Melon Transcriptome Characterization: Simple Sequence Repeats and Single Nucleotide Polymorphisms Discovery for High Throughput Genotyping across the Species

Melon (Cucumis melo L.) ranks among the highest-valued fruit crops worldwide. Some genomic tools are available for this crop, including a Sanger transcriptome. We report the generation of 689,054 C. melo high-quality expressed sequence tags (ESTs) from two 454 sequencing runs, using normalized and nonnormalized complementary DNA (cDNA) libraries prepared from four genotypes belonging to the two C. melo subspecies and the main commercial types. 454 ESTs were combined with the Sanger available ESTs and de novo assembled into 53,252 unigenes. Over 63% of the unigenes were functionally annotated with Gene Ontology (GO) terms and 21% had known orthologs of Arabidopsis thaliana (L.) Heynh. Annotation distribution followed similar tendencies than that reported for Arabidopsis thaliana, suggesting that the dataset represents a fairly complete melon transcriptome. Furthermore, we identified a set of 3298 unigenes with microsatellite motifs and 14,417 sequences with single nucleotide variants of which 11,655 single nucleotide polymorphism met criteria for use with high-throughput genotyping platforms, and 453 could be detected as cleaved amplified polymorphic sequence (CAPS). A set of markers were validated, 90% of them being polymorphic in a number of variable C. melo accessions. This transcriptome provides an invaluable new tool for biological research, more so when it includes transcripts not described previously. It is being used for genome annotation and has provided a large collection of markers that will allow speeding up the process of breeding new melon varieties.