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The genetic basis of an exaggerated sexual ornament in the stalk-eyed fly Teleopsis dalmanni

Final Report Summary - GENORN (The genetic basis of an exaggerated sexual ornament in the stalk-eyed fly Teleopsis dalmanni)

Meiotic drive, sex ratio and population structure
I collected stalk-eyed flies from the Malaysian rain forest at the start of the fellowship. DNA was extracted and I amplified 22 microsatellite loci of 1 500 samples. One part of these samples was adult flies (500) that were collected from 11 different streams in the same valley. According to the microsatellite data there is little resemblance between an ideal population, and the reality. This is expected since there is strong natural and sexual selection present in case of stalk-eyed flies that influences mating frequencies and relatedness of flies in the wild. In addition, meiotic drive may well cause distortion of linked genetic markers or genes that are modifiers of drive.

The other part of our samples (1 000) consisted of females from 6 streams and all their offspring, which were genotyped. Using microsatellites, I was able to assign sex to the offspring and establish sex ratio differences between families. This information will be related to other measures of sexual selection across these streams (e.g. harem size, male size and eye-span, population sex ratio, number of fathers).

In a third laboratory-based experiment, I used field collected males and mated them to laboratory bred females. This will provide an outstanding opportunity to link sex ratio distorting ability to microsatellite genotype and male morphological traits.

Leg hook
Early in the fellowship, I noticed that large males of Teleopsis dalmanni had leg hooks on their forelimbs. Females largely lack this trait, as do small males. Species outside the Teleopsis genus lack the trait entirely. We observed that legs are used early in contests in a 'leg-threat' in which the leg is extended laterally in front of a rival. They are also used in the final stages of fight escalation when males 'wrestle' and each male tries to grapple his opponent with his front legs in order to lift him up and throw him down.

I designed new experiments to discern the function of the leg-hook. I pioneered a protocol for filling in the leg-hook with paint and then carried out contests between painted and control males. The results suggest that the leg-hook has two possible functions: 1) It can be used to threaten the opponent during the non-contact phase. 2) It can also be used as a weapon to hold on to an opponent.

In addition, I carried out a detailed morphological analysis of the leg-hook in a sample of 500 wild flies leg-hook size strongly correlates with eye-span size and body size. Males with larger eye-span after control for body size were found to have larger leg-hooks. Females have much smaller leg-hooks, more like simple depressions and do not show the relationship with eye-span.

I carried out detailed anatomical studies of the internal structures using specialised fixation methods developed for Transmission Electron-microscopy (TEM) and Scanning Electron-microscopy (SEM). This revealed that the tendon connecting the muscle to the tibia (handle one of the nutcracker) possesses a nodule and it can slide over a lump on the femur (handle two of the nutcracker). This makes it possible to reduce the power needed for keeping the legs closed, and therefore to hold on to the opponent's leg.

Impact and use
Dr Mihaly Foldvari will become a staff member of the University of Debrecen in Hungary, where he will continue behavioural research on stalk-eyed flies based on laboratory experiments. The planned new scientific project fits very well into the profile of the Hungarian behavioural ecology research group, since they have so far only been studying bird species, and stalk-eyed flies will be an excellent addition.

The resulting extensive datasets of the present Marie Curie fellowship will be further analysed and extended where possible. Future collaboration between the present host University College London and the UD will be inevitable and frequent researcher and student visits are planned from both sides.