Accelerated differentiation of neo-W nuclear-encoded mitochondrial genes between two climate-associated bird lineages signals potential co-evolution with mitogenomes.

There is considerable evidence for mitochondrial-nuclear co-adaptation as a key evolutionary driver. Hypotheses regarding the roles of sex-linkage have emphasized Z-linked nuclear genes with mitochondrial function (N-mt genes), whereas it remains contentious whether the perfect co-inheritance of W genes with mitogenomes could hinder or facilitate co-adaptation. Young (neo-) sex chromosomes that possess relatively many N-mt genes compared to older chromosomes provide unprecedented hypothesis-testing opportunities. Eastern Yellow Robin (EYR) lineages in coastal and inland habitats with different climates are diverged in mitogenomes, and in a ~ 15.4 Mb nuclear region enriched with N-mt genes, in contrast with otherwise-similar nuclear genomes. This nuclear region maps to passerine chromosome 1A, previously found to be neo-sex in the inland EYR genome. To compare sex-linked Chr1A-derived genes between lineages, we assembled and annotated the coastal EYR genome. We found that: (i) the coastal lineage shares a similar neo-sex system with the inland lineage, (ii) neo-W and neo-Z N-mt genes are not more diverged between lineages than are comparable non-N-mt genes, and showed little evidence for broad positive selection, (iii) however, W-linked N-mt genes are more diverged between lineages than are their Z-linked gametologs. The latter effect was ~7 times stronger for N-mt than non-N-mt genes, suggesting that W-linked N-mt genes might have diverged between lineages under environmental selection through co-evolution with mitogenomes. Finally, we identify a candidate gene driver for divergent selection, NDUFA12. Our data represent a rare example suggesting a possible role for W-associated mitochondrial-nuclear interactions in climate-associated adaptation and lineage differentiation.

Does reduced mobility through fragmented landscapes explain patch extinction patterns for three honeyeaters?

Habitat loss and associated fragmentation are major drivers of biodiversity decline, and understanding how they affect population processes (e.g. dispersal) is an important conservation goal. In a large-scale test employing 10 × 10 km units of replication, three species of Australian birds, the fuscous honeyeater, yellow-tufted honeyeater and white-plumed honeyeater, responded differently to fragmentation. The fuscous and yellow-tufted honeyeaters are 'decliners' that disappeared from suitable habitat in landscapes where levels of tree-cover fell below critical thresholds of 17 and 8%, respectively. The white-plumed honeyeater is a 'tolerant' species whose likelihood of occurrence in suitable habitat was independent of landscape-level tree-cover. To determine whether the absence of the two decliner species in low tree-cover landscapes can be explained by reduced genetic connectivity, we looked for signatures of reduced mobility and gene flow in response to fragmentation across agricultural landscapes in the Box-Ironbark region of north-central Victoria, Australia. We compared patterns of genetic diversity and population structure at the regional scale and across twelve 100 km(2) landscapes with different tree-cover extents. We used genetic data to test landscape models predicting reduced dispersal through the agricultural matrix. We tested for evidence of sex-biased dispersal and sex-specific responses to fragmentation. Reduced connectivity may have contributed to the disappearance of the yellow-tufted honey-eater from low tree-cover landscapes, as evidenced by male bias and increased relatedness among males in low tree-cover landscapes and signals of reduced gene flow and mobility through the agricultural matrix. We found no evidence for negative effects of fragmentation on gene flow in the other decliner, the fuscous honeyeater, suggesting that undetected pressures act on this species. As expected, there was no evidence for decreased movement through fragmented landscapes for the tolerant white-plumed honeyeater. We demonstrated effects of habitat loss and fragmentation (stronger patterns of genetic differentiation, increased relatedness among males) on the yellow-tufted honeyeater above the threshold at which probability of occurrence dropped. Increasing extent and structural connectivity of habitat should be an appropriate management action for this species and other relatively sedentary woodland specialist species for which it can be taken as representative.

Disrupted fine-scale population processes in fragmented landscapes despite large-scale genetic connectivity for a widespread and common cooperative breeder: the superb fairy-wren (Malurus cyaneus).

