While the ultimate causes and adaptive significance of sexual size dimorphism (SSD) have been extensively studied, the developmental mechanisms behind this phenomenon have received little attention. Going through an additional larval instar may form a specific way of achieving SSD in arthropods. In the present study, the mechanisms of SSD determination of two lymantriid moths, with marked SSD, were studied. In both species, females tended to go through an additional instar compared to males, and form pupae that were more than twice the weight of the males. To reveal the role of an extra instar, larval growth was monitored in the laboratory and the growth parameters were analysed as dependent on sex and developmental type (number of instars). Prolongation of growth by means of adding an additional larval instar in females turned out to be the key mechanism in the determination of the highly female-biased SSD in the species studied. There is thus a developmental mechanism available that permits achieving a larger size by means of extending the growth period. This provides evidence against constraint-based evolutionary explanations for body sizes in insects. There was no considerable accumulation of SSD during earlier larval life when females went through more instars than males. In contrast, in those cases in which males and females had the same number of instars, SSD accumulated gradually during the course of several larval instars. Longer growing period turned out to be a crucial mechanism leading to the female-biased SSD even when instar number did not differ between sexes, although higher instantaneous relative growth rates of females also played a complementary role in the latter case. Within sexes, an additional instar was characteristic of initially smaller larvae, as predicted by the "threshold size" hypothesis.
Transgenic lines of silver birch (Betula pendula) carrying the sugar beet chitinase IV gene were used to study the effects of the heterologous expression of a transgenic chitinase on the performance of lepidopteran herbivores. The effect of wounding the leaves of birch on the performance of lepidopteran larvae and the growth of trees was also studied. Larvae of Orgyia antiqua L., Lymantriidae, and Phalera bucephala L., Notodontidae, were separately fed on the leaves of transgenic and wild-type birch, and their performance measured using nutritional indices. The relative growth rate (RGR) of O. antiqua larvae fed transgenic leaves was significantly lower than that of larvae fed wild-type leaves. Furthermore, there is little evidence that transgenic chitinase affects survival but it was lowest for the group of larvae fed leaves with the highest expression of chitinase IV. Wounding did not have a significant effect on the performance of the larvae or on the growth of the branches of the trees. The growth of branches of particular transgenic lines, however, was significantly associated with tree line. The performance of P. bucephala larvae fed leaves of transgenic and wild-type birches did not differ. The leaves used in both experiments from transgenic trees were shorter than those from wild-type trees. Using transgenic birch expressing sugar beet chitinase IV to improve the resistance of birch to fungal diseases can have negative effects on O. antiqua larvae feeding on the leaves of these birches. P. bucephala, however, was not similarly affected, which indicates that these two ecologically similar lepidopteran species may differ in their response to transgenic chitinase., Liisa Vihervuori ... [et al.]., and Obsahuje seznam literatury