Mechanisms of sympatric speciation in monophagous sawflies

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Mechanisms of sympatric speciation were investigated for Hemitaxonus japoicus comples, of which the new host race, polyblepharum race is being formed from the original race, tripteron race, in the narrow zone of about 10 km wide around 34ﾟN in the south-western part of Japan without geographical isolation. Adult females of each host race lay eggs on their own host plant. F1 females prefere Polystichum tripteron to P.polyblepharum for oviposition. Host selection may be disruptive, controlled by one gene. Two races use similar but distinct volatile chemichal substants for oviposition stimulants, of which the boiling points are between 100 and 150ﾟC/1mmHg, respectively. The newhost race larvae lose the ability to survive on the old host plant, caused by conditioning that the adult females continue to lay eggs on the new host plant. Asymmetrical mating developing between two racesis an importat post-mating reproductive isolating mechanism to preventthe gene flow from the old host race to the new. There is no remarkable differentiation in chromosome and allozyme polymorphisms between two host races, but the difference of 20 bp in the miner band of pYS family of tandemly repetitive DNA sequences between them may clear the relationship between host race formation and genetic variation. The sibling species H.sasayamensis is supposed to have speciated sympatrically from H.japonicus with host shift and to have been temporall isolated on the most parts of distribution.