Sperm design and function are important determinants of male reproductive success and are expected to be under strong selection. The way that spermatozoa phenotypes evolve is poorly understood, because there have been few studies of the quantitative genetics of sperm. Here we show, in the zebra finch Taeniopygia guttata, an extraordinary degree of inter-male variation in sperm design that is independent of sperm swimming velocity. A quantitative genetics study using data from over 900 zebra finches in a complex breeding experiment showed that sperm head, mid-piece and flagellum length are heritable, that negative genetic correlations exist between sperm traits, and that significant indirect (maternal) genetic effects exist. Selection on the zebra finch sperm phenotype may be low because sperm competition is infrequent in this species, and this, in combination with negative genetic correlations and maternal genetic effects, may account for the variation in sperm phenotype between males. These results have important implications for the evolution of sperm in other taxa.

When females are sexually promiscuous, sexual selection continues after insemination through sperm competition and cryptic female choice, and male traits conveying an advantage in competitive fertilization are selected for. Although individual male and ejaculate traits are known to influence paternity in a competitive scenario, multiple mechanisms co-occur and interact to determine paternity. The way in which different traits interact with each other and the mechanisms through which their heritability is maintained despite selection remain unresolved. In the promiscuous fowl, paternity is determined by the number of sperm inseminated into a female, which is mediated by male social dominance, and by the quality of the sperm inseminated, measured as sperm mobility. Here we show that: (i) the number of sperm inseminated determines how many sperm reach the female sperm-storage sites, and that sperm mobility mediates the fertilizing efficiency of inseminated sperm, mainly by determining the rate at which sperm are released from the female storage sites, (ii) like social status, sperm mobility is heritable, and (iii) subdominant males are significantly more likely to have higher sperm mobility than dominant males. This study indicates that although the functions of social status and sperm mobility are highly interdependent, the lack of phenotypic integration of these traits may maintain the variability of male fitness and heritability of fertilizing efficiency.

In many songbird species, females prefer males that sing a larger repertoire of syllables. Males with more elaborate songs have a larger high vocal centre (HVC) nucleus, the highest structure in the song production pathway. HVC size is thus a potential target of sexual selection. Here we provide evidence that the size of the HVC and other song production nuclei are heritable across individual males within a species. In contrast, we ¢nd that heritabilities of other nuclei in a song-learning pathway are lower, suggesting that variation in the sizes of these structures is more closely tied to developmental and environmental di¡erences between individuals. We ¢nd that evolvability, a statistical measure that predicts response to selection, is higher for the HVC and its target for song production, the robustus archistriatalis (RA), than for all other brain volumes measured. This suggests that selection based on the functions of these two structures would result in rapid major shifts in their anatomy. We also show that the size of each song control nucleus is signi¢cantly correlated with the song related nuclei to which it is monosynaptically connected. Finally, we ¢nd that the volume of the telencephalon is larger in males than in females. These ¢ndings begin to join theoretical analyses of the role of female choice in the evolution of bird song to neurobiological mechanisms by which the evolutionary changes in behaviour are expressed.

The lek paradox, in which female choice erodes genetic variation in male sexually selected traits, is a fundamental issue in sexual selection. If females gain only genetic benefits from preferentially having their ova fertilized by males with particular traits, what maintains variation in these traits? Under strong directional selection mediated through mate choice, the alleles for beneficial male traits are expected to go to fixation and exhibit little variation. A theoretical solution to the lek paradox is the genic capture hypothesis which states that: costly male traits subject to female choice are condition dependent, that male condition is dependent on genes at many loci and exhibits additive genetic variance, and that positive genetic correlations exist between sexually selected traits and condition. Using a captive population of the zebra finch Taeniopygia guttata, we tested two key predictions from this model: (1) that genetic variance exists in beak color which is a sexually selected trait, but also in condition and immune function, and (2) that positive genetic correlations exist between condition and beak color, and between beak color, condition, and immune function. Genetic parameters were estimated from a large breeding experiment involving 81 sires, 972 offspring, a pedigree of 1526 individuals, using the animal model. We employed the following index of body condition: residuals from a log-log plot of body mass on tarsus length following a standardized and extended period of exercise, in which residual mass is known to reflect fat and protein reserves. Our results were broadly consistent with the genic capture hypothesis because we found (1) additive genetic variation in beak color and immune function and condition, and (2) positive genetic correlations between condition and beak color, and between condition, beak color, and several assays of immune responsiveness. However, both of these results need qualification. In the first case we identified an important general problem in estimating the coefficient of additive genetic variance (CVA) in body condition. In the second case, although most of the genetic correlations were positive as predicted, only some were statistically significant, possibly due to our relatively small sample sizes, because genetic correlations typically have large standard errors and therefore require very large samples to be statistically significant. The statistically significant, positive genetic correlations included those between beak color and immune function (response to tetanus), and between immune function (response to tetanus) and condition, both of which indicate that females gain good genes from mating with males in good condition and/or with a redder beak color. We discuss the implications of our results for devising more rigorous but pragmatic tests of the genic capture hypothesis.

March 1999A total of 248,230 primiparous records of Holstein cows calving from 1987 to 1994, daughters of 588 sires in 3,042 herds, were used to evaluate potential genotype by environment interactions among mature equivalent milk yield (MEM), lactation mean somatic cell score (LMSCS), and conception rate at first service (CR). Herds were classified into low and high environmental groups using three different criteria: herd MEM standard deviation, a combination of herd MEM mean and herd MEM standard deviation, and the herd mean of body weight at first calving (BWFC) divided by age at first calving (AFC). Genetic parameters were modeled using multiple trait linear mixed models and fitted using the Multiple Trait Derivative Free software (MTDFREML). Heritabilities for MEM, LMSCS and CR were 0.221, 0.106, 0.015 and 0.300, 0.093, 0.009 in low and high environment herds, respectively. Genetic (and phenotypic) correlations between MEM and LMSCS, MEM and CR, LMSCS and CR were 0.277,) in low and high environment herds, respectively. The genetic correlations between pairs of traits were consistently smaller in high environment herds, suggesting that differences in management between the two environment levels lessened the antagonistic genetic association between the traits studied. Breeding programs designed to increase milk while controlling unfavorable correlated changes in LMSCS and CR must take into account the unequal genetic correlations between these traits in the two environments. Our results suggest that the relative weight ofLMSCS and CR in selection indexes, should be smaller in well managed herds than in other environments. -0.417, and -0.209, (-0.049, -0.180, and -0.040) and 0.173, -0.318, and -0.144, (-0.087, -0.166, and -0.035) in low and high environment herds, respectively. The genetic correlations between pairs of traits were consistently smaller in high environment herds, suggesting that differences in management between the two environment levels lessened the antagonistic genetic association between the traits studied. Breeding programs designed to increase milk while controlling unfavorable correlated changes in LMSCS and CR must take into account the unequal genetic correlations between these traits in the two environments. Our results suggest that the relative weight of LMSCS and CR in selection indexes, should be smaller in well managed herds than in other environments.(Key words: milk yield, conception rate, somatic cell score, management).Abbreviation key: MEM = mature equivalent milk, CR = conception rate at first service, LMSCS = lactation mean somatic cell score, FCM = fat corrected milk.