Identification of 60 Citrus Accessions Using Target SSR-seq Technology

Bud sports mutation is a DNA mutation occurred in somatic meristem, and it often displays visible morphological and other characteristic changes different from its mother plants in branches, leaves, flowers and fruits. However, the discriminating bud sports mutation from the epigenetic variation caused by environmental conditions and cultivation measures etc. external factors was still mostly depended on the molecular fingerprint detection.

This study objective was to identify citrus bud sports mutant through Target-SSR sequencing technology.

Firstly, the genome of clementine mandarin (Citrus reticulata Blanco) and satsuma (Citrus unshiu Macf.), as well as GSS and EST sequences of satsuma were used to scan SSRs loci with GMATA, and the highly polymorphic SSRs loci were screened out to design primers. The multiplex PCR with optimized primers were ampllified on 60 citrus bud sports mutants to construct high-thoughout sequencing library, and the amplification products were then sequenced on illumina Minseq platform. The clean sequencing short reads were mapped to reference target sequences to find differentiated SSRs loci presented in citrus bud sports mutants.

A total of 77 pairs of SSR primers were designed from highly polymorphic SSRs loci. The primer pair combinations were optimized and 18 multiplex PCR amplification products were sequenced. The target SSR-seq analysis showed that the genotyping data of SSRs could divided 60 citrus accessions into two groups corresponding to sweet orange and mandarin, and the mandarin group could be further subdivided into different citrus cultivars, such as Orah, ponkan etc. 11 SSR loci containing mostly ATT motif were found in 7 Tarroco blood orange mutants, 8 SSR loci containing mostly TAA motif were found in 2 ‘Wu Yue Hong’ mutants and 5 navel orange, 16 SSR loci containing mostly GA motif were found in 9 Bing Tang Cheng mutants, 9 SSR loci containing mostly AAT motif were found in 2 Sha Tang Ju mutants, and 15 SSR loci with mostly AAT motif were found in 4 satsuma mutants. This study showed that Target-SSR sequencing technology provided an excellent resolving approach to discriminate citrus bud mutations.

In this study, an effective method for citrus bud mutant identification by using Target SSR-seq technology were established, and 60 citrus germplasm accessions could be discriminated. The precision and reliability of SSR genotyping information could be utilized in citrus germplasm resources management and variety intellectual property protection.