterility locus in AA genome specie playing an important role in reproductive barriers in Oryza. Fine mapping of these QTLs would lead to understand the micro-collinearity and evolutionary relationship among Oryza species.

Fine mapping of S37, a locus responsible for pollen and embryo sac sterility in hybrids between Oryza sativa L. and O. glaberrima Steud

Hybrid sterility locus S37 between Oryza glaberrima and Oryza sativa results in both pollen and embryo sac sterility. Interspecific crossing between African cultivated rice Oryza glaberrima and Oryza sativa cultivars is hindered by hybrid sterility. To dissect the mechanism of interspecific hybrid sterility, we developed a near-isogenic line (NIL)-S37 using Dianjingyou1 (DJY1) as the recipient parent and an African cultivated rice variety as the donor parent. Empty pollen and embryo sac sterility were observed in F1 hybrids between DJY1 and NIL-S37. Cytological analyses showed that pollen abortion in the F1 hybrids occurred at the late binucleate stage due to a failure of starch accumulation in pollen grains. In addition, partial abortion of the embryo sac in the F1 hybrid was observed during function megaspore developing into mature embryo sac. Molecular analysis revealed that the semi-sterility was largely caused by the abortion of male and female gametophytes carrying the S37 allele from DJY1. A population of 25,600 plants derived from the hybrid DJY1/NIL-S37 was developed to fine map S37. Based on the physical location of molecular markers, S37 locus was finally delimited to a region of 205kb on the short arm of chromosome 1 in terms of reference sequences of cv. Nipponbare. Interestingly, an about 97-kb DNA segment was deleted in the NIL-S37 based on BAC clone information of O. glaberrima. Fifty-four open reading frames (ORF) were predicted in this 205-kb region of DJY1, whereas only 31 ORFs were in that of NIL-S37. These results are valuable for cloning of S37 gene and further breaking reproductive isolation between Oryza glaberrima and Oryza sativa cultivars, as well as marker-assisted transferring of the corresponding neutral allele in rice breeding programs.

Genome-wide mapping in a house mouse hybrid zone reveals hybrid sterility loci and Dobzhansky-Muller interactions.

Mapping hybrid defects in contact zones between incipient species can identify genomic regions contributing to reproductive isolation and reveal genetic mechanisms of speciation. The house mouse features a rare combination of sophisticated genetic tools and natural hybrid zones between subspecies. Male hybrids often show reduced fertility, a common reproductive barrier between incipient species. Laboratory crosses have identified sterility loci, but each encompasses hundreds of genes. We map genetic determinants of testis weight and testis gene expression using offspring of mice captured in a hybrid zone between M. musculus musculus and M. m. domesticus. Many generations of admixture enables high-resolution mapping of loci contributing to these sterility-related phenotypes. We identify complex interactions among sterility loci, suggesting multiple, non-independent genetic incompatibilities contribute to barriers to gene flow in the hybrid zone.