Sophisticated sperm allocation in male fowl

When a female is sexually promiscuous, the ejaculates of different males compete for the fertilization of her eggs; the more sperm a male inseminates into a female, the more likely he is to fertilize her eggs. Because sperm production is limited and costly, theory predicts that males will strategically allocate sperm (1) according to female promiscuity, (2) saving some for copulations with new females, and (3) to females producing more and/or better offspring. Whether males allocate sperm in all of these ways is not known, particularly in birds where the collection of natural ejaculates only recently became possible. Here we demonstrate male sperm allocation of unprecedented sophistication in the fowl Gallus gallus. Males show status-dependent sperm investment in females according to the level of female promiscuity; they progressively reduce sperm investment in a particular female but, on encountering a new female, instantaneously increase their sperm investment; and they preferentially allocate sperm to females with large sexual ornaments signalling superior maternal investment. Our results indicate that female promiscuity leads to the evolution of sophisticated male sexual behaviour

Sexual selection on song and cuticular hydrocarbons in two distinct populations of Drosophila montana

Sexual selection has the potential to contribute to population divergence and speciation. Most studies of sexual selection in Drosophila have concentrated on a single signaling modality, usually either courtship song or cuticular hydrocarbons (CHCs), which can act as contact pheromones. We have examined the relationship between both signal types and reproductive success using F1–3 offspring of wild- collected flies, raised in the lab. We used two populations of the Holarctic species Drosophila montana that represent different phylogeographic clades that have been separate for ca. 0.5 million years (MY), and differ to some extent in both traits. Here, we characterize the nature and identify the targets of sexual selection on song, CHCs, and both traits combined within the populations. Three measures of courtship outcome were used as fitness proxies. They were the probability of mating, mating latency, and the production of rejection song by females, and showed patterns of association with different traits that included both linear and quadratic selection. Courtship song predicted courtship outcome better than CHCs and the signal modalities acted in an additive rather than synergistic manner. Selection was generally consistent in direction and strength between the two populations and favored males that sang more vigorously. Sexual selection differed in the extent, strength, and nature on some of the traits between populations. However, the differences in the directionality of selection detected were not a good predictor of population differences. In addition, a character previously shown to be important for species recognition, interpulse interval, was found to be under sexual selection. Our results highlight the complexity of understanding the relationship between within-population sexual selection and population differences. Sexual selection alone cannot predict differences between populations.Publisher PDFPeer reviewe

The epidemiology underlying age-related avian malaria infection in a long-lived host: the mute swan Cygnus olor

Quantifying the factors that predict parasite outbreak and persistence is a major challenge for both applied and fundamental biology. Key to understanding parasite prevalence and disease outbreaks is determining at what age individuals show signs of infection, and whether or not they recover. Age-dependent patterns of the infection of a host population by parasites can indicate among-individual heterogeneities in their susceptibility to, or rate of recovery from, parasite infections. Here, we present a cross-sectional study of avian malaria in a long-lived bird species, the mute swan Cygnus olor, examining age-related patterns of parasite prevalence and modelling patterns of infection and recovery. One-hundred and fifteen swans, ranging from one to nineteen years old, were screened for infection with Plasmodium, Haemoproteus and Leucocytozoon parasites. Infections with three cytochrome-b lineages of Haemoproteus were found (pooled prevalence 67%), namely WW1 (26%), which is common in passerine birds, and two new lineages closely related to WW1: MUTSW1 (25%) and MUTSW2 (16%). We found evidence for age-related infection in one lineage, MUTSW1. Catalytic models examining patterns of infection and recovery in the population suggested that infections in this population were not life-long – recovery of individuals was included in the best fitting models. These findings support the results of recent studies that suggest hosts can clear infections, although patterns of infection-related mortality in older birds remain to be studied in more detail

Sperm design and variation in the New World blackbirds (Icteridae)

Post-copulatory sexual selection (PCSS) is thought to be one of the evolutionary forces responsible for the rapid and divergent evolution of sperm design. However, whereas in some taxa particular sperm traits are positively associated with PCSS, in other taxa, these relationships are negative, and the causes of these different patterns across taxa are poorly understood. In a comparative study using New World blackbirds (Icteridae), we tested whether sperm design was influenced by the level of PCSS and found significant positive associations with the level of PCSS for all sperm components but head length. Additionally, whereas the absolute length of sperm components increased, their variation declined with the intensity of PCSS, indicating stabilizing selection around an optimal sperm design. Given the diversity of, and strong selection on, sperm design, it seems likely that sperm phenotype may influence sperm velocity within species. However, in contrast to other recent studies of passerine birds, but consistent with several other studies, we found no significant link between sperm design and velocity, using four different species that vary both in sperm design and PCSS. Potential reasons for this discrepancy between studies are discussed

