Studies on the sympatric host shift mechanism in monophagous sawflies

1. 同域寄主小种的形成与寄主迁移发生在蕨类锯蝇分布的南部边界附近。这是在野外首次发现单食昆虫同属地形成寄主种。在Kii半岛，两种蕨类植物（Polystichum tripteron和P. polyblepharum）的成虫寄主偏好和幼虫存活率在近晚从北向南约10km范围内发生显著变化。在那里，新的宿主种族，polyblepharum种族正在形成，没有地理隔离。该锯蝇在南区为寡食性，在北区分为雷公藤属和多眼蜂属两个寄主小种，具有生殖隔离性。成虫寄主偏好可能由单个位点上的两个等位基因控制：a是指雷公藤的偏好等位基因，a是指从雷公藤中突变而来的多睑蕨等位基因。幼虫的存活也可能由单个位点上的等位基因控制，但多睑蕨特异性等位基因可能不是通过突变，而是由于在多睑蕨中产卵多代而形成的条件作用。三种隔离机制对同域寄主的形成起重要作用。破坏性选择和选择性交配是交配前的隔离，而交配偏好是交配后的隔离。鉴定与成虫寄主偏好或交配偏好有关的化合物是今后研究中需要解决的重要问题。

1. Sympatric host race formation with host shift is occurring near the southern limit of distribution in the fern sawfly Hemitaxonus japonicus. This is the first discovery that monophagous insects forming host races sympatrically in the field.2. In the Kii peninsula, the adult host preference and larval survival for two species of ferns, Polystichum tripteron and P. polyblepharum, are remarkably changing in this sawfly within about 10km from north to south near lat. 33ﾟ40'N, where the new host race, polyblepharum race is being formed without geographical isolation. This sawfly is oligophagous south of the zone, while it is divided into two host races, tripteron race and polyblepharum race, north of the zone with reproductive isolation.3. Adult host preference may be controlled by two alleles at a single locus: A means the tripteron preference allele and A means the polyblepharum allele which is a mutant from A. Larval survival may be also controlled by alleles at a single locus, but the polyblepharum-specific allele might have been derived from the oligophagous allele not by mutation but by conditioning due to oviposition to polyblepharum fern for many generations.4. Three isolation mechanisms play important roles for sympatric host race formation in this sawfly. Disruptive selection and assortative mating act as a premating isolation and mating preference works as a postmating isolation.5. Identification of chemical compounds concerned with adult host preference or with mating preference in this species complex is an important problem to solve in future studies.