Relationship between spawning mode and phylogeographic structure in mitochondrial DNA of North Atlantic capelin Mallotus villosus

摘要

Capelin Mallotus villosus spawn on beaches in Alaska and British Columbia, but spawn offshore in Icelandic waters and the Barents Sea. Both modes of reproduction CO-occur In the northwest Atlantic. The Southeast Shoal population spawns on the Grand Banks 350 km to the SE of Newfoundlandudat the same time as other stocks, all of which are beach spawners. These observat~onsg ave rise to 2 alternative hypotheses concerning the zoogeography and evolution of life cycle in capelin. First, the Southeast Shoal population was originally a beach-spawning population during the late Wisconsinian glaciation and is ancestral to all other northwest Atlantic capelin stocks. In such a case, present-day stocks from this area would represent a monophyletic group derived from a common ancestor no more than 10000 to 12000 yr ago. The alternative hypothesis is that the 2 modes of reproduction orignginally evolved inudisolatlon. Beach spawners are hypothesized to have originated in the north Pacific and recolonized Canadian Arctic waters and the northwest Atlantic following glaciation Bottom spawners originated in the North Atlantic and continued to reproduce where environmental conditions permitted. In such a case. genetic divergence among bottom-spawners and among beach-spawners from across the North Atlantic would be less than that between beach- and bottom-spawners. We tested these hypotheses by comparingudmitochondrial DNA (mtDNA) restriction fragment length polymorphisms among 6 stocks of beachspawning capelin (St. Lawrence estuary; Gulf of St. Lawrence; Placentia Bay, Conception Bay and Notre Dame Bay, Newfoundland; Nain, Labrador) and 3 stocks of bottom-spawning capelin (Southeast Shoal; Iceland; Barents Sea). We observed 2 major mtDNA genotype groups separated by a mean sequenceuddivergence of 3.42 O/O, clearly reflecting the genetic separation of the Iceland and Barents Sea stocks fromudthe northwest Atlantlc stocks. No geographical heterogeneity in the frequency of mtDNA genotypes wasudobserved among the northwest Atlantic sampling sites. However, differences in nucleon diversities among sites did not support the view that capelin form one large panmictic population in the northwest Atlantic. Although our results do not permit the identification of the Southeast Shoal stock as ancestral to northwest Atlantic capelin, these observations refute the hypothesis that the beach- and bottomspawning stocks evolved in isolation long before the end of the Wisconsinian glaciation.