Chromatin accessibility, not 5mC methylation covaries with partial dosage compensation in crows.

The evolution of genetic sex determination is often accompanied by degradation of the sex-limited chromosome. Male heterogametic systems have evolved convergent, epigenetic mechanisms restoring the resulting imbalance in gene dosage between diploid autosomes (AA) and the hemizygous sex chromosome (X). Female heterogametic systems (AAf Zf, AAm ZZm) tend to only show partial dosage compensation (0.5 < Zf:AAf < 1) and dosage balance (0.5

Evolution of Chromosomal Inversions across an Avian Radiation.

Chromosomal inversions are structural mutations that can play a prominent role in adaptation and speciation. Inversions segregating across species boundaries (trans-species inversions) are often taken as evidence for ancient balancing selection or adaptive introgression, but can also be due to incomplete lineage sorting. Using whole-genome resequencing data from 18 populations of 11 recognized munia species in the genus Lonchura (N = 176 individuals), we identify four large para- and pericentric inversions ranging in size from 4 to 20 Mb. All four inversions cosegregate across multiple species and predate the numerous speciation events associated with the rapid radiation of this clade across the prehistoric Sahul (Australia, New Guinea) and Bismarck Archipelago. Using coalescent theory, we infer that trans-specificity is improbable for neutrally segregating variation despite substantial incomplete lineage sorting characterizing this young radiation. Instead, the maintenance of all three autosomal inversions (chr1, chr5, and chr6) is best explained by selection acting along ecogeographic clines not observed for the collinear parts of the genome. In addition, the sex chromosome inversion largely aligns with species boundaries and shows signatures of repeated positive selection for both alleles. This study provides evidence for trans-species inversion polymorphisms involved in both adaptation and speciation. It further highlights the importance of informing selection inference using a null model of neutral evolution derived from the collinear part of the genome.

Occasional paternal inheritance of the germline-restricted chromosome in songbirds.

Songbirds have one special accessory chromosome, the so-called germline-restricted chromosome (GRC), which is only present in germline cells and absent from all somatic tissues. Earlier work on the zebra finch (Taeniopygia guttata castanotis) showed that the GRC is inherited only through the female line-like the mitochondria-and is eliminated from the sperm during spermatogenesis. Here, we show that the GRC has the potential to be paternally inherited. Confocal microscopy using GRC-specific fluorescent in situ hybridization probes indicated that a considerable fraction of sperm heads (1 to 19%) in zebra finch ejaculates still contained the GRC. In line with these cytogenetic data, sequencing of ejaculates revealed that individual males from two families differed strongly and consistently in the number of GRCs in their ejaculates. Examining a captive-bred male hybrid of the two zebra finch subspecies (T. g. guttata and T. g. castanotis) revealed that the mitochondria originated from a castanotis mother, whereas the GRC came from a guttata father. Moreover, analyzing GRC haplotypes across nine castanotis matrilines, estimated to have diverged for up to 250,000 y, showed surprisingly little variability among GRCs. This suggests that a single GRC haplotype has spread relatively recently across all examined matrilines. A few diagnostic GRC mutations that arose since this inferred spreading suggest that the GRC has continued to jump across matriline boundaries. Our findings raise the possibility that certain GRC haplotypes could selfishly spread through the population via occasional paternal transmission, thereby outcompeting other GRC haplotypes that were limited to strict maternal inheritance, even if this was partly detrimental to organismal fitness.

Weak antagonistic fitness effects can maintain an inversion polymorphism.

