Fitting genes:Sexual selection in the blue tit

De genen van het Major Histocompatibility Complex (MHC) spelen een belangrijke rol in de herkenning van ziekteverwekkers door het immuunsysteem. Individuen met diverse MHC genen kunnen veel verschillende ziekteverwekkers herkennen. Mijn proefschrift onderzocht de partnerkeuze van een bekende zangvogelsoort, de pimpelmees. Pimpelmeesvrouwen met diverse MHC genen bleken een partner hebben die ook diverse MHC genen heeft, terwijl vrouwen met een lage diversiteit een man met lage diversiteit hebben. Dit patroon is te verklaren doordat zowel man als vrouw een voorkeur hebben voor een partner met diverse MHC genen. Individuen met diverse MHC genen zullen succesvol zijn in hun partnerkeuze, terwijl individuen met minder diverse MHC genen het moeten doen met een partner die minder hun voorkeur genoot. Het vinden van een partner met diverse MHC genen zal leiden tot nakomelingen met diverse MHC genen en een goede gezondheid. Seksuele selectie eindigt echter niet bij het vinden van een partner. Ongeveer 11% van alle pimpelmeesjongen is namelijk buitenechtelijk. Tijdens en na de paring kunnen zowel de man als de vrouw de vaderschap beïnvloeden. Zo kan een man meer sperma insemineren als hij weet dat de vrouw met andere mannen heeft gepaard en kan de vrouw besluiten het sperma van onaantrekkelijke mannen niet op te nemen. Wij onderzochten het aantal spermacellen in het reproductieve systeem van vrouwelijke pimpelmezen. Uit onze resultaten blijkt dat vrouwtjes gepaard met een grotere man meer sperma hadden. Dat zou kunnen betekenen dat grotere mannen aantrekkelijker zijn, meer sperma produceren of dat vrouwtjes gepaard met een grotere man meer vreemdgaan

Volume of the cloacal protuberance as an indication of reproductive state in male Blue Tits <i>Cyanistes caeruleus</i>

In male passerines, the accumulation of sperm in the sperm reserves causes the cloaca to become enlarged, forming the cloacal protuberance (CP). In Blue Tits, the timing of breeding differs considerably between pairs. Hence, when catching a male during the breeding season it may be unclear whether he is in a reproductively active state (i.e. producing sperm). Here, we show in captive Blue Tits Cyanistes caeruleus that CP volume is increased in males in a reproductively active state when compared to the same males in a reproductively inactive state. However, there was some overlap in CP volume when comparing the values in reproductively active and non-active states. Measurements of CP volume at a single time point, therefore, do not allow the researcher to reliably determine an individual's reproductive status.

Polygyny and extra-pair paternity enhance the opportunity for sexual selection in blue tits

Polygyny and extra-pair paternity are generally thought to enhance sexual selection. However, the extent to which these phenomena increase variance in male reproductive success will depend on the covariance between success at these two strategies. We analysed these patterns over four breeding seasons in facultatively polygynous blue tits Cyanistes caeruleus. We found that both polygyny and extra-pair paternity increased variance in male reproductive success and that standardised variance in annual number of genetic fledglings was 2.6 times higher than standardised variance in apparent success when assuming strict monogamy. Nevertheless, male success at securing within-pair paternity was unrelated to success at gaining extra-pair paternity and, when considering the positive effect of age on extra-pair success and attracting a second female, polygynous males were no more likely to sire extra-pair fledglings. Overall, polygynous males fledged more genetic offspring than monogamous males, but first-year polygynous males lost a greater share of within-pair paternity. A literature review suggests that this adverse effect of polygyny on within-pair paternity is frequent among birds, inconsistent with the prediction that females engage in extra-pair copulation with successful males to obtain good genes. Furthermore, a male's share of paternity was repeatable between years, and among females of polygynous males within years, such that a compatibility function of extra-pair copulations was likewise unsupported. Instead, we suggest that the observed patterns are most consistent with a fertility insurance role for extra-pair copulations, which does not exclude the greater opportunity for sexual selection through differential ability of males to gain paternity

Characterization of MHC-I in the blue tit (Cyanistes caeruleus) reveals low levels of genetic diversity and trans-population evolution across European populations

The major histcompatibility complex (MHC) is a vital component of the adaptive immune system in all vertebrates. This study is the first to characterize MHC class I (MHC-I) in blue tits (Cyanistes caeruleus), and we use MHC-I exon 3 sequence data from individuals originating from three locations across Europe: Spain, the Netherlands to Sweden. Our phylogeny of the 17 blue tit MHC-I alleles contains one allele cluster with low nucleotide diversity compared to the remaining more diverse alleles. We found a significant evidence for balancing selection in the peptide-binding region in the diverse allele group only. No separation according to geographic location was found in the phylogeny of alleles. Although the number of MHC-I loci of the blue tit is comparable to that of other passerine species, the nucleotide diversity of MHC-I appears to be much lower than that of other passerine species, including the closely related great tit (Parus major) and the severely inbred Seychelles warbler (Acrocephalus sechellensis). We believe that this initial MHC-I characterization in blue tits provides an important step towards understanding the mechanisms shaping MHC-I diversity in natural populations

Fitting genes : sexual selection in the blue tit : the role of the MHC and post-copulatory effects

