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1954

The Lepidopterists' News

163

THE INHERITANCE OF HINDWING DISCAL SPOT COLOR

IN COLIAS PH1L0DICE

by Charles L. Remington

Instances of sustained conspicuous variation among the individuals of a single interbreeding population are of special interest to evolutionary biologists. Ford (1953) has recently reviewed the state of knowledge of this "balanced polymorphism" in the Lepidoptera. The conspicuous variability in species of the genus Colias, especially in North America, is familiar to butterfly collectors but has been confusing and often controversial. Some of the sym-patric variation in Colias is due to natural interspecific hybridization or to environmental effects, but there are certain characters which vary within each species and which are controlled by one or a few pairs of genes. The most familiar is the ground-color of the female, in which the "alba" form is produced by a dominant sex-limited gene and colored females are homozygous for the recessive allele. I have summarized our knowledge of genes and polymorphism in Colias elsewhere (Remington, 1954).

The color of the discal spot on the upperside of the hindwing has received very little attention, although its great variability in some species has been known for a long time. Every species of Colias and its near relative Zerene has the hindwing discal spot. In the orange species it is rather uniformly orange or orange-red except in "alba" females, where polymorphism is then visible (e.g., C. eury theme Bdv. and C. lesbia Fab.). Some yellow species have the spot consistently pale yellow (e.g., C. palasno L.) or occasionally have an orange-tinted spot (C. alexandra Edw., C. scudderi Reak., C. behrii Edw.). However, C. philodice Latr. and some other yellow species show marked polymorphism in most or all populations; individuals having the spot pale yellow fly with others having the spot deep orange, and the intermediate grades of color occur at the same time.

During our studies at Yale of Colias genetics, we have reared a number of broods of C. philodice from Connecticut which give some evidence of the inheritance of the color of the spot. The arrival of the J. H. Gerould Collection at Yale has permitted the analysis of additional broods of C. philodice from New Hampshire. These are included in the table. Other work made it not feasible to take the time required to make a photometric analysis with precise instruments, and an arbitrary color standard was established, with an index series of specimens for frequent reference. The palest spot, with no orange or red scales, was designated "yellow." The reddest spot (near "Orange Chrome" of Ridgway, 1912) was called "orange." Three evenly spaced grades between "yellow" and "orange" were designated "pale semi-orange," "semi-orange," and "deep semi-orange."

One of the first discoveries was the difference between the spot colors in males, "alba" females, and yellow females. The figure shows graphically the fact that the discal spot is much paler in males than in females having es-

Table of hindwing discal spot color in Connecticut (58 to 75A-D) and New Hampsh

broods of Colias philodice*

#Hypbenated figures show in order rhe number of $ $, of "alba" 9 9, and of yellow (aa) 9 9; thus 14 "alba" 9 9 , 6 yeilow 9 9 with the discal spot yellow. Aa = "alba"; aa = yellow 9.

1954

The Lepidopterists' News

165

sentially the same autosomal genotype and that the "alba" gene in females causes the spot to be paler than in homozygous recessive aa females. The histogram is for the largest brood available (Ft of 9 1920:); the same trend is found in the other broods (see Table). This spot-color relationship be-

v e IIow ??

1 p/fco

#f

2/

27

2*

AT

7S

J4

yellow Pale. f                  semi-

deep semi-

oronge.

semi-orange

Proportions of discal spot color types in GEROULD'S Brood 1920i. Each block shows the percentage of 3 $ or "alba" 9 9 or aa 9 9 showing the indicated color (e.g., 0.6 of all $ $ had the spot yellow).

tween males and yellow females is the opposite of that for ground-color, in which the female has distinctly paler yellowness than does the male. The following equivalents seem to be justified:

Genotype I: $ pale yellow — "alba" 9 yellow — aa 2 yellow Genotype II: $ yellow = "alba" 9 pale semi-orange = aa 9 semi-orange Genotype III: $ pale semi-orange = "alba" 9 semi-orange = aa 9 deep

semi-orange Genotype IV: S semi-orange = "alba" 9 deep semi-orange = aa 9 orange Genotype V: $ orange = "alba" 9 orange — aa 9 red-orange. Genotypes I and V seem to be rare among several hundred wild yellow {aa) females examined from eastern and western U.S.A. and Canada.

The genetic control of the different spot types is not entirely clear. The principal difficulty comes from the complete lack in my data of these important crosses: Type I X Type I; Type V X Type V; and Type I X Type V. The data available suggest that there are one or two pairs of genes principally controlling the color of the hindwing discal spot. As with the general groundcolor of the wings, it is likely that environmental effects and modifier gene complexes influence the phenotypic expression of the basic, or "switch," genes for yellowness or redness of the discal spot. These "basic" genes seem to be acting as blending factors, with alleles for both yellow and orange expressed phenotypically.

166                                   REMINGTON: Colias Discal Spot Color                      Vol.8: no.6

Komai and Ae (1953) suggested that the color of the discal spot in males of C. erate poliographus Motsch. may be controlled by a single pair of genes, with orange dominant over pale yellow. Their data were very limited, and they did not report the color of the spot in females, so it is not possible to interpret their work on the basis of the present studies.

A painstaking study of "alba" females of both C. philodice and C. eurytheme was carried out in the hope of finding discal spot differences by which heterozygous (Aa) "alba" females could be distinguished from homozygous (AA) "alba" females. This attempt was unsuccessful. However, it may be possible to find differences if a large series of absolutely certain homozygous AA "alba" females is available for comparison with known heterozygotes, such as our brood #61, which includes 32 "alba" females all known to be Aa.

No attempt has yet been made to analyze the geographic correlation of the relative frequency of the discal spot types. Hovanitz (1945) found the spot darker in Alaskan than in more southern populations of C. philodice.

It has been suggested by some that the color of the discal spot is a species-recognition character for C. philodice and its close relative, C. eurytheme. The latter has the spot red-orange in males and in aa (orange) females. The widespread occurrence of orange spots in C. philodice might be considered to have been due to introgression of the orange gene into C. philodice following recent hybridization with C. eurytheme. This possibility seems to be ruled out by the fact that individuals with orange discal spots are even more numerous than those with yellow spots among the series of New England C. philodice collected prior to the arrival of C. eurytheme in New England and now preserved at Yale and in the Museum of Comparative Zoology at Harvard.

This paper has been extensively revised since it was cited in my review (Remington, 1954). When the first version was written, the Gerould Collection was not available, and certain Connecticut broods had not yet been mounted for study. It had appeared at first that the allele for yellow is recessive to the allele for orange.

Summary

In Colias philodice from New England the yellow or orange color of the discal spot on the upper side of the hindwings appears to be controlled by one or two pairs of autosomal genes expressed as blending factors. With identical genotypes, males have phenotypically the lightest (yellowest) spot; it is deeper in "alba" females and deepest (reddest) in yellow females.

References Ford, E. B., 1953. The genetics of polymorphism in the Lepidoptera. Advances in

Genetics 5: 43-87. Hovanitz, W., 1945. The combined effects of genetic and environmental variations

upon the composition of Colias populations. Annals Ent. Soc. Amer. 38: 482-502. Komai, T. & A. S. Ae, 1953- Genetic studies of the pierid butterfly Colias hyale polio-

graphus. Genetics 38: 65-72. Remington, C. L., 1953. The genetics of Colias (Lepidoptera). Advances in Genetics

6: 403-450, 6 figs. Ridgway, R., 1912. Color standards and color nomenclature. AA pp., 53 pis. Publ. by

author, Washington.

Osborn Zoological Laboratory, Yale University, New Haven 11, Conn., U. S. A.