Although there are a number of different allopolyploids in the plant kingdom, the exact ancestral parents of some allopolyploids have not been well characterized. We propose a strategy in which virtual allopolyploid lines derived from different types of parental species are used to investigate the progenitors of an allopolyploid. The genotypes of the parental lines and the natural allopolyploid were established using a set of DNA molecular markers. The genotypes of the virtual lines were then derived from those of the parental lines, and compared extensively with that of the natural allopolyploid. We applied this strategy to investigate the progenitors of the C subgenome of Brassica napus (rapeseed, AACC). A total of 39 accessions from 10 wild and 7 cultivated types of the B. oleracea cytodeme (CC), and 4 accessions of B. rapa (AA) were used to construct 156 virtual rapeseed lines. Genetic structure was compared among natural rapeseed, virtual rapeseed lines, and their parental lines by principal component analysis and analysis of ancestry. Our data showed that the C subgenome of natural rapeseed was related closely to the genome of cultivated B. oleracea and its related wild types, such as B. incana, B. bourgeaui, B. montana, B. oleracea ssp. oleracea and B. cretica. This finding indicated that these types or their progeny might be ancestral donors of the C subgenome of rapeseed. The successful application of the strategy of virtual allopolyploidy in rapeseed demonstrates that it can possibly be used to identify the progenitors of an allopolyploid species.

In holometabolous insects, the accumulation and utilization of storage proteins (SPs), including arylphorins and methionine-rich proteins, are critical for the insect metamorphosis. SPs function as amino acids reserves, which are synthesized in fat body, secreted into the larval hemolymph and taken up by fat body shortly before pupation. However, the detailed molecular mechanisms of digestion and utilization of SPs during development are largely unknown. Here, we report the crystal structure of Bombyx mori arylphorins at 2.8 Å , which displays a heterohexameric structural arrangement formed by trimerization of dimers comprising two structural similar arylphorins. Our limited proteolysis assay and microarray data strongly suggest that papain-like proteases are the major players for B. mori arylphorins digestion in vitro and in vivo. Consistent with the biochemical data, dozens of papain cleavage sites are mapped on the surface of the heterohexameric structure of B. mori arylphorins. Hence, our results provide the insightful information to understand the metamorphosis of holometabolous insects at molecular level.