METHODS, MODELS, METAPHORICAL AIDS

Background

Scale vs. Pattern-level control in development

The biochemical events involved with the maturation and pigmentation of butterfly scales during development is relatively well-known for Papilio, a genus famous for dramatic intraspecific pattern variation, and for Precis, a model for the study of eye spot development (Koch et al. 1998, ffrench-Constant and Koch this volume). With respect to the chemistry of different scale types, Gilbert et al. (1988) and Koch (1993) showed that brown and red pigments in Heliconius are ommatins probably derived from kynurenine. These along with melanin account for blacks, many of the brown, red, and yellow scales of other butterfly groups (see ffrench-Constant and Koch this volume). However, Gilbert et al. (1988) went further to document the correlation of scale ultrastructure and pigmentation in Heliconius and the fact that white scales in this genus are structural white, lacking any pigment. A recessive allele adds 3-OH-Kynurenine to convert these scales to yellow.

The fact that the white/yellow scale morpho-type constitutes an important element of Heliconius wing pattern ground plan for mimetic evolution is critical to the discussions of genetics to follow. The model developed for scale differentiation for Heliconius by Gilbert et al. (1988, Figure 7) reflects an order of appearance of pigment types as scales mature during late development corresponding to that documented by Koch et al (1998) for other taxa. However in contrast to their findings for Papilio, our observations of Heliconius indicate that the class of pigment within a scale morpho-type does not vary progressively over time.

In general however, there may be few features that distinguish Heliconius from butterflies like Precis with respect to late events in the maturation of scales. The more difficult and over-arching issue is the question of how genes create patterns of differentiated scales across the wing. Even with the same or similar basic biochemical mechanisms for pigmenting scales at the terminus of adult development, there may be great variety in how genes determine the dominant patterns that attract us to different butterfly taxa. Thus, to know that eyespot patterns in nymphaline butterflies are somehow determined by the position of morphogen sources and sinks begs the question of how earlier acting genes determine the spatial arrangements of those critical foci. ffrench-Constant and Koch (this volume) provide an integrated and compelling model for how pattern fields and boundaries might be determined in Papilio. The question of whether the genetic control of wing pattern in Heliconius is qualitatively distinct from that proposed for other taxa will be discussed further in the conclusion of this paper.