Meiotic Chromosome Stability and Suppression of Crossover Between Non-homologous Chromosomes in xBrassicoraphanus, an Intergeneric Allotetraploid Derived From a Cross Between Brassica rapa and Raphanus sativus

Cytogenetic Study and Pollen Viability of Phalaenopsis Queen Beer 'Mantefon'

Intergeneric and interspecific hybridization has been employed for the breeding of Phalaenopsis to transfer desirable traits between species, producing novel phenotypes with improved size, color, form, and flower-bearing ability. These characteristics are often enhanced; however, many of these hybrids are triploids and have reduced or complete sterility, for example, Phalaenopsis Queen Beer 'Mantefon', an important novelty-type cultivar in Asia, particularly in China, Japan, and Republic of Korea. Despite the increasing demand for the crop for ornamental purposes, little is known about its cytogenetics, which is essential for breeding and, consequently, crop improvement. In this study, karyotyping using fluorescence in situ hybridization, meiotic chromosome behavior analysis, pollen staining, and in vitro viability germination tests were performed to understand the cause of hybrid sterility and pollen abnormality in Phalaenopsis Queen Beer 'Mantefon' from a cytogenetic perspective. Viability tests revealed pollen infertility at all flower developmental stages, confirmed by the absence of pollen tube growth. Aberrant chromosomal behavior was observed in pollen mother cells (PMCs), frequently forming univalents, chromosomal bridges, and laggards during the entire meiotic process. PMCs were also divided irregularly into sporads with varying numbers of micronuclei, which may be responsible for pollen sterility in this cultivar. Altogether, the cytogenetic analyses provided insights into the pollen development of Phalaenopsis Queen Beer 'Mantefon' and the conceivable causes of its infertility.

Identification, Characterization, and Cytological Analysis of Several Unexpected Hybrids Derived from Reciprocal Crosses between Raphanobrassica and Its Diploid Parents

Interspecific hybridization and accompanying backcross between crops and relatives have been recognized as a powerful method to broaden genetic diversity and transfer desirable adaptive traits. Crosses between radish (Raphanus sativus, RR, 2n = 18) and Brassica oleracea (CC, 2n = 18), which formed allotetraploid Raphanobrassica (RRCC, 2n = 36), initiated the construction of resynthetic allopolyploids. However, these progenies from the backcrosses between Raphanobrassica and the two diploid parents have not been well deciphered. Herein, thousands of backcrosses using both Raphanobrassica and the two diploid parents as pollen donors were employed. Several hybrids with expected (2n = 27) and unexpected chromosome numbers (2n = 26 and 2n = 36) were obtained. Fluorescence in situ hybridization (FISH) analysis with R-genome-specific sequences as probes demonstrated that the genome structures of the two expected hybrids were RRC and CCR, and the genome structures of the three unexpected hybrids were RRRC, CCCR, and RRC' (harbouring an incomplete C genome). The unexpected hybrids with extra R or C genomes showed similar phenotypic characteristics to their expected hybrids. FISH analysis with C-genome-specific sequences as probes demonstrated that the unexpected allotetraploid hybrids exhibited significantly more intergenomic chromosome pairings than the expected hybrids. The expected and unexpected hybrids provide not only novel germplasm resources for the breeding of radish and B. oleracea but also very important genetic material for genome dosage analysis.

