Callum S. McDiarmid

An emerging question in evolutionary research is the extent to which mitochondrial variation across species and populations drives functional differences in fitness‐related traits. The asymmetric introgression and narrow mitochondrial clines seen in long‐tailed finch ( Poephila acuticauda ) subspecies is suggestive of selection but there is no direct evidence of fitness related differences in admixed birds. Asymmetric introgression of the eastern P. a. hecki mito‐type into P. a. acuticauda , suggests that the former may possess a selective energetic advantage such as superior escape flight performance. We employed a spatial tracking software, Argus, to quantify the take‐off flight performance of 158 wild‐derived, captive long‐tailed finches as they were released repeatedly, from which we obtained nine flight metrics. Average force and maximum vertical distance travelled were repeatable across all individuals and significantly lower in P. a. hecki compared to P. a. acuticauda males. Hence, contrary to predictions, P. a. hecki did not display escape flight performance superior to P. a. acuticauda suggesting take‐off flight performance in wild long‐tailed finches is not related to the observed asymmetry in the introgression of mito‐type between the subspecies. Furthermore, in intraspecific female hybrids, in which we might theoretically expect a mismatch between mitochondrial haplotype and mitonuclear genes, we also found no evidence of impaired flight take‐off performance, relative to those with matched mitochondrial and mitonuclear genes (here: female P. a. acuticauda ). Finally, male and female hybrids did not differ in the average force and maximum vertical distance travelled, again suggesting that take‐off performance is not significantly worse in the heterogametic sex, as predicted by Haldane's rule. The angle of the take‐off flight was a key determinant of our flight performance metrics and consequently, future work could benefit from better directing the flight path of tracked birds.

Spermatozoa exhibit striking morphological variation across the animal kingdom. In passerine birds, sperm exhibit considerable variation in size, yet the basic sperm phenotype is highly conserved; sperm are filiform, the head is corkscrew-shaped, and the midpiece is elongated and twisted around the flagellum. A significant departure from this typical sperm morphology has been reported in the sister species, the Eurasian bullfinch (Pyrrhula pyrrhula) and Azores bullfinch (P. murina). Here, we report a second evolutionary shift in passerine sperm phenotype in the nominate subspecies of the red-browed finch (Neochmia temporalis temporalis); sperm are nonfiliform, with an ellipsoid head and an extremely short midpiece restricted to the nuclear–axoneme junction. Additionally, we show that the sperm phenotype of the red-browed finch is similar to the putatively neotenous sperm described in the two bullfinch species. Using whole-genome data, we found no evidence that the unusual sperm phenotype of the red-browed finch is associated with reduced genetic variation or a population bottleneck. In contrast, using data on relative testes size, we find some support for the hypothesis that relaxed postcopulatory sexual selection, via a lack of sperm competition, may, at least in part, explain the unusual sperm of the red-browed finch. We also discuss the possible roles of mutation, genetic drift, and genetic hitchhiking in the evolutionary origins and maintenance of neotenous sperm phenotypes. Finally, we suggest that these dramatic evolutionary shifts in sperm phenotype warrant further investigation and highlight the need for a greater understanding of the developmental and genomic basis of sperm phenotype.

Carotenoid pigmentation produces the yellow and red coloration of birds and other vertebrates, but our understanding of the genetic architecture of carotenoid ornamentation is largely limited to studies of novel color variants observed in captively bred populations. The complexity of carotenoid-based color evolution in nature remains poorly characterized. Here, we examine the long-tailed finch Poephila acuticauda , an Australian songbird with two hybridizing subspecies that differ in bill coloration: yellow in western subspecies acuticauda and red in eastern subspecies hecki . We characterize the carotenoid composition of each subspecies and find that yellow bills can be explained by the loss of C(4)-oxidation, thus blocking yellow dietary pigments from being metabolized to red. Combining linked-read genomic sequencing and reflectance spectrophotometry measurements of bill color collected from wild-sampled finches and laboratory crosses, we identify four loci that together explain 53% of variance in this trait. The two loci of largest effect contain the genes CYP2J19 , an essential enzyme for the ketolation via C(4)-oxidation of dietary carotenoids, and TTC39B , an enhancer of ketocarotenoid production. Evolutionary genealogy reconstruction indicates that the red-billed phenotype is ancestral and yellow alleles at both CYP2J19 and TTC39B arose and fixed in acuticauda approximately 100 kya. Yellow alleles then introgressed into hecki less than 5 kya. Across all four loci, acuticauda derived variants show evidence of selective sweeps, implying that yellow bill coloration has been favored by natural selection. Our study suggests that the frequent adaptive evolutionary transitions between red and yellow ornamentation in nature can have a simple genetic basis. Significance: We studied variation in carotenoid ornamentation of an Australian songbird with two hybridizing subspecies that differ in bill color: one yellow and the other red. We identified a single metabolic process, C(4)-oxidation, underlying the distinct carotenoid composition of these two bill colors. Genetic association mapping revealed four major effect loci that explained most of the observed variation the trait, including the oxidative ketolation enzyme CYP2J19 and the carotenoid ketolation enhancer gene TTC39B . Evolutionary reconstruction indicates that yellow alleles are derived, ancient (~100 kya), and under positive selection. This has driven their recent (<5 kya) adaptive introgression across the hybrid zone. These findings have important implications for understanding the role of natural selection in phenotypic evolution in natural systems.

