Hybrid Sterility Research Articles

Current advance on molecular genetic regulation of rice fertility

Hybrid rice has contributed greatly to global food security. Cytoplasmic male sterility (CMS) and photo/ thermo sensitive genic male sterility (P/TGMS) are genetic bases for three-line and two-line hybrid rice production, respectively. In contrast, (sub-) specific hybrid sterility (HS) is a major barrier for utilization of hybrid vigor of distant hybrid rice. Therefore, understanding the molecular regulatory mechanism of rice fertility is a key technical issue for hybrid rice industry, and a long-standing basic scientific issue for nuclear-cytoplasmic interaction and reproductive isolation. Chinese geneticists of plant sciences have made tremendous contributions on the molecular genetic basis of rice fertility related to hybrid rice production. Here, we review the development of hybrid rice production systems in China and summarize current advance on genetic basis and molecular mechanism of CMS, P/TGMS, and HS involved in hybrid rice. We also discuss problems of hybrid rice production in China and point out new direction for future utilization of heterosis in rice.

Development of KASP marker for cytoplasmic male sterility in Nicotiana tabacum and utilization in trait introgression

Cytoplasmic male sterility (CMS) is an important trait for large-scale hybrid seed production which avoids manual emasculation and undesired horizontal spread of pollen. Rearrangements in mitochondrial genome in terms of deletions and insertions are frequent causes leading to CMS. Mitochondrial ATP synthase is a multisubunit molecular machine which is involved in synthesis of ATP. In this study, three mutations in ATPase subunit 6 were identified and their cosegregation with male sterility was established using tobacco male sterile hybrids and Nicotiana suvaolensis. A breeder friendly Kompetitive allele specific polymerase chain reaction (KASP) SNP marker was developed for high throughput and quick genotyping. Introgression of this trait into selected germplasm lines (n = 9) was achieved based on foreground for CMS and background selection for recurrent parent using KASP marker and 50K custom tobacco SNP array, respectively. Analysis of genotyping data from SNP array revealed the presence of 88-99% of recurrent parent genome in BC3F1 plants. The selected BC3F1 plants exhibit CMS and are indistinguishable from the fertile recurrent parent (germplasm) in terms of plant morphology.

Neuroaxonal Dystrophy in a mule: first case reported

ABSTRACT A 12-month-old mule (sterile hybrid equine species) presented unspecific neurological changes (symmetric ataxia, dysmetria, conscious proprioceptive deficit and weakness). Due to poor prognosis and to the fact that a sibling from the previous generation exhibited similar clinical signs that were not definitively diagnosed, the animal was euthanized. Diagnosis of neuroaxonal dystrophy was confirmed by anatomohistopathological analysis. This is the first clinical case of neuronal dystrophy in a mule reported in the world. The clinical and histopathological characteristics of this disease were very similar to those reported for several equine breeds. Therefore, the disease should also be considered in the diagnosis of neurological conditions in mules and donkeys.

Recurrent gene co-amplification on Drosophila X and Y chromosomes.

Y chromosomes often contain amplified genes which can increase dosage of male fertility genes and counteract degeneration via gene conversion. Here we identify genes with increased copy number on both X and Y chromosomes in various species of Drosophila, a pattern that has previously been associated with sex chromosome drive involving the Slx and Sly gene families in mice. We show that recurrent X/Y co-amplification appears to be an important evolutionary force that has shaped gene content evolution of sex chromosomes in Drosophila. We demonstrate that convergent acquisition and amplification of testis expressed gene families are common on Drosophila sex chromosomes, and especially on recently formed ones, and we carefully characterize one putative novel X/Y co-amplification system. We find that co-amplification of the S-Lap1/GAPsec gene pair on both the X and the Y chromosome occurred independently several times in members of the D. obscura group, where this normally autosomal gene pair is sex-linked due to a sex chromosome—autosome fusion. We explore several evolutionary scenarios that would explain this pattern of co-amplification. Investigation of gene expression and short RNA profiles at the S-Lap1/GAPsec system suggest that, like Slx/Sly in mice, these genes may be remnants of a cryptic sex chromosome drive system, however additional transgenic experiments will be necessary to validate this model. Regardless of whether sex chromosome drive is responsible for this co-amplification, our findings suggest that recurrent gene duplications between X and Y sex chromosomes could have a widespread effect on genomic and evolutionary patterns, including the epigenetic regulation of sex chromosomes, the distribution of sex-biased genes, and the evolution of hybrid sterility.

