SSR markers have been widely used to differentiate mango cultivars and identify genetic diversity[11, 20], distinguish some synonymous (identical SSR genotypes with different cultivar names) and homonymous (different SSR genotypes with the same cultivar name) accessions[24], genetic relatedness[10] and so on. All these SSR markers will be used in the future studies of genetic diversity, genetic map construction and marker-assisted selection and utilized for breeding programs in mango. In previous study, we carried out transcriptome analysis of ‘Zill’ mango fruits[25], and 93 EST-SSR markers were developed based on transcriptome sequence data[23]. In this study, 34 pairs of primers with different bands between ‘Jinhuang’ and ‘Renong No.1’ were selected for the screening of hybrid identification primers from 93 SSR markers, of which eight pairs of SSR marker primers with polymorphism between parents and easy to amplify and distinguish bands were screened.
Cross breeding is an important way to breed new mango varieties. However, the difficulties in artificial hybridization, parent compatibility selection, and population construction of target traits have limited the progress of breeding new high quality mango varieties. At present, SSR molecular markers have been widely used in hybrid identification of various crops [8, 26, 27]. Eight SSR primers showed higher polymorphism between parents in combination with polyacrylamide gel electrophoresis were used to identify authenticity of ‘Jinhuang’ × ‘Irwin’ hybrids, and a total of 184 F1 seedlings were identified as true hybrids[28]. These results showed that it was feasible to identify the authenticity of fruit tree hybrids by SSR molecular markers. However, the traditional SSR molecular marker technology has many disadvantages, such as heavy workload, low precision, time-consuming and incapability for the detection of a large number of samples. Compared with polyacrylamide gel electrophoresis detection, fluorescent capillary electrophoresis has many advantages, such as higher resolution, as fine as 1 bp. In addition, electrophoresis results are displayed in a peak plot corresponding to the size of the amplified fragments, which can be directly collected to achieve digitization of the results in a form, which is easy to record, analyze and save. The application of high-throughput, multi-PCR and 10-fold capillary electrophoresis detection technology has improved the detection efficiency by 33-fold[29]. Briefly, SSR fluorescence labeled capillary electrophoresis detection method has the advantages of high-throughput, sensitiveness, cost-effectiveness and high accuracy. Wang et al[14] used eight polymorphic SSR markers to identify hybrids of 117 bayberry hybrid progenies, and successfully identified 83 true hybrids. Cheng et al.[15] screened a pair of SSR primer for seed purity identification of watermelon hybrid 'Wunong No.8' by using fluorescent marker SSR technology, and the purity was 99%, which was consistent with results of morphological identification of 'Wunong No.8' in the field. Lei et al.[30] used 17 pairs of SSR primers to identify true and false hybrids of two F1 hybrid populations in tea, and successfully identified 41 and 35 true hybrids, with hybrid rates of 85.42% and 79.55%, respectively.
However, when SSR molecular markers were used to identify the authenticity of hybrid progenies, the results of different primers were inconsistent. Han et al.[7] thought that different SSR primers had different identification efficiency for homozygous dominant markers and heterozygous dominant markers. Theoretically, only one homozygous dominant marker is required to identify all hybrid offspring, while for heterozygous dominant markers, at least five parent-specific markers are required to identify 97% of true hybrids. Yin et al.[31] found the first type of AA×BB, AB×CD and AB×CC marker, and there is no same allele locus between parents. Theoretically, only one SSR primer is needed to identify all true hybrids, and the identification rate is high. For the second type of AA × AB and AB × BC type marker, because there is one identical allele locus between parents, the identification ability is poor, which needs further verification. In this study, eight pairs of SSR markers with high polymorphism were used to identify the authenticity of 65 mango F1 hybrids. A total of 62 true hybrids were identified, and the hybrid rate was 95.38%. The results showed that the fluorescent SSR markers could be applied for the identification of true and false mango hybrids. The identification process was simple, fast and low cost, and the results were stable and reliable. It is a useful supplement to molecular biology identification technology of mango hybrids, which could be widely used in mango germplasm innovation, hybrid breeding and other fields in the future.
Mango breeding is a long-term activity, complicated by a heterozygous genome, juvenility, low fruit set and retention rates, long evaluation periods, polyembryony and outcrossing behavior. These factors make genetic improvement through conventional parental selection and breeding slow and unpredictable. It is difficult to obtain genetic materials and has a long growth cycle, which makes mango genetic research far behind other fruit crops. The results of previous studies showed that genetic composition of ‘Jinhuang’ and ‘Renong No.1’ was quite different, the progenies were widely separated, and strong heterosis was observed in hybrids for most of the traits examined. The quantitative traits such as average fruit weight, total soluble solids, and descriptive traits such as peel color, pulp color and fruit flavor of hybrid offspring showed rich diversity[32], which could be used as excellent gene resources for subsequent breeding. The number of hybrid population constructed in this study was small, so it is necessary to expand the population number to lay a foundation for construction of mango molecular marker linkage map and subsequent genetic analysis. The adoption of molecular markers and genomics-based breeding strategies will likely improve predictability and breeding efficiency.