Understanding genetic incompatibilities and genetic introgression between incipient species are major goals in evolutionary biology. Mitochondrial genes evolve rapidly and exist in dense gene networks with coevolved nuclear genes, suggesting that mitochondrial respiration may be particularly susceptible to disruption in hybrid organisms. Mitonuclear interactions have been demonstrated to contribute to hybrid dysfunction between deeply divergent taxa crossed in the laboratory, but there are few empirical examples of mitonuclear interactions between younger lineages that naturally hybridize. Here, we use controlled hybrid crosses and high‐resolution respirometry to provide the first experimental evidence in a bird that inter‐lineage mitonuclear interactions impact mitochondrial aerobic metabolism. Specifically, respiration capacity of the two mitodiscordant backcrosses (with mismatched mitonuclear combinations) differs from one another, although they do not differ significantly from the parental groups or mitoconcordant backcrosses as we would expect of mitonuclear disruptions. In the wild hybrid zone between these subspecies, the mitochondrial cline centre is shifted west of the nuclear cline centre, which is consistent with the direction of our experimental results. Our results therefore demonstrate asymmetric mitonuclear interactions that impact the capacity of cellular mitochondrial respiration and may help to explain the geographic discordance between mitochondrial and nuclear genomes observed in the wild.

Understanding genetic incompatibilities and genetic introgression between incipient species are major goals in evolutionary biology. Mitochondrial genes evolve rapidly and exist in dense gene networks with coevolved nuclear genes, suggesting that mitochondrial respiration may be particularly susceptible to disruption in hybrid organisms. Mitonuclear interactions have been demonstrated to contribute to hybrid disfunction between deeply divergent taxa crossed in the laboratory, but there are few empirical examples of mitonuclear interactions between younger lineages that naturally hybridise. Here we use experimental crosses and high resolution respirometry to provide the first evidence in a bird that inter-lineage mitonuclear interactions impact mitochondrial aerobic metabolism. Specifically, respiration capacity of the two paternal backcrosses (with mismatched mito-nuclear combinations) differ from one another, although they do not differ to the parental groups or maternal backcrosses as we would expect of mitonuclear disruptions. In the wild hybrid zone between these subspecies the mitochondrial cline centre is shifted west of the nuclear cline centre, which is consistent with the direction of our experimental results. Our results therefore demonstrate asymmetric mitonuclear interactions that impact the capacity of cellular mitochondrial respiration and may help to explain the geographic discordance between mitochondrial and nuclear genomes observed in the wild.

Sperm traits are important in determining fertilisation success and are metabolically expensive to produce. There is little known about how energy acquisition and allocation affect sperm traits in avian taxa. This study assesses the impact of experimentally manipulated diet on long‐tailed finch sperm by comparing the length of sperm components (head, midpiece, flagellum and total sperm) between the treatment groups (homogenous diet and diverse diet) and across time points (before and after the experiment). Diet homogeneity was manipulated as this may impact diet quality in a species that is usually fed a diversity of seed species to thrive in captivity. Our results showed no impact of diet on the length of different sperm components. These results contrast with findings of a similar recent study of the zebra finch which found a significant level of diet‐affected plasticity in midpiece length. For both diet treatment groups in this study there was a slight but significant increase in head, flagellum and total sperm length over the treatment period. Despite this, all sperm components were highly repeatable, consistent with the finding from observational studies in passerine birds. The efficacy of our dietary manipulation was confirmed by a significant reduction in the saturation of bill colour in the long‐tailed finches as a result of the homogenous diet, and a difference in body mass across the treatment groups.

