Heredity 65 (1990) 163-168 The Genetical Society of Great Britain Received 6 February 1990 Selection for changes in the incidence of wing dimorphism in Gryllus firmus D. A. Roff Department of Biology, McGill University, 1205 Dr Penfield Avenue, Montreal, Quebec, Canada, H3A IB1. Previous studies suggested that in the cricket Gryllus firmus the heritability of wing dimorphism is approximately 0-65. This estimate was based on a full sib analysis and hence may be confounded by non additive effects. To confirm this estimate a selection experiment was undertaken, lines being selected for increased and decreased incidence of macroptery. The response to selection was asymmetric, response for decreased percentage macroptery being faster than that for increased proportion macroptery. The realized heritability in the "up" line was approximately 0-25, and in the "down" line approximately 0-95, the mean across all lines being 0-6: this agrees very well with the estimate of 0-67 obtained from a full sib analysis. Two possible reasons for the asymmetric response are suggested. In the related cricket species, Gryllus rubens, the genes controlling wing morph appear to be on the sex chromosome while in C. firmus no sex linkage is evident. A new method of estimating realised heritability for threshold traits is presented. INTRODUCTION of reproduction and a reduction in fecundity (Roff, 1984, 1986a; Denno et a!., 1989; Roff and Fair- Migrationis an important component of the lives bairn, 1990). Thus there is a trade-off between flight of many insects permitting them to colonize and capability and reproduction. persist in temporary habitats (Southwood, 1962; To understand how the incidence of a par- Johnson, 1969; Harrison, 1980; Dingle, 1985). But ticular wing morph will evolve under changing though there are long term benefits to migration environmental conditions we must understand the there are also costs such as increased mortality and genetic basis of the traits involved in this trade-off decreased reproduction (Southwood, 1962; Roff, and the genetic correlations between them. Herita- 1977), and hence it is not surprising that migratory bility estimates of wing dimorphism for two polymorphisms are common (Harrison, 1980; crickets, Gryllusfirmus and Allonemobiusfasciatus, Dingle, 1985). Many insect species are wing suggest that a significant fraction of the variation dimorphic, some individuals within the population (>50 per cent) can be attributed to additive genetic or a family having wings and being capable of effects (Roff, 1986b; Mousseau and Roff, 1989; flight and others having either no wings or reduced maternal effects were insignificant in the study of wings and being incapable of flight. Within these G.firmus but could not be examined in the analysis species there is a clear dichotomy between the of A. fasciatus). However, in both cases heritability morph capable of migration and that which is not. estimates were obtained by full sib analysis and But an individual with wings does not have to fly thus contain an unknown fraction of non additive and hence the existence of wing dimorphic species effects. To overcome this deficiency a selection suggests that there is a cost not only to migration experiment was undertaken using Gryllus firmus, itself but also to possessing the capability of migra- lines being selected for increased and decreased tion, viz wings, wing muscles and associated struc- incidence of macroptery. The results of this experi- tures. This hypothesis has been confirmed in a wide ment and a new method for estimating realized variety of insects, in which it has been found that heritability of a threshold trait are presented in the flight-capable morphs show a delay in the onset this paper. 164 D. A. ROFF MATERIALS AND METHODS maintained to provide further estimates of herita- bility. One "within family" macropterous and two Experimental protocol micropterous lines were so established, designated Detailsof the species and rearing methods are as WL1, WS1 and WS2, respectively. given in Roll (1986b), and only the salient For each of the lines Li, Si and Cl, six cages points are presented here. The stock of G. firmus containing 60 newly hatched nymphs per cage, and used in the present study was derived from for the within family lines (WL1, WS1 and WS2), approximately 40 individuals (approximate sex two cages of 60 nymphs/cage were established. ratio 1: 1) from a single location in northern Males and females of a desired morph (macrop- Florida in 1981. They are maintained in diapause terous for Li, micropterous for Si and both for averting conditions (25-30°C, no set photoperiod Cl), were selected upon eclosion into adults until but the laboratory lights ensure a relatively long approximately 100 (and never less than 50) of each light period), with a breeding stock of between sex were obtained. In the case of WL1, WSi and 100-300 individuals. For the