Sex linkage in salmonids: evidence from a hybridized genome of brook trout and Arctic charr.

In second-generation sparctics (Salvelinus fontinalis x Salvelinus alpinus) backcrossed to S. fontinalis, we have identified tight classical linkage of phenotypic sex with Ldh-1, Aat-5, and Gpi-3. We designate this locus Sex-1 and suggest that it may be the primary sex-determining locus in salmonids. Cumulative salmonid gene-to-centromere map distances for the three biochemical loci put the order as centromere--Ldh-1--(Aat-5 and Gpi-3), with the latter two loci being tightly linked. An absence of association of phenotypic sex (presumably Sex-1) with these same three loci and other loci known to be linked to these loci is shown in splakes (S. fontinalis x Salvelinus namaycush) and cutbows (Salmo gairdneri x Salmo clarki). These data imply that the linkage of Sex-1 with these loci is found only in S. alpinus and support the view that Sex-1 lies across the centromere from these three loci in S. alpinus, representing a Robertsonian fusion not found in any of the other four species. A similar specific Robertsonian fusion is argued for S. gairdneri, where Sex-1 may be linked across a centromere to another biochemical locus (Ha). These linkage results and chromosomal observations of other investigators suggest that Sex-1 lies on an information-depauperate arm.