A putative RA-like region in the brain of the scale-backed antbird, Willisornis poecilinotus (Furnariides, Suboscines, Passeriformes, Thamnophilidae)

Abstract

The memorization and production of song in songbirds share important parallels with the process of speech acquisition in humans. In songbirds, these processes are dependent on a group of specialized telencephalic nuclei known as the song system: HVC (used as a proper name), RA (robust nucleus of arcopallium), LMAN (lateral magnocellular nucleus of the nidopallium) and striatal Area X. A recent study suggested that the arcopallium of the Sayornis phoebe, a non vocal learner suboscine species, contains a nucleus with some properties similar to those of songbird RA, suggesting that the song system may have been present in the last common ancestor of these groups. Here we report morphological and gene expression evidence that a region with some properties similar to RA is present in another suboscine, the Amazonian endemic Willisornis poecilinotus. Specifically, a discrete domain with a distinct Nissl staining pattern and that expresses the RA marker RGS4 was found in the arcopallium where the oscine RA is localized. Our findings, combined with the previous report on the S. phoebe, suggest that an arcopallial region with some RA-like properties was present in the ancestor of both Suboscines infraorders Tyranni and Furnarii, and is possibly an ancestral feature of Passeriformes.

Keywords: RGS4; in situ hybridization; song nuclei; vocal learning.

Figure 1. The song control system of the Pale-breasted Thrush, Turdus leucomelas. (A) Schematic representation of the song control system in an oscine brain, showing the song production pathway (dashed arrows) and the song learning pathway (arrows) (Adapted from Nottebohm, 2005). (B) Nissl stained brain section of T. leucomelas, the arcopallium is located within the dashed line box. (C) The archopallium is demarcated by white dashed line, with arrowheads indicating the location of the RA nucleus. Scale bars are 20 μm (B) and 5 μm (C).

Figure 2. An RA-like region in the brain of the antbird Wllisornis poecilinotus. (A) Nissl stained section of the W. poecilinotus brain, with the arcopallium region denoted by a dashed line box. (B) RA like nucleus characterized by the presence of large neuronal cells that are distinct from the surrounding cells. (C) Expression pattern of the song nuclei marker PVALB in a brain section of W. poecilinotus and (D) within the arcopallium. (E) Expression pattern of the song nuclei marker RGS4 in a brain section of W. poecilinotus and (F) within the arcopallium. Arrowheads indicate the location of the RA-like nucleus. Scale bars are 20 μm (A, C, E) and 5 μm (B, D, F).

Figure 3. Phylogenetic tree of the suborder suboscines. New world suboscines are divided into the infraorders Tyranni and Furnarii. Putative RA-like nuclei have been found in S. phoebe (Liu et al., 2013) and in Willisornis poecilinotus (this study).