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Biomedical subjects

T R Birkhead

Publications and source records attributed to T R Birkhead.

At least 19 recordsLinked to original sources
PubMed2005

Genetic effects on sperm design in the zebra finch.

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.

Animals↗
PubMed2004

Evolutionary ecology of the prezygotic stage.

The life cycles of sexually reproducing animals and flowering plants begin with male and female gametes and their fusion to form a zygote. Selection at this earliest stage is crucial for offspring quality and raises similar evolutionary issues, yet zoology and botany use dissimilar approaches. There are striking parallels in the role of prezygotic competition for sexual selection on males, cryptic female choice, sexual conflict, and against selfish genetic elements and genetic incompatibility. In both groups, understanding the evolution of sex-specific and reproductive traits will require an appreciation of the effects of prezygotic competition on fitness.

Animals↗
PubMed2004

Nontransitivity of paternity in a bird.

In many animals reproductive success is determined after insemination by the interaction of male and female processes. While sperm competition is reasonably well understood in some taxa, other processes, such as cryptic female choice and differential early embryo mortality resulting from genetic incompatibilities, are less well understood. The relative importance of these different factors contributing to reproductive success is difficult to assess. Here we control for male-mediated effects (which are often considerable and can mask more subtle processes) through the artificial insemination of known numbers of sperm in the domestic fowl to reveal that male reproductive success is nontransitive across females: the success of a particular male depends on the background against which his sperm compete for fertilization. Two potential processes could account for this effect: cryptic female choice (sperm choice) or differential early embryo mortality. Regardless of the mechanism, nontransitivity of male reproductive success has important evolutionary consequences, including the maintenance of variation in male fitness.

Analysis of Variance↗
PubMed2002

Sperm viability and sperm competition in insects.

Sperm quality plays an important role in vertebrates in determining which male has the advantage when two or more males compete to fertilize a female's ova. In insects, however, the importance of sperm quality has never been considered, despite sperm competition being widespread and well studied in this group. We tested the hypothesis that sperm viability, measured as the proportion of live sperm, covaried with the intensity of sperm competition in insects. In a pairwise comparison of seven closely related species pairs, each comprising a monandrous and a polyandrous species (i.e., with and without sperm competition, respectively), we found that in all cases the polyandrous species had a higher proportion of live sperm in their sperm stores. The distribution of the percentage of live sperm showed considerable inter- and intraspecific variation, suggesting that, all else being equal, males will vary in their ability to fertilize ova on the basis of sperm viability alone. Our results suggest that sperm viability is one of a suite of male adaptations to sperm competition in insects.

Animals↗
PubMed2002

The sexually-selected sperm hypothesis: sex-biased inheritance and sexual antagonism.

When females are inseminated by more than one male (polyandry) sexual selection continues after insemination in the form of sperm competition and cryptic female choice. The sexually-selected sperm hypothesis proposes that, under the risk of sperm competition, additive variation in male traits determining fertilising efficiency will select for female propensity to be polyandrous in order to increase the probability of producing sons with superior fertilising efficiency. Two factors complicate this prediction: sex-biased transmission of male fertilising efficiency traits and sexual antagonism of sex-limited traits, fostered by sex-biased inheritance. Here, we (i) review the evidence that male traits contributing towards fertilising efficiency are heritable through sex-biased mechanisms, and (ii) explore the evolutionary implications for male and female reproductive strategies caused by both sex-biased transmission and sexual antagonism of fertilising efficiency traits. Many male fertilising efficiency traits are heritable through sex-biased mechanisms and may not necessarily increase female fitness. The predictions of the sexually-selected sperm hypothesis change dramatically under these different mechanisms of inheritance of fertilising efficiency traits, and different fitness pay-offs derived by females from the expression of such traits. Both sex-biased control of fertilising efficiency and sexual antagonism may also be important in explaining the maintenance of the genetic variance and selection potential of fertilising efficiency. We propose that a useful approach to test the sexually-selected sperm hypothesis is to combine studies which identify behavioural and physiological mechanisms explaining variation in reproductive success with artificial selection experiments to infer the underlying evolutionary patterns.

Animals↗
PubMed2002

Pre- and post-insemination episodes of sexual selection in the fowl, Gallus g. domesticus.

