Okay, I lied. There is no such thing as "the language gene." Language is the cumulative corollary of a multitude of genes.
A gene known as FoxP2, however, plays a very important role in the development of language. Mutations in this gene are associated with a condition known as developmental verbal dysparxia (DVD), a condition which causes deficits in both the expressive and receptive aspects of language. DVD patients have structural and functional abnormalities in the striatum of the basal ganglia (a region in the brain), which expresses high levels of FoxP2.
Zebra finches, a type of songbird, also express this gene heavily in Area X, a basal ganglia region, while in the process of learning their song, or singing directed songs. Area X is a part of a basal ganglia-forebrain circuit, the AFP, which bears similarities with mammalian cortical-basal ganglia loops. 25 days after hatching, zebra finch males begin to learn songs from an adult tutor. They imitate their tutor with remarkable fidelity around 90 days after hatching.
I am currently working in one of my old professor's labs whose research is centered around this gene. In December of '07, a paper came out in PLOS Biology which paralleled what my professor is attempting to do. The summary of the paper follows.
The paper intends to show that "zebra finches with reduced FoxP2 expression levels in Area X imitated tutor songs incompletely and inaccurately." To do this, the research team created a viral vector known as shFoxP2. shFoxP2 was injected into the zebra finch on the 23rd day after hatching. Once shFoxP2 was injected, it integrated with the host's DNA and was thusly expressed throughout the birds life. The purpose of shFoxP2 was to lower the expression of the FoxP2 gene in Area X. The research team also created a control virus, known as shControl, which did not target any zebra finch gene. All viral constructs (shControl and shFoxP2) expressed a GFP gene, which allowed infected brain to be detected by fluorescence microscopy.
On the 30th day after hatching, each juvenile zebra finch, from now on referred to as a pupil, was placed with an adult male zebra finch as a tutor. What follows is an analysis of the song development of the birds injected with the viruses.
The images, namely A, B, and especially G, are quite revealing. Image A shows a 50 nano-meter brain section from a male zebra finch. Area X is emphasized by the white arrows. Area B shows the same image through fluorescent microscopy. Note, the green section in image B shows the expression of GFP. Essentially, green shows the part of the brain that is infected with the virus. Another thing to note is that the green region in B corresponds with the outline of Area X in A. Image G is important image. Cells that express FoxP2 appear red. Those infected with the virus appear green. Note, that in the picture showing GFP expression, the density of infected cells is about the same for both shControl and shFoxP2. On the other hand, shFoxP2 has a much lower level of FoxP2 expression than shControl.
Now, to analyze the songs of the birds themselves. Zebra finch songs consist of different sound elements, called syllables. Syllables are interrupted by intervals of silence. Syllables are sung in a sequential order, which constitute a motif. During a song, a variable number of motifs are sung in short succession.
Images A-C show a moitif. Tutor syllables are indicated by black letters. Imprecisely copied pupil syllables are shown by red letters. Figure D analyzes the similarity of the bird songs quantitatively (shGFP can be thought of as another control), but a simple glimpse at the sonograms will show the drastic differences in the motifs sung by pupils injected with shControl and shFoxP2. Note, how similar the song of an shControl injected pupil is to that of an shFoxP2 injected pupil. According to computer analysis, the shControl pupil copied each syllable in the proper sequential order well enough for the computer to analyze and identify the syllable. In shFoxP2 injected pupils, syllables were out of order and some were too haphazard for the computer to identify as the same syllable sung by the tutor.
This picture is also quite revealing. Each vertical column shows a syllable. Syllables I, II, and III in A were imitated from the same tutor as the corresponding syllables in B. In each column, two vertical lines mark the beginning and the end of the longest rendition of each syllable. Note the variability in those infected with shFoxP2, especially in columns II and IV. shControl infected birds had relatively invariable duration times. Also note that the acoustic structure is far more variable again the birds infected with shFoxP2 than those infected with shControl. Image C shows the variability in birds infected with different viruses (once again, shGFP is another control), along with the tutor. It is interesting to see that birds injected with shControl have an almost indistinguishable variability in their song from the variability in the song of tutors. Image D analyzes syllable duration variation. Once again a similar phenomenon is observed. shFoxP2 birds have a much more variable duration of their syllables, whereas the control birds are almost indistinguishable from the tutors.
All in all, we can conclude from the results that the FoxP2 gene plays an important role in song imitation in zebra finches. It could be possible that inhibiting FoxP2 expression affects motor control of singing, however given that syllables with similar spectral feature could be learned or omitted by the same pupil this seems unlikely. It would be interesting to see whether "dysparxic song" is percieved as different by other finches and interferes with communication, as DVD does in humans. Considering that female songbirds prefer males with well-learned songs, this is what is to be expected. The fact that a reduction in FoxP2 affects the outcome of both song learning in birds and speech development in humans provides evidence that during evolution ancestral genes adapted in the human brain and gave rise to the capacity for language.
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