The study of chromosomal inversion polymorphisms has received much recent attention, particularly in cases where inversions have drastic effects on phenotypes and fitness (e.g. lethality of homozygotes). Less attention has been paid to the question of the maintenance of inversion polymorphisms that show only weak effects. Here, we study the maintenance of such an inversion polymorphism that links 250 genes on chromosome Tgu11 in the zebra finch (Taeniopygia guttata). Based on data from over 6000 captive birds, we estimated the effects of this inversion on a wide range of fitness-related traits. We found that, compared with the ancestral allele A, the inverted allele D had small additive beneficial effects on male siring success and on female fecundity. These fitness-enhancing effects may explain the initial spread of the derived D allele (allele frequency 53%). However, individuals that were homozygous for D had a slightly lower survival rate, which may explain why the D allele has not spread to fixation. We used individual-based simulations to examine how an inversion polymorphism with such antagonistic fitness effects behaves over time. Our results indicate that polymorphisms become stabilized at an intermediate allele frequency if the inversion links an additively beneficial allele of small effect size to a recessive weakly deleterious mutation, overall resulting in weak net heterosis. Importantly, this conclusion remains valid over a wide range of selection coefficients against the homozygous DD (up to lethality), suggesting that the conditions needed to maintain the polymorphism may frequently be met. However, the simulations also suggest that in our zebra finch populations, the estimated recessive deleterious effect of the D allele (on survival in captivity) is not quite large enough to prevent fixation of the D allele in the long run. Estimates of fitness effects from free-living populations are needed to validate these results.

Individual variation and plasticity in the infant-directed communication of orang-utan mothers.

Between-individual variation in behavioural expression, such as social responsiveness, has been shown to have important eco-evolutionary consequences. However, most comparative research on non-human primate communication has focused on species- or population-level variation, while among- and within-individual variation has been largely ignored or considered as noise. Here, we apply a behavioural reaction norm framework to repeated observations of mother-offspring interactions in wild and zoo-housed orang-utans (Pongo abelii, P. pygmaeus) to tease apart variation on the individual level from population-level and species-level differences. Our results showed that mothers not only differed in the composition of their infant-directed gestural repertoires, but also in communicative tactics, such as gestural redoings (i.e. persistence) and responsiveness to infants' requests. These differences remained after controlling for essential moderators, including species, setting, parity and infant age. Importantly, mothers differed in how they adjusted their behaviour across social contexts, making a strong case for investigating within-individual variation. Our findings highlight that partitioning behavioural variation into its within-individual, between-individual and environmental sources allows us to estimate the extent of plastic responses to the immediate environment in great ape communication.

Violating the normality assumption may be the lesser of two evils.

When data are not normally distributed, researchers are often uncertain whether it is legitimate to use tests that assume Gaussian errors, or whether one has to either model a more specific error structure or use randomization techniques. Here we use Monte Carlo simulations to explore the pros and cons of fitting Gaussian models to non-normal data in terms of risk of type I error, power and utility for parameter estimation. We find that Gaussian models are robust to non-normality over a wide range of conditions, meaning that p values remain fairly reliable except for data with influential outliers judged at strict alpha levels. Gaussian models also performed well in terms of power across all simulated scenarios. Parameter estimates were mostly unbiased and precise except if sample sizes were small or the distribution of the predictor was highly skewed. Transformation of data before analysis is often advisable and visual inspection for outliers and heteroscedasticity is important for assessment. In strong contrast, some non-Gaussian models and randomization techniques bear a range of risks that are often insufficiently known. High rates of false-positive conclusions can arise for instance when overdispersion in count data is not controlled appropriately or when randomization procedures ignore existing non-independencies in the data. Hence, newly developed statistical methods not only bring new opportunities, but they can also pose new threats to reliability. We argue that violating the normality assumption bears risks that are limited and manageable, while several more sophisticated approaches are relatively error prone and particularly difficult to check during peer review. Scientists and reviewers who are not fully aware of the risks might benefit from preferentially trusting Gaussian mixed models in which random effects account for non-independencies in the data.

Extra-pair paternity as a strategy to reduce the costs of heterospecific reproduction? Insights from the crow hybrid zone.

Within hybrid zones of socially monogamous species, the number of mating opportunities with a conspecific can be limited. As a consequence, individuals may mate with a heterospecific (social) partner despite possible fitness costs to their hybrid offspring. Extra-pair copulations with a conspecific may thus arise as a possible post hoc strategy to reduce the costs of hybridization. We here assessed the rate of extra-pair paternity in the hybrid zone between all-black carrion crows (Corvus (corone) corone) and grey hooded crows (C. (c.) cornix) and tested whether extra-pair paternity (EPP) was more likely in broods where parents differed in plumage colour. The proportion of broods with at least one extra-pair offspring and the proportion of extra-pair offspring were low overall (6.98% and 2.90%, respectively) with no evidence of hybrid broods having higher EPP rates than purebred nests.

