Papilio Dardanus, Brown, and Papilio Glaucus, Linn
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THE GENETICS OF SOME MIMETIC FORMS OF PAPILIO DARDANUS, BROWN, AND PAPILIO GLAUCUS, LINN. By C. A. CLARKE AND P. M. SHEPPARD The Universit),, Liverpool. (Received, june 3, 1957) Bates (I862) put [brward the hypothesis that some anilnals obtain protection from ~heir predators by resembling or mimicking unpalatable or otherwise protected species and in consequence are mistaken for them by the predators. Mtiller (1879) suggested that even protected species would gain by resembling one another. These two hy-po- theses have been discussed and enlarged upon by many people but comparatively little work has been done either to determine the extent of the protection so afforded or to ascertain the evolutionary steps by which the mimicry has been brought about. The genetic work that has been done in this field (m.ostly with butterflies) suggests that the differences between various mimetic and non-mimetic forms of polymorphic butterflies are controlled by simple mendelian mechanisms, often a single allelomorphic difference. This has led some people, notably Punnett (I915) and more recently Goldschmidt (i945) to maintain that, because the allelomorph must have arisen at a single step by mutation, the mimetic resemblance must also have arisen fully developed from the beginning. This view leads to ~various theoretical difficulties which have been discussed by Ford (1953). Because of these, both Fisher (1930) and Ford (Carpenter and Ford 1933, Ford 1937) take the view that when a mutant pro- ducing some mimetic resemblance is established in a population the resemblance is improved by selection for a gene-complex in which the original effect of the gene is altered towards more perfect mimicry. The truth of these hypotheses can be tested by suitable race crosses as has been pointed out by Ford (L953). Goldschmidt's view leads to the conclusion that when a butterfly resembles different subspecies of a model in different areas, the various forms will be controlled by a multiple alIelomorphic series, whereas in Fisher and Ford's view the same alldomorph will be responsible in all the areas but the mutant's effect will be modified by the presence of a different gene-complex in each region. Thus in P. dardanus Brown there are two very similar female forms, hippocoon in West Afi'ica mimicking Amauris niavius niavius and in South and East Aft'lea hippocoonides mimicking Amauris niavius dominicanus. On Goldschmidt's view the difference between the two would be due to a different allelomorph of a single gene, whereas on Fisher and Ford's the same gene would control these forms and the difference wouId be due to modifiers at other loci. To distinguish between the two hypotheses unequivocally, it is first necessary to know the mode of inheritance of the mimetic tbrms within the subspecies to be crossed and to develop stocks of known genetic constitution. This paper" reports the results of an investigation into the genetics of some of the forms of the mimetic butterflies Papilio dardanus and :f~apilio glaucus. C. A. CLARKE AND P.M. SHEPPARD 237 ~"IA'I.'ERIALS AND METtlODS I. Papilio dardanus In this butterlly we have been concerned onl.v with race ceaea, which inhabits South Aft'ira northwards to Delagoa Bay. Four principal female forms occur :--1) f. cenea, mimicking Amqtlris albimaculala, 2) Ji hippocoonides, mimicldng Amauris niavius dominicanus, 3).f. trophonius, mimicking Danais chr),sippus, 4)f leighi, non-mimetic although tailless and not resembling the males. [There are also "two other forms, natalica and salaami, which are considered to be rarities, but Wells (see below) estimates that natalica is as high as 10% in his district.] The males in race cenea, as in all others, are always tailed and monomorphic. (see Plate 1). DISTRIBUTION In the southern part of South Africa, cenea is much the commonest, while north- wards hippocoonides is most frequently found. Our material was obtained from Natal, in the districts round Durban, Pietermaritzburg and Eshowe respectively. In these areas there appears to be considerable variation in the proportions of the female forms. Thus Leigh (Poulton 1928) gives the following distribution for Durban, obtained by breeding fi-om wild larvae :-- cenea 129 (85 %) hippocoonides 14 (9%) trophonius 6 (4%) l~i~hi 3 (2%) On the other hand, Dickson (personal communication 1955) reports that in Durban in recent years sometimes cenea and sometimes hippocoonides appears tobe the com- moner, that tr@honius is scarce and that hc has never seen leighi. [With regard to fi leighi it is of interest that Poulton thought Leigh's figure of 2% much too high.] In Pietermaritzburg, Wells (personal communication 1955) estimates the proportion of the female