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Lots of news coverage today about a paper in Nature (subscription needed?) about butterfly speciation. Reuters headline is typical Scientists create hybrid butterfly species in labOthers use "recreate", which is more accurate in terms of the intent of the paper. The Telegraph writes: A butterfly has been reproduced by scientists, revealing for the first time the colourful past of a striking wild species.The process is "Homoploid hybrid speciation - hybridization without change in chromosome number". This is where a new species arises from the interbreeding of two distinct species. Most pairs of animal species don't interbreed at all (it's part of the operational definition of species, after all). Hybrids, like mules, are usually sterile. In cases where the two parents have very different arrangements of genes on different numbers of chromosomes, this is not surprising; getting a complete genome into a gamete becomes nontrivial, and getting two complementing genomes together becomes too improbable. Hybridization between two species with the same number of chromosomes is rare partly because when animals are that close genetically, they're less likely to become reproductively isolated than plants. The phenomenon is also rare in plants, according to the paper. If two populations are not reproductively isolated, then we don't consider them species, even if there are variations in appearance. What's cool about the paper from Mavárez et al. is that they have an example where two species can hybridize to make a third, instead of three species arising by a parent species branching, and then one of the branches branching again. Heliconius cydno is black with white and yellow marks, whereas H. melpomene is black with red, yellow and orange marks. Both species exhibit strong positive assortative mating based on their wing colour patterns and also differ in habitat use18 and host plant preference19, but inter-specific hybrids do occur at low frequency in the wild15. Heliconius heurippa has an intermediate wing pattern, which has led to the suggestion that this is a hybrid species1, 20. Its hindwing is indistinguishable from that of sympatric H. m. melpomene, whereas the yellow band on its forewing is similar to that of parapatric H. cydno cordula. Ecologically, H. heurippa is most similar to H. cydno, which it replaces geographically in the eastern Andes of Colombia.Mavárez et al. used DNA polymorphisms to show first that H. heurippa really qualifies as a species. It's reproductively isolated from H. cydno and H. melpomene . Then they showed that there is a pathway to H. heurippa from interbreeding H. cydno and H. melpomene . Crosses between H. cydno and H. melpomene give offspring that can't be bred further, as with mules...but only the females are sterile. The males can backcross to either parent species. After a couple of rounds of backcrossing to H. cydno cordula Mavárez et al. got fertile butterflies that bred true with wing patterns that were indistinguishable from H. heurippa...and these laboratory animals can interbreed with H. heurippa. Mavárez et al. also found in a wild population of sympatric H. m. melpomene and H. cydno cordula in San Cristóbal, Venezuela, we observed natural hybrids at an unusually high frequency (8%), including some individuals very similar to our laboratory-produced H. heurippa-like butterflies (Fig. 2b). Microsatellite data show that these individuals have genotypes indistinguishable from that of H. cydno and must therefore be at least fifth-generation backcrosses (Supplementary Fig. 4). This shows that multiple generations of backcrossing can occur in the wild and that female hybrid sterility is not a complete barrier to introgressive hybridization.They then asked why these different butterflies are reproductively isolated. After confirming that there is a dramatic difference in mate preference, they found Experiments with dissected wings showed that both elements of the forewing colour pattern of H. heurippa were necessary for the stimulation of courtship (Fig. 3). H. heurippa males were less than half as likely to approach and court the H. m. melpomene or the H. cydno cordula pattern than their own (Fig. 3). When either the red or yellow bands were experimentally removed from the H. heurippa pattern, this led to a similar reduction in its attractiveness, demonstrating that both hybrid elements are necessary for mate recognition by male H. heurippa(Fig. 3).I'm not an expert in evolutionary biology, but it's interesting to think about this as an illustration of the grey areas in the species concept, and it's interesting to note this in the context of theories about human-chimpanzee speciation. Carl Zimmer (who has moved to ScienceBlogs) has more...it's his usual lucid writing, but I don't like his Frankenstein analogy, however. The hybrids are made by breeding, not surgery, much less reanimation. I was even less impressed with this from the Telegraph . The new study shows that hybrids can thrive and may have wider implications, for instance for GM organisms, said Mr Jiggins.
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