Allelic Variation Of TT8 Genes Regulating Seed Color In Brassica Juncea

Rapeseed-mustard or oilseed Brassica is one of the most important oil crops in the world.Yellow seed has higher oil and protein contents,lower pigments and thinner seed coat compared with black seed under the same genetic background in Brassica species.Previous studies have shown that The TT8 gene is responsible for seed color,and the mutation of TT8 gene leads to yellow seed trait in Brassica.We collected B.juncea accessions from 39 countries for identification and geographic distribution of TT8 alleles to elucidate the evolution of the various TT8 alleles and yellow seed in B.juncea.The results obtained are as follow.1.For different TT8 genes on the chromosomes A09 and B08 in B.juncea,specific primers were designed and used for PCR amplification.Seven Bju A09.TT8 alleles and six Bju B08.TT8 alleles were identified from worldwide 749 mustard accessions tested.Compared to the wild type,Bju A09.TT8.a1-a5 and Bju B08.TT8.b1-b4 alleles had large insertions while Bju A09.TT8.a6 was a deleted mutation and Bju B08.TT8.b5 had only a base substitution.2.The sequences of the B.juncea TT8 alleles were aligned with the annotated library of oilseed rape(B.napus)genome by the software Repeatmasker.The results showed that Bju A09.TT8.a1-a4 and Bju B08.TT8.b1-b4 alleles contained class I transposons,while a few alleles carried insertion of class II or Helitron-like transposons.3.The statistics of B.juncea TT8 allele and haplotype frequencies showed the haplotype Bju A09.TT8.a4-Bju B08.TT8.b5 was the most prevalent,accounting for 89.49%(247/276)of the yellow seed accessions analyzed,followed by Bju A09.TT8.a4-Bju B08.TT8.b3 haplotype with 6.52%.4.Analysis of geographical distribution of B.juncea TT8 alleles revealed that the frequency of mutated alleles was significantly higher from China,especially Xinjiang than from other countries.Combined with historical records,it is inferred that Xinjiang,China is the place where yellow-seeded mustard originates from.The above results provide a basis for the selection of superior gene resources for breeding yellow-seeded rapeseed-mustard.