From Preliminary to Saturated Linkage Maps

The early generation of genetic maps include only morphological and isozyme markers, and later they were integrated through cytological markers and anchor markers shared by different maps. In switchgrass, the first publicly available genetic maps possesses totally 102 RFLP markers, among which 45 are on female map, and 57 on male map (Missaoui et al. 2005b). Then a higher density male and female framework maps were constructed, and their lengths were 1,645 and 1,376 cM with the genome estimated to be within 10 cM of a mapped marker in both maps (Okada et al. 2010). The present longest linkage map is 2,085 cM, with an average marker interval of 4.2 cM (Liu et al. 2012). The recent map consists of 18 linkage groups and is arranged into nine homoeologous pairs (Liu et al. 2012) (Table 1). ESTs from sequencing of switchgrass tissues, including young crowns and roots, were produced and made publicly available, which provide the resource for new marker development (Srivastava et al. 2010; Palmer et al. 2012). Recently, over 50,000 genomic-SSRs (Sharma et al. 2012) and 2,000 EST-SSRs (Wang et al. 2012) were respectively identified through high-resolution sequencing, and some of them were validated by PCR experiment for testing effective amplification and product-length polymorphism (Liu et al. 2013b). High throughput marker technologies including SNPs are available (Ersoz et al. 2012) and genotyping by sequencing is under development. Thus, the availability of an ultra high-density map in switchgrass is possible in the near future if more markers are added.