Integrating genomes, brain and behavior in the study of songbirds

Abstract

Songbirds share some essential traits but are extraordinarily diverse, allowing comparative analyses aimed at identifying specific genotype-phenotype associations. This diversity encompasses traits like vocal communication and complex social behaviors that are of great interest to humans, but that are not well represented in other accessible research organisms. Many songbirds are readily observable in nature and thus afford unique insight into the links between environment and organism. The distinctive organization of the songbird brain will facilitate analysis of genomic links to brain and behavior. Access to the zebra finch genome sequence will, therefore, prompt new questions and provide the ability to answer those questions.

Figure 1. Zebra finches

A zebra finch male (center) flanked by two females. Zebra finches are highly social, maintaining lifelong monogamous pair bonds in large colonies.

Figure 2. Overview of songbird phylogeny

Note the position of family Estrildidae (which includes the zebra finch) as one of the ‘vocal learner’ families, in contrast to the ‘non-learner’ suboscines. Some families and species from each superfamily are listed as representatives (in total there are 122 families within the five major superfamilies of the Passerida). Drawings of a male of one species contained within the representative families are included (drawings not to scale). Estrildidae: zebra finch (Taeniopygia guttata); Fringillidae: purple finch (Carpodacus purpureus); Icteridae: Brewer’s blackbird (Euphagus cyanocephalus); Sittidae: red-breasted nuthatch (Sitta canadensis); Troglodytidae: winter wren (Troglodytes troglodytes); Muscicapidae: vivid niltava (Niltava vivida); Sturnidae: European starling (Sturnus vulgaris); Paridae: yellow-cheeked tit (Parus spilonotus); Phylloscopidae: Tickell’s leaf warbler (Phylloscopus affinis); Corvidae: western scrub jay (Aphelocoma californica); Paradisaeidae: lesser bird-of-paradise (Paradisaea minor); Pipridae: golden-collared manakin (Manacus vitellinus); Tyrannidae: brown-backed chat-tyrant (Ochthoeca fumicolor).

Figure 3. Core elements of the ‘song circuit’

Several brain centers responsible for the production of learned vocal signals in songbirds are shown. Red lines indicate the motor output pathway, descending from the nucleus known as HVC (used as a proper name) to the robust nucleus of the arcopallium (RA). Blue lines indicate an anterior forebrain loop needed for song learning, which begins with projections from a distinct set of neurons in HVC to a specialized region within the basal ganglia known as Area X. Area X projects to the medial dorsolateral nucleus of the anterior thalamus (DLM), which projects back up to the lateral portion of the magnocellular nucleus of the anterior nidopallium (LMAN). LMAN projects back onto neurons in RA that also receive input from HVC; this projection also sends collaterals to Area X.

Figure 4. Spectrograms of songs from several songbird species

(A) Songs of three unrelated male zebra finches. Each zebra finch sings a unique song, observable by the distinct structure of each song element, or syllable, and by the ordering of those syllables. Note the typical ‘harmonic stack’ structure to the songs (visible as alternating black and white horizontal bars), individual variation in syllable structure, number of syllables, and total duration of song. (B) Canary song featuring pure-tone like syllables, syllable repetition, a multitude of different syllables, and a long song duration. (C) Tonal whistles of the northern cardinal, a species in which both sexes sing. (D) Song sparrow song highlighting syllable diversity; tones and harmonic stacks are visible. Elements can be repeated, and there are several types of syllables that comprise the song. (D) Songs of brood parasitic Cameroon indigobirds. There are two types of song: ‘non-mimicry’ songs, which are indigobird-specific, and ‘mimicry’ songs, which have syllables that are copied from the host species song. Note that the conspecific non-mimicry song is more complex than the mimicry song; it is composed of several distinct syllables types, many with rapid frequency modulation, and is longer in duration than the mimicry song.

Figure 5. Genome–brain–behavior relationships

The genome influences behavior of individuals indirectly by encoding proteins and RNAs, which are components of cells and circuits, which make up the neural systems responsible for behavioral control. Individual behavior is subject to natural selection, leading over generations to the establishment of gene sequences associated with successful behavioral phenotypes. Environment affects behavior in physiological time via the responses of neural systems, and it affects the genome in evolutionary time as a component of natural selection. Physiological processes can lead to epigenetic modifications of the genome. An individual’s environment includes the behavior of other interacting individuals. Social organization emerges out of the behavioral interactions of individuals.