Mapping centromeres of microchromosomes in the zebra finch (Taeniopygia guttata) using half-tetrad analysis.

Centromeres usually consist of hundreds of kilobases of repetitive sequence which renders them difficult to assemble. As a consequence, centromeres are often missing from assembled genomes and their locations on physical chromosome maps have to be inferred from flanking sequences via fluorescence in situ hybridization (FISH). Alternatively, centromere positions can be mapped using linkage analyses in accidentally triploid individuals formed by half-tetrads (resulting from the inheritance of two chromatids from a single meiosis). The current genome assembly of the zebra finch (Taeniopygia guttata) comprises 32 chromosomes, but only for the ten largest chromosomes centromere positions have been mapped using FISH. We here map the positions of most of the remaining centromeres using half-tetrad analyses. For this purpose, we genotyped 37 zebra finches that were triploid or tetraploid due to inheritance errors (and mostly died as embryos) together with their parents at 64 microsatellite markers (at least two per chromosome). Using the information on centromere positions on the ten largest chromosomes, we were able to identify 12 cases of non-disjunction in maternal meiosis I and 10 cases of non-disjunction in maternal meiosis II. These 22 informative cases allowed us to infer centromere positions on additional 19 microchromosomes in reference to the current genome assembly. This knowledge will be valuable for studies of chromosome evolution, meiotic drive and species divergence in the avian lineage.

Association mapping of morphological traits in wild and captive zebra finches: reliable within, but not between populations.

Identifying causal genetic variants underlying heritable phenotypic variation is a long-standing goal in evolutionary genetics. We previously identified several quantitative trait loci (QTL) for five morphological traits in a captive population of zebra finches (Taeniopygia guttata) by whole-genome linkage mapping. We here follow up on these studies with the aim to narrow down on the quantitative trait variants (QTN) in one wild and three captive populations. First, we performed an association study using 672 single nucleotide polymorphisms (SNPs) within candidate genes located in the previously identified QTL regions in a sample of 939 wild-caught zebra finches. Then, we validated the most promising SNP-phenotype associations (n = 25 SNPs) in 5228 birds from four populations. Genotype-phenotype associations were generally weak in the wild population, where linkage disequilibrium (LD) spans only short genomic distances. In contrast, in captive populations, where LD blocks are large, apparent SNP effects on morphological traits (i.e. associations) were highly repeatable with independent data from the same population. Most of those SNPs also showed significant associations with the same trait in other captive populations, but the direction and magnitude of these effects varied among populations. This suggests that the tested SNPs are not the causal QTN but rather physically linked to them, and that LD between SNPs and causal variants differs between populations due to founder effects. While the identification of QTN remains challenging in nonmodel organisms, we illustrate that it is indeed possible to confirm the location and magnitude of QTL in a population with stable linkage between markers and causal variants.

A prezygotic transmission distorter acting equally in female and male zebra finches Taeniopygia guttata.

The two parental alleles at a specific locus are usually inherited with equal probability to the offspring. However, at least three processes can lead to an apparent departure from fair segregation: early viability selection, biased gene conversion and various kinds of segregation distortion. Here, we conduct a genome-wide scan for transmission distortion in a captive population of zebra finches (Taeniopygia guttata) using 1302 single-nucleotide polymorphisms (SNPs) followed by confirmatory analyses on independent samples from the same population. In the initial genome-wide scan, we found significant distortion at three linked loci on chromosome Tgu2 and we were able to replicate this finding in each of two follow-up data sets [overall transmission ratio = 0.567 (95% CI = 0.536-0.600), based on 1101 informative meioses]. Although the driving allele was preferentially transmitted by both heterozygous females [ratio = 0.560 (95% CI = 0.519-0.603)] and heterozygous males [ratio = 0.575 (95% CI = 0.531-0.623)], we could rule out postzygotic viability selection and biased gene conversion as possible mechanisms. Early postzygotic viability selection is unlikely, because it would result in eggs with no visible embryo and hence no opportunity for genotyping, and we confirmed that both females and males heterozygous for the driving allele did not produce a larger proportion of such eggs than homozygous birds. Biased gene conversion is expected to be rather localized, while we could trace transmission distortion in haplotypes of several megabases in a recombination desert. Thus, we here report the rare case of a prezygotically active transmission distorter operating equally effectively in female and male meioses.

