Morphology and Physiology of Larval Color Variation in the White-Lined Sphinx Moth, Hyles Lineata

Abstract

Animal color patterns occur for a variety of reasons and can have a broad impact on fitness. Additionally, the presence of multiple color phenotypes in a species can be ecologically advantageous. One must first understand the physiological mechanism behind color patterning and variable morphs to understand how this variation affects fitness or provides an ecological advantage. Multiple phenotypes produced by one genotype, including multiple color morphs, can be genetic or plastic and may evolve very quickly as an adaptive response to changing environments. Phenotypes that are genetically based, called polymorphisms, can be controlled by one to many genes, are heritable and may have genetic links to other traits. Multiple color morphs may also be determined by environmental conditions, such as temperature and photoperiod, or phenotypic plasticity. Here, I sought to determine the physiological mechanisms underlying the multiple color phenotypes of an abundant and widespread lepidopteran larva. Hyles lineata has a primary, basal coloration of yellow and green that can be observed throughout the larval body, most noticeably during the final instar. Also, this larva may have dark lateral dorsal stripes that vary in thickness during the juvenile stage. These stripes can be as minimal as a thin dotted line or so thick that the entire larval body appears black. The variation in both basal coloration and dorsal striping can be observed in the field and in the laboratory and seem to be independent of each other. I designed a series of experiments to determine which, if any, of these color patterns is genetically controlled and which, if any, is phenotypically plastic. To do so, I conducted several single-pair inheritance crosses to document the yellow to green larval color ratios expressed by their offspring. From these crosses, I was able to determine that the yellow and green coloration observed in larval H. lineata is, indeed, genetically based. Further, this trait is controlled by a single two-allele gene, green allele dominant, with contributions from modifier genes. A series of experiments testing variations in environments was also conducted. This included conditions of variable temperatures, photoperiod and combinations of the two. Larvae were exposed to these conditions from the second instar and the appearance of the dark dorsal patterning was noted upon the fifth instar. This patterning was found to be phenotypically plastic. While there was no variation in this trait observed under varying temperature conditions, there was an increase in stripe thickness with decreased photoperiod as well as with combinations of photoperiod and temperature. To identify and measure this variation in pigmentation, various methods were employed, from visual scoring and histological observation to chemical extraction. These methods were described and reviewed. The dark, plastic patterning observed as dorsal stripes in H. lineata was determined to be produced by the pigment melanin. Further, larvae that visually appeared to have greater dark patterning also had a higher percentage darkness when quantified with image analysis as well as a higher volume of extracted melanin, identified and quantified with the use of spectrophotometry. The mechanisms of the color variation observed in H. lineata are now better understood; this organism has a basal yellow/green coloration genetic polymorphism as well as a phenotypically plastic melanic dorsal lateral patterning. As it is known that both of these forms of phenotype variation can be linked to fitness and/or be advantageous, future research can focus on the potential adaptive consequences of these traits.