Non-homologous chromosome pairing during meiosis in haploid Brassica rapa.

KEY MESSAGE: Cytological observations of chromosome pairing showed that evolutionarily genome duplication might reshape non-homologous pairing during meiosis in haploid B. rapa. A vast number of flowering plants have evolutionarily undergone whole genome duplication (WGD) event. Typically, Brassica rapa is currently considered as an evolutionary mesohexaploid, which has more complicated genomic constitution among flowering plants. In this study, we demonstrated chromosome behaviors in haploid B. rapa to understand how meiosis proceeds in presence of a single homolog. The findings showed that a diploid-like chromosome pairing was generally adapted during meiosis in haploid B. rapa. Non-homologous chromosomes in haploid cells paired at a high-frequency at metaphase I, over 50% of examined meiocytes showed at least three pairs of bivalents then equally segregated at anaphase I during meiosis. The fluorescence immunostaining showed that the cytoskeletal configurations were mostly well-organized during meiosis. Moreover, the expressed genes identified at meiosis in floral development was rather similar between haploid and diploid B. rapa, especially the expression of known hallmark genes pivotal to chromosome synapsis and homologous recombination were mostly in haploid B. rapa. Whole-genome duplication evolutionarily homology of genomic segments might be an important reason for this phenomenon, which would reshape the first division course of meiosis and influence pollen development in plants.

Cytological atlas at meiosis reveals insights into pollen fertility in synthetic Brassica allotriploids between allotetraploid B. carinata and diploid B. rapa.

The formation of allopolyploid crops basically depends on the successful interspecific hybridization and polyploidization, which generally involves in a combination of distinct but related genomes from independent species. But cytological analysis of these initially synthesized allohaploids immediately after genome merging is poorly explored in regards to anther and pollen development to date. In this study, Brassica trigenomic allohaploids (ABC) were produced to investigate the immediate effects of the genome combinations on pollen fertility during anther development via crosses between natural allotetraploid B. carinata (BBCC) and diploid B. rapa (AA). The results showed that in the synthetic Brassica allotriploids (ABC), the anther development was completely disrupted, and the pollen grains were mostly inviable with varied genetic complements. In addition, the meiosis course was aberrantly altered and eccentric chromosomal configurations including multivalent, bridges and lags occurred frequently during metaphase I to anaphase II. Genomic in situ hybridization (GISH) further revealed that B genome of homoeology was frequently apt to interact with A and C genomes, and cytoskeletal organizations was improperly distributed during meiosis in these synthetic Brassica allotriploids. Furthermore, we also confirmed that the expression of typical meiosis-related genes was obviously repressed during anther development in these Brassica allotriploids. Taken together, our results provide a detailed cytology for insights into pollen development in the synthetic allotriploid hybrids, which are conventionally considered as a useful genetic resource for polyploid Brassica breeding.