Although much attention has been recently directed to sexual selection arising after insemination from sexual promiscuity, little is known about the mechanisms determining reproductive success after insemination, and the way these mechanisms interact with each other and with selective mechanisms occurring before insemination: mate choice and mate acquisition. Here, we briefly review the findings of an on-going study investigating the mechanisms generating variation in reproductive success at both a pre- and a post-insemination stage in the domestic fowl. Female preference consistently favours socially dominant males before and after insemination. However, although social status mediates the number of sperm that a male inseminates into a female, dominant males may inseminate sperm of lower fertilising quality than their subordinates. We argue that mitochondrial genes may contribute to determine sperm quality, and speculate that the maternal control of mitochondrial genes may prevent sexual selection from operating on males, thus explaining both the lack of a positive correlation between social dominance and sperm quality and the maintenance of variation in male quality in the fowl.

Animals↗
PubMed2001

Ejaculate allocation by male sand martins, Riparia riparia.

Males of many species allocate sperm to ejaculates strategically in response to variation in the risk and intensity of sperm competition. The notable exception is passerine birds, in which evidence for strategic allocation is absent. Here we report the results of a study testing for strategic ejaculate allocation in a passerine bird, the sand martin (Riparia riparia). Natural ejaculates were collected from males copulating with a model female. Ejaculates transferred in the presence of a rival male contained significantly more sperm than ejaculates transferred in the absence of a rival male. There was no evidence that this difference was due to the confounding effects of the year of ejaculate collection, the identity of the model female, the colony, the stage of season or the period of the day in which ejaculates were collected. A more detailed examination of the ejaculate patterns of individual males, achieved by the DNA profiling of ejaculates, provided additional evidence for strategic allocation of sperm.

Animals↗
PubMed2000

Female feral fowl eject sperm of subdominant males.

Paternity is often determined by competition between the ejaculates of different males. Males can also use particular behaviours or structures to manipulate how females use sperm. However, the ability of females to bias sperm utilization in favour of preferred males independently of male manipulation has not been demonstrated. Females are predicted to respond differentially to the sperm of different males when the reproductive interests of the sexes differ and when females are coerced into copulating. Here we show that in female feral fowl most copulations are coerced, and that females consistently bias sperm retention in favour of the preferred male phenotype. Females prefer to copulate with dominant males, but when sexually coerced by subordinate males, they manipulate the behaviour of dominant males to reduce the likelihood of insemination. If this fails, females differentially eject ejaculates according to male status in the absence of any male manipulation and preferentially retain the sperm of dominant males.

Animals↗
PubMed1999

No evidence for killer sperm or other selective interactions between human spermatozoa in ejaculates of different males in vitro.

This study examines one of the possible mechanisms of sperm competition, i.e. the kamikaze sperm hypothesis. This hypothesis states that sperm from different males interact to incapacitate each other in a variety of ways. We used ejaculates from human donors to compare mixes of semen in vitro from the same or different males. We measured the following parameters: (i) the degree of sperm aggregation, velocity and proportion of morphologically normal sperm after 1 and 3 h incubation in undiluted semen samples, (ii) the proportion of viable sperm plus the same parameters as in (i) in 'swim-up' sperm suspensions after 1 and 3 h incubation, (iii) the degree of self and non-self sperm aggregation using fluorescent dyes to distinguish the sperm of different males, and (iv) the extent of sperm capacitation and acrosome-reacted sperm in mixtures of sperm from the same and different males. We observed very few significant changes in sperm aggregation or performance in mixtures of sperm from different males compared with mixtures from the same male and none that were consistent with previously reported findings. The incapacitation of rival sperm therefore seems an unlikely mechanism of sperm competition in humans.

Acrosome Reaction↗
PubMed1999

Sperm mobility determines the outcome of sperm competition in the domestic fowl.

The aim of this study was to establish whether the mobility of sperm of the domestic fowl, as measured by an in vitro assay, predicted the outcome of sperm competition. Thirteen pairs of New Hampshire roosters, comprising one male categorized as having high-mobility sperm and the other as having average-mobility sperm, were used. Each male provided 25 x 10(6) sperm, which were mixed and artificially inseminated into between four and seven New Hampshire hens, each of which produced 2-11 offspring. The experiment was conducted twice, such that the same pair of males inseminated the same females. Paternity was assigned by using microsatellite markers. There was a clear effect of sperm-mobility phenotype on the outcome of sperm competition: in all 13 pairs the high-mobility male fathered the majority of offspring (75.3% overall; p < 0.0001). The proportion of offspring fathered by the high-mobility male within pairs varied significantly between male pairs (p < 0.0005). This effect was associated with the difference in sperm-mobility scores between males within pairs; there was a significant positive relationship between the proportion of offspring fathered by the high-mobility male and the ratio of mobility scores between males (p < 0.05). In addition, compared with their success predicted from the non-competitive situation, in the competitive situation high-mobility males were disproportionately successful in fertilizing eggs compared with average-mobility males. This may occur because female sperm storage is limited in some way and a greater proportion of high-mobility sperm gain access to the female's sperm storage tubules. There was no evidence that female effects accounted for any of the variation in paternity.