Distribution and taxonomy of <i>Isoetes tuckermanii</i> subsp. <i>acadiensis</i>, comb. nov. (Isoetaceae) in North America

Isoetes acadiensis is an emergent aquatic lycophyte of freshwater shores found in a narrow range along the Atlantic coast of northeastern North America where it frequently coexists with Isoetes tuckermanii (sensu stricto [s. str.]). Apparently fertile plants with intermediate morphology occur commonly in mixed populations. No sterile hybrids between the two taxa have been detected. Although I. acadiensis maintains a distinctive geographic distribution (within and smaller than that of I. tuckermanii [s. str.]), exhibits molecular evidence of genetic distinctiveness, and has morphologically distinctive features in most populations, the weight of evidence suggests it is not distinct from I. tuckermanii at a species level. Accordingly, I. tuckermanii subsp. acadiensis, comb. nov. is proposed as the appropriate designation for this biogeographically important Acadian endemic.

MENTHA PIPERITA L. - A PROMISING DENTAL CARE HERB MAINLY AGAINST CARIOGENIC BACTERIA

Oral diseases are considered from the major health problems and are not limited to dental caries and periodontal diseases but to various autoimmune conditions. Itsayurvedic therapy includes different plants used in management of toothache, sore throat, mouth sores, abscess, broken tooth and jaw, tooth sensitivity, mouth thrush, dental caries, gingivitis, tooth bleaching, dental anxiety, dental phobia and plants used for dental extraction. Peppermint (Menthapiperita L.), a sterile hybrid of the species M. aquatica L. and M. spicata, L. is considered one of the important aromatic herbs containing high amount of volatile oil used in dental care. The peppermint leaves have a characteristic, aromatic, strong odor and an aromatic, warm, pungent taste followed by a cooling sensation.The medicinal parts are the essential oil extracted from the aerial parts of the flowering plant, the dried leaves, the fresh flowering plant and the whole plant. M. piperita L.is a perennial 50–90 cm high, normally quadrangular and a prototypical member of the mint family. The essential oil of M.piperitaL.leaves is characterized by the presence of high percent of menthol (29-48%) in addition tomenthone (20-31%), and the different isomers of menthol in addition to other constituents.M. piperitaL. is one of most promising species with antibacterial potential against cariogenic bacteria as Streptococcus mutans. Peppermint oil and leaves posses several other biological effects as antiseptic in oral preparations, antibacterial,antiviral, antifungal, antioxidant and antispasmodic effects. It is also used as a flavoringagent in food and pharmaceutical industry and oral preparations.

An asymmetric allelic interaction drives allele transmission bias in interspecific rice hybrids

Hybrid sterility (HS) between Oryza sativa (Asian rice) and O. glaberrima (African rice) is mainly controlled by the S1 locus. However, our limited understanding of the HS mechanism hampers utilization of the strong interspecific heterosis. Here, we show that three closely linked genes (S1A4, S1TPR, and S1A6) in the African S1 allele (S1-g) constitute a killer-protector system that eliminates gametes carrying the Asian allele (S1-s). In Asian–African rice hybrids (S1-gS1-s), the S1TPR-S1A4-S1A6 interaction in sporophytic tissues generates an abortion signal to male and female gametes. However, S1TPR can rescue S1-g gametes, while the S1-s gametes selectively abort for lacking S1TPR. Knockout of any of the S1-g genes eliminates the HS. Evolutionary analysis suggests that S1 may have arisen from newly evolved genes, multi-step recombination, and nucleotide variations. Our findings will help to overcome the interspecific reproductive barrier and use Asian–African hybrids for increasing rice production.

Dominance effects strengthen premating hybridization barriers between sympatric species of grasshoppers (Acrididae, Orthoptera).