De genen van het Major Histocompatibility Complex (MHC) spelen een belangrijke rol in de herkenning van ziekteverwekkers door het immuunsysteem. Individuen met diverse MHC genen kunnen veel verschillende ziekteverwekkers herkennen. Mijn proefschrift onderzocht de partnerkeuze van een bekende zangvogelsoort, de pimpelmees. Pimpelmeesvrouwen met diverse MHC genen bleken een partner hebben die ook diverse MHC genen heeft, terwijl vrouwen met een lage diversiteit een man met lage diversiteit hebben. Dit patroon is te verklaren doordat zowel man als vrouw een voorkeur hebben voor een partner met diverse MHC genen. Individuen met diverse MHC genen zullen succesvol zijn in hun partnerkeuze, terwijl individuen met minder diverse MHC genen het moeten doen met een partner die minder hun voorkeur genoot. Het vinden van een partner met diverse MHC genen zal leiden tot nakomelingen met diverse MHC genen en een goede gezondheid. Seksuele selectie eindigt echter niet bij het vinden van een partner. Ongeveer 11% van alle pimpelmeesjongen is namelijk buitenechtelijk. Tijdens en na de paring kunnen zowel de man als de vrouw de vaderschap beïnvloeden. Zo kan een man meer sperma insemineren als hij weet dat de vrouw met andere mannen heeft gepaard en kan de vrouw besluiten het sperma van onaantrekkelijke mannen niet op te nemen. Wij onderzochten het aantal spermacellen in het reproductieve systeem van vrouwelijke pimpelmezen. Uit onze resultaten blijkt dat vrouwtjes gepaard met een grotere man meer sperma hadden. Dat zou kunnen betekenen dat grotere mannen aantrekkelijker zijn, meer sperma produceren of dat vrouwtjes gepaard met een grotere man meer vreemdgaan. The genes of the Major Histocompatibility Complex (MHC) play a key role in immune recognition. Individuals with diverse MHC genes are able to recognize and resist many different infections. My thesis investigated mate choice in a common passerine bird, the blue tit. My results show that females with diverse MHC genes had a partner with diverse MHC genes, while females with MHC genes with low diversity had a partner with low diversity. This pattern can be explained by both males and females choosing a partner with diverse MHC genes. Individuals with diverse MHC genes will end up with their preferred partner, while individuals with low MHC diversity may have to settle for a less preferred partner. Obtaining a partner with high MHC diversity will lead to offspring with diverse MHC genes and good health. Sexual selection does not end when a partner has been found. In the blue tit, about 11% of all offspring result from cuckoldry. During and after copulation both the male and the female may affect paternity. A male may for instance inseminate more sperm when he knows he competes with other males, while a female may eject the sperm of unattractive males. We investigated the number of sperm in the reproductive tract of female blue tits. Our results demonstrate that females paired to a larger male had more sperm. This may indicate that larger males are more attractive, produce more sperm or that females paired to a larger male copulate more with other males.

Ultraviolet crown colouration affects contest outcomes among male blue tits, but only in the absence of prior encounters

P>1. Static animal colour patches may function among competitors to minimize conflict escalation over resources, by serving as a signal of resource holding potential or aggressiveness. Empirical evidence for the use of colour patches in conflict resolution is largely restricted to pigment-based colours (melanins and carotenoids) and rarely defines the context in which the signals are used. 2. Here we test whether structural-based ultraviolet (UV) crown colouration functions in conflict resolution among dyads of first-year male blue tits Cyanistes caeruleus in captivity, in a context that is known to favour aggressive individuals. 3. We found that on first encounter in a pairwise context, experimentally UV-reduced males were significantly more likely to lose to control-treated opponents than expected by chance. However, this disparity was less pronounced when conflicts were settled with physical fighting or when the opponent was considerably smaller in size. 4. When the same dyads were tested again several weeks later, but with the UV treatment reversed among opponents, none of the effects remained significant, but instead the winner was most likely to be the individual that won at their first encounter. 5. Our results suggest that structural-based UV colouration can affect the outcome of an interaction, but that size differences and the outcome of initial interactions between opponents can override the influence of this signal in conflict resolution. Whether there is a functional basis to maintain a link between aggressiveness and colouration may thus be highly dependent on the general context under which individuals compete in the wild

Reduced extrapair paternity in response to experimental stimulation of earlier incubation onset in blue tits

Although the causes and consequences of extrapair paternity (EPP) have been studied extensively in birds, little is known about the regulation of extrapair copulation (EPC) behavior and how it may tie in with other aspects of avian reproduction. In birds, the presence of eggs stimulates incubation and, subsequently, the cessation of egg production. We propose that the same mechanism also regulates female motivation to engage in EPCs. To test this idea, we simulated the earlier onset of laying in blue tits (Cyanistes caeruleus), by adding model eggs to nests before natural laying commenced. Most females accepted these eggs, covering them with nest material in the natural way. As predicted, these broods hatched more asynchronously than control broods, revealing an earlier onset of incubation, and were less likely to contain extrapair offspring (EPO) suggesting that stimulation from eggs also inhibits motivation to seek EPCs. Egg stimulation is thought to cause cessation of laying a fixed number of days before clutch completion, after a certain hormonal threshold is exceeded. Similarly, a lower threshold may inhibit engagement in EPCs relative to clutch completion, explaining the proportional increase in EPO toward the extremes of clutch size, which we also observed, and would be consistent with a fertility or compatibility insurance function for EPCs. Our findings may represent the best experimental evidence for female-mediated effects on the timing of EPCs and suggest how EPP may be integrated within the regulatory mechanism of avian reproduction.