No Evidence for Pre-Copulatory Sexual Selection on Sperm Length in a Passerine Bird

There is growing evidence that post-copulatory sexual selection, mediated by sperm competition, influences the evolution of sperm phenotypes. Evidence for pre-copulatory sexual selection effects on sperm traits, on the other hand, is rather scarce. A recent paper on the pied flycatcher, Ficedula hypoleuca, reported phenotypic associations between sperm length and two sexually selected male traits, i.e. plumage colour and arrival date, thus invoking pre-copulatory sexual selection for longer sperm. We were unable to replicate these associations with a larger data set from the same and two additional study populations; sperm length was not significantly related to either male plumage colour or arrival date. Furthermore, there was no significant difference in sperm length between populations despite marked differences in male plumage colour. We also found some evidence against the previously held assumption of longer sperm being qualitatively superior; longer sperm swam at the same speed as shorter sperm, but were less able to maintain speed over time. We argue that both empirical evidence and theoretical considerations suggest that the evolution of sperm morphology is not primarily associated with pre-copulatory sexual selection on male secondary sexual traits in this or other passerine bird species. The relatively large between-male variation in sperm length in this species is probably due to relaxed post-copulatory sexual selection

Sperm Length Variation as a Predictor of Extrapair Paternity in Passerine Birds

The rate of extrapair paternity is a commonly used index for the risk of sperm competition in birds, but paternity data exist for only a few percent of the approximately 10400 extant species. As paternity analyses require extensive field sampling and costly lab work, species coverage in this field will probably not improve much in the foreseeable future. Recent findings from passerine birds, which constitute the largest avian order (∼5,900 species), suggest that sperm phenotypes carry a signature of sperm competition. Here we examine how well standardized measures of sperm length variation can predict the rate of extrapair paternity in passerine birds.We collected sperm samples from 55 passerine species in Canada and Europe for which extrapair paternity rates were already available from either the same (n = 24) or a different (n = 31) study population. We measured the total length of individual spermatozoa and found that both the coefficient of between-male variation (CV(bm)) and within-male variation (CV(wm)) in sperm length were strong predictors of the rate of extrapair paternity, explaining as much as 65% and 58%, respectively, of the variation in extrapair paternity among species. However, only the CV(bm) predictor was independent of phylogeny, which implies that it can readily be converted into a currency of extrapair paternity without the need for phylogenetic correction.We propose the CV(bm) index as an alternative measure to extrapair paternity for passerine birds. Given the ease of sperm extraction from male birds in breeding condition, and a modest number of sampled males required for a robust estimate, this new index holds a great potential for mapping the risk of sperm competition across a wide range of passerine birds

Variation in testis size and sperm morphology in the bluethroat, Luscinia svecica svecica

In this study I examined intra-specific variation in primary sex traits in male bluethroats (Luscinia svecica svecica), a passerine species with a high intensity of sperm competition, The single most important predictor of the size of testes and seminal glomera was male age (yearlings versus older). This finding suggests that older males have higher sperm production rates, which may allow for higher copulation rates and/or larger ejaculates than in younger males. Previous findings of older males having a higher extra-pair fertilization success, and similar paternity loss in own nest as younger males despite less intense mate guarding, contribute to a general pattern of age-dependent sperm competition investments in male bluethroats. None of the measured conditional variables (body mass, haemoglobin, haematocrit), nor the body size variables (wing length, tarsus length, scull length) correlated significantly with testis size. The bluethroat sperm were relatively long (216.4 µm, n=46, SD=3.1µm) as expected for a species with intense sperm competition. Between-male variation in average sperm length was considerably larger than the within-male variation in individual sperm length. However, the between-male coefficient of variation in mean sperm length was considerably lower than that reported recently for another passerine with low intensities of sperm competition. More species should be examined to test for a possible relationship between inter-male variation in sperm length and the intensity of sperm competition

Sperm morphology and function in passerine birds: insights from intra- and interspecific studies

