Cytoplasmic male sterility (CMS) is important for exploiting heterosis in crop plants and also serves as a model for investigating nuclear–cytoplasmic interaction. The molecular mechanism of cytoplasmic male sterility and fertility restoration was investigated in several important economic crops but remains poorly understood in the Welsh onion. Therefore, we compared the differences between the CMS line 64-2 and its maintainer line 64-1 using transcriptome sequencing with the aim of determining critical genes and pathways associated with male sterility. This study combined two years of RNA-seq data; there were 1504 unigenes (in May 2013) and 2928 unigenes (in May 2014) that were differentially expressed between the CMS and cytoplasmic male maintainer Welsh onion varieties. Known CMS-related genes were found in the set of differentially expressed genes and checked by qPCR. These genes included F-type ATPase, NADH dehydrogenase, cytochrome c oxidase, etc. Overall, this study demonstrated that the CMS regulatory genes and pathways may be associated with the mitochondria and nucleus in the Welsh onion. We believe that this transcriptome dataset will accelerate the research on CMS gene clones and other functional genomics research on A. fistulosum L.

Transcriptome analysis is an efficient way to explore molecular markers in plant species, for which genome sequences have not been published. To address the limited number of markers published for the Welsh onion, this study found 6486 loci of genic simple sequence repeats (SSR), which consisted of 1–5 bp repeat motifs, based on next-generation sequencing (NGS) technology and the RNA-Seq approach. The most abundant motif was mononucleotide (52.33%), followed by trinucleotide (31.96%), and dinucleotide (14.57%). A total of 2525 primer pairs were successfully designed, and 91 out of 311 tested primers were polymorphisms. Overall, 38 genic SSR markers were randomly selected to further validate the degree of genetic diversity, and 22 genic SSR markers (57.89%) showed high levels of polymorphism. The average polymorphism information content (PIC) value and the number of alleles (Na) were 0.63 and 5.27, respectively, and the unweighted pair-group method with arithmetic average (UPGMA) cluster analysis grouped the 22 Allium accessions into three groups with Nei’s similarity coefficients ranging from 0.37 to 0.99. This result suggested that these genic SSR markers could be used to develop a higher resolution genetic map and/or to analyze the phylogenetic relationships among Allium plants in the near future.