The genotypic control of homoeologous chromosome association in Lolium temulentum � Lolium perenne interspecific hybrids

Nondisjunction and unequal spindle organization accompany the drive of Aegilops speltoides B chromosomes

B chromosomes (Bs) are supernumerary chromosomes, which are often preferentially inherited. When transmission rates of chromosomes are higher than 0.5, not obeying the Mendelian law of equal segregation, the resulting transmission advantage is collectively referred to as 'chromosome drive'. Here we analysed the drive mechanism of Aegilops speltoides Bs. The repeat AesTR-183 of A. speltoides Bs, which also can be detected on the Bs of Aegilops mutica and rye, was used to track Bs during pollen development. Nondisjunction of CENH3-positive, tubulin interacting B sister chromatids and an asymmetric spindle during first pollen grain mitosis are key for the accumulation process. A quantitative flow cytometric approach revealed that, independent of the number of Bs present in the mother plant, Bs accumulate in the generative nuclei to > 93%. Nine out of 11 tested (peri)centromeric repeats were shared by A and B chromosomes. Our findings provide new insights into the process of chromosome drive. Quantitative flow cytometry is a useful and reliable method to study the drive frequency of Bs. Nondisjunction and unequal spindle organization accompany during first pollen mitosis the drive of A. speltoides Bs. The prerequisites for the drive process seems to be common in Poaceae.

Homoeologous chromosome pairing across the eukaryote phylogeny

During the past quarter century, molecular phylogenetic inferences have significantly resolved evolutionary relationships spanning the eukaryotic tree of life. With improved phylogenies in hand, the focus of systematics will continue to expand from estimating species relationships toward examining the evolution of specific, fundamental traits across the eukaryotic tree. Undoubtedly, this will expose knowledge gaps in the evolution of key traits, particularly with respect to non-model lineages. Here, we examine one such trait across eukaryotes-the regulation of homologous chromosome pairing during meiosis-as an illustrative example. Specifically, we present an overview of the breakdown of homologous chromosome pairing in model eukaryotes and provide a discussion of various meiotic aberrations that result in the failure of homolog recognition, with a particular focus on lineages with a history of hybridization and polyploidization, across major eukaryotic clades. We then explore what is known about these processes in natural and non-model eukaryotic taxa, thereby exposing disparities in our understanding of this key trait among non-model groups.

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.

Cytological affinities and interfertilities between Lolium temulentum and L. persicum (Poaceae) accessions

Interspecific crossing between L. temulentum L. and L. persicum Boiss. & Hohen. ex Boiss. was performed to clarify their interfertility based on the results of chromosome pairing, pollen fertility and seed set. Both parents were normal with a high percentage of chromosome association of ring bivalents in contrast to rod bivalents at metaphase I, pollen fertility and seed set, but F1 hybrids showed different proportions of them for each crossing combination. Chromosome affinity expressed by pairing was certainly a factor affecting the pollen fertility or seed set in F1 hybrids, but it was not the most important. The positive correlation was generally found between pollen fertility and seed set of F1 hybrids. The L. persicum accession with relatively high interfertility with L. temulentum was supposed to be derived from natural hybridization between L. temulentum and L. persicum. The degree of cytogenetic differentiation between L. temulentum and L. persicum existed because of lower chromosomal pairing, pollen fertility and seed set, but their F1 hybrids were partially fertile.

Crop-to-wild gene flow, introgression and possible fitness effects of transgenes

Crop-to-wild gene flow has received close attention over the past ten years in connection with the development and cultivation of transgenic crops. In this paper, we review key examples of crop/wild sympatry and overlapping flowering phenology, pollen and seed dispersal, the barriers to hybridisation and introgression, the evolution and fate of interspecific hybrids, their fitness, and the potential cost of transgenes. We pay particular attention to ways in which the evolution and divergence between crops and their wild relatives may interfere with these successive steps. Our review suggests that crop-to-weed gene flow is highly idiosyncratic and that crop gene dispersion will certainly be very difficult to preclude totally. Future directions for research should thus focus on the long-term establishment and effects of transgenes on natural communities.