Testing the predictions of sex allocation hypotheses in dimorphic, cooperatively breeding riflemen

Evolutionary theory predicts that parents should invest equally in the two sexes. If one sex is more costly, a production bias is predicted in favour of the other. Two well-studied causes of differential costs are size dimorphism, in which the larger sex should be more costly, and sex-biased helping in cooperative breeders, in which the more helpful sex should be less costly because future helping ‘repays’ some of its parents’ investment. We studied a bird species in which both processes should favour production of males. Female riflemen Acanthisitta chloris are larger than males, and we documented greater provisioning effort in more female-biased broods indicating they are likely costlier to raise. Riflemen are also cooperative breeders, and males provide more help than females. Contrary to expectations, we observed no male bias in brood sex ratios, which did not differ significantly from parity. We tested whether the lack of a population-wide pattern was a result of facultative sex allocation by individual females, but this hypothesis was not supported either. Our results show an absence of adaptive patterns despite a clear directional hypothesis derived from theory. This appears to be associated with a suboptimal female-biased investment ratio. We conclude that predictions of adaptive sex allocation may falter because of mechanistic constraint, unrecognised costs and benefits, or weak selection

Conflict-reducing innovations in development enable increased multicellular complexity

Obligately multicellular organisms, where cells can only reproduce as part of the group, have evolved multiple times across the tree of life. Obligate multicellularity has only evolved when clonal groups form by cell division, rather than by cells aggregating, as clonality prevents internal conflict. Yet obligately multicellular organisms still vary greatly in ‘multicellular complexity’ (the number of cells and cell types): some comprise of a few cells and cell-types, while others have billions of cells and thousands of types. Here, we test whether variation in multicellular complexity is explained by two conflict suppressing mechanisms, namely a single cell bottleneck at the start of development, and a strict separation of germline and somatic cells. Examining the life-cycles of 129 lineages of plants, animals, fungi and algae, we show using phylogenetic comparative analyses that an early segregation of the germline stem-cell lineage is key to the evolution of more cell types, driven by a strong correlation in the Metazoa. In contrast, the presence of a strict single cell bottleneck was not related to either the number of cells or cell types, but was associated with early germline segregation. Our results suggest that segregating the germline earlier in development enabled greater evolutionary innovation, although whether this is a consequence of conflict reduction or other non-conflict effects, such as developmental flexibility, is unclear

Stallion semen quality depends on major histocompatibility complex matching to teaser mare.

The major histocompatibility complex (MHC) has repeatedly been found to influence mate choice of vertebrates, with MHC-dissimilar mates typically being preferred over MHC-similar mates. We used horses (Equus caballus) to test whether MHC matching also affects male investment into ejaculates after short exposure to a female. Semen characteristics varied much among stallions. Controlling for this variance with a full-factorial within-subject experimental design, we found that a short exposure to an MHC-dissimilar mare enhanced male plasma testosterone and led to ejaculates with elevated sperm numbers as compared to exposure to an MHC-similar mare. Sperm velocity seemed not affected by the treatment. Overall genetic similarity between stallions and mares (determined from polymorphic microsatellites on 20 different chromosomes) played no significant role here. The MHC type of the teaser mare also affected characteristics of cold-stored sperm after 24 and 48 hr. As expected from ejaculate economics, sperm viability was elevated after exposure to an MHC-dissimilar mare. However, oxidative stress and the percentage of sperm with a high DNA fragmentation were mostly increased after exposure to an MHC-dissimilar mare, depending also on whether the teaser mare was in oestrous or not. We conclude that males can quickly adjust ejaculate quality relative to a female's MHC, and that this male reaction to the social environment can also affect important characteristics of cold-stored semen

What is inclusive fitness theory, and what is it for?

Inclusive fitness theory is a cornerstone of modern evolutionary biology, yet critics contend it is not general but subject to serious limitations, and is ripe for replacement, for example by multilevel selection theory. These critics also question empirical predictions made using inclusive fitness theory, such as on sex allocation, and the use of statistical concepts in understanding responses to selection. Here I summarise recent resolutions of these criticisms, then discuss what inclusive fitness theory actually is and why it is useful for evolutionary biology. In doing so I focus on recent developments in evaluating causal explanations for social evolution, and the role of inclusive fitness theory in explaining group adaptations, including the major transitions to obligate eusociality and eukaryotic multicellularity