Project description:The genome of the white-throated sparrow (Zonotrichia albicollis) contains an inversion polymorphism on chromosome 2 that is linked to predictable variation in a suite of phenotypic traits including plumage color, aggression, and parental behavior. Differences in gene expression between the two color morphs, which represent the two common inversion genotypes (ZAL2/ZAL2 and ZAL2/ZAL2m), are therefore of potential interest toward understanding the molecular underpinnings of these phenotypes. To identify genes that are differentially expressed between the two morphs and correlated with behavior, we quantified both behavior and gene expression in a population of free-living white-throated sparrows. We quantified behavioral responses to simulated territorial intrusions (STIs) early during the breeding season. In the same birds, we then performed a transcriptomewide analysis of gene expression in two behaviorally relevant brain regions, the medial amygdala and hypothalamus. Using network analyses, we identified modules of genes that were correlated with both morph and STI-induced singing behavior. The majority of these genes were located within the inversion, demonstrating the profound effect the inversion has on the expression of genes captured by the rearrangement. Gene pathway analyses revealed that in the medial amygdala, the most prominent pathways were those related to steroid hormone receptor activity. Within these pathways, the only gene encoding such a receptor was ESR1 (estrogen receptor alpha). Our results thus suggest that ESR1 and related genes are important for behavioral differences between the morphs.
Project description:BackgroundThe level of nucleotide diversity observed across the genome is positively correlated with the local rate of recombination. Avian karyotypes are typified by large variation in chromosome size and the rate of recombination in birds has been shown to be negatively correlated with chromosome size. It has thus been predicted that nucleotide diversity is negatively correlated with chromosome size in aves. However, there is limited empirical evidence to support this prediction.ResultsHere we sequenced 27 autosomal and 12 sex chromosome-linked loci in the white-throated sparrow (Zonotrichia albicollis) to quantify and compare patterns of recombination, linkage disequilibrium (LD), and genetic diversity across the genome of this North American songbird. Genetic diversity on the autosomes varied up to 8-fold, with the lowest diversity observed on the macrochromosomes and the highest diversity on the microchromosomes. Genetic diversity on the sex chromosomes was reduced compared to the autosomes, the most extreme difference being a ~300-fold difference between the W chromosome and the microchromosomes. LD and population structure associated with a common inversion polymorphism (ZAL2/2m) in this species were found to be atypical compared to other macrochromosomes, and nucleotide diversity within this inversion on the two chromosome arrangements was more similar to that observed on the Z chromosome.ConclusionsA negative correlation between nucleotide diversity and autosome size was observed in the white-throated sparrow genome, as well as low levels of diversity on the sex chromosomes comparable to those reported in other birds. The population structure and extended LD associated with the ZAL2/2m chromosomal polymorphism are exceptional compared to the rest of the white-throated sparrow genome.
Project description:Inversion polymorphisms have been linked to a variety of fundamental biological and evolutionary processes. Yet few studies have used large-scale genomic sequencing to directly compare the haplotypes associated with the standard and inverted chromosome arrangements. Here we describe the targeted genomic sequencing and comparison of haplotypes representing alternative arrangements of a common inversion polymorphism linked to a suite of phenotypes in the white-throated sparrow (Zonotrichia albicollis). More than 7.4 Mb of genomic sequence was generated and assembled from both the standard (ZAL2) and inverted (ZAL2(m)) arrangements. Sequencing of a pair of inversion breakpoints led to the identification of a ZAL2-specific segmental duplication, as well as evidence of breakpoint reusage. Comparison of the haplotype-based sequence assemblies revealed low genetic differentiation outside versus inside the inversion indicative of historical patterns of gene flow and suppressed recombination between ZAL2 and ZAL2(m). Finally, despite ZAL2(m) being maintained in a near constant state of heterozygosity, no signatures of genetic degeneration were detected on this chromosome. Overall, these results provide important insights into the genomic attributes of an inversion polymorphism linked to mate choice and variation in social behavior.
Project description:Chromosomal inversions have been of long-standing interest to geneticists because they are capable of suppressing recombination and facilitating the formation of adaptive gene complexes. An exceptional inversion polymorphism (ZAL2(m)) in the white-throated sparrow (Zonotrichia albicollis) is linked to variation in plumage, social behavior and mate choice, and is maintained in the population by negative assortative mating. The ZAL2(m) polymorphism is a complex inversion spanning > 100 Mb and has been proposed to be a strong suppressor of recombination, as well as a potential model for studying neo-sex chromosome evolution. To quantify and evaluate these features of the ZAL2(m) polymorphism, we generated sequence from 8 ZAL2(m) and 16 ZAL2 chromosomes at 58 loci inside and 4 loci outside the inversion. Inside the inversion we found that recombination was completely suppressed between ZAL2 and ZAL2(m), resulting in uniformly high levels of genetic differentiation (F(ST)=0.94), the formation of two distinct haplotype groups representing the alternate chromosome arrangements and extensive linkage disequilibrium spanning ~104 Mb within the inversion, whereas gene flow was not suppressed outside the inversion. Finally, although ZAL2(m) homozygotes are exceedingly rare in the population, occurring at a frequency of < 1%, we detected evidence of historical recombination between ZAL2(m) chromosomes inside the inversion, refuting its potential status as a non-recombining autosome.
Project description:White-throated sparrows increase fat deposits during pre-migratory periods and rely on these fat stores to fuel migration. Adipose tissue produces hormones and signaling factors in a rhythmic fashion and may be controlled by a clock in adipose tissue or driven by a master clock in the brain. The master clock may convey photoperiodic information from the environment to adipose tissue to facilitate pre-migratory fattening, and adipose tissue may, in turn, release adipokines to indicate the extent of fat energy stores. Here, we present evidence that a change in signal from the adipokines adiponectin and visfatin may act to indicate body condition, thereby influencing an individual's decision to commence migratory flight, or to delay until adequate fat stores are acquired. We quantified plasma adiponectin and visfatin levels across the day in captive birds held under constant photoperiod. The circadian profiles of plasma adiponectin in non-migrating birds were approximately inverse the profiles from migrating birds. Adiponectin levels were positively correlated to body fat, and body fat was inversely related to the appearance of nocturnal migratory restlessness. Visfatin levels were constant across the day and did not correlate with fat deposits; however, a reduction in plasma visfatin concentration occurred during the migratory period. The data suggest that a significant change in the biological control of adipokine expression exists between the two migratory conditions and we propose a role for adiponectin, visfatin and adipose clocks in the regulation of migratory behaviors.