Chromosome pairing in Lolium perenne x L. temulentum diploid hybrids: genetic and cytogenetic evaluation

A Lolium perenne genotype (E5/2/5/10), which had been selected for low chiasma frequency over a number of generations and which was suspected of containing one or two heterozygous dominant genes with a significant effect on chiasma frequency, was crossed with L. temulentum (Ba3081) to create a hybrid population of 47 diploid plants. The mean chiasma or paired arm (PA) frequency of homoeologous chromosomes at meiosis in the population was 9.1/cell (1.3 PA/chromosome pair) with a distribution skewed towards high PA frequency. More than 90% of the hybrid chromosomes paired at meiosis in spite of the disparity in chromosome length and DNA quantity between the two species. Overall, the distribution of PAs between chromosomes for a given number of PAs/cell favoured the production of rod bivalents over ring bivalents and univalents, indicating that there is a mechanism present that maximizes the total number of bivalent associations formed. Molecular marker analysis using AFLPs and isoenzymes did not identify any clear major gene effect on PA frequency in the hybrid population. It was concluded that the control of PA frequency in E5/2/5/10 was not a simple genetic mechanism.

Genetic control of synapsis and recombination in Lolium amphidiploids

Homologous bivalent formation in amphidiploids of Lolium is promoted during meiosis by diploidising genes carried by A-chromosomes, and by supernumerary B-chromosomes. The site and mode of action of these diploidising factors were investigated by comparing the relative frequencies of pairing configurations at meiotic prophase and metaphase I in several different hybrid genotypes. The results indicate that diploidising genes act predominantly by increasing the stringency of synapsis at early stages of meiotic prophase. By contrast, B-chromosomes appear to promote bivalent formation by ensuring that homoeologously paired chromosome segments within multivalents do not crossover. The results show that the additive effects of diploidising genes and B-chromosomes are to a certain extent separable in terms of their mode of action and timing during meiosis.

Chromosome pairing in a Lolium temulentum X Lolium perenne diploid hybrid with a low chiasma frequency

Despite an average difference of about 50% in DNA amount, homoeologous chromosomes pair effectively at first metaphase in the diploid interspecific hybrid between Lolium temulentum and Lolium perenne. However, in the presence of accessory B chromosomes and "diploidising genes" pairing at metaphase I is severely reduced. Reconstruction of serial electron micrographs through pollen mother cell nuclei show that synaptonemal complexes are formed at pachytene between not only homoeologous but also non-homologous chromosome segments resulting in multivalent formation. These associations are largely ineffective in terms of chiasma formation and degenerate at late pachytene. It is highly probable that the pairing determinants exercise their control on chromosome pairing largely by prohibiting the siting of crossovers in homoeologously paired chromosome segments.

The genetics of meiotic chromosome pairing in Lolium temulentum x Lolium perenne tetraploids

The degree of preferential pairing of homologous chromosomes was estimated in a series of tetraploid hybrids of Lolium temulentum x Lolium perenne by means of cytological and genetic analyses. The correlations between the frequency of bivalents at first metaphase of meiosis in the hybrid tetraploids and the degree of preferential pairing calculated from the segregation pattern of isozyme alleles in a test cross was extremely high. The results showed clearly that suppression of heterogenetic pairing in these Lolium tetraploids is achieved by a genetic system involving the A chromosomes as well as the B chromosome system which has been known for some time. Certain similarities with the genetic system controlling pairing in polyploid wheats are discussed.

Cytogenetics of Lolium perenne : Part 1: Chiasma frequency variation in inbred lines

A number of inbred lines of Lolium perenne have been developed as far as the fifth inbred generation and are being used for basic studies on the cytogenetics of the species. An analysis on variation in the pattern of chiasma frequency and distribution, including the parent plants and all generations down to the S5, reveals that the effect of inbreeding is to reduce chiasma frequency and to increase both the cell and bivalent variances. Evidence is presented for a genetic basis of polygenic control of chiasma formation and distribution, and a model is suggested for control over the three related components of variation which involves a two-stage level of regulation.