Sexual selection can lead to the rapid evolution of premating hybridization barriers and allows accelerated diversification and speciation within an evolutionary lineage. Especially during early stages of divergence, hybridization may impede further divergence, which strongly depends on the reproductive success of hybrids. Behavioural sterility of hybrids can limit or even prevent homogenizing gene flow. In this study, we investigated the attractiveness of male courtship songs for females of the grasshopper species Chorthippus biguttulus and C. brunneus and their interspecific F1 and F2 hybrids. Song preferences of females of both species are highly species specific and differ in three parameters: shape of the preference function, preference for syllable pattern and phrase duration. F1 hybrid females of both reciprocal crosses as well as F2 hybrid females resembled closely pure C. biguttulus females in respect of shape of the preference function and preference for syllable pattern, while preference for phrase duration showed an intermediate expression. This resulted in song preferences of hybrid females that closely resembled those of one parental species, that is C. biguttulus females. Such strong dominance effects were rarely reported so far. They represent an effective barrier limiting gene flow between the two species, since hybrid females will backcross to only one parental species and discriminate against hybrid males, which are behaviourally sterile. Such taxon‐specific modes of inheritance may have facilitated the rapid divergence of acoustically communicating grasshoppers of the species group of Chorthippus biguttulus. Our findings have novel implications on the expression of neuronal filters and the evolution of complex courtship signals.

Aberrant Meiotic Configurations Cause Sterility in Clone-Origin Triploid and Inter-Group Hybrid Males of the Dojo Loach, Misgurnus anguillicaudatus

Gonochoristic wild-type dojo loaches (Misgurnus anguillicaudatus) are diploid (2n = 50) and reproduce bisexually. However, sympatric clonal diploids generate unreduced diploid isogenic eggs that develop gynogenetically. Clone-origin triploidy arises following the incorporation of a haploid wild-type sperm nucleus into the diploid egg. Triploid females produce fertile haploid eggs by meiotic hybridogenesis, while triploid males are sterile. Clonal loaches arose from past hybridization event(s) between genetically diverse groups, A and B. Artificial hybrid females between the 2 groups produce unreduced and/or aneuploid eggs, but the hybrid males are sterile. In this study using FISH, we analyzed chromosome pairing in meiotic cells of clone-origin triploid and inter-group hybrid males to clarify the cytogenetic mechanisms underlying the male-specific sterility. We used a repetitive sequence probe to identify group B-derived chromosomes and a 5.8S + 28S rDNA probe to identify pairs of homologous chromosomes. We found that asynapsis and irregular synapsis occur in triploid and hybrid males containing 2 different genomes and that this may cause the formation of sterile germ cells. These results will help us to understand hybrid sterility from the viewpoint of synapsis behavior.

Vegetative and micropropagation of leucaena

Keynote paper presented at the International Leucaena Conference, 1‒3 November 2018, Brisbane, Queensland, Australia.To effectively utilize sterile hybrids of leucaena, efficient protocols for vegetative propagation are needed that meet different user requirements and capabilities. We developed and compared methods for propagating several sterile hybrids of leucaena and compared them with each other and with propagation via seeds for variety K636. Methods included air-layers, rooted cuttings, grafting and tissue culture (micropropagation). All methods required 14‒20 weeks from generation of new shoots on the stock plant to production of rooted plantlets ready to outplant as compared with 6‒8 weeks for seedlings of K636. Successful rooting was highest for air-layers and rooted cuttings. Grafting had lower success owing to a higher skill requirement for the propagator. Tissue culture showed promise, but use of field-grown material was limited by microbial contamination of propagation media. Rooted cuttings are the best option presently for operational-scale propagation, but the method requires a mist system or a carefully controlled non-mist environment. If an effective method can be developed, grafting of young shoots onto a seedling rootstock is an alternative that retains the advantages of a seedling tap root and requires fewer resources than rooted cuttings or tissue culture.

The Parasitic Wasp, Cotesia congregata (Say), Consists of Two Incipient Species Isolated by Asymmetric Reproductive Incompatibility and Hybrid Inability to Overcome Host Defenses

