Heredity 78 (1997) 277—283 Received 12 April 1996 Genetic basis for different host use in Epilachna pustulosa, a herbivorous ladybird beetle HIDEKI UENO*, NAOYUKI FUJIVAMAt & HARUO KATAKURAt Laboratory of Environmental Medicine and Inform at/cs, Division of Bioscience, Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060 and tLaboratory of Systematics and Evolution, Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo 060, Japan Thegenetic basis for different host plant use was studied in a herbivorous ladybird beetle, Epilachna pustulosa, that exhibits interpoputational variation in host plant utilization. It usually depends on thistle, but one of the local forms occurs on both thistle and blue cohosh, which differ at the infraclass taxonomic level. In this local population, genetic association between the developmental performance on the two plants was neutral, suggesting genetic independ- ence across the host plants. The form of reaction norms indicated some changes in relative rank position. Genetic variation among individuals on each host plant was detected. These results suggest that different genotypes are selected on different host plants and that a substantial proportion of the overall phenotypic plasticity is contributed by genotype-depend- ent environmental effects. Keywords:Epilachnapustulosa, genetic correlations, host plants, reaction norms. ential on the second character, resulting in changes Introduction in their joint evolution (Lande, 1979; Via & Lande, Theevolution of herbivore—plant interrelation has 1985). Even in the absence of direct selection, been a particularly contentious problem in ecology characters can undergo evolutionary changes as a and genetics (Futuyma & Peterson, 1985; Jaenike, result of selection on correlated characters (Lande 1990; Via, 1990). Much attention has been paid to & Arnold, 1983; Falconer, 1989). how the host range of a herbivorous insect is deter- By regarding the fitness component on one host mined, for this is supposed to be a clear example of plant as a character and the fitness component on the adaptation process. Adaptation to host plants is another host as a different character, genetic corre- dependent on the presence and nature of genetic lation between the performance across the host variation in behaviour and physiological perform- plants can be estimated (Via, 1984; Falconer, 1989). ance. Therefore the elucidation of the genetic basis The evolution of insect—plant relationships is signifi- for difference in habitat use, and habitat-related cantly influenced by this genetic correlation; with a fitness components, is a key to understanding the negative genetic correlation, selection for an course of insect—plant interaction. increase in a fitness component character on one Because selection operates on the whole pheno- host plant would result in a decrease in the type rather than on individual characters, the genetic analogous component character on the other plant covariance among characters substantially influences species. This trade-off in the performance may the overall course of evolution. In the case where further lead to the evolution of host-specialized both characters are under selection pressure, selec- genotypes, which are more fit in one particular tion on one character can produce a selection differ- habitat than other genotypes. On the other hand, with a positive genetic correlation, selection would lead to a population constitution where genotypes *Correspondence. Present address: Laboratory of Biology, Faculty of Education, Niigata University, 8050 Ikarashi Ni-No- with high performance on one host would also show Cho, Niigata 950-21, Japan. E-mail: [email protected] a high performance on the other. Thus, a difference 1997 The Genetical Society of Great Britain. 277 278 H. UENOETAL. in host habitat would cause little difference in at least three copulations for each male and female performance among genotypes. pair. This was to ensure last-male paternity; last- The form of reaction norms, patterns of pheno- male sperm precedence (P2) in this species was type distributions produced by a single genotype reported as 0.651—0.827 (Nakano, 1985). Assuming across environments, also has important implications the same sperm precedence pattern for the last for the course of insect—plant interaction. If geno- three successive copulations, the expected fertiliza- typic responses to the environment differ consider- tion success of previous males is 0.349 =0.0425at ably, then norms of reaction may cross, changing the most and 0.173 =0.00518at least. ranking of phenotypes (Stearns, 1992). Under direc- For each female, 30 eggs were collected from tional selection for higher growth performance, several egg clutches. In the following statistical some genotypes would therefore be favoured in one calculations they were regarded as full-sib progenies, plant environment and different genotypes in the assuming complete last-male paternity. Of these 30 other. Thus, in this way, different genotypes may be eggs, 15 were reared on thistle and 15 on blue selected in different host plant habitats. cohosh. Larvae were reared individually. Fresh host The present study uses the herbivorous ladybird plant leaves were supplied every other day. Larval beetle, Epilachna pustulosa, to investigate (1) development period was checked daily. On the day whether different genotypes are favoured in each of pupation, fresh weight was measured to 0.0001 g host plant environment, and (2) how characters are with a microbalance. After eclosion adult body genetically correlated across host plants, i.e. whether length was measured under a microscope to the their genetic correlations are negative and whether nearest 0.01 mm. possible trade-off relations are implicated. A mixed-model ANOVA was performed to detect Epilachna pustulosa exhibits interpopulational the contribution of factors to the total variation in variation in host plant utilization (Katakura, 1981; larval performance, setting family and interaction as Hoshikawa, 1983). It usually depends on the thistle random, and sex and host as fixed effects. A mixed (Cirsium kamtschaticum, Asteraceae), but one of the ANOVA decomposes observed variances into variance local forms, the Sapporo form, occurs on both thistle components as shown in Table 2. This analysis was and blue cohosh (Caulophyllum robustum, Berber- performed on three variables, larval period, pupal idaceae), which differ at the infraclass taxonomic weight and adult body length. The analysis also level. Of the previous studies concerned with insect provides statistical tests of genetic correlations for genetic systems on closely related plant species, few the traits across the hosts. With the present statis- detected negative genetic association (Rausher, tical procedure, the between-family variance directly 1984; Hare & Kennedy, 1986; Fox, 1993). However, detects the covariance association term of the trait different evolutionary consequences might be across the host environments. Mean squares with expected when host shifts involve plants from F-values above 97.5 per cent indicate significant different families (Bush, 1969). The particular inter- positive genetic covariation, whereas F-values below est in this study therefore was to ascertain the 2.5 per cent indicate significant negative genetic genetic relationship between growth performance on covariation, at the 5 per cent level (Fry, 1992). totally different host environments in Sapporo popu- Two further ANOVAS were performed to evaluate lations, where variation in host plant utilization is the between-family variance of performance on each most apparent. host plant separately. Between full-sib family variances contain nonaddi- Materialsand methods tive sources of genetic variation, including domi- nance and maternal effects. Furthermore, these Epilachnapustulosa is a univoltine species. Over- estimates may possibly be inflated by the variances wintered adults were collected on thistles in a suburb of the common microenvironment in the rearing of Sapporo City in May and June 1994. Rearing conditions. These values should therefore be conditions were 22°C and 16L-8D throughout this regarded as approximations of genetic properties but experiment. Pairs each consisting of one male and they provide an upper limit of genetic variation. one female were confined in separate plastic cases. Because genetic properties are not necessarily the Virtually all females would have copulated and same for males and females, and some phenotypic stored sperm before overwintering (Katakura, 1982) differences were actually observed for the relation- and additional copulations could have been possible ship between developmental period and adult body in late spring to early summer before collection. size, genetic correlations were estimated separately Hence we started the experiment after we confirmed for each sex. Genetic correlation of the same trait The Genetical Society of Great Britain, Heredity, 78, 277—283. GENETIC BASIS FOR HOST USE 279 across the hosts was estimated as r,y = Coy (X,Y)! significantly larger in body length and heavier in [Var(X)Var(Y)]"2, where Cov(X,Y) is the covariance pupal weight when reared on cohosh than were their of family mean of the trait reared on one host plant siblings raised on thistles. This between-plant differ- and family mean of the same trait reared on the ence in overall tendency was observed in both males other host, and Var(X) is