Eric Jenczewski

The first meiosis of resynthesized Brassica napus, a genome blender

Polyploidy promotes the restructuring of merged genomes within initial generations of resynthesized Brassica napus, possibly caused by homoeologous recombination at meiosis. However, little is known about the impact of the first confrontation of two genomes at the first meiosis which could lead to genome exchanges in progeny. Here, we assessed the role of the first meiosis in the genome instability of synthetic B. napus. We used three different newly resynthesized B. napus plants and established meiotic pairing frequencies for the A and C genomes. We genotyped the three corresponding progenies in a cross to a natural B. napus on the two homoeologous A1 and C1 chromosomes. Pairing at meiosis in a set of progenies with various rearrangements was scored. Here, we confirmed that the very first meiosis of resynthesized plants of B. napus acts as a genome blender, with many of the meiotic-driven genetic changes transmitted to the progenies, in proportions that depend significantly on the cytoplasm background inherited from the progenitors. We conclude that the first meiosis generates rearrangements on both genomes and promotes subsequent restructuring in further generations. Our study advances the knowledge on the timing of genetic changes and the mechanisms that may bias their transmission.

Insight on segregation distortions in two intraspecific crosses between annual species of Medicago (Leguminosae)

Abstract About 40% (α=0.05) of the PCR-derived markers scored in a Medicago truncatula and M. tornata intraspecific cross departed from Mendelian expectations at α=0.05. This proportion is among the highest ever documented in the literature, notably for intraspecific crosses. Estimations of DNA amount were also implemented for the parental genotypes or parental lines, and significant variations were observed. Our results suggest that the parental genotypes have diverged for quite a while, and we propose that the level of distortion we documented is correlated with the genome size difference we measured.

Pairing and recombination at meiosis of Brassica rapa (AA) × Brassica napus (AACC) hybrids

Interspecific crosses contribute significantly to plant evolution enabling gene exchanges between species. The efficiency of interspecific crosses depends on the similarity between the implicated genomes as high levels of genome similarity are required to ensure appropriate chromosome pairing and genetic recombination. Brassica napus (AACC) is an allopolyploid, resulting from natural hybridization between Brassica rapa (AA) and Brassica oleracea (CC), both being diploid species derived from a common ancestor. To study the relationships between genomes of these Brassica species, we have determined simultaneously the pairing and recombination pattern of A and C chromosomes during meiosis of AAC triploid hybrids, which result from the interspecific cross between natural B. napus and B. rapa. Different AAC triploid hybrids and their progenies have been analysed using cytogenetic, BAC-FISH, and molecular techniques. In 71% of the pollen mother cells, homologous A chromosomes paired regularly, and usually one chromosome of each pair was transmitted to the progeny. C chromosomes remained mainly univalent, but were involved in homoeologous pairing in 21.5% of the cells, and 13% of the transmitted C chromosomes were either recombined or broken. The rate of transmission of C chromosomes depended on the identity of the particular chromosome and on the way the hybrid was crossed, as the male or as the female parent, to B. napus or to B. rapa. Gene transfers in triploid hybrids are favoured between A genomes of B. rapa and B. napus, but also occur between A and C genomes though at lower rates.

Genetic regulation of meiosis in polyploid species: new insights into an old question.

Precise chromosome segregation is vital for polyploid speciation. Here, we highlight recent findings that revitalize the old question of the genetic control of diploid-like meiosis behaviour in polyploid species. We first review new information on the genetic control of autopolyploid and allopolyploid cytological diploidization, notably in wheat and Brassica. These major advances provide new opportunities for speculating about the adaptation of meiosis during polyploid evolution. Some of these advances are discussed, and it is suggested that research on polyploidy and on meiosis should no longer be unlinked.

Homeologous Recombination Plays a Major Role in Chromosome Rearrangements That Occur During Meiosis of Brassica napus Haploids

Chromosomal rearrangements can be triggered by recombination between distinct but related regions. Brassica napus (AACC; 2n = 38) is a recent allopolyploid species whose progenitor genomes are widely replicated. In this article, we analyze the extent to which chromosomal rearrangements originate from homeologous recombination during meiosis of haploid B. napus (n = 19) by genotyping progenies of haploid × euploid B. napus with molecular markers. Our study focuses on three pairs of homeologous regions selected for their differing levels of divergence (N1/N11, N3/N13, and N9/N18). We show that a high number of chromosomal rearrangements occur during meiosis of B. napus haploid and are transmitted by first division restitution (FDR)-like unreduced gametes to their progeny; half of the progeny of Darmor-bzh haploids display duplications and/or losses in the chromosomal regions being studied. We demonstrate that half of these rearrangements are due to recombination between regions of primary homeology, which represents a 10- to 100-fold increase compared to the frequency of homeologous recombination measured in euploid lines. Some of the other rearrangements certainly result from recombination between paralogous regions because we observed an average of one to two autosyndetic A–A and/or C–C bivalents at metaphase I of the B. napus haploid. These results are discussed in the context of genome evolution of B. napus.

The molecular biology of meiosis in plants.

Meiosis is the cell division that reshuffles genetic information between generations. Recently, much progress has been made in understanding this process; in particular, the identification and functional analysis of more than 80 plant genes involved in meiosis have dramatically deepened our knowledge of this peculiar cell division. In this review, we provide an overview of advancements in the understanding of all aspects of plant meiosis, including recombination, chromosome synapsis, cell cycle control, chromosome distribution, and the challenge of polyploidy.

