Studies on the sympatric host shift mechanism in monophagous sawflies

1.在蕨类叶蜂的分布南界附近发生了共域寄主小种的形成和寄主移位。这是首次发现单食性昆虫在田间形成共生的寄主小种。在Kii半岛，从北到南近10公里的范围内，两种蕨类植物--三叶蕨属和多眼蕨属的成虫寄主选择性和幼虫存活率发生了显著的变化。33ﾟ40‘N，新的寄主小种多眼小种正在形成，没有地理上的隔离。该带以南为稀食性小种，而在带北分为三翅小种和多眼小种两个寄主小种，并有生殖隔离现象。成虫对寄主的偏好可能受同一基因座上的两个等位基因控制：A指的是Tripteron偏爱等位基因，A指的是A突变后的多眼睑等位基因。幼虫的存活也可能是由单基因座上的等位基因控制的，但多眼睑特异的等位基因可能不是通过突变获得的，而是由于多代产卵而产生的条件性。三种隔离机制对白叶蜂共域寄主小种的形成起重要作用。破坏性选择和分类交配是一种前向隔离，而交配偏好则是一种后向隔离。在该物种复合体中鉴定与成虫寄主偏好或交配偏好有关的化合物是未来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.