Recently, I attended a conference about hybridization in Hamburg (Germany). When I saw the line-up of speakers (among others James Mallet, Nick Barton and Simon Martin), I registered right away. And my high expectations were certainly met. Here is a personal overview of three days of hybrid science!
What the F*ck are Species?
The symposium started on Tuesday 12th of June with a plenary talk by James Mallet. He has been studying hybridization between mimicry races of Heliconius butterflies for some time and provided a nice overview of this work. Recent genomic analyses of these South American butterflies resulted in a surprising conclusion: only 2 percent of the genome follows the species tree whereas the remaining 98 percent is flowing freely. This result complicates the question of what species actually are. No wonder James’ Twitter-account is called @WTF_R_species…
The second day of the symposium opened with another familiar name in the hybrid community: Nick Barton. He has produced some classical papers on hybridization and hybrid zones, such as “Analysis of Hybrid Zones” and “The Role of Hybridization in Evolution.” A large part of his work focuses on the theoretical analyses of hybrid zones (read: a lot of mathematics). When I heard him asking for some chalks, I prepared myself for a bombartment of mathematical formulas on the blackboard. However, he mainly talked about recent work on the Antirrhinum flowers hybrid zone where they found several islands of divergence in the genome, which were formed by a combination of gene flow and multiple selective sweeps. Read more about this work here.
The next talk also focused on flowers. Mario Vallejo-Marin explained how hybridization and whole genome duplication work together to shape the evolution of monkeyflowers (genus Mimulus). Interestingly, whole genome duplication can save fertility in hybrids that are normally sterile.
The final speaker of the morning session, Kenneth Whitney, presented the latest results of an experimental approach to hybridization research. His group assessed the fitness of sunflower hybrids (genus Helianthus). Field experiments showed that hybrids evolve faster and have increased fitness compared to “pure” species. This suggests that hybridization can speed up adaptation. However, genetic analyses indicated that the hybrids have acquired genetic material from local plants, which might explain why they adapted so quickly. Kenneth thinks that hybrids are more likely to pick up genes from other plants because they have less self-incompatibility alleles. But this hypothesis remains to be tested. You can read more about these finding here.
The three types of flowers from this session: Antirrhinum (left), Mimilus (topright) and Helianthus (bottomright).
Fish, Snails and Ants
After the coffee break, Arne Nolte kicked off with the evolution of sex determination in Cottus fish. In contrast to mammals and birds, these fish do not have separate sex chromosomes. It turns out that in hybrids, sex is largely determined by dominant alleles at multiple loci that act side by side. About 10% is driven by additive effects at other loci.
The next talk, by Ingo Schlupp, also revolved around fish, namely the Amazon Molly. This peculiar species is of hybrid origin and reproduces clonally. However, in order to reproduce the females need sperm, which they obtain from other species. This system is known as gynogenesis.
Anja Marie Westram stayed closed to the water and presented her work on Littorina snails. These animals come in two ecotypes, a “wave” form that is adapted to life on the rocky shores where waves come crashing in and a “crab” form that resides between the rocks where it hides from predators (i.e. crabs). These ecotypes interbreed along hybrid zones. Anja Marie compared several of these hybrid zones and found mainly concordant patterns. A nice example of replicating hybrid zones.
The final talk before lunch required extra attention, because Jurgen Gadau introduced us into the wonderful world of harvester ants. When these ants mate with the same genetic lineage, they produce queens. But when they mate with a different lineage, you get workers. Jurgen is trying to figure out how such a system evolved and can be maintained.
Red harvester ants
Molecular Plant Biology
Because I am not really a plant person, the next session was more challenging for me: three talks on the molecular mechanisms in plant hybrids. Definitely interesting but not always easy to follow. First, Luca Comai talked about chromosome remodelling. Apparently, some hybrids eleminate half of their genome, partioning it into micronuclei. Some of these fragments curl up into ring chromosomes (as already described by Barbara McClintock) and could potentially transfer genes between species. Second, Paul Grini introduced the AGAMOUS-like (AGL) transcription factors. Some of these genes are misexpressed in Arabidopsis hybrids, resulting in dead seeds. Detailed analyses showed that things go wrong during the endosperm cellularization. Finally, I learned a bit more about how plants can become polyploid (i.e. having more than two pairs of chromosomes). The goup of Arp Schnittger has figured out that this is due to problems at the Spindle Assembly Checkpoint (SAC) during cell division. The cell tries to separate the chromosomes but eventually gives up and continues, resulting in polyploidy.
The model system plant: Arabidopsis thaliana.