Characterization and distribution of de novo mutations in the zebra finch.

Germline de novo mutations (DNMs) provide the raw material for evolution. The DNM rate varies considerably between species, sexes and chromosomes. Here, we identify DNMs in the zebra finch (Taeniopygia guttata) across 16 parent-offspring trios using two genome assemblies of different quality. Using an independent genotyping assay, we validate 82% of the 150 candidate DNMs. DNM rates are consistent between both assemblies, with estimates of 6.14 × 10-9 and 6.36 × 10-9 per site per generation. We observe a strong paternal bias in DNM rates (male-to-female ratio ɑ ≈ 4), but this bias is in transition mutations only, leading to a transition-to-transversion ratio of 3.18 and 3.57. Finally, we find that DNMs tend to be randomly distributed across chromosomes, not associated with recombination hotspots or genic regions. However, the sex chromosome chrZ shows a roughly fourfold increased DNM rate compared to autosomes, which is more than the expected increase due to chrZ spending two-thirds of its time in males. Overall, our results further enhance our understanding of DNMs in passerine songbirds.

Wild and captive immature orangutans differ in their non-vocal communication with others, but not with their mothers.

Abstract: In many group-living species, individuals are required to flexibly modify their communicative behaviour in response to current social challenges. To unravel whether sociality and communication systems co-evolve, research efforts have often targeted the links between social organisation and communicative repertoires. However, it is still unclear which social or interactional factors directly predict communicative complexity. To address this issue, we studied wild and zoo-housed immature orangutans of two species to assess the impact of the socio-ecological setting on the production of non-vocal signal repertoires. Specifically, we compared repertoire size, dyadic repertoire similarity, and number of social goals (i.e. observer's estimate of the signaller's intended interaction outcome) for communicative interactions with mothers versus other conspecifics, controlling for critical individual and environmental factors. In this small sample of immature orangutans, wild-captive contrasts were statistically significant only for other-directed repertoires, but not for mother-directed repertoires, and not for the number of social goals that immatures communicated towards. While the repertoires of individuals living in the same research setting were more similar than those living in contrasting settings, this difference was most pronounced for other-directed repertoires of the less socially tolerant orangutan species. These results suggest that the boosted interactional opportunities in captivity rather than mere differences in environmental affordances or communicative needs drive the wild-captive contrast in orangutan communicative repertoires. Overall, this fine-grained analysis of repertoires further underscores that not only a species' social organisation but also the targeted audience may have a profound impact on communicative behaviour. Significance statement: Navigating a dynamic social environment often requires flexible signal use. While it has repeatedly been shown that the social organisation and structure of species predict the complexity of their communication systems, the mechanisms underlying these relationships are largely unknown. Because targeted studies to assess this issue in great apes are difficult, we take an alternative approach here: we compare the same species living in the wild and in artificial habitats in captivity. This contrast allows a direct test of how repertoires respond to the relevant difference in socio-ecological conditions. Our results show that the diversity of interaction partners (i.e. social opportunities), but not the diversity of social goals (i.e. possible interaction outcomes) or the broader physical opportunities (i.e. safe ground use), predict the size and consistency of wild and captive signalling repertoires. Supplementary Information: The online version contains supplementary material available at 10.1007/s00265-023-03426-3.

QTL and quantitative genetic analysis of beak morphology reveals patterns of standing genetic variation in an Estrildid finch.

The intra- and interspecific diversity of avian beak morphologies is one of the most compelling examples for the power of natural selection acting on a morphological trait. The development and diversification of the beak have also become a textbook example for evolutionary developmental biology, and variation in expression levels of several genes is known to causally affect beak shape. However, until now, no genomic polymorphisms have been identified, which are related to beak morphology in birds. QTL mapping does reveal the location of causal polymorphisms, albeit with poor spatial resolution. Here, we estimate heritability and genetic correlations for beak length, depth and width and perform a QTL linkage analysis for these traits based on 1404 informative single-nucleotide polymorphisms genotyped in a four-generation pedigree of 992 captive zebra finches (Taeniopygia guttata). Beak size, relative to body size, was sexually dimorphic (larger in males). Heritability estimates ranged from 0.47 for beak length to 0.74 for beak width. QTL mapping revealed four to five regions of significant or suggestive genome-wide linkage for each of the three beak dimensions (nine different regions in total). Eight out of 11 genes known to influence beak morphology are located in these nine peak regions. Five QTL do not cover known candidates demonstrating that yet unknown genes or regulatory elements may influence beak morphology in the zebra finch.

