The maintenance of species differences across a Heliconius hybrid zone

Abstract

A contact zone between Heticonius erato and H. himera in southern Ecuador provides an opportunity to study the transition from races to species in Heliconiine butterflies. Genetic differentiation at 30 allozyme loci (D = 0.28) is five times greater between himera and erato than among races of erato (D <0.062). Analysis of restriction fragment length polymorphisms in an 800-bp region of the mitochondrial genome shows fixed differences between the species.Despite 5-10 per cent hybridization per generation, these mtDNA, allozyme and colour pattern differences are in almost complete linkage disequilibrium throughout the contact zone. In mixed populations, there was no consistent evidence for convergence of himera and erato allozyme allele frequencies, and only four individuals out of 383 examined showed evidence for interspecific mtDNA gene flow. Linkage analysis of backcross broods showed that the 11 allozyme and two colour pattern loci, which are markedly divergent between the species, map to eight of 21 chromosomal linkage groups. Therefore, 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 allozyme 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 the study of speciation.

Genetic Evidence for Assortative Mating Between 13-Year Cicadas and Sympatric "17-Year Cicadas With 13-Year Life Cycles" Provides Support for Allochronic Speciation

“…The M. tredecim/neotredecim contact zone could be similar to that observed by Jiggins et al (1997) for Heliconius erato and H. himera. In these two species, parental genotypes at a number of unlinked gene loci remain in tact despite some hybridization (5-10%) and no apparent hybrid inviability.…”

Natural hybridization in heliconiine butterflies: the species boundary as a continuum

“…In three cases, H. cydno × H. pachinus , H. erato × H. himera , H. charithonia × H. peruvianus , the hybridizing taxa are parapatric. We consider these to be species pairs operationally on the grounds that intermediates are rare in areas of contact compared with parental forms (Additional File 2) [37,46,51-55]. …”

“…Although the initial hybridization can be difficult due to strong assortative mating, genes from hecale , atthis , ismenius , melpomene and cydno in the melpomene-cydno- silvaniform group can apparently be mixed together at will in the laboratory (Additional Files 1, 2; [47]). The similarity of allelic frequencies at some loci, and the strong differences at others in H. himera and H. erato can be explained by selective gene flow at some loci [51]. Two recent studies [54,55] have demonstrated sharing of some, but not all molecular markers between Heliconius melpomene , Heliconius cydno , and H. pachinus .…”

Tests of reproductive isolation among species in the Fundulus notatus (Cyprinodontiformes: Fundulidae) species complex

“…Reproductive barriers are typically classified based on the stage in the life cycle that they occur (prezygotic or postzygotic) or the factors that act to reduce hybrid fitness (endogenous or exogenous selection) (Jiggins et al 1997;Coyne and Orr 1998;Jiggins and Mallet 2000;Bronson et al 2003;Hurt et al 2005). Some have suggested strong mate selection (Mendelson 2003) or body size differences (McKinnon et al 2004) will facilitate development of prezygotic barriers.…”