A composite linkage map from two crosses for the species complex Picea mariana × Picea rubens and analysis of synteny with other Pinaceae

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Abstract
Four individual linkage maps were constructed from two crosses for the species complex Picea mariana (Mill.) B.S.P. × Picea rubens Sarg in order to integrate their information into a composite map and to compare with other Pinaceae. For all individual linkage maps, 12 major linkage groups were recovered with 306 markers per map on average. Before building the composite linkage map, the common male parent between the two crosses made it possible to construct a reference linkage map to validate the relative position of homologous markers. The final composite map had a length of 2,319 cM (Haldane) and contained a total of 1,124 positioned markers, including 1,014 AFLPs, 3 RAPDs, 53 SSRs, and 54 ESTPs, assembled into 12 major linkage groups. Marker density of the composite map was statistically homogenous and was much higher (one marker every 2.1 cM) than that of the individual linkage maps (one marker every 5.7 to 7.1 cM). Synteny was well conserved between individual, reference, and composite linkage maps and 94% of homologous markers were colinear between the reference and composite maps. The combined information from the two crosses increased by about 24% the number of anchor markers compared to the information from any single cross. With a total number of 107 anchor markers (SSRs and ESTPs), the composite linkage map is a useful starting point for large-scale genome comparisons at the intergeneric level in the Pinaceae. Comparisons of this map with those in Pinus and Pseudotsuga allowed the identification of one breakdown in synteny where one linkage group homoeologous to both Picea and Pinus corresponded to two linkage groups in Pseudotsuga. Implications for the evolution of the Pinaceae genome are discussed.