A sex chromosome inversion is associated with copy number variation of mitochondrial DNA in zebra finch sperm.

The propulsion of sperm cells via movement of the flagellum is of vital importance for successful fertilization. While the exact mechanism of energy production for this movement varies between species, in avian species energy is thought to come predominantly from the mitochondria located in the sperm midpiece. Larger midpieces may contain more mitochondria, which should enhance the energetic capacity and possibly promote mobility. Due to an inversion polymorphism on their sex chromosome TguZ, zebra finches (Taeniopygia guttata castanotis) exhibit large within-species variation in sperm midpiece length, and those sperm with the longest midpieces swim the fastest. Here, we test through quantitative real-time PCR in zebra finch ejaculates whether the inversion genotype has an effect on the copy number of mitochondrial DNA (mtDNA). We find that zebra finches carrying the derived allele (correlated with longer sperm midpieces) have more copies of the mtDNA in their ejaculates than those homozygous for the ancestral allele (shorter midpieces). We suggest downstream effects of mtDNA copy number variation on the rate of adenosine triphosphate production, which in turn may influence sperm swimming speed and fertilization success. Central components of gamete energy metabolism may thus be the proximate cause for a fitness-relevant genetic polymorphism, stabilizing a megabase-scale inversion at an intermediate allele frequency in the wild.

Assortative mating and epistatic mating-trait architecture induce complex movement of the crow hybrid zone.

Hybrid zones provide a window into the evolutionary processes governing species divergence. Yet, the contribution of mate choice to the temporal and spatial stability of hybrid zones remains poorly explored. Here, we investigate the effects of assortative mating on hybrid-zone dynamics by means of a mathematical model parameterized with phenotype and genotype data from the hybrid zone between all-black carrion and gray-coated hooded crows. In the best-fit model, narrow clines of the two mating-trait loci were maintained by a moderate degree of assortative mating inducing pre- and postzygotic isolation via positive frequency-dependent selection. Epistasis between the two loci induced hybrid-zone movement in favor of alleles conveying dark plumage followed by a shift in the opposite direction favoring gray-coated phenotypes ∼1 200 generations after secondary contact. Unlinked neutral loci diffused near-unimpeded across the zone. These results were generally robust to the choice of matching rule (self-referencing or parental imprinting) and effects of genetic drift. Overall, this study illustrates under which conditions assortative mating can maintain steep clines in mating-trait loci without generalizing to genome-wide reproductive isolation. It further emphasizes the importance of the genetic mating-trait architecture for spatio-temporal hybrid-zone dynamics.

A test for meiotic drive in hybrids between Australian and Timor zebra finches.

Meiotic drivers have been proposed as a potent evolutionary force underlying genetic and phenotypic variation, genome structure, and also speciation. Due to their strong selective advantage, they are expected to rapidly spread through a population despite potentially detrimental effects on organismal fitness. Once fixed, autosomal drivers are cryptic within populations and only become visible in between-population crosses lacking the driver or corresponding suppressor. However, the assumed ubiquity of meiotic drivers has rarely been assessed in crosses between populations or species. Here we test for meiotic drive in hybrid embryos and offspring of Timor and Australian zebra finches-subspecies that have evolved in isolation for about two million years-using 38,541 informative transmissions of 56 markers linked to either centromeres or distal chromosome ends. We did not find evidence for meiotic driver loci on specific chromosomes. However, we observed a weak overall transmission bias toward Timor alleles at centromeres in females (transmission probability of Australian alleles of 47%, nominal p = 6 × 10-5). While this is in line with the centromere drive theory, it goes against the expectation that the subspecies with the larger effective population size (i.e., the Australian zebra finch) should have evolved the more potent meiotic drivers. We thus caution against interpreting our finding as definite evidence for centromeric drive. Yet, weak centromeric meiotic drivers may be more common than generally anticipated and we encourage further studies that are designed to detect also small effect meiotic drivers.