Animals↗
PubMed1998

Sperm transport in the reproductive tract of female zebra finches (Taeniopygia guttata).

Aspects of sperm transport in the oviducts of female zebra finches were examined by recording the decline in the number of spermatozoa on the outer perivitelline layer of successively laid eggs. Data from a single clutch of eggs from 32 females indicates that the mean per capita rate of loss of spermatozoa was estimated to be 0.0170 +/- 0.002 SEM spermatozoa h-1. However, individual females showed no consistency in the rate at which spermatozoa were lost from their oviduct over five successive clutches. Models of the mechanism of sperm competition in birds assume that the rate of loss of spermatozoa does not differ between inseminations made before or after the start of egg laying. This assumption was found to be valid: the instantaneous per capita rate of loss of spermatozoa did not differ significantly between females inseminated either before (0.01445 +/- 0.0028 SEM spermatozoa h-1) or after (0.01674 +/- 0.0023 SEM spermatozoa h-1) the onset of oviposition. The rate of sperm transport through the infundibulum was determined to be slower than that between the utero-vaginal sperm storage tubules and the infundibulum by comparing the number of spermatozoa associated with the perivitelline layers of eggs laid after a day on which no eggs were laid. Eggs with < 20 spermatozoa on the outer perivitelline layer were found to have a 50% probability of being infertile. The results are compared with data for domestic fowl and turkeys.

Animals↗
PubMed1998

Sperm competition in birds.

Sperm competition in birds occurs when a female is inseminated by more than one male during a single breeding cycle. Despite most birds being socially monogamous, sperm competition is widespread and results in frequent extra-pair paternity. Sperm competition is a fundamental part of sexual selection since it results in differential reproductive success among males. Male adaptations to sperm competition include relatively large testes, large sperm stores and long spermatozoa, mate guarding and frequent pair copulations. Females show no obvious morphological adaptations to sperm competition but, by controlling whether copulations are successful, they probably determine its frequency and extent. Despite this, the evolutionary benefits females acquire from extra-pair fertilizations are poorly understood. Experiments in which females are inseminated with equal numbers of spermatozoa from two males usually show last male sperm precedence. Understanding the mechanism of sperm competition requires understanding of why the last male to inseminate a female fertilizes a disproportionate number of eggs. The data from sperm competition studies on the domestic fowl, turkeys and zebra finches are consistent only with a passive sperm loss model of sperm competition. The mechanism is as follows: after insemination, spermatozoa enter the sperm storage tubules located in the oviduct, from which they are lost at a constant rate over days or weeks. All else being equal, the interval between two inseminations determines the probability of fertilization: the second of two inseminations fertilizes most eggs simply because, by the time fertilization occurs, fewer of these spermatozoa have been lost. Other factors also affect the outcome of sperm competition: the timing of insemination relative to oviposition, the differential fertilizing capacity of males and differences in the numbers of spermatozoa inseminated; as a consequence, last male sperm precedence is not automatic. On the basis of the mechanism of sperm competition, the optimal strategy for both males and females to maximize their likelihood of extra-pair fertilization is to copulate with an extra-pair partner as close as possible to the onset of oviposition.

Adaptation, Physiological↗
PubMed1995

Male phenotype and ejaculate quality in the zebra finch Taeniopygia guttata.

We tested the idea that female preference for relatively attractive extra-pair males arises because the morphological and behavioural features that females find attractive covary with ejaculate features: Sheldon's (Proc. R. Soc. Lond. B 257 25-30 (1994) phenotype-linked fertility insurance hypothesis. Two phenotypic traits that female zebra finches find attractive in males are song rate and symmetry of chest band plumage, but we found neither of these to be significantly related to any of the following ejaculate features: number of sperm, percentage of live sperm, absolute number of sperm, sperm length or sperm swimming velocity. Furthermore, and surprisingly, we did not find the predicted negative relationship between male song rate and fluctuating asymmetry of chest band plumage. Because most ejaculate features (except sperm numbers in rested males) show low levels of repeatability, it is unlikely that female zebra finches could reliably obtain a better quality ejaculate by choosing to copulate with a more attractive male. There was thus no evidence for the phenotype-linked fertility insurance hypothesis. Nor did we obtain evidence for the more general fertility insurance hypothesis: we found that female zebra finches paired to a vasectomized male, and hence receiving no sperm, were no more likely to seek an extra-pair copulation than females paired to an intact male.

