Salmonid chromosome evolution as revealed by a novel method for comparing RADseq linkage maps

Whole genome duplication (WGD) can provide material for evolutionary innovation. Assembly of large, outbred eukaryotic genomes can be difficult, but structural rearrangements within such taxa can be investigated using linkage maps. RAD sequencing provides unprecedented ability to generate high-density linkage maps for non-model species, but can result in low numbers of homologous markers between species due to phylogenetic distance or differences in library preparation. Family Salmonidae is ideal for studying the effects of WGD as the ancestral salmonid underwent WGD relatively recently, around 65 million years ago, then rediploidized and diversified. Extensive synteny between orthologous chromosomes occurs in extant salmonids, but each species has both conserved and unique chromosome arm fusions and fissions. Here we generate a high-density linkage map (3826 markers) for the Salvelinus genera (Brook Charr S. fontinalis), and then identify orthologous chromosome arms among the other available salmonid high-density linkage maps, including six species of Oncorhynchus, and one species for each of Salmo and Coregonus, as well as the sister group for the salmonids, Esox lucius for homeolog designation. To this end, we developed O_SCPCAPMC_SCPCAPO_SCPLOWAPC_SCPLOWO_SCPCAPCC_SCPCAPO_SCPLOWOMPC_SCPLOW, a program that identifies identical and proximal markers between linkage maps using a reference genome of a related species as an intermediate. This approach increases the number of comparable markers between linkage maps by 5-fold, enabling a characterization of the most likely history of retained chromosomal rearrangements post-WGD, and identifying several conserved chromosomal inversions. Analyses of RADseq-based linkage maps from other taxa will also benefit from O_SCPCAPMC_SCPCAPO_SCPLOWAPC_SCPLOWO_SCPCAPCC_SCPCAPO_SCPLOWOMPC_SCPLOW, available at: https://github.com/enormandeau/mapcomp/