Understanding how habitat fragmentation affects population processes (e.g. dispersal) at different spatial scales is of critical importance to conservation. We assessed the effects of habitat fragmentation on dispersal and regional and fine-scale population structure in a currently widespread and common cooperatively breeding bird species found across south-eastern Australia, the superb fairy-wren Malurus cyaneus. Despite its relative abundance and classification as an urban tolerant species, the superb fairy-wren has declined disproportionately from low tree-cover agricultural landscapes across the Box-Ironbark region of north-central Victoria, Australia. Loss of genetic connectivity and disruption to its complex social system may be associated with the decline of this species from apparently suitable habitat in landscapes with low levels of tree cover. To assess whether reduced structural connectivity has had negative consequences for genetic connectivity in the superb fairy-wren, we used a landscape-scale approach to compare patterns of genetic diversity and gene flow at large (landscape/regional) and fine (site-level) spatial scales. In addition, using genetic distances, for each sex, we tested landscape models of decreased dispersal through treeless areas (isolation-by-resistance) while controlling for the effect of isolation-by-distance. Landscape models indicated that larger-scale gene flow across the Box-Ironbark region was constrained by distance rather than by lack of structural connectivity. Nonetheless, a pattern of isolation-by-resistance for males (the less-dispersive sex) and lower genetic diversity and higher genetic similarity within sites in low-cover fragmented landscapes indicated disruption to fine-scale gene flow mechanisms and/or mating systems. Although loss of structural connectivity did not appear to impede gene flow at larger spatial scales, fragmentation appeared to affect fine-scale population processes (e.g. local gene flow mechanisms and/or mating systems) adversely and may contribute to the decline of superb fairy-wrens in fragmented landscapes in the Box-Ironbark region.

Easy-to-use R functions to separate reduced-representation genomic datasets into sex-linked and autosomal loci, and conduct sex assignment.

Identifying sex-linked markers in genomic datasets is important because their presence in supposedly neutral autosomal datasets can result in incorrect estimates of genetic diversity, population structure and parentage. However, detecting sex-linked loci can be challenging, and available scripts neglect some categories of sex-linked variation. Here, we present new R functions to (1) identify and separate sex-linked loci in ZW and XY sex determination systems and (2) infer the genetic sex of individuals based on these loci. We tested these functions on genomic data for two bird and one mammal species and compared the biological inferences made before and after removing sex-linked loci using our function. We found that our function identified autosomal loci with ≥98.8% accuracy and sex-linked loci with an average accuracy of 87.8%. We showed that standard filters, such as low read depth and call rate, failed to remove up to 54.7% of sex-linked loci. This led to (i) overestimation of population FIS by up to 24%, and the number of private alleles by up to 8%; (ii) wrongly inferring significant sex differences in heterozygosity; (iii) obscuring genetic population structure and (iv) inferring ~11% fewer correct parentages. We discuss how failure to remove sex-linked markers can lead to incorrect biological inferences (e.g. sex-biased dispersal and cryptic population structure) and misleading management recommendations. For reduced-representation datasets with at least 15 known-sex individuals of each sex, our functions offer convenient resources to remove sex-linked loci and to sex the remaining individuals (freely available at https://github.com/drobledoruiz/conservation_genomics).

Genes and song: genetic and social connections in fragmented habitat in a woodland bird with limited dispersal.