Genetic Regulation of Meiotic Cross-Overs between Related Genomes in Brassica napus Haploids and Hybrids

Although the genetic regulation of recombination in allopolyploid species plays a pivotal role in evolution and plant breeding, it has received little recent attention, except in wheat (Triticum aestivum). PrBn is the main locus that determines the number of nonhomologous associations during meiosis of microspore cultured Brassica napus haploids (AC; 19 chromosomes). In this study, we examined the role played by PrBn in recombination. We generated two haploid × euploid populations using two B. napus haploids with differing PrBn (and interacting genes) activity. We analyzed molecular marker transmission in these two populations to compare genetic changes, which have arisen during meiosis. We found that cross-over number in these two genotypes was significantly different but that cross-overs between nonhomologous chromosomes showed roughly the same distribution pattern. We then examined genetic recombination along a pair of A chromosomes during meiosis of B. rapa × B. napus AAC and AACC hybrids that were produced with the same two B. napus genotypes. We observed significant genotypic variation in cross-over rates between the two AAC hybrids but no difference between the two AACC hybrids. Overall, our results show that PrBn changes the rate of recombination between nonhomologous chromosomes during meiosis of B. napus haploids and also affects homologous recombination with an effect that depends on plant karyotype.

Mutations in AtPS1 (Arabidopsis thaliana Parallel Spindle 1) Lead to the Production of Diploid Pollen Grains

Polyploidy has had a considerable impact on the evolution of many eukaryotes, especially angiosperms. Indeed, most—if not all—angiosperms have experienced at least one round of polyploidy during the course of their evolution, and many important crop plants are current polyploids. The occurrence of 2n gametes (diplogametes) in diploid populations is widely recognised as the major source of polyploid formation. However, limited information is available on the genetic control of diplogamete production. Here, we describe the isolation and characterisation of the first gene, AtPS1 (Arabidopsis thaliana Parallel Spindle 1), implicated in the formation of a high frequency of diplogametes in plants. Atps1 mutants produce diploid male spores, diploid pollen grains, and spontaneous triploid plants in the next generation. Female meiosis is not affected in the mutant. We demonstrated that abnormal spindle orientation at male meiosis II leads to diplogamete formation. Most of the parents heterozygosity is therefore conserved in the Atps1 diploid gametes, which is a key issue for plant breeding. The AtPS1 protein is conserved throughout the plant kingdom and carries domains suggestive of a regulatory function. The isolation of a gene involved in diplogamete production opens the way for new strategies in plant breeding programmes and progress in evolutionary studies.

Differentiation between natural and cultivated populations of Medicago sativa (Leguminosae) from Spain: analysis with random amplified polymorphic DNA (RAPD) markers and comparison to allozymes

The conservation of a crop’s wild relatives as genetic resources requires an understanding of the way genetic diversity is maintained in their populations, notably the effect of crop‐to‐wild gene flow. In this study, the amount of differentiation between natural and cultivated populations of Medicago sativa was analysed using random amplified polymorphic DNA (RAPD) markers and an extension of the amova procedure adapted to autotetraploid organisms. Simulations of structured populations were performed to test whether amova provides estimates of population structure in autotetraploids that can be directly compared to those obtained for allozyme data. Simulations showed that φ‐statistics allow a good estimation of population differentiation when unbiased allelic frequencies are used to correct the conditional expectations of squared genetic distances. But such unbiased estimates can not be practically guaranteed, and population structure is notably overestimated when some populations are fixed for the presence of amplified fragments. However, removing fixed loci from the data set improves the statistical power of the test for population structure. The genetic variation of 15 natural and six cultivated populations of M. sativa was analysed at 25 RAPD loci and compared to estimates computed with allozymes on the same set of populations. Although RAPD markers revealed less within‐population genetic diversity than allozymes, the quantitative and qualitative patterns of population structure were in full agreement with allozymes. This confirmed the conclusions drawn from the allozymic survey: crop‐to‐wild gene flow occurred in many locations, but some other mechanisms opposed cultivated traits to be maintained into natural populations.

Evidence for gene flow between wild and cultivated Medicago sativa (Leguminosae)based on allozyme markers andquantitative traits

Genetic differentiation between co-occurring crops and their wild relatives will be greatly modified by crop-to-weed gene flow and variation between human and natural selective pressures. The maintenance of original morphological features in most natural populations of Medicago sativa in Spain questions the relative extent of these antagonistic forces. In this paper, we measured and compared the pattern of population differentiation within and among the wild and cultivated gene pool with respect to both allozymes and quantitative traits. Patterns of diversity defined three kinds of natural populations. First, some populations were intermediate with respect to both allozymes and quantitative traits. This suggests that crop-to-weed gene flow may have created hybrid populations in some locations. Second, some populations were different from all the cultivated landraces with respect to both allozymes and quantitative traits. This probably results from variable gene flow in space and in time, due to demographic stochasticity in either natural or cultivated populations. Third, differentiation from cultivated landraces was only achieved for the quantitative traits but not for allozymes in two populations. This suggests that natural selection in some locations may oppose gene flow to establish cultivated traits into the natural introgressed populations.