METHODS, MODELS, METAPHORICAL AIDS

Background

Scale vs. Pattern-level control in development

Heliconius wing as a computer screen

Regulatory Genes and Computer Screens

Tool Box Part 2. Windows, Shutters, and Walls: the wing patterns of the MCS Clade

What about the silvaniforms, the "S" in MCS ?

Although species in the silvaniform group are equal in distastefulness to other Heliconius (Chai, 1990), their wing pattern evolution is asymmetrically influenced, much as Batesian mimics would be, by pattern themes of the more distasteful ithomiine butterflies (see Plate 1b in Mallet and Gilbert 1995). As a consequence, some silvaniforms superficially appear to be using quite different rules for wing pattern from other members of the MCS clade. While genetic contact between this group and melpomene/cydno is probably very rare, F1 crosses of H. ismenius and both pachinus and cydno suggest that they are using the same tool box for pattern (Gilbert 1984, Plate 1A). A recent cross of Costa Rican ismenius and Ecuadorian cydno produced credible mimics of an Ecuadorian race of the pupal mating species H. hecalesia in the F1 generation (Figure 7 middle right). Needless to say, such quantum events, even though extremely rare, could move mimetic evolution along much more rapidly than gradual substitutions of new mutant alleles. Backcrosses of the F1 males of this cross to pure cydno indicates the possibility of introgression between silvaniforms and cydno/melpomene (personal observation).

Note that although F1 products of melpomene crosses with silvaniforms like H. hecale (Figure 1 top b and H. ethilla (not shown) are known, successful backcrosses are not known to me. Yet the link between cydno and the silvaniforms could provide a route for melpomene genes to move into the silvaniforms. The red gene of melpomene expressed in Costa Rican H. hecale (not shown) indicates that on the FW, the homologue to cydno/melpomene window/shutter system is restricted to a thin band just distal to the end of the discal cell (these hybrids received from R. Boender, Butterfly World). Based on what one observes in products of cydno X melpomene pseudo F2 broods (Figure 7, top, f and h, note both FW and HW), reducing the FW window in ancestors of Costa Rican hecale would account for the expanded marginal zone expressing standard nymphaline ground plan i.e., the white/yellow spots needed to mimic toxic ithomiines. Several H. cydno races in South America, including weymeri and hermogenes may track the Apocynaceae-feeding ithomiine, Elzunia (see Linares 1997a) by this mechanism. The variety of wing patterns in H. ethilla, H. hecale, H. ismenius, and especially the highly polymorphic H. numata (Brown and Benson 1975), suggests that the same pattern tool box, along with introgression, may be functioning to create novelty, mimicry, and rapid diversification across the entire MCS clade.