forms as follows :--cenea 40%,, kippocoonides 35/o,o~ lrophonius 15% and natalica 10%. In Eshowe, Swanepoel (personal communication 1955) writes that l: hippocoonides is by far the commonest, ceuea comes next, and that gr@honius is very scarce. The original wild stock was sent to us in England early in 1954 by air mail and the occasional material received subsequently has helped to overcome the dangers of inbreeding. (These families are indicated in Table I by the word "wild".) It was found that eggs and caterpillars travelled we/l, but that pupae often died, apparently beiag abnormally sensitive to changes of temperature. In England the butterflies were bred in a g,'eenhouse at a ininimum temperature of 60~ and under these condi- tions they were continuously brooded throughout the year, the approximate time from egg to perfect insect being 2~ months. Pairing was always elTected by the method ofhandmating, (see Clarke and Sheppard 1956) and everlr effort was made to Inate the same male with more than one form of female. It was tbund that the males 238 Genetics of Papilio glaucus and P. dardanus usually copulated more readily one to two days after emergence. After mating, the females were confined in large sleeves of black silk organza tied on to branches of growing citrus plants; the insects were fed on sugar and water every two days. Laying began a day or two after copulation and the best results were obtained at a temperature of between 70~ and I00~ alternating with periods of cooler conditions. Some of the insects did best in the evening under artificial light, but in general nothing stimu- lated them so much as sunshine. There was very great variability in the fertility of the females; sdme did not lay at all, and when they did the numbers of eggs varied fi'om very few up to about 80. The caterpillars were fed on citrus plants, but in many broods there was a high mortality among small larvae, which died before even starting to eat. H. Papilio glaucus This butterfly has two forms of female; one is yellow and non-mimetic and the other is black, mimicking Battus philenor (see Plate 2). The males are always monomorphic and yellow (see Plate 3). We obtained the original stock of pupae of this butterfly fi'om the Chicago district where Dluhy (personal communication 1955) states that about 90~ of the females are black and 10% yellow; northwards there is a steep dine into Canada, where 100% of the females are yellow. To the South very nearly all become black, except in FIoi-ida, where there is a race consisting almost entirely of yellow females. P. glaucus does not occur west of the Rocky Mountains except perhaps in the extreme North where its western relatives are not found. From the western U.S.A. we were able to obtain a few pupae of two of these monomorphic relatives, P. rutuhts and P. eurymedon, neither of which has a black female form. We bred the butterfly at room temperature or a little above throughout the spring and summer months but overwintering pupae were kept in an unheated attic. Under these conditions tlae butterfly was double-brooded in 1955, but in 1956, owing to the late start to the season, we only obtained a partial summer brood. Liriodendron (tulip tree) w;as tbund to be by far the best food-plant for the larvae, but magnolia may also be satisfactory, though we have as yet insufficient evidence on this point. When lilac was used the larval mortality was always high. Pairing was invariably carried out by the method of hand-mating and we made every effort to mate the same male both to black and to yellow forms of female. The females laid on Liriodendron and magnolia and after several techniques had been tried it was found that sleeving them on the food plant, as with P. dardamts, was the best method. There was again marked variability in the fertility of the females, but neither form appeared more fertile than tlle other. TEXT--TABLE 1 The table gives the fbrm (C=f. cenea, H,=f. hippocoonides, L=f. ldghi, T=f. trophonius), the brood number of the female, and Ihe brood number of the male where known. When the male has been successfillly mated more than once it is given a Greek letter, so that broods having a common father can be recognised,. Where wild parenls have been used the letter "w" is entered against them. Where the insect is of unknown parentage the letter "u" is used, C. A. CLARKE AND P. M. SI-IEPPARD Table I. Breeding data in Papilio dardanus C ~l: 6e'tlga~ H=f. hippocoonides, L=L leighi, T--l: trophonius, w=wild, u-~unknown. Greek letter denotes multiple mating of males. OFFSPRING OFFSPRING O 0 6 o~ 5 "o ~'O ~ G"a- ~ O+ Z ZC Z~ ~-o ~O'" ~o ~o ~ -~ o ~ o o ,'- wild H 440 larvae 2 0 0 1 693 601 601 2 0 0 0 wild H 460 Iarvae 1 i 0 0 694 601 601 2 0 2 0 C H 467 w w 5 0 0 3 702 609 u ? 0 0 3 C H 469 w w 0 0 1 0 705 609 601 12 0 4.