/-/eredity7l (1993) 242—251 Received 3 December 1992 Genetical Society of Great Britain Polymorphism and evolution in the butterfly Danaus chrysippus (L.) (Lepidoptera: Danainae) DAVID A. S. SMITH, DENIS F. OWEN*, IAN J. GORDON & AGOROACHAI M. OWINY Department of Biology, Eton College, Windsor SL4 6EW, tSchoo/ of Biological and Mo/ecu/ar Sciences, Oxford Brookes University, Headington, Oxford 0X3 OBP, U.K., Department of Zoology, University of Nairobi, P0 Box 30197, Nafrobi, Kenya and §Department of Zoology, Makerere University, P0 Box 7062, Kampala, Uganda Analysisof the genetic structure of a sample of the polymorphic butterfly Danaus chiysippus from Kampala, Uganda shows that the population is undergoing substantial evolutionary change. Comparison with samples from the same area going back to before 1900, indicate that the frequency of form alcippus has increased from 16 per cent to 71 per cent (1909—91) while f. dorip- pus has decreased from 14 per cent to 2 per cent, f. aegyptius from 66 per cent to 24 per cent and f. albinus from 4 per cent to 3 per cent. Genotype frequency differences between the sexes at two of the three loci examined suggest that a balanced polymorphism is maintained by opposing selective forces acting on males and females. Non-gametic (genotypic) disequilibrium between two pairs of unlinked loci indicates that natural selection is involved, again with sex differences. It is suggested that the polymorphism originated after hybridization of allopatric races which evolved during the Pleistocene but are now maintained sympatrically. The selective agents have not been identified but mimetic relationships, both Batesian and Müllerian, are almost certainly involved. Keywords:allopatricevolution, D. chiysippus, mimicry, non-gametic disequilibrium, polymorph- ism, sympatric evolution. Introduction gators in the East African region), alcippus, dorippus and albinus (Fig. 1). Colour plates can be found in Inmuch of Africa the butterfly Danaus chrysippus (L.) Rothschild etal. (1975) and Smith(1980). (Nymphalidae: Danainae) is polymorphic for colour Many alcippus have only a small white patch in the and pattern. As it is both a Batesian model for several hindwing or simply white scaling lining the veins; these unrelated species, belonging to four different butterfly we refer to as weak alcippus (also known as alcip- families (or subfamilies depending on the authority) poides); similarly there are weak albinus (also known as and also a Müllerian mimic sharing in rings with several semialbinus). Many dorippus have a variable number of other unrelated unpalatable species, it is a remarkable 'aegyptius' subapical spots on the underside of the fore- exception to the rule that both Batesian models and wing; these are also called transiens. In all four forms Müllerian mimics are monomorphic (Owen, 1971; the orange ground colour may be completely or Smith, 1980; Gordon, 1984). The polymorphism is partially replaced by brown. best developed in East and Central Africa, especially in Forms aegyptius, alcippus, dorippus and albinus are Uganda; there are also large areas, both within and involved in a Müllerian mimicry complex with Acraea outside Africa, where the butterfly is monomorphic encedana Pierre and A. encedon (L.) (Acraeinae) and and this is the general rule, with only local exceptions, are also models for many presumed Batesian mimics, throughout its extensive Palaearctic, Oriental and particularly Hypolimnas misippus (L.) (Nymphalidae). Australasian range. Maps showing the distribution of the forms of D. In East Africa there are four main colour forms, still chrysippus in Africa are given in Owen & Chanter referred to by their traditional Latin names: aegyptius (1968), Owen (1971), Rothschild et al. (1975) and (incorrectly called chiysippus by many previous investi- Pierre (1973, 1980). The switch between orange and brown ground colour phenotypes (which have not *Correspondence received Latin names) probably has little impact on 242 POLYMORPHISM AND EVOLUTION IN D. CHRYSIPPUS 243 Ib) Hardy—Weinberg equilibrium, which they are unlikely to be (see below). At Kampala, penetrance of a is nearly equal in the two sexes whereas at Dar es Salaam it is 24 per cent higher in males than in females. The B locus controls ground colour, B giving brown and bb orange. Many heterozygotes are detectable but they are variable: Bb genotypes always have a brown forewing and a little brown on the costal margin of the hindwing but the hindwing may be entirely brown as in BB homozygotes. The B locus was first identified by Clarke et al. (1973) and confirmed by Smith (1975a). The C locus controls the forewing pattern, C giving dorippus (or albinus when combined with a white (d) hindwing), and cc giving aegyptius (or alcippus when combined with a white hindwing). Many