Formation of the synaptonemal complex in a gynogenetic allodiploid hybrid fish

Introduction: The correct pairing and separation of homologous chromosomes during meiosis is crucial to ensure both genetic stability and genetic diversity within species. In allodiploid organisms, synapsis often fails, leading to sterility. However, a gynogenetic allodiploid hybrid clone line (GDH), derived by crossing red crucian carp (Carassius auratus ♀) and common carp (Cyprinus carpio ♂), stably produces diploid eggs. Because the GDH line carries 100 chromosomes with 50 chromosomes from the red crucian carp (RCC; ♀, 2n = 2x = 100) and 50 chromosomes from the common carp (CC; C. carpio L., ♂, 2n = 2x = 100), it is interesting to study the mechanisms of homologous chromosome pairing during meiosis in GDH individuals. Methods: By using fluorescence in situ hybridization (FISH) with a probe specific to the red crucian carp to label homologous chromosomes, we identified the synaptonemal complex via immunofluorescence assay of synaptonemal complex protein 3 (SCP3). Results: FISH results indicated that, during early ovarian development, the GDH oogonium had two sets of chromosomes with only one set from Carassius auratus, leading to the failure formation of normal bivalents and the subsequently blocking of meiosis. This inhibition lasted at least 5 months. After this long period of inhibition, pairs of germ cells fused, doubling the chromosomes such that the oocyte contained two sets of chromosomes from each parent. After chromosome doubling at 10 months old, homologous chromosomes and the synaptonemal complex were identified. Discussion: Causally, meiosis proceeded normally and eventually formed diploid germ cells. These results further clarify the mechanisms by which meiosis proceeds in hybrids.

Admixture of divergent genomes facilitates hybridization across species in the family Brassicaceae

Hybridization and polyploidization are pivotal to plant evolution. Genetic crosses between distantly related species are rare in nature due to reproductive barriers but how such hurdles can be overcome is largely unknown. Here we report the hybrid genome structure of xBrassicoraphanus, a synthetic allotetraploid of Brassica rapa and Raphanus sativus. We performed cytogenetic analysis and de novo genome assembly to examine chromosome behaviors and genome integrity in the hybrid. Transcriptome analysis was conducted to investigate expression of duplicated genes in conjunction with epigenome analysis to address whether genome admixture entails epigenetic reconfiguration. Allotetraploid xBrassicoraphanus retains both parental chromosomes without genome rearrangement. Meiotic synapsis formation and chromosome exchange are avoided between nonhomologous progenitor chromosomes. Reconfiguration of transcription network occurs, and less divergent cis-elements of duplicated genes are associated with convergent expression. Genome-wide DNA methylation asymmetry between progenitors is largely maintained but, notably, B. rapa-originated transposable elements are transcriptionally silenced in xBrassicoraphanus through gain of DNA methylation. Our results demonstrate that hybrid genome stabilization and transcription compatibility necessitate epigenome landscape adjustment and rewiring of cis-trans interactions. Overall, this study suggests that a certain extent of genome divergence facilitates hybridization across species, which may explain the great diversification and expansion of angiosperms during evolution.

Prdm9 deficiency of rat oocytes causes synapsis among non-homologous chromosomes and aneuploidy

Aneuploidy (abnormal chromosome number) accompanies reduced ovarian function in humans and mice, but the reasons behind this concomitance remain underexplored. Some variants in the human gene encoding histone-3-lysine-4,36-trimethyltransferase PRDM9 are associated with aneuploidy, and other variants with ovarian function reduced by premature ovarian failure (POF), but no link between POF and aneuploidy has been revealed. SHR/OlaIpcv rat females lacking PRDM9 manifest POF-a reduced follicle number, litter size, and reproductive age. Here, we explored this model to test how POF relates to oocyte euploidy. The mutant rat females displayed increased oocyte aneuploidy and embryonic death of their offspring compared to controls. Because rat PRDM9 positions meiotic DNA breaks, we investigated the repair of these breaks. Fertile control rodents carry pachytene oocytes with synapsed homologous chromosomes and repaired breaks, while sterile Prdm9-deficient mice carry pachytene-like oocytes with many persisting breaks and asynapsed chromosomes. However, most PRDM9-lacking rat oocytes displayed a few persisting breaks and non-homologous synapsis (NHS). HORMAD2 protein serves as a barrier to sister-chromatid repair and a signal for the synapsis and DNA repair checkpoints. NHS but not asynapsis was associated with HORMAD2 levels similar to the levels on rat pachytene chromosomes with homologous synapsis. NHS was accompanied by crossing-over decreased below the minimum that is essential for euploidy. We argue that the increased mutant rat aneuploidy is due to NHS, which allows some oocytes to pass meiotic checkpoints without one crossing-over per chromosomal pair, leading to segregation errors, and thereby NHS links POF to aneuploidy.

