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Hybrid sterility, Haldane's rule and speciation in Heliconius cydno and H. melpomene
Most genetic studies of Haldane’s rule, in which hybrid sterility or inviability affects the heterogametic sex preferentially, have focused on Drosophila. It therefore remains unclear to what extent the conclusions of that work apply more generally, particularly in female-heterogametic taxa such as birds and Lepidoptera. Here we present a genetic analysis of Haldane’s rule in Heliconius butterflies. Female F1 hybrids between Heliconius melpomene and H. cydno are completely sterile, while males have normal to mildly reduced fertility. In backcrosses of male F1 hybrids, female offspring range from completely sterile to fully fertile. Linkage analysis using the Z-linked triose-phosphate isomerase locus demonstrates a “large X” (Z) effect on sterility. Expression of female sterility varies among crosses in this and a previous study of Heliconius. Sterility may result from the production of normal but infertile eggs, production of small infertile eggs, or from a complete failure to develop ovarioles, which suggests multiple routes to the evolution of hybrid sterility in these Heliconius species. These results conform to the expectations of the “dominance” rather than “faster male” theories of Haldane’s rule and suggest that relatively few loci are responsible. The two species are broadly sympatric and hybridize in the wild, so that female hybrid sterility forms one of several strong but incomplete barriers to gene flow in nature. The effect of female sterility is comparable to that of selection against non-mimetic hybrids, while mate choice forms a much stronger barrier to gene transfer.
Phylogeography of Heliconius cydno and its closest relatives: Disentangling their origin and diversification
The origins of the extraordinary diversity within the Neotropics have long fascinated biologists and naturalists. Yet, the underlying factors that have given rise to this diversity remain controversial. To test the relative importance of Quaternary climatic change and Neogene tectonic and paleogeographic reorganizations in the generation of biodiversity, we examine intraspecific variation across the Heliconius cydno radiation and compare this variation to that within the closely related Heliconius melpomene and Heliconius timareta radiations. Our data, which consist of both mtDNA and genome-scan data from nearly 2250 amplified fragment length polymorphism (AFLP) loci, reveal a complex history of differentiation and admixture at different geographic scales. Both mtDNA and AFLP phylogenies suggest that H. timareta and H. cydno are probably geographic extremes of the same radiation that probably diverged from H. melpomene prior to the Pliocene-Pleistocene boundary, consistent with hypotheses of diversification that rely on geological events in the Pliocene. The mtDNA suggests that this radiation originated in Central America or the northwestern region of South America, with a subsequent colonization of the eastern and western slopes of the Andes. Our genome-scan data indicate significant admixture among sympatric H. cydno/H. timareta and H. melpomene populations across the extensive geographic ranges of the two radiations. Within H. cydno, both mtDNA and AFLP data indicate significant population structure at local scales, with strong genetic differences even among adjacent H. cydno colour pattern races. These genetic patterns highlight the importance of past geoclimatic events, intraspecific gene flow, and local population differentiation in the origin and establishment of new adaptive forms. © 2014 John Wiley and Sons Ltd.
Speciation by hybridization in Heliconius butterflies
Speciation is generally regarded to result from the splitting of a single lineage. An alternative is hybrid speciation, considered to be extremely rare, in which two distinct lineages contribute genes to a daughter species. Here we show that a hybrid trait in an animal species can directly cause reproductive isolation. The butterfly species Heliconius heurippa is known to have an intermediate morphology and a hybrid genome1, and we have recreated its intermediate wing colour and pattern through laboratory crosses between H. melpomene, H. cydno and their F1 hybrids. We then used mate preference experiments to show that the phenotype of H. heurippa reproductively isolates it from both parental species. There is strong assortative mating between all three species, and in H. heurippa the wing pattern and colour elements derived from H. melpomene and H. cydno are both critical for mate recognition by males.
Genome-wide patterns of divergence and gene flow across a butterfly radiation
The Heliconius butterflies are a diverse recent radiation comprising multiple levels of divergence with ongoing gene flow between species. The recently sequenced genome of Heliconius melpomene allowed us to investigate the genomic evolution of this group using dense RAD marker sequencing. Phylogenetic analysis of 54 individuals robustly supported reciprocal monophyly of H. melpomene and Heliconius cydno and refuted previous phylogenetic hypotheses that H. melpomene may be paraphylectic with respect to H. cydno. Heliconius timareta also formed a monophyletic clade closely related but distinct from H. cydno with Heliconius heurippa falling within this clade. We find evidence for genetic admixture between sympatric populations of the sister clades H. melpomene and H. cydno/timareta, particularly between H. cydno and H. melpomene from Central America and between H. timareta and H. melpomene from the eastern slopes of the Andes. Between races, divergence is primarily explained by isolation by distance and there is no detectable genetic population structure between parapatric races, suggesting that hybrid zones between races are not zones of secondary contact. Our results also support previous findings that colour pattern loci are shared between populations and species with similar colour pattern elements. Furthermore, this pattern is almost unique to these genomic regions, with only a very small number of other loci showing significant similarity between populations and species with similar colour patterns. © 2012 Blackwell Publishing Ltd.