Animals↗
PubMed1995

Sperm competition: evolutionary causes and consequences.

The interaction between functional and mechanistic approaches to sperm competition and between male and female perspectives are described and illustrated by a study of the zebra finch, Taeniopygia guttata. Sperm competition experiments in the laboratory show that last male sperm precedence occurs (as it does in many other taxa) although the mechanism is unknown (as in most other taxa). Empirically-derived values were used to construct a mathematical model of sperm competition in the zebra finch. The model indicates that precedence occurs as a consequence of: (i) the temporal pattern of pair copulations; (ii) the rate at which sperm are lost from the female tract; and (iii) more sperm being transferred during extra-pair copulations than during pair copulations. The latter effect is a consequence of males seeking extra-pair copulations after their own pair copulation period has ended. The effect of sperm numbers on the pattern of sperm precedence may be further increased by: (i) extra-pair males increasing ejaculate size (sperm numbers) (for which there is no evidence); (ii) extra-pair males being of a better quality and transferring more sperm or better quality sperm (for which there is some evidence); and (iii) cryptic female choice. Females eject over 99% of sperm following insemination; if they eject fewer sperm from males chosen as extra-pair copulation partners, the potential for cryptic female choice is considerable. However, this is still being investigated. The model also predicts the optimal time for an extra-pair copulation to occur (from either a male or female perspective). A comparison between the predicted and observed pattern suggests that the optimal timing of extra-pair copulations is constrained in both sexes.

Animals↗
PubMed1994

A comparative study of sperm-egg interactions in birds.

A comparative study was made of the number of spermatozoa trapped on the outer perivitelline layer and the number of spermatozoa penetrating the inner perivitelline layer of the eggs of 27 species of bird. The total number of spermatozoa (trapped spermatozoa plus holes made by spermatozoa) varied between 29 and 164,000 per egg among species and was significantly and positively correlated with size of the ovum. In most species, holes formed a 'halo' around the germinal disc area and the density of holes was much greater in this region than elsewhere, especially in passerine birds. In some species, a high proportion of holes occurred at some distance from the germinal disc. This seems to be an artefact due to the fact that some spermatozoa trapped in the outer perivitelline layer undergo proteolytic activity between fertilization and oviposition and create additional holes in the inner perivitelline layer both at and away from the germinal disc. Across all species and within most individual species, the number of trapped spermatozoa was positively correlated with the number of holes in the inner perivitelline layer. Decreases in the total number of spermatozoa on successive eggs of a clutch provided an index of the rate at which spermatozoa were used from the sperm storage tubules.

Animals↗
PubMed1993

Selection and utilization of spermatozoa in the reproductive tract of the female zebra finch Taeniopygia guttata.

The numbers and proportion of spermatozoa reaching different parts of the female reproductive tract after a single natural insemination were investigated in zebra finches Taeniopygia guttata. The number of spermatozoa transferred during a single, natural copulation was estimated by comparing the number of spermatozoa in the seminal glomera of males that had performed a single copulation with control males. The mean number of spermatozoa per ejaculate was 5.8 x 10(6) +/- 1.80 x 10(6) SEM. The mean number of spermatozoa stored in the sperm storage tubules in the uterovaginal junction following a single, natural insemination was 6027 +/- 1874, 0.104% of those inseminated. The mean number of spermatozoa reaching the infundibulum and trapped on the perivitelline layer of all eggs of the clutch after a single copulation was 45.6 +/- 9.18 and a further 36 penetrated the perivitelline layer of the ovum, i.e. 82 in total (1.4% of the spermatozoa in the sperm storage tubules and 0.001% of spermatozoa in the ejaculate). Female zebra finches that completed a natural breeding cycle with a mean of 12 copulations had 404 +/- 111 spermatozoa trapped on the perivitelline layer of all eggs of the clutch, and an estimated further 173 spermatozoa penetrated the perivitelline layer. A smaller proportion of spermatozoa was trapped on the perivitelline layer of zebra finch eggs, than in chicken or turkey eggs.

Animals↗