Parasitic wasps are highly diverse and play a major role in suppression of herbivorous insect pest populations. Several previously identified species of parasitic wasps have been found to be complexes of cryptic species resulting from adaptations to specific hosts or host foodplants. Cotesia congregata (Say) (Hymenoptera: Braconidae), which has long served as a model system for host-parasitoid interactions, can be used for investigating the process of diversification among sympatric populations that differ in host and host foodplant usage. Two incipient species of C. congregata have been identified in the USA mid-Atlantic region, “MsT wasps” originate from Manduca sexta (L.) (Lepidoptera: Sphingidae) on tobacco and “CcC wasps” originate from Ceratomia catalpae (Boisduval) (Lepidoptera: Sphingidae) on catalpa. Both wasp sources can develop in either host species. Hybrids resulting from MsT♂xCcC♀ crosses are fertile, whereas hybrids from CcC♂xMsT♀ crosses are typically sterile. In this study, we compared relative expression in vivo of seven C. congregata bracovirus (CcBV) genes among MsT and CcC parental and hybrid crosses. Also, we established hybrid crosses between MsT and CcC wasps and four additional host foodplant sources of C. congregata. Patterns of relative expression in vivo of MsT and CcC CcBV genes differed; a few were not expressed in hosts parasitized by CcC wasps. Overall, relative expression of CcBV genes from MsT and CcC wasps did not differ with respect to the host species parasitized. Low or absent expression of CcBV genes was found in hosts parasitized by sterile hybrids. For the most part, the other four host-foodplant wasp sources were reproductively compatible with either MsT or CcC wasps and hybrid crosses with the alternative wasp source were asymmetrically sterile. Crosses involving CcC males or MsT females produced sterile hybrids that lacked mature ovaries. Cumulatively, results indicate that C. congregata is composed of two sympatric incipient species that can utilize multiple host species rather than several host-associated races or cryptic species.

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.

Cytological and transcriptome analysis reveal that interaction at Sb pollen sterility locus cause down-regulation of important meiosis-related genes associated with high pollen sterility in autotetraploid rice hybrids

Polyploidy could increase the interactions of pollen sterility loci and Sb locus interaction cause higher pollen abortion than other loci. Therefore, we focused on the interaction at Sb pollen sterility locus in autotetraploid rice compared to diploid rice hybrid using the near-isogenic lines in the present study. Cytological observations indicated that interaction at Sb locus cause high pollen sterility (69.9%) and abnormal chromosome behavior (37.02%) at Metaphase II in autotetraploid rice hybrid. A total of 139 meiosis-related or meiosis stage-specific genes were detected in the autotetraploid rice hybrid harboring interaction at Sb locus and 27 of these meiosis-related or specific genes displayed significant down-regulation, including four pollen fertility related genes (Rad51, XRI1, PSS1 and MIL1). These results revealed a stronger interaction at Sb pollen sterility locus than other loci, which cause down-regulation of many important meiosis-related genes that were associated with higher pollen sterility in autotetraploid rice hybrids.

A new biological species in the Mercurialis annua polyploid complex: functional divergence in inflorescence morphology and hybrid sterility.

Background and AimsPolyploidy has played a major role in the origin of new plant species, probably because of the expansion of polyploid populations in the species’ ecological niche, and because reproductive isolation can be established between a new polyploid population and its diploid progenitor species. It is well established that most polyploid species are polyphyletic, with multiple independent origins, and that polyploid genomes may undergo rapid change after their duplication and hybridization associated with their origin. We considered whether multiple independent origins and rapid genomic change might lead to reproductive isolation between polyploid populations of the same ploidy but with potentially different evolutionary histories.MethodsWe tested our hypothesis by assessing differences in DNA content and morphology, the evolution of reproductive isolation, and the phylogenetic placement of two broadly sympatric hexaploid lineages of the wind-pollinated annual plant Mercurialis annua hitherto regarded as populations of the same species.Key ResultsThe two hexaploid lineages of M. annua have slightly divergent DNA content, and distinct inflorescence morphology. They also fall into largely different clades of a chloroplast phylogeny and are reproductively isolated from one another.ConclusionsThe distinct evolutionary histories of the two hexaploid lineages of M. annua have contributed to the remarkable reproductive diversity of the species complex. It seems likely that reproductive interference between them will eventually lead to the displacement of one lineage by the other via pollen swamping. Thus, whereas polyploidization can contribute to speciation, diversification might also be compromised by reproductive interference.

Comparison of Y-chromosome-linked TSPY, TSPY2, and PRAMEY genes in Taurus cattle, yaks, and interspecific hybrid bulls

