A sex‐linked locus ( Mpi ) in the opossum shrimp Mysis relicta : implications for early postglacial colonization history

Strong and persistent associations between sex and genotype frequencies at the Mpi allozyme locus (mannose‐6‐phosphate isomerase) were found in three lacustrine populations of Mysis relicta sp. I in eastern Finland. Almost all females were homozygotes 100 / 100 , whereas most males were heterozygotes 107/100 . This disequilibrium suggests a complete linkage between Mpi and a sex‐determining factor in a male heterogametic system and provides the first evidence for a genetic sex‐determining mechanism in the crustacean order Mysida. However, no disequilibria were found in other parts of the species range. Potential mechanisms involved in generating the disequilibria are considered (sex‐specific selection, recombination modifiers, hitchhiking, population bottlenecks). The restricted geographical distribution of the sex– Mpi association can be related to the early postglacial geological evolution of the lakes studied. A detailed scenario for the colonization history and genetic changes in the populations is presented. This involves initial immigration using temporary connections among ice‐marginal lakes in eastern Finland approximately 10000  bp , associated population bottlenecks, and the origin and local selective spread of a nonrecombining Y‐ Mpi 107 chromosome in the Sotkamo area c. 9300  bp . Soon after the isolation of the three headwater lakes ( c. 9100  bp ), a new recombination modifier caused a breakdown of the association in populations downstream.