All Ways Lead to Rome—Meiotic Stabilization Can Take Many Routes in Nascent Polyploid Plants

Newly formed polyploids often show extensive meiotic defects, resulting in aneuploid gametes, and thus reduced fertility. However, while many neopolyploids are meiotically unstable, polyploid lineages that survive in nature are generally stable and fertile; thus, those lineages that survive are those that are able to overcome these challenges. Several genes that promote polyploid stabilization are now known in plants, allowing speculation on the evolutionary origin of these meiotic adjustments. Here, I discuss results that show that meiotic stability can be achieved through the differentiation of certain alleles of certain genes between ploidies. These alleles, at least sometimes, seem to arise by novel mutation, while standing variation in either ancestral diploids or related polyploids, from which alleles can introgress, may also contribute. Growing evidence also suggests that the coevolution of multiple interacting genes has contributed to polyploid stabilization, and in allopolyploids, the return of duplicated genes to single copies (genome fractionation) may also play a role in meiotic stabilization. There is also some evidence that epigenetic regulation may be important, which can help explain why some polyploid lineages can partly stabilize quite rapidly.

Male meiosis and pollen morphology in diploid Indonesian wild bananas and cultivars

Breeding of banana is hampered by its genetic complexity, structural chromosome rearrangements and different ploidy levels. Various scientific disciplines, including cytogenetics, linkage mapping, and bioinformatics, are helpful tools in characterising cultivars and wild relatives used in crossing programs. Chromosome analysis still plays a pivotal role in studying hybrid sterility and structural and numerical variants. In this study, we describe the optimisation of the chromosome spreading protocol of pollen mother cells focusing on the effects of standard fixation methods, duration of the pectolytic enzyme treatment and advantages of fluorescence microscopy of DAPI stained cell spreads. We demonstrate the benefits of this protocol on meiotic features of five wild diploid Musa acuminata bananas and a diploid (AA) cultivar banana “Rejang”, with particular attention on pairing configurations and chromosome transmission that may be indicative for translocations and inversions. Pollen slides demonstrate regular-shaped spores except “Rejang”, which shows fertile pollen grains of different size and sterile pollen grains, suggesting partial sterility and unreduced gamete formation that likely resulted from restitutional meiotic divisions.

Reduced fertility caused by meiotic defects and micronuclei formation during microsporogenesis in xBrassicoraphanus

Background

Hybridization and polyploidization events are important driving forces in plant evolution. Allopolyploids formed between different species can be naturally or artificially created but often suffer from genetic instability and infertility in successive generations. xBrassicoraphanus is an intergeneric allopolyploid obtained from a cross between Brassica rapa and Raphanus sativus, providing a useful resource for genetic and genomic study in hybrid species.

Objective

The current study aims to understand the cause of hybrid sterility and pollen abnormality in different lines of synthetic xBrassicoraphanus from the cytogenetic perspective.

Methods

Alexander staining was used to assess the pollen viability. Cytogenetic analysis was employed to monitor meiotic chromosome behaviors in pollen mother cells (PMCs). Origins of parental chromosomes in xBrassicoraphanus meiocytes were determined by genome in situ hybridization analysis.

Results

The xBrassicoraphanus lines BB#4 and BB#6 showed high rates of seed abortion and pollen deformation. Abnormal chromosome behaviors were observed in their PMCs, frequently forming univalents and inter-chromosomal bridges during meiosis. A positive correlation also exists between meiotic defects and the formation of micronuclei, which is conceivably responsible for unbalanced gamete production and pollen sterility.

Conclusion

These results suggest that unequal segregation of meiotic chromosomes, due in part to non-homologous interactions, is responsible for micronuclei and unbalanced gamete formation, eventually leading to pollen degeneration and inferior fertility in unstable xBrassicoraphanus lines.