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1958

The Lepidopterists' News

7

SEXUAL MOSAICS IN PIERIS

by S. R. Bowden

The green-veined wThite butterfly, Pieris napi L., has had attached to it by nature and lepidopterists (working apparently in close collaboration) a wide variety of problems, which will hardly reach final solution in the near future. One of these problems is set by its local forms, its more or less distinct subspecies and the separate species closely allied to it. These are related at the present day in such a manner that decisions on status depend entirely on definitions, and speculation on past history is unavoidable.

Here we have, in Europe alone, perhaps about ten subspecies of the Pieris napi-bryonice group. The taxonomists, quite reasonably, would like to regard the whole of them as subspecies of Pieris napi, for though P. Bryonia Och-senheimer is biologically almost entirely distinct in the central Alps, hybridization has certainly been going on elsewhere and one can find no sufficient reason for drawing a firm line in one place rather than another. This does not mean that it is useless to investigate the degrees of relationship of the various local races or subspecies — quite the contrary is true. But one cannot be surprised if different tests give contradictory or at least inconsistent results.

Pieris napi is so easily paired in captivity that an obvious line of approach is through crossings of the various forms. Persistent inability to obtain F2 hybrids would indicate, with fair certainty, full specific separation. It can be said at once that none of the European races with which we have worked is distinct enough from any other to produce this unequivocal result. The writer has carried the following subspecies crosses as far as the F3, though with some rather alarming fluctuations in fertility:

adalwinda Fruhstorfer (Lapland) X napi (Britain)                =AN

bryonies (Swiss Alps) X adalwinda                                             =BA

neobryonice Sheljuzhko (Karnten) X adalwinda                       =KA

flavescens Wagner (nr. Vienna) X bryonies                               =FB

However, the cross

bryonies (Swiss Alps) X napi (Britain)                                     =BN

could not be made to yield an F3 brood, though it might have done so under slightly more favourable conditions. In general, it was impossible to get fertile eggs from the F2 hybrid females and we were forced to back-cross the F2 hybrid males to bryonice; this back-cross was then carried as far as F3 in its own line, that is to

[bryonia X (bryonia X napi)2]3                               =[B.(BN)2]3.

We have described elsewhere (1956) the general course of this breeding work up to 1954, with remarks on fertility, inheritance of markings and other matters. The quite unanticipated tendency of the females in certain hybrid broods to emerge before the males is dealt with in three papers (1957). Here we wish to summarize our experience of the incidence of mosaic gy-nandromorphism.

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Vol.12: nos.1-2

Since expense will forbid the accompaniment of this note by photographic illustrations, it may be well to make clear what is meant in the present connection by mosaic gynandromorphism. Halved gynandromorphs, in which the left side of the insect is entirely of one sex and the right side of the other sex, are comparatively easily explained, as for example by Ford (1945: 193). They are also rare, both in the wild and among bred insects, and have never occurred in the writer's broods. Sexual mosaics, on the other hand, carry apparently random patchworks of wholly male and wholly female areas, and range from insects almost entirely male to insects with only a small streak of male scaling. In typical Pieris napi, female areas on an otherwise male butterfly are revealed by the different distribution of black scales, but in bryonies, adalwinda, and their hybrids with napi an ochreous yellow ground-colour, as well as dusky shading, may throw female areas into prominence.

The relatively frequent occurrence of "gynandromorphs" among species-hybrids was noted long ago, for example by Standfuss (1900), writing of experiments made among a number of genera, including Smerinthus, Bombyx, Saturnia, Actias and Zygana:

"Besides males capable of limited and individually varying degree of reproduction, and females mostly sterile.....there appeared among certain secondary hybrids

[back-crosses, etc.] a relatively large number of gynandromorphous specimens......

llIf we count all the secondary hybrids together, they amount to 282, among which was the considerable number of 27 gynandromorphic specimens, which were divided

over at least 20 broods. When we think that after careful calculation......A.

Speyer came to the conclusion that there was only one gynandromorphic specimen to

every 30,000 typical ones in nature......it would be absurd to regard this high

percentage......as a mere matter of chance.

"The percentage of gynandromorphic specimens among primary hybrids is without the slightest doubt infinitesimal, compared with their occurrence among secondary hybrids. Tt is, however, relatively higher than among individuals of pure origin."

It is now widely accepted that even race-crosses within a species may show "intersexuality": differences in balance between the autosomes (or the cytoplasm) and the X-chromosomes cause an insect originally of one sex to change over to the other during development. The best-known publications on intersexuality in Lepidoptera are those of Goldschmidt (1933), who worked on Asiatic and European races of the gipsy moth, Lymantria dispar L. He reported that intersexual females (i.e. intersexes with an XY constitution) had wing markings of male colour unless the degree of intersexuality was very slight indeed, but that intersexual males (with XX constitution) had wings on which streaks and patches of male and female scales formed an unblended mosaic. Further, a similar mosaic appearance was presented even by female intersexes when they carried a gene found only near Gifu, Japan. No final explanation of the Gifu XY intersexes was offered and that given for the equally random distribution of male and female parts in supposedly XX intersexes was hardly satisfactory. These two classes appear indistinguishable in Goldschmidt's figures, though dissection, or examination of the insect in the pupal stage, furnished a distinction when interpreted.

