Expressed sequence tags (ESTs) from
Coffea canephora leaves and fruits were used to search for types and frequencies of simple sequence repeats (EST–SSRs) with a motif length of 1–6 bp. From a non-redundant (NR) EST set of 5,534 potential unigenes, 6.8% SSR-containing sequences were identified, with an average density of one SSR every 7.73 kb of EST sequences. Trinucleotide repeats were found to be the most abundant (34.34%), followed by di- (25.75%) and hexa-nucleotide (22.04%) motifs. The development of unique genic SSR markers was optimized by a computational approach which allowed us to eliminate redundancy in the original EST set and also to test the specificity of each pair of designed primers. Twenty-five EST–SSRs were developed and used to evaluate cross-species transferability in the
Coffea genus. The orthology was supported by the amplicon sequence similarity and the amplification patterns. The >94% identity of flanking sequences revealed high sequence conservation across the
Coffea genus. A high level of polymorphic loci was obtained regardless of the species considered (from 75% for
C. liberica to 86% for
C. canephora). Moreover, the polymorphism revealed by EST–SSR was similar to that exposed by genomic SSR. It is concluded that
Coffea ESTs are a valuable resource for microsatellite mining. EST-SSR markers developed from
C. canephora sequences can be easily transferred to other
Coffea species for which very little molecular information is available. They constitute a set of conserved orthologous markers, which would be ideal for assessing genetic diversity in coffee trees as well as for cross-referencing transcribed sequences in comparative genomics studies.
Keywords SSR mining - EST–SSR -
Coffea
- Transferability - Genetic variation