Understanding the processes leading to population declines in fragmented landscapes is essential for successful conservation management. However, isolating the influence of disparate processes, and dispersal in particular, is challenging. The Grey Shrike-thrush, Colluricincla harmonica, is a sedentary woodland-dependent songbird, with learned vocalizations whose incidence in suitable habitat patches falls disproportionally with decline in tree cover in the landscape. Although it has been suggested that gaps in tree cover might act as barriers to its dispersal, the species remains in many remnants of native vegetation in agricultural landscapes, suggesting that it may have responded to habitat removal and fragmentation by maintaining or even increasing dispersal distances. We quantified population connectivity of the Grey Shrike-thrush in a system fragmented over more than 120 years using genetic (microsatellites) and acoustic (song types) data. First, we tested for population genetic and acoustic structure at regional and local scales in search of barriers to dispersal or gene flow and signals of local spatial structuring indicative of restricted dispersal or localized acoustic similarity. Then we tested for effects of habitat loss and fragmentation on genetic and acoustic connectivity by fitting alternative models of mobility (isolation-by-distance [the null model] and reduced and increased movement models) across treeless vs. treed areas. Birds within -5 km of each other had more similar genotypes and song types than those farther away, suggesting that dispersal and song matching are limited in the region. Despite restricted dispersal detected for females (but not males), populations appeared to be connected by gene flow and displayed some cultural (acoustic) connectivity across the region. Fragmentation did not appear to impact greatly the dispersal of the Grey Shrike-thrush: none of the mobility models fit the genetic distances of males, whereas for females, an isolation-by-distance model could not be rejected in favor of the models of reduced or increased movement through treeless gaps. However, dissimilarities of the song types were more consistent with the model of reduced cultural connectivity through treeless areas, suggesting that fragmentation impedes song type sharing in the Grey Shrike-thrush. Our paper demonstrates that habitat fragmentation hinders important population processes in an Australian woodland bird even though its dispersal is not detectably impacted.

Perched at the mito-nuclear crossroads: divergent mitochondrial lineages correlate with environment in the face of ongoing nuclear gene flow in an Australian bird.

Relationships among multilocus genetic variation, geography, and environment can reveal how evolutionary processes affect genomes. We examined the evolution of an Australian bird, the eastern yellow robin Eopsaltria australis, using mitochondrial (mtDNA) and nuclear (nDNA) genetic markers, and bioclimatic variables. In southeastern Australia, two divergent mtDNA lineages occur east and west of the Great Dividing Range, perpendicular to latitudinal nDNA structure. We evaluated alternative scenarios to explain this striking discordance in landscape genetic patterning. Stochastic mtDNA lineage sorting can be rejected because the mtDNA lineages are essentially distinct geographically for > 1500 km. Vicariance is unlikely: the Great Dividing Range is neither a current barrier nor was it at the Last Glacial Maximum according to species distribution modeling; nuclear gene flow inferred from coalescent analysis affirms this. Female philopatry contradicts known female-biased dispersal. Contrasting mtDNA and nDNA demographies indicate their evolutionary histories are decoupled. Distance-based redundancy analysis, in which environmental temperatures explain mtDNA variance above that explained by geographic position and isolation-by-distance, favors a nonneutral explanation for mitochondrial phylogeographic patterning. Thus, observed mito-nuclear discordance accords with environmental selection on a female-linked trait, such as mtDNA, mtDNA-nDNA interactions or genes on W-chromosome, driving mitochondrial divergence in the presence of nuclear gene flow.

Predicting landscape-genetic consequences of habitat loss, fragmentation and mobility for multiple species of woodland birds.

Inference concerning the impact of habitat fragmentation on dispersal and gene flow is a key theme in landscape genetics. Recently, the ability of established approaches to identify reliably the differential effects of landscape structure (e.g. land-cover composition, remnant vegetation configuration and extent) on the mobility of organisms has been questioned. More explicit methods of predicting and testing for such effects must move beyond post hoc explanations for single landscapes and species. Here, we document a process for making a priori predictions, using existing spatial and ecological data and expert opinion, of the effects of landscape structure on genetic structure of multiple species across replicated landscape blocks. We compare the results of two common methods for estimating the influence of landscape structure on effective distance: least-cost path analysis and isolation-by-resistance. We present a series of alternative models of genetic connectivity in the study area, represented by different landscape resistance surfaces for calculating effective distance, and identify appropriate null models. The process is applied to ten species of sympatric woodland-dependant birds. For each species, we rank a priori the expectation of fit of genetic response to the models according to the expected response of birds to loss of structural connectivity and landscape-scale tree-cover. These rankings (our hypotheses) are presented for testing with empirical genetic data in a subsequent contribution. We propose that this replicated landscape, multi-species approach offers a robust method for identifying the likely effects of landscape fragmentation on dispersal.