Genómica de la diversificación de Heliconius cydno (Lepidóptera: Nymphalidae) en el norte de Suramérica: patrones geográficos y fenotípicos
Major geological events, such as the uplift of the Andes and the formation of inter-Andean valleys, have played a key role in shaping Neotropical biodiversity. This study investigates their influence on the recent adaptive radiation (∼1.5 Ma) of Heliconius cydno, a mimetic and aposematic butterfly widely distributed from Central America to northern South America, which exhibits notable variation in wing color patterns. This phenotypic diversity is controlled by a modular genetic architecture, in which a few key genes and cis-regulatory elements regulate differences among morphotypes in this and other lepidopteran species. Using whole-genome sequencing of 113 individuals comprising all described subspecies, we investigated (1) the role of Andean orogeny in the diversification of H. cydno and (2) the genetic basis of variation in one of its distinctive phenotypic traits. Results show that nine of the fourteen H. cydno subspecies correspond to independent lineages that originated in northwestern South America and later expanded into Central America and the Colombian inter-Andean valleys. Genetic structure reflects geography, forming five main clusters: Central America, Venezuela, the Magdalena Valley, the Cauca Valley, and Gorgona Island. Some populations, such as the insular subspecies H. c. subsp. nov., show strong genetic structure, low diversity, and bottleneck signatures—unlike continental subspecies, which exhibit high levels of shared ancestry and gene flow. Additionally, the microRNA miRNA193, derived from the lncRNA Ivory, was identified as the locus underlying variation in the white submarginal band on the hindwing—a trait whose genetic basis had remained unknown in Heliconius. Evidence of introgression suggests that this trait was transferred from H. cydno to H. melpomene, contributing to the establishment of a novel mimicry ring with H. erato. Altogether, these findings highlight the roles of geography, genomic architecture, and introgression in the diversification of H. cydno.
Sharp genetic discontinuity across a unimodal Heliconius hybrid zone
Hybrid zones are powerful natural systems to study evolutionary processes to gain an understanding of adaptation and speciation. In the Cauca Valley (Colombia), two butterfly races, Heliconius cydno cydnides and Heliconius cydno weymeri, meet and hybridize. We characterized this hybrid zone using a combination of mitochondrial DNA (mtDNA) sequences, amplified fragment length polymorphisms (AFLPs), microsatellites and sequences for nuclear loci within and outside of the genomic regions that cause differences in wing colour pattern. The hybrid zone is largely composed of individuals of mixed ancestry. However, there is strong genetic discontinuity between the hybridizing races in mtDNA and, to a lesser extent, in all nuclear markers surveyed. The mtDNA clustering of H. c. cydnides with the H. cydno race from the Magdalena Valley and H. c. weymeri with the H. cydno race from the pacific coast suggests that H. c. cydnides colonized the Cauca Valley from the north, whereas H. c. weymeri did so by crossing the Andes in the southern part, implying a secondary contact origin. Colonization of the valley by H. cydno was accompanied by mimicry shift. Strong ecological isolation, driven by locally adaptive differences in mimetic wing patterns, is playing an important role in maintaining the hybrid zone. However, selection on wing pattern alone is not sufficient to explain the genetic discontinuity observed. There is evidence for differences in male mating preference, but the contribution of additional barriers needs further investigation. Overall, our results support the idea that speciation is a cumulative process, where the combination of multiple isolation barriers, combined with major phenotypic differences, facilitates population divergence in face of gene flow. See also the Perspective by Mallet and Dasmahapatra © 2012 Blackwell Publishing Ltd.
A narrow Heliconius cydno (Nymphalidae; Heliconiini) hybrid zone with differences in morph sex ratios
In the western cordillera of the Cauca valley, Colombia, there is a narrow hybrid zone between two races of Heliconius cydno , one of which is polymorphic. Three large broods show that most of the phenotypic variation observed can be explained by four loci of major effect, named Sb , Yb , L, and K . Two further loci, G and Wo , were identified that control more minor phenotypic variation. Sb, Yb, and Wo are linked and the latter is differentially expressed between the sexes. The transition between H. c. cydnides in the north and H. c. weymeri to the south occurs over approximately 15 km. Collections from a single site near the center of the hybrid zone show that gene frequencies have been stable over the 10 yr from 1991 to 2001 and that color?pattern genes are in Hardy–Weinberg equilibrium, with little evidence for linkage disequilibrium. This suggests that mating is random between color?pattern forms. Nonetheless, there is evidence for variation in the sex ratio, with parental phenotypes and the most melanic hybrid phenotypes showing a fairly even sex ratio compared to a strong male bias observed in the remaining hybrids. We hypothesize that this might be explained by differential selection between sexes and phenotypes, phenomena that could have important implications for hybrid zone analysis and the evolution of Müllerian mimicry.