1958                                             The Lepidopterists' News                                                  9

TABLE 1 — Mosaics in Subspecific Hybrids

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Bowden: Sexual mosaics

Vol.12: nos.1-2

Goldschmtdt's Lymantria broods provided amply for dissection of genital and other structures at all stages — work which the present writer was not competent to carry out on his relatively scarce and less predictable material. Whether the Pieris mosaics are or are not intersexes in Goldschmidt's sense must be left undetermined for the present.

Jt will be convenient to submit a numerical summary (Table I) relating to all the inter-subspecific hybrids obtained from our broods of 1951 to 1955 inclusive. For shorthand purposes, the various subspecies are represented by capital letters as indicated earlier, and the female parent is placed first; the symbols are then combined pseudo-algebraically: (AN)3 represents the F,3 hybrid and thus differs from (AN)2.AN. According to some authors the British populations which we are here calling "napi" belong to one or two subspecies distinct from the central European Pieris napi napi.

Though for the present purpose it is immaterial whether the butterflies did or did not succeed in emerging from their pupae, in general only emerged butterflies have been counted. Only in one known case (one of those recorded for the brood 1955-/?) did a sexual mosaic fail to emerge from its pupa.

There is some reason to think that mosaics may generally occur more rarely among bryonice-napi YA hybrids than our figures would suggest. Such hybrids have been raised by Main (1909), Hesselbarth (1952), Easton (1953), Petersen & Tenow (1954), Meyer (1956) and no doubt by many others, but no mosaics are mentioned in the accounts that we have seen.

The figures show in the four Y1 broods marked with an asterisk a significant preponderance of males. Among hybrids there is often a deficiency of the heterogametic sex, but it does not here occur equally in all broods of similar origin. There is a hardly significant deficiency of females also in the B.(BN)2 brood 1953-/, but in all the other broods of this group the balance falls the other way.

The (BN)2. BN brood, 1952-£, is worthy of particular note. This was the only brood produced by a (BN)2 female; many eggs failed to hatch, but there were few losses afterwards. Apart from a very small patch of male scaling on one butterfly, it appeared to be an all-female brood. One is inclined to wonder whether some of the insects should have been male and were in fact male intersexes, or quasi-gynandromorphs which had lost X-chromosomes over their whole bodies instead of in isolated patches of tissue.

What conclusions can be drawn from the occurrence of these mosaics among our hybrids?

Firstly, the opinion of Standfuss that "gynandromorphs" are apt to occur in hybrids, and particularly in "secondary" hybrids, is amply confirmed in Pieris. We need not venture far into cytological speculation, but it is clear that the F2 genotypes are likely to be worse balanced than the Ft, while the F3 and back-crosses offer endless opportunities for disequilibrium.

Secondly, only the napi-adalwinda crosses produced no mosaics. This, for what it is worth, tends to confirm Petersen's opinion (1947, 1949) that adalwinda is not a separate species.

1958

The Lepldopterists' News

11

On the other hand bryonice-flavescens crosses produced mosaics in the F3) although flavescens has been regarded as a subspecies of bryonite. However, it does seem likely that there is a great deal more napi in the make-up of the remarkable population called flavescens than Muller and Kautz (1938) would admit.

The hybrids of adalwlnda, long called "northern bryonice", with Swiss bryonies produced nine mosaics, eight of which occurred in one F3 brood accompanied only by a single male. Its hybrids with the multi-brooded subspecies neobryonice also produced one mosaic in the third generation. Can we adduce this as additional evidence that adalwlnda is specifically distinct from bryonite? Unfortunately for the argument, mosaics apparently similar occur at times within a group of closely related populations.

Since 1948 we have maintained a purely British stock carrying the re-cessives albino (evanescent black markings) and Head's yellow (Head, 1939). Each of three yellow females was crossed with Hertfordshire wild stock; in one line a sexual mosaic appeared among the F2 broods. Other yellow females were paired with reputed Scottish males; two sexual mosaics appeared in one of the F2 broods. We have since used a reticulate system of breeding which results in complex relationships while avoiding excessive inbreeding; only those broods relevant to the present discussion appear in Fig. 1. After a further five or six generations in which no mosaics were detected we obtained (again in an F2) the brood 195 I-o, which could be expected to produce half yellows and one quarter albinos — and did actually yield the homo-zygotes in approximately these proportions. Twenty-two butterflies emerged the same summer, 5 females + 2 males + 15 sexual mosaics; 66 pupae went into diapause. At this point twenty-odd pupae were sent to Oxford for genetic research, but unfortunately the stock could not be maintained there. From the remainder emerged, the following spring and summer, 12 females + 7 males + 9 mosaics. Eighteen pupae died. The deficiency of males is hardly sufficient to suggest that the mosaics may have been genetically of this sex. Some of them had indeed largely male wings, but in these the genitalia appeared abnormal (no dissections were made) ; in one butterfly entirely male markings were accompanied by abnormal genitalia. There is, of course, no reason to expect mosaic structure to be confined to the wings. In all, about half the insects were visibly mosaics. It proved possible to pair the normal insects among themselves and several broods were raised, but none of these nor in their successors have mosaics again appeared.