selection experiments WS2, 20 females and ten males were selected. The individuals were raised in batches of 60 individuals relatively large number of parents was used to per disposable mouse cage, as described in Roff prevent inbreeding depression. The mass selection (1986b). Food was provided, ad libitum and com- procedure outlined above was followed on all gen- prised Purina© rabbit chow and fresh lettuce erations subsequent to the first. To provide a repli- leaves. cate, a second series of lines was set up some Previous studies were conducted at 30°C and months after the first, the protocol followed being a photoperiod of 17 h L; 7 h D: under these condi- identical except that a mass selection design was tions the proportion of macropterous males and followed from the outset and no initial group of females is about 76 and 64 per cent, respectively single pair matings constructed. These lines will (Roll, 1986b). To reduce this percentage to around be designated L2, S2 and C2. 50 per cent in the females, in the present experi- In G. rubens wing morph is controlled to a meni crickets were reared at 28°C, 15 h L: 9 h D. large extent by sex-linked genes (Walker, 1987; An e;timate of the heritability of wing dimorphism Gryllus have an XO system with the female under these conditions was obtained in the first being XX); to test for this possibility in G. selection experiment as follows: eggs were firmus, the following crosses were made in obtained from the stock culture and the nymphs generation 10 of the first experiment, Li x SI, raised under the experimental conditions. From LIxWS1, LixWS2, WL1xS1, WL1xWS2. In this group 20 pairs were extracted comprising 10 each cross two cages, each comprising ten adult pairs LW x LW (macropterous x macropterous) females from one line and five adult males from and 10 pairs SWxSW (micropterousx another, were set up and 180 nymphs collected micropterous). Estimates of the heritability of wing from these for rearing. All crosses were reciprocal dimDrphism were made from these crosses for the with respect to sex. founding population. Since these matings do not include mixed crosses, estimates of heritability have to be corrected for assortative mating (dis- Statistical cussed below). analysis To establish a macropterous line (hereafter Theestimation of heritability from full sib data referred to as the Li line) 200 adults (100 males, for a threshold trait is summarised in Roff (i986b, 100 females) from the 10 LW x LW matings were typographical corrections to formulae given in this mixed together, with approximately equal rep- paper are presented in Mousseau and Roff, 1989). resentation from each family. Similarly, a In the present analysis the situation is complicated micropterous line (Si) was started by mixing by the fact that the matings are assortative. To together the offspring from the SW x SW crosses. calculate the uncorrected estimate of heritability, A Control line (Cl) was formed by mixing the H, we require an estimate of the mean proportion offspring from all crosses. Based on initial esti- of micropterous individuals per family, p. The pro- mates of percentage macroptery in each family, 3 portion p is normally estimated by (i /n where "extreme" families were selected and matings nisthe number of families and p, is the proportion made between siblings within each family, 20 of micropterous individuals in family i (Roff, females and ten males per family: final data on i986b). Since in the present case mating is assorta- these families indicated that they were not tive, p was estimated from the parental population. "extreme" (see Results) but the lines were The heritability estimate can be corrected using SELECTION AND WING DIMORPHISM 165 the formula given by Falconer (1981, P. 164), The probability that an individual in generation i + 1 will be micropterous is, /12 ={—1+../[1 +4rH]}/(2r) (1) z÷1)2)dx =P±1.(3) where 112isthe narrow sense heritability, H is the (1/)J exp (—05(x — uncorrected estimate of heritability, and r is the phenotypic correlation between parents measured The likelihood of obtaining the observed series on the underlying continuous scale. This cannot be of micropterous individuals in the selection experi- directlyestimated from the dichotomous ment is, phenotypes of the parents (macropterous or N micropterous). However, given the proportions of H CP(1-P' (4) macropterous males and females in the population from which the parents are derived, and assuming where n, is the number of offspring in generation that the dichotomous trait is a consequence of an i, r- is the number of micropterous offspring in interaction between a threshold and a continuously generation i, and N is the number of generations. varying character (Falconer, 1981), r can be esti- Taking logs, we obtain mated by Monte Carlo simulation. The base popu- lation comprised 51 per cent macropterous females LLcC {r, log (P)+(n —ri) log (1 —P)}.