Discovery and population genomics of structural variation in a songbird genus.

Structural variation (SV) constitutes an important type of genetic mutations providing the raw material for evolution. Here, we uncover the genome-wide spectrum of intra- and interspecific SV segregating in natural populations of seven songbird species in the genus Corvus. Combining short-read (N = 127) and long-read re-sequencing (N = 31), as well as optical mapping (N = 16), we apply both assembly- and read mapping approaches to detect SV and characterize a total of 220,452 insertions, deletions and inversions. We exploit sampling across wide phylogenetic timescales to validate SV genotypes and assess the contribution of SV to evolutionary processes in an avian model of incipient speciation. We reveal an evolutionary young (~530,000 years) cis-acting 2.25-kb LTR retrotransposon insertion reducing expression of the NDP gene with consequences for premating isolation. Our results attest to the wealth and evolutionary significance of SV segregating in natural populations and highlight the need for reliable SV genotyping.

Epistatic mutations under divergent selection govern phenotypic variation in the crow hybrid zone.

The evolution of genetic barriers opposing interspecific gene flow is key to the origin of new species. Drawing from information on over 400 admixed genomes sourced from replicate transects across the European hybrid zone between all-black carrion crows and grey-coated hooded crows, we decipher the interplay between phenotypic divergence and selection at the molecular level. Over 68% of plumage variation was explained by epistasis between the gene NDP and a ~2.8-megabase region on chromosome 18 with suppressed recombination. Both pigmentation loci showed evidence for divergent selection resisting introgression. This study reveals how few, large-effect loci can govern prezygotic isolation and shield phenotypic divergence from gene flow.

Traffic noise exposure depresses plasma corticosterone and delays offspring growth in breeding zebra finches.

The impact of human activity on the acoustic environment is overwhelming, with anthropogenic noise reaching even remote areas of the planet. The World Health Organization has identified noise pollution as one of the leading environmental health risks in humans, and it has been linked to a myriad of short- and long-term health effects in exposed individuals. However, less is known about the health effects of anthropogenic noise exposure on animals. We investigated long- and short-term effects of traffic noise on zebra finches breeding in small communal aviaries, using a repeated measures design. Birds bred in both noise and no-noise conditions, and we measured baseline plasma glucocorticoid levels before, during and after breeding. In addition, we assayed immune function, measured reproductive success and offspring growth and compared rates of extra-pair paternity of breeding adults. Breeding birds had significantly lower baseline plasma corticosterone levels when exposed to traffic noise than when they were not exposed to noise playback. In addition, the nestlings reared during noise exposure were lighter than nestlings of the same parents when breeding in control conditions. Our results suggest that traffic noise poses a more severe hurdle to birds at more vulnerable stages of their life history, such as during reproductive events and ontogeny. While chronic exposure to traffic noise in our birds did not, by itself, prove to be a sufficient stressor to cause acute effects on health or reproductive success in exposed individuals, it did result in disruptions to normal glucocorticoid profiles and delayed offspring growth. However, animals living in urban habitats are exposed to a multitude of anthropogenic disturbances, and it is likely that even species that appear to be thriving in noisy environments may suffer cumulative effects of these multiple disturbances that may together impact their fitness in urban environments.

A sex-chromosome inversion causes strong overdominance for sperm traits that affect siring success.

Male reproductive success depends on the competitive ability of sperm to fertilize the ova, which should lead to strong selection on sperm characteristics. This raises the question of how heritable variation in sperm traits is maintained. Here we show that in zebra finches (Taeniopygia guttata) nearly half of the variance in sperm morphology is explained by an inversion on the Z chromosome with a 40% allele frequency in the wild. The sperm of males that are heterozygous for the inversion had the longest midpieces and the highest velocity. Furthermore, such males achieved the highest fertility and the highest siring success, both within-pair and extra-pair. Males homozygous for the derived allele show detrimental sperm characteristics and the lowest siring success. Our results suggest heterozygote advantage as the mechanism that maintains the inversion polymorphism and hence variance in sperm design and in fitness.