Post-copulatory intra- and intersexual selection at fertilization in a polygamous fish
Final Report Abstract
The threespine stickleback, Gasterosteus aculeatus, with its conspicuous male sexual traits, is a well-known model for pre-copulatory sexual selection. Threespine sticklebacks are also often engaged in sneak fertilizations, inducing sperm competition and post-copulatory sexual selection. It is thus an ideal species to investigate both episodes of sexual selection and their interaction. In this DFG-supported project, we have made significant advances in understanding functional variation in stickleback sperm traits. There was a positive link between sperm size and sperm linearity, which in turn was associated with an increased swimming speed. Differences in sperm velocity correlated with competitive fertilization success. In sperm competition experiments with equal sperm numbers of rival males, head-to-tail-length ratio of a male’s sperm determined paternity. When one of the two rivals ejaculated more sperm or ejaculated before the other male, his paternity share increased. In a series of experimental studies, the effects of several intrinsic (age, relatedness) and extrinsic factors (food quality and quantity, temperature, ultraviolet radiation) on sperm traits and breeding coloration were determined. Relatedness only influenced pre-copulatory sexual selection. Sperm and testis traits were not generally impaired by inbreeding, and this result was not changed by a second/third generation of brother–sister matings. A comparison of reproductively active brothers of the first and second breeding season showed that older males produced more but smaller sperm, which might be of lower quality. Exposure to enhanced ambient UV-A levels as an environmental stressor had detrimental effects on both male breeding coloration and sperm velocity providing evidence that UV radiation affects traits targeted by pre- and post-copulatory sexual selection. Both sperm velocity and linearity significantly increased whereas the percentage of motile sperm decreased at higher temperatures (25 °C vs. 15 °C). At higher temperatures, a lower number of sperm was sufficient to fertilize 50 % of the eggs but in the end more eggs remained unfertilized at the higher temperature. The observed results stress the role of current and future levels of naturally occurring ambient water temperatures as an important environmental factor influencing fitness related traits. Sperm traits (velocity) correlated with breeding coloration (red intensity), thus supporting the phenotype-linked fertility hypothesis. Feeding experiments showed that there were intriguing links between pre- and post-copulatory sexual selection. Under food restriction, sperm swimming speed and coloration were traded off. Food-restricted males suffered from a higher mortality and a reduced body growth but had a more intense breeding coloration, faster sperm and outcompeted their well-fed carotenoid-supplemented brothers during competitive fertilization trials. Among stickleback species, there was significant variation in sperm number and sperm morphology. Future studies should concentrate on the function of this variation with respect to mating system and intensity of post-copulatory sexual selection. In conclusion, extrinsic factors like water temperature, UV-A radiation, food quantity and quality had profound effects on sperm traits, secondary sexual traits and their interaction. Apart from possible genetic effects, the effects of intrinsic factors like inbreeding and age seem to have less impact on pre- and postcopulatory sexually selected traits in sticklebacks. It was remarkable that redder stickleback males also had greater sperm quality, and that both traits were traded-off under food restriction. We were also surprised by the great within and among individual, populational and interspecific variation in sperm traits. Unexpected were furthermore the small effects of inbreeding and age.
Publications
- (2012). Inbreeding in three-spined sticklebacks (Gasterosteus aculeatus L.): effects on testis and sperm traits. Biol J Linn Soc 107: 510-520
Mehlis, M, Frommen, JG, Rahn, AK & Bakker, TCM
(See online at https://doi.org/10.1111/j.1095-8312.2012.01950.x) - (2013). Attractive males have faster sperm in three-spined sticklebacks Gasterosteus aculeatus. Curr Zool 59: 761-768
Mehlis, M, Hilke, LK & Bakker, TCM
(See online at https://doi.org/10.1093/czoolo/59.6.761) - (2013). Male reproductive traits of full-sibs of different age classes in threespined sticklebacks (Gasterosteus aculeatus). SpringerPlus 2: 175
Mehlis, M & Bakker, TCM
(See online at https://doi.org/10.1186/2193-1801-2-175) - (2014). Functional variation of sperm morphology in sticklebacks. Behav Ecol Sociobiol 68: 617-627
Bakker, TCM, Hollmann M, Mehlis, M & Zbinden, M
(See online at https://doi.org/10.1007/s00265-013-1676-5) - (2014). Sexually selected traits and reproductive performance in male three-spined sticklebacks (Gasterosteus aculeatus). PhD thesis, University of Bonn, Germany
Mehlis, M
- (2014). The influence of ambient UV light on sperm quality and sexual ornamentation in three-spined sticklebacks (Gasterosteus aculeatus). Oecologia 174: 393-402
Rick, IP, Mehlis, M, Eßer, E & Bakker, TCM
(See online at https://doi.org/10.1007/s00442-013-2773-x) - (2014). The influence of ambient water temperature on sperm performance and fertilization success in three-spined sticklebacks (Gasterosteus aculeatus). Evol Ecol 28: 655-667
Mehlis, M & Bakker, TCM
(See online at https://doi.org/10.1007/s10682-014-9707-x) - (2015). Dynamic resource allocation between pre- and postcopulatory episodes of sexual selection determines competitive fertilization success. Proc R Soc B 282:20151279
Mehlis, M, Rick, IP & Bakker, TCM
(See online at https://doi.org/10.1098/rspb.2015.1279) - (2015). Sperm quality but not relatedness predicts sperm competition success in threespine sticklebacks (Gasterosteus aculeatus). BMC Evol Biol 15: 74
Mehlis, M, Rahn, AK & Bakker, TCM
(See online at https://doi.org/10.1186/s12862-015-0353-x)