Domestic yaks (Bos grunniens) and domestic Taurus cattle (Bos taurus) are closely related. An interesting phenomenon in interspecific crossings is male sterility in the F1 hybrid (yattle) and F2 backcross, with no late meiotic cells or spermatids in the seminiferous tubules. The mammalian Y chromosome is crucial for spermatogenesis and male fertility. This study investigated the copy number variations and mRNA of Y-transitional region genes TSPY2 (testis specific protein, Y-linked 2 and testis-specific Y-encoded protein 3-like) and PRAMEY (preferentially expressed antigen in melanoma, Y-linked), and Y-ampliconic region genes TSPY (testis-specific Y-encoded protein 1-like), ZNF280BY (zinc finger protein 280B, Y-linked) and HSFY (heat-shock transcription factor, Y-linked) in mature testes from Taurus cattle, yaks, and yattle. Phylogenetic trees divided 33 copies of TSPY into major 2 types (TSPY-T1 and TSPY-T2), 19 copies of TSPY2 into 2 types (TSPY2-T1 and T2), and 8 copies of PRAMEY into 4 types (PRAMEY-T1 to T4). Searching by the Basic Local Alignment Search Tool of the TSPY2 coding sequences in GenBank revealed that TSPY2 was conserved in Bovidae. The TSPY2-T2 sequences were absent, whereas PRAMEY-T2 and PRAMEY-T4 were amplified on the yak Y chromosome. The average copy numbers of TSPY-T2 and ZNF280BY were significantly different between cattle and yaks. The TSPY-T2, TSPY2, PRAMEY, ZNF280BY, and HSFY genes were uniquely or predominantly expressed in testes. Reverse-transcription quantitative PCR showed that the TSPY-T2, PRAMEY-T2, HSFY, ZNF280BY, protamine 1 (PRM1), and protamine 2 (PRM2) genes were almost not expressed in yattle. The PRM1 and PRM2 genes are used as positive markers for spermatozoa. Thus, our results showed that the genomic structure of the Y-transitional and Y-ampliconic region differed between Taurus cattle and yaks. Dysregulated expression of Y-ampliconic region genes TSPY-T2, HSPY, ZNF280BY, and Y-transitional region gene PRAMEY-T2 may be associated with hybrid male sterility in yattle.

RECONSTRUCTIVE AND STABILIZING ABILITY OF THE MAIZE LINES IN STERILE CYTOPLASM “M” AND “C” TYPES OF CMS

Seeds of maize hybrids in the Russian Federation are obtained using CMS, therefore it is necessary to transfer the developed hybrids to a sterile basis to introduce them into production. The purpose of the work is to study the reaction of self-pollinated maize lines to various types of CMS. There was estimated the response of 26 self-pollinated maize lines to the “M” type of CMS and the response of 23 lines to the “C” type of CMS. The simple sterile hybrids “Milena M”, “Madonna M”, “Krucha M”, “Istok C”, “Nimfa C”, “Malvina C”, which are the mother forms of the new three-linear maize hybrids have been used as the testers of the sources of sterility. The self-pollinated lines have been classified according to the response to the “M” type of CMS as sterility fixers (11), incomplete sterility fixers (7), fertility reducing agents (6), non-uniform in reconstructing ability (2). In relation to the “C” type of CMS there were identified 10 sterility fixers, 3 incomplete sterility fixers, 7 fertility reducing agents, 3 non-uniform in reconstructing ability. The reducing agents of Moldavian type of CMS “FC 18 A”, “T 22 A”, “DS 768/83-3”, “DS 768/85-4”, “DS 768/85”, “DS 768/85-5” are of the greatest practical interest as they can be used as natural reducing agents (paternal forms) in the developed hybrid combinations. The constant natural reducing agents of the “C” type of CMS (“T 22 A”, “PLS 61”, “DK 22/325”, as paternal forms of hybrids grown on sterile cytoplasm of the “C” type) are of practical value. The use of sterile testers in hybridization allows simultaneously with the development of new hybrids to start the first stage of their transfer to a sterile basis, namely to evaluate the response of the lines to the sterile cytoplasm.

Carbohydrate metabolism and fertility related genes high expression levels promote heterosis in autotetraploid rice harboring double neutral genes