Some More Plants
After a long and exciting poster session, we re-assembled for the final two talks of the day. Jeffrey Chen kicked off with stories about the hybrid origin of cotton and the balance between stress and growth in Arabidopsis hybrids. There are too many details to cover here, so I will redirect you to the original publications in Genome Biology and Nature Communications.
The final talk of the day was given by Richard Abbott. He explained how climate change can lead to more hybridization events as species expand into new areas. He gave examples of secondary contact between willow trees and possible adaptive introgression in Chinese cypresses. Climate change can also lead to habitat disturbance which could facilitate hybridization (as already pointed out by Edgar Anderson in the 1940s). It turns out that the UK and Ireland are the perfect places to test this idea. Many hybrids have managed to establish themselves here, as exemplified by the book Hybrid Flora of the British Isles. Richard told us the fascinating story of Senecio plants in the UK. It turned out that one species (S. squalidus) was actually a hybrid that originated in Italy on the slopes of mount Etna. Who knew?
A hybrid plant species: Senecio squalidus.
Butterflies and Beetles
The third day began later than planned because Zacharia Gompert could not make it. The first talk of the day was by Simon Martin who introduced us to a peculiar study system in Africa: the butterfly Danaus chrysippus. These butterflies come in several mimicry morphs that are determined by three loci. They hybridize in a contact zone in eastern Africa where some weird things are going on. First, males are killed by the endosymbiont Spiroplasma, leading a biased sex ratio with mostly females. Second, some individuals have evolved a new sex-chromosome by fusing two other ones. Simon is trying to figure out how these three processes – hybridization, male killing and neo-sex-chromosome – interact with each other. Exciting work.
The next talk also revolved around insects and their bacteria. The work of Susanne Dobbler deals with beetles that have been infected with three different strains of Wolbachia. The outcome is the same as with the butterflies: no males are produced by infected females. However, the uninfected females also mostly produce females. The exact reason behind this pattern remains elusive but it might have something to do with mito-nuclear incompatibilities.
A colorful specimen of Danaus chrysippus.
From Plants to Yeast
The next session was an all-female line-up, prompting the chair (Stefan Hoth) to wonder if some male killing occurred. The session was started by Eunyoung Chae who explained how deleterious interactions between particular immune receptors (NLRs) in hybrid plants result in an auto-immine response in plants, culminating in hybrid necrosis.
Kirsten Bomblies introduced another species of Arabidopsis, namely A. arenosa, which occurs in diploid and autotetraploid lineages. Kirsten managed to retrace the gene flow patterns between several populations. For example, genes flowed from the western Carpatians through the Baltic into a railway site in Berchtesgaden, helping the latter adapt to local conditions. A nice example of adaptive introgression.
The next speaker, Wei Yuan, switched back to everyone’s favorite model system: Arabidopsis thaliana. She analyses transcriptomic data in hybrids and uncovered a set of more than 1000 non-additively expressed genes that can be divided over five functional modules. She is now planning to disentangle the patterns within and between these modules.
Susanne Edelmann presented similar work for Brassica napus. But instead of working with transcriptomic data, she focused on methylation patterns (which correlate with gene expression levels). She found less methylation in hybrids which could explain higher growth rates.
The final speaker, Rike Stelkens, turned away from plants and presented her work on experimental evolution in yeast. Her lab crosses two species (Saccharomyces cerevisiae and S paradoxus) that are 14 percent divergence. That is ten times as divergent as humans and chimps. Several experiments show that the hybrids do better than their parental species, probably because of particular recombination patterns. You can find more information about this work in this pre-print.
A closer look at Saccharomyces cerevisiae
Patterns of Introgression
The final session of the symposium was kicked off by Gerard Heckel who studies the radiation of Microtus voles. These small rodents are infected with the Tula-virus and there seems to be co-divergence between the voles and the virus. Gerard nicely illustrated this process with a Russian doll cartoon: when the voles diverge, their pathogens (i.e. the Russian dolls inside) hitchhike along and also diverge. Next, Diethard Tautz presented a new method to quantify phylogenetic discordance. Applying this method to mouse and human data uncovered some interesting patterns of introgression.
Before the plenary talk by Rosemary Grant, the poster prizes were distributed. To my big surprise, I was awarded third place. I really enjoyed the poster session but I would not have expected to win something. And Rosemary’s talk? That was a wonderful story about her work (with her partner Peter Grant) on the Darwin’s Finches. I will not try to provide a short summary here. Just read their papers and books. I can definitely recommend The Beak of the Finch by Jonathan Weiner.
The winners of the poster prizes (I am standing in the middle).