Cc hetero- zygotes can be recognized phenotypically by the presence of pale subapical spots on the underside of the forewing (transiens). At Dar es Salaam, Tanzania, the expressivity of c in Cc genotypes is influenced by the B locus, 45 per cent (N= 129) of bbCc hetero- zygotes being transiens compared with 74.6 per cent (N= 173)of B— Cc (Smith, 1980). The B and C loci are closely linked with a recombin- ation value of 1.9 per cent (3.8 per cent in males only as meiosis is probably achiasmatic in females). The A locus assorts independently of B and C at Dar es Salaam (Smith, 1975a, 1980) and also did so in crosses Fig. 1 Colour forms (f.) of Danaus ch,ysippus. (a) between alcippus (aa B —)fromSierra Leone and 1. aegyptius. (b) f. alcippus. (c) 1. dorippus. (d) f. albinus. The black and white areas are as shown; the stippled areas are subspecies petilia (AA bb) from Australia (Clarke et al., either orange or brown. 1973). Here we report on genotype frequency in a sample of D. chrysippus obtained at Kampala, Uganda, in generalized mimetic resemblance (Gordon, 1984), December 1991 and present evidence indicating that especially as many of the mimics are similarly variable; powerful natural selection is operating at the present Smith (1980) has suggested that the orange—brown time and that a considerable evolutionary change has polymorphism relates to climatic adaptation, possibly occurred in the past 91 years (approximately 1092 through thermoregulation, influencing activity levels breeding generations). and, in particular, courtship behaviour. The genetics of the polymorphism in D. ch,ysippus Materials and methods involves three loci, A, B and C (Smith, 1975a). The A locus controls the pattern of the hindwing, the A allele Source of material giving brown or orange aegyptius and aa a large white patch (alcippus). Aa heterozygotes vary between D. chrysippus is an abundant butterfly and large having a reduced white patch, white scaling lining the samples are easily obtained by using an ordinary veins (both scored as weak alcippus) or no trace of butterfly net. In December 1991 we collected a sample white (scored as aegyptius); hence not all heterozygotes in the Kampala area at sites where previous samples are phenotypically distinguishable from AA homo- had been obtained by us (1964—66, Owen & Chanter zygotes. Estimates for the penetrance of a in hetero- (1968); 1972, A.M.O., unpublished data). We also zygotes, based on breeding at Dar es Salaam, Tanzania examined two samples in the Hope Entomological are 74.4 per cent (N= 47) (Smith, 1975a) and 65.6 per Collections, Oxford, which had been randomly cent (N=66) (Smith, 1980) with an average of 70 per obtained on the instructions of Professor E. B. Poulton. cent. An estimate of 69.7 per cent penetrance for the One of these, obtained by C. A. Wiggins in 1909—12, is Kampala sample (N =211)suggests good agreement from the same area as the more recent samples, with Dar es Salaam but assumes the alleles are in whereas the other comes from Kome Island (just off 244 D. A. S. SMITH ETAL. Table 1 Frequency (percentage) of phenotypes and genotypes in a sample of Danaus chrysippus collected in the Kampala region, Uganda, in December 1991 Form and colour Genotype Male FemalePopulation alcippus, orange aa bb cc 46.7 24.0 35.5 alcippus,brown aaB— cc 3.7 7.7 5.7 weak alcippus (= alcippoides),orangeAa bb cc 26.2 24.0 25.1 weak alcippus (= alcippoides),brown Aa B —cc 0.9 8.7 4.7 aegyptiust orange A— bbcc 15.9 24.0 19.9 aegyptiust, brown A —B—cc 1.9 5.8 3.8 albinus, orange aa bb C— 0.9 0.0 0.5 albinus-transiens, orange aa bb Cc 3.7 0.0 1.9 weak albinus( =semialbinus),orangeAa bb C— 0.0 1.0 0.5 dorippus, orange A —bbC— 0.0 1.9 0.9 dorippus-transiens, orange A —bbCc 0.0 2.9 1.4 Sample size A— bbCc 107 104 211 tScored as chiysippus by some earlier workers, this name should be restricted to the similar but distinct Oriental and Palaearctic subspecies. Entebbe in Lake Victoria), was obtained in 1946, and Table 2 Frequencies of dominant and recessive phenotypes is included because D. chrysippus ranges widely and in a sample of 211 D. chiysippus from Kampala, Uganda the colour forms (at a given time) are stabilized over relatively large geographical areas (Owen, 1971). Male Female Three earlier large collections, all mainly pre-1900, D R D R were examined: two in the Natural History Museum, Locus London and one in the Hope Entomological Collec- (%) (%) (%) (%) Z1) tions, Oxford. Although museum samples are notor- A 44.9 55.1 68.3 31.7 10.820** iously non-random with rare phenotypes generally B 6.5 93.5 22.1 77.9 9.249** over-represented, in this case the phenotype frequen- C 4.7 95.3 5.8 94.2 0.002 cies in the three collections are in fact, surprisingly, formally homogeneous and this increases our confid- D, dominant; R, recessive.