BackgroundAutotetraploid rice hybrids have great potential to increase the production, but hybrid sterility is a major hindrance in the utilization of hybrid vigor in polyploid rice, which is mainly caused by pollen abortion. Our previous study showed that double pollen fertility neutral genes, Sa-n and Sb-n, can overcome hybrid sterility in autotetraploid rice. Here, we used an autotetraploid rice line harboring double neutral genes to develop hybrids by crossing with auto- and neo-tetraploid rice, and evaluated heterosis and its underlying molecular mechanism.ResultsAll autotetraploid rice hybrids, which harbored double pollen fertility neutral genes, Sa-n and Sb-n, displayed high seed setting and significant positive heterosis for yield and yield-related traits. Cytological observations revealed normal chromosome behaviors and higher frequency of bivalents in the hybrid than parents during meiosis. Transcriptome analysis revealed significantly higher expressions of important saccharides metabolism and starch synthase related genes, such as OsBEIIb and OsSSIIIa, in the grains of hybrid than parents. Furthermore, many meiosis-related and specific genes, including DPW and CYP703A3, displayed up-regulation in the hybrid compared to a parent with low seed setting. Many non-additive genes were detected in the hybrid, and GO term of carbohydrate metabolic process was significantly enriched in all the transcriptome tissues except flag leaf (three days after flowering). Moreover, many differentially expressed genes (DEGs) were identified in the yield-related quantitative trait loci (QTLs) regions as possible candidate genes.ConclusionOur results revealed that increase in the number of bivalents improved the seed setting of hybrid harboring double pollen fertility neutral genes. Many important genes, including meiosis-related and meiosis-specific genes and saccharides metabolism and starch synthase related genes, exhibited heterosis specific expression patterns in polyploid rice during different development stages. The functional analysis of important genes will provide valuable information for molecular mechanisms of heterosis in polyploid rice.

Transcriptome profiling of the fertile parent and sterile hybrid in tea plant flower buds

BackgroundThe tea plant is a crucial economic crop. The floral organ development consumes a large amount of nutrients, which affects the leaf yield. To understand the mechanism by which the tea plant produces sterile floral buds, we obtained a sterile tea plant by artificial hybridization. RNA-sequencing based transcriptome analysis was implemented in three samples to determine the differentially expressed genes (DEGs) related to flower development.ResultsIn this study, a total of 1991 DEGs were identified; 1057 genes were up-regulated and 934 genes were down-regulated in sterile hybrid floral buds. These were mainly distributed in the regulation of biological and metabolic processes. Significantly, auxin biosynthesis genes YUCCA, AUX1 and PIN were dramatically down-regulated, and ARF gene was up-regulated in the sterile hybrid floral buds, and flower development-related genes AP1, AP2 and SPL were changed. A total of 12 energy transfer-related genes were significantly decreased. Furthermore, the expression of 11 transcription factor genes was significantly different.ConclusionThe transcriptome analysis suggested that the production of sterile floral buds is a complex bioprocess, and that low auxin-related gene levels result in the formation of sterile floral buds in the tea plant.

Testes Proteases Expression and Hybrid Male Sterility Between Subspecies of Drosophila pseudoobscura.

Hybrid male sterility (HMS) is a form of postmating postzygotic isolation among closely related species that can act as an effective barrier to gene flow. The Dobzhansky-Muller model provides a framework to explain how gene interactions can cause HMS between species. Genomics highlights the preponderance of non-coding DNA targets that could be involved in gene interactions resulting in gene expression changes and the establishment of isolating barriers. However, we have limited knowledge of changes in gene expression associated with HMS, gene interacting partners linked to HMS, and whether substitutions in DNA regulatory regions (cis) causes misexpression (i.e., expression of genes beyond levels found in parental species) of HMS genes in sterile hybrids. A previous transcriptome survey in a pair of D. pseudoobscura species found male reproductive tract (MRT) proteases as the largest class of genes misregulated in sterile hybrids. Here we assay gene expression in backcross (BC) and introgression (IG) progeny, along with site of expression within the MRT, to identify misexpression of proteases that might directly contribute to HMS. We find limited evidence of an accumulation of cis-regulatory changes upstream of such candidate HMS genes. The expression of four genes was differentially modulated by alleles of the previously characterized HMS gene Ovd.

Fertility analyses of interspecific hybrids between Lagerstroemia indica and L. speciosa

Interspecific crosses play an important role in gene introgression, plant improvement and speciation. However, poor fertility of F<sub>1</sub> generation was commonly found, which hampered backcrossing and ideal progeny generation. To explore useful materials for further breeding programs, sterile hybrids (DD1, FD1, ZD3) from different cross combinations of Lagerstroemia indica and L. speciosa and the fertile hybrid (ZD6) were selected. The results showed that pollen grains of sterile hybrids had no germination ability while ZD6 showed 25.90% pollen germination rate. The morphology of stigmas and their papilla cells showed no apparent difference. Normal pollen tubes could be detected in ovaries of ZD6 and ZD3 24 h after pollination. However, the enlarged ovaries of ZD3 began to abscise at 72 h after pollination, which suggested that the barriers occurred during post-fertilization phases. As a consequence, ZD6 can be used as either male or female parent for further Lagerstroemia breeding programs, while the sterile hybrids may be used as female parent through embryo rescue culture.