A conserved supergene locus controls colour pattern diversity in Heliconius butterflies

Abstract

We studied whether similar developmental genetic mechanisms are involved in both convergent and divergent evolution. Mimetic insects are known for their diversity of patterns as well as their remarkable evolutionary convergence, and they have played an important role in controversies over the respective roles of selection and constraints in adaptive evolution. Here we contrast three butterfly species, all classic examples of Müllerian mimicry. We used a genetic linkage map to show that a locus, Yb, which controls the presence of a yellow band in geographic races of Heliconius melpomene, maps precisely to the same location as the locus Cr, which has very similar phenotypic effects in its co-mimic H. erato. Furthermore, the same genomic location acts as a "supergene", determining multiple sympatric morphs in a third species, H. numata. H. numata is a species with a very different phenotypic appearance, whose many forms mimic different unrelated ithomiine butterflies in the genus Melinaea. Other unlinked colour pattern loci map to a homologous linkage group in the co-mimics H. melpomene and H. erato, but they are not involved in mimetic polymorphism in H. numata. Hence, a single region from the multilocus colour pattern architecture of H. melpomene and H. erato appears to have gained control of the entire wing-pattern variability in H. numata, presumably as a result of selection for mimetic "supergene" polymorphism without intermediates. Although we cannot at this stage confirm the homology of the loci segregating in the three species, our results imply that a conserved yet relatively unconstrained mechanism underlying pattern switching can affect mimicry in radically different ways. We also show that adaptive evolution, both convergent and diversifying, can occur by the repeated involvement of the same genomic regions.

Figure 1. Colour Pattern Diversity of H. numata, H. melpomene, and their Respective Co-Mimics

The upper half of the figure shows five sympatric forms of H. numata from northern Peru (second row, left to right: H. n. f. tarapotensis, H. n. f. silvana, H. n. f. aurora, H. n. f. bicoloratus, and H. n. f. arcuella) with their distantly related comimetic Melinaea species (Nymphalidae: Ithomiinae) from the same area (first row: M. menophilus ssp. nov., M. ludovica ludovica, M. marsaeus rileyi, M. marsaeus mothone, and M. marsaeus phasiana) [20]. The lower half of the figure shows five colour pattern races of H. melpomene, each from a different area of South America (third row: H. m. rosina, H. m. cythera, H. m. aglaope, H. m. melpomene, and H. m. plesseni) with their distantly related comimetic H. erato races from the same areas (fourth row: H. e. cf. petiveranus, H. e. cyrbia, H. e. emma, H. e. hydara, and H. e. notabilis). H. m. aglaope and H. e. emma are known as rayed forms, whereas H. m. rosina, H. m. melpomene, and co-mimics are known as postman forms. H. melpomene and H. erato are from divergent clades of Heliconius and are identified in the field using minor morphological characters, such as the different form of the red rays on the hindwing between H. m. aglaope and H. e. emma (third from left) or the arrangement of red versus white patches in H. m. plesseni and H. e. notabilis (first from right). Co-mimics H. numata and Melinaea spp. belong to different subfamilies of the Nymphalidae and have very different body morphology and wing venation. The phylogram on the left is a maximum-likelihood tree based on 1,541 bases of mitochondrial DNA (scale bar in substitutions per site, all bootstrap values over 99).

Figure 2. Crosses Used for Mapping the Yb, P, and Cr Loci

(A) Crossing scheme in H. melpomene showing segregation of tightly linked loci Yb and Sb (hindwing yellow bar and white margin, present in H. m. cythera, Yb^cYb^c Sb^cSb^c) in brood B033. Genotypes are shown on the figure. The hindwing image in the box has been manipulated to highlight the shadow hindwing bar characteristic of heterozygote genotypes. Segregation of the linked N locus controlling the yellow forewing band was followed in a different set of crosses not shown here (Table S1; Materials and Methods). (B) Crossing scheme in H. erato showing segregation of Cr alleles in brood CH-CH5; Cr controls the forewing yellow band (absent in H. e. cyrbia, Cr^cCr^c), and the hindwing yellow bar and white margin (present in H. e. cyrbia). The red-patterning gene D also segregates in this cross, but is unlinked to Cr; only progeny with a D^hiD^hi genotype are shown on the figure (Table S2; see also [24] for a figure of a similar cross showing all nine possible genotypes). (C) Crossing scheme in H. numata showing segregation of the P alleles in intercross B502. F1 parents are heterozygous for different alleles, thus producing four different genotypes in the progeny. P switches the entire colour pattern, with strong dominance between sympatric alleles. Other broods (not shown) segregating for the very same P^ele and P^sil alleles were sired by the same male or its full brother (Table S3).

Figure 3. Chromosomal Maps for Linkage Group Homologues in H. melpomene (LG15), H. numata (LG15), and H. erato (LG02)

Distances are in Haldane centimorgans. The alternative orders for P and a41 relative to Hm01 in H. numata are not significantly different (ΔLnL = −1.40). Similarly, most orders of N, Sb, Yb, and a41 in H. melpomene are not significantly different (from ΔLnL = −0.15 for the order a41–Yb–N–Sb– to ΔLnL = −0.77 for a41–N–Yb–Sb–). Finally, the two orders for Cr and GerTra in H. erato are also equally significant. Therefore, we here show the most likely gene orders but cannot exclude that the colour loci are on the other side of the anchor loci a41, Fox, or GerTra. In contrast, anchor loci order GerTra–RpP40–Hm01–Hm08 is robust, with alternative orders significantly worse (ΔLnL < −2), although the relative placement of RpL22 and eIF3-S9 is uncertain in H. melpomene and H. erato (ΔLnL > −2).

Figure 4. Annotation of Clone AEHM-41C10 from the Heliconius melpomene BAC Library

The region is situated on LG15 in the H. melpomene genome [22]. The sequence contains open reading frames of strong homology to 12 reported genes, three of which appear to be retrotransposon-associated coding regions (dotted boxes). Also highlighted in double frames are the a41 marker, which was used in H. numata and H. melpomene crosses and to isolate the clone from the library, and the Rabgeranylgeranyl transferase gene, used as a marker in H. erato crosses.