Interview with the author: Arms races with mitochondrial genome soft sweeps in a gynodioecious plant, Plantago lanceolata

Professor Deborah Charlesworth discusses her research on the origin of mitochondrial male sterility mutations. Some hermaphroditic plant species have a fraction of individuals with flowers that are male sterile and only express the female function. This reproductive system is known as gynodioecy, and it can create conflicts between genes favouring male or female function. In a paper recently published in Molecular Ecology, Bergero et al. explore the persistence of gynodioeccy in the Plantago system and test two fundamental hypothesis for the maintenance of gynodioeccy in nature. Deborah Charlesworth chatted with us and gave us some wonderful insights as to how this research started and where it is going now. Don’t miss it!

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Plantago lanceolata populations studied. The locations are indicated by small black dots for the NES populations, connected by arrows with pie charts showing the frequencies of the two major haplotypes described in the text (the key shows the colours denoting the two haplotypes, which are visible in the online version), and by large pink dots for the non‐NES populations, all of which have only haplotype 1. The sample sizes can be ascertained from Figure S1) [Colour figure can be viewed at wileyonlinelibrary.com]

What led to your interest in this topic / what was the motivation for this study? 
The phenomenon of male-sterility in plants has been a puzzle for a long time, and the discovery that mitochondrial mutations are involved only deepened the mysteries further, because it became clear that the mutations were not the usual kinds, such as point mutations in or deletions of coding sequences. I hoped that getting sequences of the mitochondrial genome might be helpful

What difficulties did you run into along the way? 
There were no major difficulties in getting the sequence data, though because plant mitochondrial genomes are very larger, we could sequence only coding regions of genes. Of course, some genes proved not to include any sequence variants, so the number of genes with useful data was reduced, compared with our initial hoped.

What is the biggest or most surprising finding from this study? 
A surprising finding was that some populations that contain females, and therefore certainly have male-sterility mutations present, did not have variants in the mitochondrial genes we sequenced. Because male-sterility is generally cytoplasmically inherited, it seems unlikely that in this species it is due to nuclear male-sterility mutations (especially as earlier excellent genetic studies in our study species strongly support cytoplasmic inheritance in populations in the Netherlands). It was also unexpected to find that, in most European populations sampled, the only variants were pretty rare, so that the entire mitochondrial genome behaved almost as a single genetic “allele” or “haplotype”, whereas populations from one region in northern UK had two different haplotypes, with variants seen in several genes.  

Moving forward, what are the next steps for this research? 
The next steps will have to be taken by others, as I am too old to continue (I now work on sex chromosome evolution in a fish, and one project is enough at my age). But I hope that others will add mitochondrial sequence data to see whether other plants with cytoplasmic male-sterility have mitochondrial haplotypes that are associated with the femaleness factors. It will also be good to test whether, as in our study species, multiple sequence variants are associated into haplotypes, or not. It is known that mitochondrial genomes can undergo a process of genetic recombination, and studying sequence variants can tell us whether this occurs often in natural populations, or not. If it occurs often, then it should be possible to identify sequence variants that are associated with male-sterility. If, however, exchanges between haplotypes occur very rarely, it could prove to be very difficult to find the actual mutations involved. Another intriguing possibility is that male-sterility is caused by parts of the genome other than the protein-coding genes, and that, even if those genes do not undergo recombination, other parts of the genome might do so, perhaps producing the variants that lead to male-sterility. It has long been known that mitochondrial genome rearrangements seem to be involved in male-sterility, rather than point mutations, and I have sometimes wondered whether perhaps this phenotype occurs when parts of the genome are duplicated, which might lead to down-regulation of genes that suddenly have an extra copy, or partial copy, as occurs in transgenic plants.

What would your message be for students about to start their first research projects in this topic? 
Find something easier unless you are really clever. On the other hand, my experience has been that one often gets puzzling results, and they often prompt one to think about the question in a different way. In fact, I would say that one almost never starts by asking the right questions, but that these tend to emerge as one sees the data and thinks about them.

What have you learned about science over the course of this project? 
You never know what you will turn up, and whatever it is, it often raises more puzzles than it solves.

Describe the significance of this research for the general scientific community in one sentence.
This was the first multi-gene sequence data collected to study mitochondrial male sterility in a plant, and it showed that, in our study species, this genome recombines rarely, at least in the parts we could study, and, by extension, across the entire mitochondrial genome.

Describe the significance of this research for your scientific community in one sentence.
This study is just a small step in attempts to understand the puzzle of how mitochondrial male sterility mutations may arise.

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