One point of significance in connection with the brood 1951-0 is that mosaic inlays always resembled the rest of the insect in respect of albinism and ground-colour: there were no mosaics of black-albino markings nor of yellow-white ground. The patchwork was one of sex only.

How much of the originally Irish blood remained in the yellow stock by 1948 (after Head's death) we have no means of knowing, but allowing only for Hertfordshire males introduced by ourselves the autosomal con-

12

Bowden: Sexual mosaics

Vol.12: nos.1-2

I94S

1946

t?49

1949

1949

1950

1950

1951

1951

Fig. 1. Relationship of British Pieris napi broods containing mosaics (shown by combined $ and 9 symbols).

stitution of 1951-o should have been 1/32 Scottish, % Irish and the remainder English. The cytoplasm of 1951-0, like that of both 1949-6* and 1949-or, was Irish (ex Head's stock). The occurrence of mosaics in an F2 generation could be connected with the fact that after a cross has been made a further generation is required to bring the male line's XX into the female line's cytoplasm. On the other hand, both grandparents of 195l-o had "Irish" cytoplasm.

We cannot suppose that the British races present total genetic differences, or differences of genetic valency, of the same order as the various European subspecies that we have crossed; if they did so, we should expect more of our broods to have included mosaics. Though the simple operation of some recessive mosaic gene can be ruled out when the proportion is as high as 50 per cent, we have to consider whether some accessory gene, sex-linked or autosomal, may be required to provide a more complete explanation.

1958

The Lepidopterists' News

13

SUMMARY

Hybrids (Ft, F2, F3 and various back-crosses) between five European subspecies of the Pieris napi-bryon'ue group have been raised in the course of investigation of subspecific relationships. Twenty sexual mosaics have occurred, spread over eleven out of seventy broods; apart from three among Ft bryonice X napi and one in F2 bryonue X adalwinda the mosaics have appeared in the F3 or in more-or-less complex back-crosses. One small brood contained eight mosaics, one male and no females; another brood contained fifteen females, one almost entirely female insect and no males.

The occurrence of at least twenty-four sexually abnormal insects among fifty in a single brood of British P. napi is reported ; these were mosaics in sex only, though autosomal recessive genes were present capable of producing dichroism of ground-colour and of markings.

The results are tentatively discussed, but the relation of mosaic gynan-dromorphism in Pieris to intersexuality (in Goldschmidt's sense) is left open.

References

Bowden, S. R., 1956. Hybrids within the European Pieris napi L. species-group. Proc.

So. London ent. nat. Hist. Soc. 1954-55: 135-159. ______ 1957. Diapause in female hybrids: Pieris napi adalwinda and related subspecies.

Entomologist 90: 247-254; 273-281. [Includes references to earlier papers.] Easton, N. T., 1953. Private communication. Ford, E. B., 1945. Butterflies. Collins, London. Goldschmidt, R., 1933. Lymantria. Bibliogr. genet. 11: 1-186. [This paper, in English,

includes a bibliography of the author's earlier writings.] Harrison, A., & H. Main, 1909. Experiments in crossing British Pieris napi with Swiss

Pieris napi, var. bryonue. Trans, ent. Soc. Tendon 1908: Proc. 87-88. Head, H. W., 1939. The history of Pieris napi var. citronea. Entomologist 72: 179-180. Hesselbarth, G., 1952. Bemerkungen zu Pieridenzuchten 1950-1951. Trans. 9th int.

Congr. Ent. 1: 172-176. Main, H. — see Harrison & Main. Meyer, J. H., 1956. Private communication.

Miiller, L., & H. Kautz, 1938. Pieris bryoniae O. and Pieris napi L. Vienna. Petersen, B., 1947. Die geographische Variation einiger fennoscandischer Lepidop-

teren. Zool. Bidr. Uppsala 26: 330-531.

------....., 1949. On the evolution of Pieris napi L. Evolution 3: 269-278.

Petersen, B., & O. Tenow, 1954. Studien am Rapsweissling und Bergweissling.

Zool. Bidr. Uppsala 30: 169-198. Standfuss, M., 1900. Synopsis of experiments in hybridization and temperature made

with Lepidoptera up to the end of 1898. [transl. from German]. Entomologist 33:

161 etc.; 34: 11 etc.

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