Abstract: 【Objective】Starch grain density affects maize kernel test weight. In this study, we use a mutant Mrd of maize to identify and fine map the gene controlling starch grain density, which are helpful for the cloning and function verification of the related grain test weight gene.【Method】The Mrd is a starch grain density mutant which was identified during maize breeding practices. Scanning electron microscopy and near infrared spectroscopy (NIR) analyzer were used to observe changes of chemical composition in the Mrd kernels. The segregation population F₂ and BC₁ were derived from the cross of Mrd and B73, which were planted in Zhengzhou and Yuanyang, Henan Province and Sanya, Hainan Province, and used for genetic analysis. BSA (Bulked Segregation Analysis) was used to identify linkage markers selected from 1 000 pairs of SSR primers from maizeGDB (http://www.maizegdb.org) of target gene. The BC₁ segregation population of 38 thousand individuals were used to fine map the target gene tw1. The candidate genes were sequenced and functional predicted by bioinformatics. Allelism test was performed for tw1 and su2. 【Result】Compared with the normal seed, the Mrd had smaller grain size and no change in grain length and increased specific gravity. The crude protein content in mutants decreased, the content of crude starch did not change significantly, but the Mrd has irregular shape and smaller starch grain, increased density, and increased grain test weight. Observations of the starch grain structure inside the kernel in different days after pollination showed that the density of the mutant kernel starch kernel increased with the progress of development and was always higher than that of the normal kernel. The genetic analysis of F₂ and BC₁ population showed that grain test weight mutation was controlled by a single recessive gene tw1 which was firstly located on chromosome 6 between SSR marker umc1105 and bnlg1154. After screening and analysis the recombinant from the BC₁ segregation population, the gene was located between B3 and A47. There are three protein-coding genes in the 0.2 Mb candidate physical interval. Allelism test excluded su2 gene and sequence analysis of the other two candidate genes verified that GRMZM2G042607 should be the primary candidate gene of tw1, which encodes protein with a carbohydrate recognition domain and deposits deposition of carbohydrates in the seed. 【Conclusion】The tw1 gene is fine mapped and find the candidate gene, GRMZM2G042607, encodes beta-1,3 galactosyltransferase.

Key words: maize, test weight, starch grain density, tw1, fine mapping