Sperm cells are the most variable animal cells, and a tremendous variation in sperm phenotypes exists among species, from minute amoeboid sperm to giant sperm in some species of fruit flies. Much effort has been devoted to the study of sperm evolution. Sperm competition, when sperm from two or more males compete over fertilization of a set of ova, is thought to be the major force driving the evolution of sperm. There are several ways in which sperm competition can effect the evolution of sperm traits, e.g. through increased sperm production, sperm motility and size. This thesis concerns variation among and function of sperm in passerine birds. At three levels, within males, between males of the same species and between males of different species, co-workers and I have studied variation in sperm morphology and motility of selected species of passerine birds. Sperm traits, such as sperm numbers, sperm size and sperm motility are potentially important for male fertilization success. Hence, sperm traits are expected to be under strong selection due to evolutionary forces such as e.g. sperm competition. Thus, it is surprising how scarce the available information on sperm traits (size, variation, motility) is in passerine birds. In an intraspecific study on bluethroats Luscinia svecica, we found that primary sexual characters, but not sperm size or motility, were age-dependent. These results suggest that increased sperm production in older males can help explain the age-dependent patterns in fertilization success observed in this species. Further, we documented that between-male variation in sperm size is higher than within-male in bluethroats and willow warblers Phylloscopus trochilus, and through resampling procedures we suggested appropriate sample sizes for spermatozoa measured per male and number of males that should be sampled to obtain precise estimates of sperm size and sperm size variation. Moreover, we found that variation in sperm size is closely and negatively related to risk of sperm competition, in a comparative analysis of 22 passerine species. Our interpretation of these results is that increased risk of sperm competition leads to stabilizing selection on sperm size. In another comparative study on 42 passerine species, we documented that sperm swimming speed and sperm length is positively related to risk of sperm competition. However, we found no relationship between sperm swimming speed and sperm size. Thus, it seems that sperm competition leads to both increased sperm swimming speed and increased sperm length, independent of each other. In addition, sperm swimming speed was negatively related to clutch size, a proxy for female sperm storage. Finally, we found in an intraspecific study on tree swallows Tachycineta bicolor, that both sperm quantity and quality were significantly related to fertilization success. This result has great implications as the first study linking sperm traits and fertilization success in a free living passerine

Deep sympatric mitochondrial divergence without reproductive isolation in the common redstart Phoenicurus phoenicurus

Mitochondrial DNA usually shows low sequence variation within and high sequence divergence among species, which makes it a useful marker for phylogenetic inference and DNA barcoding. A previous study on the common redstart (Phoenicurus phoenicurus) revealed two very different mtDNA haplogroups (5% K2P distance). This divergence is comparable to that among many sister species; however, both haplogroups coexist and interbreed in Europe today. Herein, we describe the phylogeographic pattern of these lineages and test hypotheses for how such high diversity in mtDNA has evolved. We found no evidence for mitochondrial pseudogenes confirming that both haplotypes are of mitochondrial origin. When testing for possible reproductive barriers, we found no evidence for lineage-specific assortative mating and no difference in sperm morphology, indicating that they are not examples of cryptic species, nor likely to reflect the early stages of speciation. A gene tree based on a short fragment of cytochrome c oxidase subunit 1 from the common redstart and 10 other Phoenicurus species, showed no introgression from any of the extant congenerics. However, introgression from an extinct congeneric cannot be excluded. Sequences from two nuclear introns did not show a similar differentiation into two distinct groups. Mismatch distributions indicated that the lineages have undergone similar demographic changes. Taken together, these results confirm that deeply divergent mitochondrial lineages can coexist in biological species. Sympatric mtDNA divergences are relatively rare in birds, but the fact that they occur argues against the use of threshold mtDNA divergences in species delineation. Assortative mating, mtDNA, reproductive isolation, sympatric divergencepublishedVersio

Early diversification of sperm size in the evolutionary history of the old world leaf warblers (Phylloscopidae)

