ChIP-cloning analysis uncovers centromere-specific retrotransposons in Brassica nigra and reveals their rapid diversification in Brassica allotetraploids.

Centromeres are indispensable functional units of chromosomes. The evolutionary mechanisms underlying the rapid evolution of centromeric repeats, especially those following polyploidy, remain unknown. In this study, we isolated centromeric sequences of Brassica nigra, a model diploid progenitor (B genome) of the allopolyploid species B. juncea (AB genome) and B. carinata (BC genome) by chromatin immunoprecipitation of nucleosomes containing the centromere-specific histone CENH3. Sequence analysis detected no centromeric satellite DNAs, and most B. nigra centromeric repeats were found to originate from Tyl/copia-class retrotransposons. In cytological analyses, six of the seven analyzed repeat clusters had no FISH signals in A or C genomes of the related diploid species B. rapa and B. oleracea. Notably, five repeat clusters had FISH signals in both A and B subgenomes in the tetraploid B. juncea. In the tetraploid B. carinata, only CL23 displayed three pairs of signals in terminal or interstitial regions of the C-derived chromosome, and no evidence of colonization of CLs onto C-subgenome centromeres was found in B. carinata. This observation suggests that centromeric repeats spread and proliferated between genomes after polyploidization. CL3 and CRB are likely ancient centromeric sequences arising prior to the divergence of diploid Brassica which have detected signals across the genus. And in allotetraploids B. juncea and B. carinata, the FISH signal intensity of CL3 and CRB differed among subgenomes. We discussed possible mechanisms for centromeric repeat divergence during Brassica speciation and polyploid evolution, thus providing insights into centromeric repeat establishment and targeting.

[Organization, function and genetic controlling of synaptonemal complex].

The synaptonemal complex (SC) is a super protein lattice that connects paired homologous chromosomes in most meiotic systems. This special organization is related to the meiosis processes such as homologous chromosomes pairing, synapsis, recombination, segregation, etc. Flaws of it would lead the meiocytes to apoptosis, which contributes to sterility. In recent years, the study of this complex has been a hotspot in meiosis research, but little was known about its exact mechanism. This review summarized the organization, function, and genetics of this complex with recent advances. Prospects of its further study were also briefly discussed..