Genetic control of synapsis and recombination in Lolium amphidiploids

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.

[Monosomic alien addition lines: a new tool for studying and using plant genomics]

Interspecies hybridization and backcrossing is a means to transfer desirable genes from one species to another in breeding programs of higher plants. Monosomic alien addition lines (MAALs) can be produced via addition of single chromosome of an alien donor species to the entire chromosome complement of the recipient species. MAALs are powerful tools for elucidating genome structure and transferring genes. Backcrossing of MAALs to the recipient parent results in plants containing short overlapping introgressions, which cover the entire donor genome. These introgression lines can be used as vectors of alien genomic libraries in a recipient genetic background. In addition, a complete set of MAALs also serves as a library of the donor genome dissected into individual chromosome entities, which facilitates high-throughput marker allocation to individual donor chromosomes, and marker assignments and syntenic relationships can be compared between the donor chromosomes and the respective orthologous recipient chromosomes. Meanwhile, MAALs can be used to study the introgression mechanism and the pairing status of homologous chromosomes. In this review, we presented the production and properties of MAALs and highlighted their advantages for genetic breeding and fundamental researches.

Production of alien chromosome additions and their utility in plant genetics

Breeding programs aiming at transferring desirable genes from one species to another through interspecific hybridization and backcrossings often produce monosomic and disomic additions as intermediate crossing products. Such aneuploids contain alien chromosomes added to the complements of the recipient parent and can be used for further introgression programs, but lack of homoeologous recombination and inevitable segregation of the alien chromosome at meiosis make them often less ideal for producing stable introgression lines. Monosomic and disomic additions can have specific morphological characteristics, but more often they need additional confirmation of molecular marker analyses and assessment by fluorescence in situ hybridization with genomic and chromosome-specific DNA as probes. Their specific genetic and cytogenetic properties make them powerful tools for fundamental research elucidating regulation of homoeologous recombination, distribution of chromosome-specific markers and repetitive DNA sequences, and regulation of heterologous gene expression. In this overview we present the major characteristics of such interspecific aneuploids highlighting their advantages and drawbacks for breeding and fundamental research.

Transgene escape: what potential for crop-wild hybridization?

To date, regional surveys assessing the risk of transgene escape from GM crops have focused on records of spontaneous hybridization to infer the likelihood of crop transgene escape. However, reliable observations of spontaneous hybridization are lacking for most floras, particularly outside Europe. Here, we argue that evidence of interspecific reproductive compatibility derived from experimental crosses is an important component of risk assessment, and a useful first step especially where data from field observations are unavailable. We used this approach to assess the potential for transgene escape via hybridization for 123 widely grown temperate crops and their indigenous and naturalized relatives present in the New Zealand flora. We found that 66 crops (54%) are reproductively compatible with at least one other indigenous or naturalized species in the flora. Limited reproductive compatibility with wild relatives was evident for a further 12 crops (10%). Twenty-five crops (20%) were found to be reproductively isolated from all their wild relatives in New Zealand. For the remaining 20 crops (16%), insufficient information was available to determine levels of reproductive compatibility with wild relatives. Our approach may be useful in other regions where spontaneous crop-wild hybridization has yet to be well documented.

Relationship between pachytene synapsis, metaphase I associations, and transmission of 2B and 4B chromosomes in rye

2B rye plants selected for high (H) or low (L) B transmission rate were studied at pachytene and metaphase I of meiosis to determine the relationship between synapsis, bivalents at metaphase I, and B transmission rate. The results show that the 2 B chromosomes (Bs) form bivalents at pachytene in both the H and L lines, whereas the frequency of bivalents at metaphase I is much higher in the H than in the L line. This demonstrates that B transmission is mainly related to the proper association of Bs at metaphase I, as well as that synapsis of the 2 Bs in the L line is normal, but the bivalent is not consolidated by a chiasma in most cases. Crosses were made between 2B plants of the H and L lines in all combinations (H × H, H × L, L × H, and L × L) to obtain 4B plants. Similarly, bivalent formation at pachytene and metaphase I was studied. The results show that 4B plants of the H × H and L × L classes differ significantly at pachytene and metaphase I since the former forms more bivalents. The heterozygous 4 Bs of the H × L and L × H classes show intermediate values. The relation H × H > H × L > L × H > L × L was consistently found for the variables transmission rate, bivalents at pachytene, bivalents at metaphase I, and B mean chiasma frequency. A maternal effect was also found. Our data suggest that there are two separate mechanisms acting upon synapsis and chiasma formation in H and L B chromosomes: (i) there is variable efficiency of the control of synapsis at early stages of meiosis; and (ii) there is variable efficiency of the control of the number of chiasmata.

Key words: B chromosomes, synaptonemal complex, Secale cereale.