Species v hybrid. More fuel for the debate

By their nature, conflicting ideas about evolution aren’t easily resolved. Some – such as the role of hybridisation in evolution – have played out over centuries.

New findings just published in the journal Science add fuel to the hybridisation fire.

An international research team analysed the genomes of 20 species of passion vine butterflies (Heliconius spp), which have similar, predator-warning colour patterns.

Earlier studies have found that one reason for this is that the different species share parts of their DNA, thanks to hybridisation that occurred in the distant past.

The new study discovered a surprisingly high amount of gene flow, or gene migration, among the butterflies – even between species that are distantly related.

The findings point to hybridisation as a key process in the emergence of biological diversity.

To understand how genes were passed to other species by hybridising – a process known as introgression – the researchers analysed new genome assemblies of the 20 species.

“The cool thing about making genome assemblies instead of simple genome ‘resequencing’ is that it’s not just the DNA bases that change: the entire structures of genomes can change through evolutionary time,” says lead author Nate Edelman, from Harvard University in the US. “And using the assemblies, we can detect those changes.”

Harvard co-author James Mallett says the study’s aim was to “probe deeper into the phylogenetic tree”.

“What we found is really astonishing: introgression even among species that are distantly related,” he says. “Species are simply not what we thought they were, and now we have the data to show it. The evolutionary tree of butterflies is a complete morass of inter-connectedness – every bit of the butterfly genome seems to have a different tree.”

The new genome assemblies were built by sequencing short fragments of DNA, then assembling them in the proper order.

Upon analysing the assemblies, the researchers found evidence that some genes were capable of moving between species, and others were far more resistant to the process. One of the key factors determining a gene’s capacity to move is a biological process called “recombination”.

“In humans and most animals, every individual inherits two copies of their genome, one from her mother, and one from her father,” says Mallet.

“The reason you differ genetically from your sibling is due to recombination. Your father contributed to you a newly scrambled, recombined copy of his own parents’ genomes, as did your mother with her parents’ genomes. So the combination of components from each parent is different in every individual.”

It’s believed that recombination is an advantage if the goal is to create diverse genotypes for future generations. The system of recombination described in the study suggests that it also occurs during gene flow between species.

The study authors say this could provide a possible route for adaptive genes to be passed occasionally between species, as well within species.

“It might seem that useful genes are more likely to be transferred between species,” says Michael Miyagi, also from Harvard. “That’s true, but there are also more mundane structural issues with the genome that mean some regions are more likely to have genes go back and forth.”

The team was able to identify a key gene that acts to switch colour patterns as one that moved between species.

“We found that in one particular region of the genome, there are about 500,000 base pairs that have been inverted relative to the ancestral sequence,” says Miyagi.

“And smack in the middle of that inversion is that gene that we know controls colour pattern. When you have an inversion like that, it means you’re keeping all the things within it together, so they can’t recombine.”

The study concludes that hybridisation is one way for species to derive their genomes, and that it may be a key process in creating the natural diversity we see today.

“In nature, it’s very unlikely that any individual will mate with a member of another species,” says Mallet.

“But over evolutionary time, it does happen. It probably only happens in the ‘youngest’ groups of species – species that are rapidly evolving. Most of the diversity of life is probably created in these rapid radiations.

“They are involved in events such as the origin of mammals. During these radiations… hybridisation and introgression… could be an important means of shuffling variation and recombining adaptations from different lineages.”