Two PhD chapters submitted!

Well done to Pierre Lacoste and Riccardo Poloni who submitted their first manuscripts from their PhDs ! Both on polymophic Lepidoptera.

Lacoste et al. shows the results of mate choice experiments in the tropical butterfly Heliconius numata in French Guiana. They show an asymmetrical pattern of disassortative mating, with the morphs bearing the inversion allele displaying disassortative preferences, while the morph bearing the standard allele shows no preference. This contrasts with the pattern found in Peru, where disassortative mating was more or less symmetrical. Together with a simulations and analytical models, this leads to the prediction that mimetic benefits associated with the inversion is key to the stability of the polymorphism.

Thanks to Melanie McClure for cosupervising Pierre’s experiments in her lab in Cayenne, to Mathieu Chouteau for sharing experience and data, and to Ludovic Maisonneuve for his contribution to the modelling.

Lacoste P, Chouteau M, Maisonneuve L, Joron M* & McClure M* (submitted) « Asymmetry in disassortative mating sheds light on the processes maintaining inversion polymorphism ».

Poloni et al. reports the results of a predation experiment on the invasive box tree moth Cydalima perspectalis, using blue tits as predators. The papers shows that the white and dark morphs of this moth are under different predation regimes. The dark morph is cryptic and benefits from this especially when rare. The white morph is highly conspicuous and suffers more from predation, but benfits from a dilution effect when common, which is a rare finding for a highly palatable moth. Combined together the two forces may contribute to the maintenance of the polymorphism.

Thanks to Jonna Mappes and Ossi Nokelainen for their help and for hosting and training Riccardo and Marina at the Konnevesi research station in Jyvaskyla Finland.

Poloni R, Dhennin M, Mappes J*, Joron M* & Nokelainen O* (submitted) « Positive and negative frequency-dependent selection acting on polymorphism in a palatable moth ».

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Inversions come with breakpoints!

Congratulations to Romain Villoutreix for this thorough review of supergene structure. Supergenes are classically thought to be formed by the capture of several previously recombining functional mutations. An alternative model is to consider rearrangement breakpoints as a major mutation that comes automatically in LD with the mutations on the DNA strand on which the rearragement occurs. The latter model may overcome some theoretical difficulties of the classic model necessitating several mutations to segregate in the population prior to being locked into an inversion, and acknowledges the functional effects of many breakpoints.

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A key role for deleterious mutations

Congratulations to Paul Jay and Mathieu Chouteau for their amazing study now published in Nature Genetics !

The chromosomal inversions bringing great mimicry benefits in Heliconius numata are full of deleterious mutations and insertions of transposable elements. Individual with a homozygous inversion genotype shave very poor larval survival. This means those inversion only bring their mimicry advantage when heterozygous. Therefore, selection caused by these deleterious effects maintains the standard (non-inverted) arrangements in the population, and balances the amazing wing-pattern polymorphism observed.

Figure: Larval survival according to inversion genotype. Both chromosome types bearing inversions have a low survival rate. All heterozygotes have good survival, similar to individuals lack inversions altogether.

This illustrates the dual effect of recombination suppression associated with inversions: they lock together beneficial mutations, forming greatly favourable haplotypes, but are also prone to capturing and accumulating deleterious variation, acting against their fixation. This could be a very general mechanism underlying inversion polymorphisms throughout the tree of life.

Nature Genetics. doi:10.1038/s41588-020-00771-1
Read online:

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Welcome to Pierre Lacoste

Pierre is joining us for a PhD project on the ecology of mate choice (how do females choose males disassortatively?) and on the functional bases of heterozygote advantage (what’s wrong with inversion homozygotes?). Plenty of field and insectary work ahead. Welcome Pierre!

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Are more toxic prey better protected?

This is the question addressed in Mathieu Chouteau’s paper, just published in Animal Behaviour (vol 153:49-59).

How do bird predators consider the natural range of variation in toxin load among butterfly species in a community? Mathieu asked chicks what they thought of the chemically-defended butterflies composing the mimicry communities of Eastern Peru. He measured how fast they would learn to avoid butterfly-flavoured food crumbs, as opposed to tasty crumbs. Surprisingly, Mathieu found that only a few butterfly species with low loxin content can improve their aversiveness to chicks. Most well-defended prey elicited similar learning rates, irrespective of the wide variation in their concentration in cyanogenic or other distasteful compounds.

Then, why should butterflies invest in high toxicity ? Some can be really nasty ! If natural predators such as jacamars or flycatchers learn like chicks, the benefits to butterflies in loading themselves with toxins may lie elsewhere than in the education of predators. Perhaps in accessing competition-free hostpants, or in making better pheromones, or in elicting immediate taste-rejection ?

Another important consequence is that differences in toxicty among mimetic butterflies may not represent a major determinant of their roles as mimics vs. models. On the contrary, if all toxic species are considered equally aversive by predators, their roles in predator education should be determined primarily by their relative abundances. Fritz Muller was right again!

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Two positions available – Apply now

Two positions are open in my group, to work on the ecology and evolutionary genomics of mimicry supergenes in Heliconius butterflies.

1- A 3-year PhD studentship on the ecology of inversions and balancing selection. The project aims at deciphering the evolution of a complex of chromosomal inversions forming a supergene in H. numata. The PhD student will study the selective factors (e.g. viability trade-offs, deleterious variations, etc.) and the mating signals (visual, olfactory) contributing to the maintenance of inversion polymorphism.
Full announcement :
Deadline for applications : 12/06/2019.

2- A 2-year postdoc position on the population genomics of supergenes and inversions. The project aims at improving our understanding of how inversions have established and spread in populations and through the Amazonian range, and how inversion polymorphism is associated with changes in the connectivity and demography of populations, likely to affect ecological and evolutionary trajectories of polymorphic populations.
Full announcement :
Deadline for applications : 20/06/2019

Please do circulate those ads around your labs and do not hesitate to contact me for more details on the project or the application procedure.

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Alien inversion

May 2018: Congratulations to Paul Jay, Annabel Whibley, Angeles de Cara, Lise Frézal and Reuben Nowell (!) for their paper showing that the supergene was formed via the introgression of an inversion. Supergene alleles evolved their differences and coadaptation in different species. That’s now in Current Biology!

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Polymorphism not maintained by mimicry

July 2017: Congratulations to Mathieu Chouteau for his paper in PNAS, showing how H. numata mate disassortatively with respect to mimetic wing pattern. Polymorphism is not maintained by mimicry, really!

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