Two highly informative dinucleotide SSR multiplexes for the conifer Larix decidua (European larch)

We have designed two highly polymorphic microsatellite multiplexes for Larix decidua Mill (European larch), a coniferous tree species with a fragmented distribution across Europe. The multiplexes combine microsatellites previously designed for the sister species L. kaempferi and newly identified microsatellites obtained by pyrosequencing of an enriched microsatellite library and subsequent marker candidate selection. As we wanted to target highly polymorphic markers, only microsatellite motifs with a high number of repeats (=12) were selected. An important proportion of the marker candidates presented multiple bands, bad amplification or insufficient polymorphism. Such difficulties were expected owing to the large genome size of the studied species. Our strategy for marker validation followed most recent recommendations for microsatellite development, for example verifying marker quality in terms of polymorphism and accurate allele binning before multiplexing. The most promising loci were combined in two multiplexes, a 7-plex and a 6-plex. These were tested on a sample of 413 individuals from 18 populations distributed across the natural range. The 13 loci had from 9 to 36 alleles. Markers were successfully tested in another laboratory, confirming robustness of the marker protocols. We also tested transferability on six other larch species from Asia and North America. Overall, this study shows that, even in species with large genome size and relatively low overall polymorphism, microsatellites can be successfully developed using next-generation sequencing technologies, provided that some additional precautions are taken compared to species lacking these characteristics.