What can hybrid zones tell us about speciation? The case of Heliconius erato and H. himera (Lepidoptera: Nymphalidae)

Article (PDF Available)inBiological Journal of the Linnean Society 59(3) · November 1996with80 Reads
DOI: 10.1111/j.1095-8312.1996.tb01464.x
Abstract
To understand speciation we need to study the genetics and ecology of intermediate cases where interspecific hybridization still occurs. Two closely related species of Heliconius butterflies meet this criterion: Heliconius himera is endemic to dry forest and thorn scrub in southern Ecuador and northern Peru, while its sister species, H. erato, is ubiquitous in wet forest throughout south and central America. In three known zones of contact, the two species remain distinct, while hybrids are found at low frequency. Collections in southern Ecuador show that the contact zone is about 5 km wide, half the width of the narrowest clines between colour pattern races of H. erato. The narrowness of this cline argues that very strong selection (s ≈ 1) is maintaining the parapatric distributions of these two species. The zone is closely related with a habitat transition from wet to dry forest, which suggests that the narrow zone of parapatry is maintained primarily by ecological adaptation. Selection on colour pattern loci, assortative mating and hybrid inviability may also be important. The genetics of hybrids between the two species shows that the major gene control of pattern elements is similar to that found in previous studies of H. erato races, and some of the loci are homologous. This suggests that similar genetic processes are involved in the morphological divergence of species and races. Evidence from related Heliconius supports a hypothesis that ecological adaptation is the driving force for speciation in the group.
    • "Reproductive isolation (either pre-or post-zygotic) can also be reinforced (assortative mating) when hybridization alters reproductive performance (Bull & Possingham, 1995). Despite these variations, recent studies suggest that hybridization is frequently observed in contact zones (Jiggins et al., 1996; Mallet, 2005). The genetic introgression can differentially involve mitochondrial and nuclear genomes (Dufresnes et al., 2015; Mitchell, Muehlbauer & Freedberg, 2016). "
    [Show abstract] [Hide abstract] ABSTRACT: Western European vipers are well-defined species with parapatric distributions that reflect contrasting thermal niches and climatic adaptations. Contact zones are usually narrow, coincide with steep ecological gradients and are associated with clear habitat segregation. Natural hybridization has been demonstrated between several species but has not been detected in others. The cold-adapted adder (V. berus, subgenus Pelias) is not known to hybridize with the warm-adapted aspic viper (V. aspis subgenus Vipera). For over 12 years, we have monitored sympatric populations of V. berus and V. aspis in western France where the two species exhibit very similar life cycles. We tested for possible hybridization because individuals with intermediate morphological traits have been reported in the past and were recently detected in the study population. Our results demonstrate that hybridization actually occurs and is directional since it involves females V. aspis viper and male V. berus in all analyzed cases. We discuss our results in the frame of previous findings on contact zones to evaluate in which conditions hybridization may occur.
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    • "Indeed, the upper Río Mayo location of the same hybrid zones is particularly interesting because it is not so obviously associated with ecotones or climatic features, raising the intriguing possibility that it may be free to move. Some previous studies of H. erato have concluded that hybrid zones associate with ecological adaptation leading to parapatric speciation to distinct habitats (Jiggins et al. 1996; Arias et al. 2008; Blum 2008). However, wing pattern differences do not necessarily constitute adaptations to the local environment. "
    File · Data · May 2016 · BMC Evolutionary Biology
    • "(Additional file 1:Table S2). This is a relatively high level of divergence given that H. himera is a recent incipient species and is known to hybridize frequently with H. erato in nature (Figs. 2 and 3,161718 ). These patterns suggest that divergence may accumulate rapidly in the early stages of speciation, even if reproductive isolation is incomplete and gene flow is still occurring. "
    [Show abstract] [Hide abstract] ABSTRACT: A key to understanding the origins of species is determining the evolutionary processes that drive the patterns of genomic divergence during speciation. New genomic technologies enable the study of high-resolution genomic patterns of divergence across natural speciation continua, where taxa pairs with different levels of reproductive isolation can be used as proxies for different stages of speciation. Empirical studies of these speciation continua can provide valuable insights into how genomes diverge during speciation. We examine variation across a handful of genomic regions in parapatric and allopatric populations of Heliconius butterflies with varying levels of reproductive isolation. Genome sequences were mapped to 2.2-Mb of the H. erato genome, including 1-Mb across the red color pattern locus and multiple regions unlinked to color pattern variation. Phylogenetic analyses reveal a speciation continuum of pairs of hybridizing races and incipient species in the Heliconius erato clade. Comparisons of hybridizing pairs of divergently colored races and incipient species reveal that genomic divergence increases with ecological and reproductive isolation, not only across the locus responsible for adaptive variation in red wing coloration, but also at genomic regions unlinked to color pattern. We observe high levels of divergence between the incipient species H. erato and H. himera, suggesting that divergence may accumulate early in the speciation process. Comparisons of genomic divergence between the incipient species and allopatric races suggest that limited gene flow cannot account for the observed high levels of divergence between the incipient species. Our results provide a reconstruction of the speciation continuum across the H. erato clade and provide insights into the processes that drive genomic divergence during speciation, establishing the H. erato clade as a powerful framework for the study of speciation.
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