THE MAINTENANCE OF SPECIES-DIFFERENCES ACROSS A HELICONIUS HYBRID ZONE

A contact zone between Heliconius erato and H. himera in southern Ecua dor provides an opportunity to study the transition from races to spec ies in Heliconiine butterflies. Genetic differentiation at 30 allozyme loci (D = 0.28) is five times greater between himera and erato than a mong races of erato (D<0.062). Analysis of restriction fragment length polymorphisms in an 800-bp region of the mitochondrial genome shows f ixed differences between the species. Despite 5-10 per cent hybridizat ion per generation, these mtDNA, allozyme and colour pattern differenc es are in almost complete linkage disequilibrium throughout the contac t zone. In mixed populations, there was no consistent evidence for con vergence of himera and erato allozyme allele frequencies, and only fou r individuals out of 383 examined showed evidence for interspecific mt DNA gene flow. Linkage analysis of backcross broods showed that the 11 allozyme and two colour pattern loci, which are markedly divergent be tween the species, map to eight of 21 chromosomal linkage groups. Ther efore, barriers to gene flow are not restricted to just a few strongly selected loci. Although analysis of population structure shows little evidence of interspecific gene flow, strong differences between alloz yme loci in levels of divergence suggest that selection and gene flow may affect loci in different ways. Hybrid zones such as this, in which divergent genotypes coexist, should provide good model systems for th e study of speciation.