Mating behavior can play a key role in speciation by inhibiting or facilitating gene flow between closely related taxa. Hybrid zones facilitate a direct examination of mating behavior and the traits involved in establishing species barriers. The long‐tailed finch (Poephila acuticauda) has two hybridizing subspecies that differ in bill color (red and yellow), and the yellow bill phenotype appears to have introgressed ~350 km eastward following secondary contact. To examine the role of mate choice on bill color introgression, we performed behavioral assays using natural and manipulated bill colors. We found an assortative female mating preference for males of their own subspecies when bill color was not manipulated. However, we did not find this assortative preference in trials based on artificially manipulated bill color. This could suggest that assortative preference is not fixed entirely on bill color and instead may be based on a different trait (e.g., song) or a combination of traits, or alternatively may be due to lower statistical power alongside the bill manipulations being unconvincing to the female choosers. Intriguingly, we find a bias in the inheritance of bill color in captive bred F1 hybrid females. Previous modeling suggests that assortative mate preference and this kind of partial dominance in the underlying genes may together contribute to introgression, making the genetic architecture of bill color in this system a priority for future research.

Sperm traits can influence fertilisation success, but there is still much we do not understand about sperm condition dependence, that is, how much sperm traits depend on the male's energy acquisition and allocation. This is especially pronounced in avian taxa, despite extensive observational studies and sampling in wild populations. In this study, we collected sperm samples before and after experimentally reducing diet quality of wild-derived captive zebra finches in small mixed-sex groups, which we compared with individuals on a control diet. We measured the length of sperm components (head, midpiece, flagellum and total sperm length), the proportion of sperm with normal morphology, the proportion of sperm that were progressively motile and sperm swimming velocity (curvilinear velocity; VCL). The only sperm trait we found to be impacted by reduced diet quality was a significant decrease in sperm midpiece length. This is consistent with emerging evidence in other non-model systems, as well the fact that diet can alter mitochondrial density and structure in other tissue types. There was also a significant decrease in sperm velocity and the proportion of motile sperm over the course of the experiment for both experimental groups (i.e. unrelated to diet). This decrease in sperm velocity with largely unchanged sperm morphology emphasizes that there are other important determinants of sperm velocity, likely including seminal fluid composition.

Spermatozoa exhibit remarkable levels of morphological diversification among and within species. Among the passerine birds, the zebra finch (Taeniopygia guttata) has become a model system for studies of sperm biology, yet studies of closely related Estrildidae finches remain scarce. Here, we examine sperm morphology in the masked finch (Poephila personata) and place the data into the broader context of passerine sperm morphology using data for an additional 189 species. The masked finch exhibited high levels of within- and among-male variation in total sperm length and in specific sperm components. Furthermore, among-male variance in sperm length was significantly greater in estrildid (N = 12) compared with non-estrildid species (N = 178). We suggest that the high variation in sperm morphology in the masked finch and other estrildid species is likely to be linked to low levels of sperm competition, hence relaxed or weak selection on sperm length, in the clade. Our findings highlight that the highly variable sperm of the masked finch and widely studied zebra finch are ‘typical’ for estrildid species and stress the relevance of studying groups of closely related species. Finally, we suggest that further studies of Estrildidae will enhance our understanding of sperm diversity and avian diversity more generally.

The exploration of hybrid zones and the intergenomic conflicts exposed through hybridization provide windows into the processes of divergence and speciation. Sex chromosomes and mitonuclear incompatibilities have strong associations with the genetics of hybrid dysfunction. In ZW sex‐determining systems, maternal co‐inheritance of the mitochondrial and W chromosomes immediately exposes incompatibilities between these maternal contributions of one species and the Z chromosome of another. We analyze mitochondrial and Z chromosome admixture in the long‐tailed finch (Poephila acuticauda) of Australia, where hybridizing subspecies differ prominently in Z chromosome genotype and in bill color, yet the respective centers of geographic admixture for these two traits are offset by 350 km. We report two well‐defined mitochondrial clades that diverged ∼0.5 million years ago. Mitochondrial contact is geographically co‐located within a hybrid zone of Z chromosome admixture and is displaced from bill color admixture by nearly 400 km. Consistent with Haldane's rule expectations, hybrid zone females are significantly less likely than males to carry an admixed Z chromosome or have mismatched Z‐mitochondrial genotypes. Furthermore, there are significantly fewer than expected mitonuclear mismatches in hybrid zone females and paternal backcross males. Results suggest a potential for mitonuclear/sex chromosome incompatibilities in the emergence of reproductive isolation in this system.