Summary of research
Selection and the nature of the species boundary in Heliconius 
23 April 2001
Research group:
Margarita Beltrán
Vanessa Bull
Dr. Chris Jiggins
Dr. Mauricio Linares
Prof. James Mallet
Russell Naisbit
Collaborators: 
Dr. Biff Bermingham
Dr. Mauricio Linares
Dr. W.Owen McMillan
Dr. David Heckel
 
Grant support: Natural Environment Research Council


Our overall objectives were to investigate speciation by measuring the natural selection keeping compatible species apart.  This research is fundamental to an understanding of the origin of species, and of the maintenance of biological diversity in general.  Because of our research, Heliconius has become an important model system for speciation research.

A pair of sister species were investigated: Heliconius melpomene and Heliconius cydno.  These butterflies are well studied ecologically and genetically, and, because they form part of a rapidly radiating tropical group, are excellent model organisms for studies of the ecology and genetics of speciation.  These species remain distinct although they overlap and hybridize occasionally throughout the rainforests of Central America and western South America. We have answered the following questions:

1) What is the genealogy of Heliconius cydno, H. melpomene, and related species? We have used nuclear and mitochondrial DNA sequences to develop a well-supported phylogeny of the group that disproves an earlier estimate.  Between closely related species that hybridize, such as cydno and melpomene, there is evidence for “naturally occurring genetic modification”.  In particular, the mannose phosphate isomerase gene appears to have been transferred across the species boundary relatively recently.

2) How does mating discrimination between species evolve?  Heliconius cydno and melpomene differ strongly in their colour pattern: they each belong to different “mimicry rings” of butterflies that are unpalatable to birds.  We have shown for the first time that the evolution of mimicry is a major cause of speciation, due to the effect of colour pattern mimicry on mating discrimination.  We have also demonstrated that the two species have diverged more in mate recognition where they overlap than in non-overlapping populations, a pattern expected if there has been adaptive evolution to avoid deleterious hybridization (a process known as “reinforcement”).  Reinforcement has been shown previously only in a handful of convincing cases.

3) What is the nature and strength of selection against hybrids?  F1 hybrid females between the two species are sterile, while all male hybrids are fertile.  In butterflies, the females are chromosomally  heterogametic (WZ; equivalent to XY males in humans, for example), and our result thus conforms to Haldane’s Rule, named after JBS Haldane who first noted that the heterogametic sex suffers more incompatibilities in crosses between species. To our surprise, we have also uncovered F1 female hybrid sterility (in only one direction of cross) between geographic populations within one of the species, H. melpomene.  Both male and female hybrids between cydno and melpomene suffer in sexual selection: they are discriminated against by individuals of both species.  We are currently performing field and laboratory studies to measure predator selection against hybrids due to colour pattern, which is non-mimetic in hybrids.

4) Where do species differences map in the Heliconius genome?  We have shown that Haldane’s Rule sterility of hybrid females maps to the sex (Z-) chromosome, a result which supports the Muller/Turelli/Orr “dominance theory” of Haldane’s Rule.  We are developing a genome map of Heliconius which will enable genomic studies of colour pattern loci, sterility genes, loci involved in mate choice, and ecological traits involved in speciation.



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