Sperm morphological traits are highly variable among species and are commonly thought to evolve by post-copulatory sexual selection. However, little is known about the evolutionary dynamics of sperm morphology, and whether rates of evolutionary change are variable over time and among taxonomic groups. Here, we examine sperm morphology from 21 species of Old World leaf warblers (Phylloscopidae), a group of generally dull, sexually monochromatic birds, which are known to have high levels of extra-pair paternity. We found that sperm length differs markedly across species, spanning about 40% of the range observed across a larger selection of passerine birds. Furthermore, we found strong support for an ‘early-burst’ model of trait evolution, implying that the majority of divergence in sperm length has occurred early in the evolutionary history of this clade with subsequent evolutionary stasis. This large early divergence matches the early divergence reported in ecological traits (i.e. body size and feeding behaviour). Our findings demonstrate that rates of evolution in sperm morphology can change over time in passerine taxa, and that evolutionary stasis in sperm traits can occur even in species exhibiting characteristics consistent with moderate-to-high levels of sperm competition. It remains a major challenge to identify the selection mechanisms and possible constraints responsible for these variable rates of sperm evolution. The final version of this reseach has been published in the Journal of Evolutionary Biology. © 2016 Wile

Salinity-induced phenotypic plasticity in threespine stickleback sperm activation

acceptedVersio

Supplementary_data_sperm_activation_in_stickleback

Sperm velocity score numbers for threespine stickleback originating from saltwater and freshwater tested in saltwater and freshwater. Please see Supplementary 1 for a more detailed description

When Older Males Sire More Ofspring—Increased Attractiveness or Higher Fertility?

In birds with extrapair mating, older males usually have higher fertilization success than younger males. Two hypotheses can potentially explain this pattern: 1) females prefer older, and often more ornamented males, or 2) older males invest more in reproduction and fertility than younger males. Here we studied factors associated with age-related male fertilization success in a population of barn swallows Hirundo rustica in Canada. We document that male fertilization success increased gradually up to a minimum age of four-year old. The age effect was especially strong for the number of extrapair offspring obtained and the occurrence of a second brood. The higher fertilization success of older males was also associated with an early start of breeding in spring. The length of the elongated outermost tail feathers, a postulated male ornament preferred by females, also increased with age (in both sexes), but it was not a significant predictor of male fertilization success within age classes. Male fertility traits, especially testis size, but also sperm motility and sperm velocity, increased significantly across age groups. Our results suggest that the higher fertilization success by older males is due to their higher reproductive investments and that their longer tails are an adaptation to early arrival on the breeding grounds

Weak geographical structure in sperm morphology across the range of two willow warbler Phylloscopus trochilus subspecies in Scandinavia

Sperm morphology is highly diversified among species and at higher taxonomic levels. In birds, there is also increasing evidence of geographical differentiation in sperm traits within species, especially in those with strong sperm competition. Geographical divergences in sperm traits might imply the formation of a reproductive barrier in a speciation process. Here we study sperm morphology variation of willow warblers Phylloscopus trochilus in a geographical context in Scandinavia, across the range of two subspecies that are differentiated in certain genetic markers, morphology and migratory routes. We describe geographical patterns in genotypes (two previously described single-nucleotide polymorphism (SNP) markers and four polymorphic microsatellites); in wing length, tarsus length and body mass; and in sperm traits of 330 male willow warblers sampled at 33 localities across Norway (58°N–69°N). Birds were on average larger and longer-winged in the north (spp. acredula) than in the south (spp. trochilus), and showed a sigmoid change in the SNP allele frequencies and body morphology around 65°N. We found no evidence of genetic structuring in the microsatellites. There was no geographical variation in sperm traits across Norway, except that sperm heads were on average longer in the south. Sperm head length was also associated with the two SNP markers, with longer sperm heads for the southern alleles, and midpiece length was weakly associated with one of the SNP markers. Similar among-male variances in total sperm length among the 33 sampling sites indicate uniform levels of sperm competition across Norway. We conclude that sperm morphology remains a rather undifferentiated trait between the two willow warbler subspecies in Scandinavia, which is consistent with a pattern of a shallow genetic divergence. This indicates that sperm morphology is not a reproductive barrier maintaining the narrow hybrid zone. The final version of this research has been published in Journal of Avian Biology. © 2016 Wile