Lawrence Gibbs and Orley R. Taylor
Department of Entomology
University of Kansas
Though one's vision of the Monarch butterfly conjures up an image of a brilliant orange and black winged-insect, there exists a rare variation of the Monarch that "pales" in comparison. A white Monarch, named nivosus by Lepidopterists, is grayish white in all areas of the wings that are normally orange (Vane-Wright 1993). Several authors (Stimson and Meyers, 1984; Vane-Wright 1993) have assumed that the white form results from the inability of the butterfly to synthesize the normal orange pigment, but this hypothesis has never been tested. The white Monarchs appear to be normal in all other respects giving rise to many questions. Why is this form so rare? Is nivosus selectively eaten by predators? Does it have difficulty finding mates? Is the genetic basis for this form the same in all populations? These, and many other questions, need to be answered. We hope to have answers to some of these questions in the coming year.
Distribution and Origin
White Monarchs (nivosus) have been found throughout the world, including Australia, New Zealand, Indonesia and the United States. Generally, nivosus is extremely rare with only a few being reported each year. The exception is Hawaii where its frequency has recently reached 10% (Vane-Wright 1997). Nivosus appears to have been in Hawaii since, at least, the end of the last century, and the allele responsible may have been present in the founder population of the mid 1800s (Vane-Wright 1993). White Monarchs have been reported elsewhere, from Washington, D.C., in 1896; Missouri, 1908; Pennsylvania, 1921; the coast of California, ca. 1980; Florida, 1996 (Minno 1996); Brisbane, Australia, 1980; North Island, New Zealand, 1985; and Ambon and Seram (in the Moluccas), 1906-1916 (Vane-Wright 1993).
The existence of white forms within other species of Danaus, such as the Austro-Oriental D. melanippus and all the races of the Wallacean species D. ismare, suggests that nivosus could easily have arisen independently in the widely separated Monarch (Danaus plexippus) populations (Vane-Wright 1986). An alternative hypothesis is that the nivosus Monarch found in Hawaii, Australia, New Zealand and Moluccas might have been the result of recombination of a rare but homologous allele, inherited directly from their American ancestors (Vane-Wright 1986).
In Hawaii, the nivosus phenotype (white morph) is inherited as an autosomal recessive (Stimson and Meyers, 1984). White-white crosses yield only white offspring, while crosses between these offspring and orange wild type Monarchs result in orange offspring which are presumed to be heterozygotes. Furthermore, as expected for an autosomal recessive allele, crosses between white Monarchs and heterozygous offspring yielded both white and wild type Monarchs at frequencies close to the predicted 1:1 ratio (Stimson and Meyers 1984). In most Monarch populations, the recessive allele for nivosus is extremely rare and most individuals carrying this allele are heterozygotes. The probability of a mating between two such heterozygotes which would yield progeny in a ratio of 3 wild type to 1 nivosus is so uncommon, unless there are brother/sister matings (Vane-Wright 1997), that nivosus is seldom encountered in the wild outside of Hawaii.
The color of a Monarch's wings is due entirely to its wing scales. The cuticle of the wing itself is usually colorless, and the color pattern is essentially a finely tiled mosaic of monochrome scales (Nijhout 1991). Each scale gains its respective color due to the presence of various pigments found within the scale. These pigments can be categorized as melanins, ommochromes, pterins, flavonoids, and bile pigments. The most common pigments are melanins, tyrosine derivatives (Nijhout 1991), and they range in color from the typical black to yellow, brown and even red (Kayser 1985). Ommochromes are derived from tryptophan and characterized by a red-to-brown color (Nijhout 1991). Pterins are white and yellow-to-red pigments derived from guanosine triphosphate, and have yet to be found in any family except for the Pieridae (Nijhout 1991). Flavonoids are very diverse in color, and likely to be derived from food plants by the larvae and ultimately deposited in the wings of the adult. Flavonoids are thought to be limited to mainly the more primitive butterfly families (Kayser 1985). Lastly, the bile pigments (like bilins and biliverdin) are derived from glycine and are typically blue in color (Kayser 1985). Although there are numerous studies on scale pigments, the pigment-forming pathway that is being affected in the nivosus Monarch remains to be shown.
Due to the ingestion of cardenolides from milkweed host plants as larvae, the adult Monarchs appear to be unpalatable to all but a few predatory animals. The orange and black color pattern is often thought to be a warning signal to predators. The rarity of the white morph in North America may be due to a case of mistaken identity by vertebrate predators who confuse the white Monarchs for palatable butterflies (Stimson and Meyers, 1984).
However, the levels of cardiac glycosides in the tissues of Monarchs in Hawaii are low, apparently because leaves of the major host plant, Calotropis gigantea (crown flower) contain a low amount of these compounds. Possibly because of the low cardenolid content, both the white and orange forms are preyed upon by birds called bulbuls, Pycnonotus jacosus and Pycnonotus cafer on the Hawaiian island of Oahu (Stimson and Meyers, 1984). Curiously, the frequency of the white morph in Oahu, has increased in the last 20 years and now approaches 10% in some areas (Stimson and Meyers, 1984; Stimson and Berman, 1990). This increase could be due to predation. At Monarch feeding and ovipositing sites more orange Monarch wings have been found than expected based on the frequency of the orange form in the population. This suggests that predation is lower for white morphs (Stimson and Bergman, 1990) and raises the possibility that the white form is more cryptic (hard to see) to the bulbuls than the orange form. This interpretation is supported by observations from other Hawaiian islands which lack bulbuls. The white form occurs at a relatively low frequency or is lacking on islands without bulbuls (Stimson and Bergman, 1990). Additional studies are needed to determine whether selective predation of white vs. orange Monarchs is due to levels of cardiac glycosides or the ability of the bulbuls to "see" the white form. We should also keep in mind that the increase in the frequency in nivosus could be due to other, as yet unrecognized, selective factors.
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