Cell death in the avian brain with emphasis on the development and plasticity of the song control system

A daily rhythm of cell proliferation in a songbird brain

Neurogenesis is an active process of creating new neurons in the neurogenic zone. It is influenced by many factors, including the circadian system, which is synchronized by light. Neurogenesis in laboratory rodents peaks at night, and the rodents are nocturnal, contrary to humans that are active during the day. Here, we studied whether proliferation and apoptosis exhibit a daily rhythm in the brain of the diurnal songbird zebra finch (Taeniopygia guttata) and whether the cell proliferation peaks during the dark phase of the day, as in rodents. We injected the birds with the cell proliferation marker 5-ethynyl-2´-deoxyuridine (EdU; thymidine analog), quantified the number of dividing cells in the neurogenic ventricular zone (VZ), and measured mRNA expression of clock genes as well as genes indicating cell proliferation or apoptosis. First, we confirmed the daily rhythms of the clock genes. Next we found that proliferation along the whole VZ did not exhibit a daily rhythm. However, proliferation in the central ventral part of the VZ, i.e. "the hot-spot" area, showed a daily rhythm of proliferation. The highest number of newborn cells was detected in the dark phase of the day. The relative expression of the apoptotic genes caspase 3, Bcl-2, and Bax as well as the proliferating cell nuclear antigen (PCNA) did not show any rhythm. In summary, our results show that cell proliferation in the "hot-spot" region of the VZ in diurnal songbirds shows rhythmic activity over a period of 24 h and that the maximum cell proliferation occurs in the passive phase. This study may have implications for understanding the mechanisms underlying the daily regulation of brain cell proliferation in different species.

Differences in vocal brain areas and astrocytes between the house wren and the rufous-tailed hummingbird

The house wren shows complex song, and the rufous-tailed hummingbird has a simple song. The location of vocal brain areas supports the song's complexity; however, these still need to be studied. The astrocytic population in songbirds appears to be associated with change in vocal control nuclei; however, astrocytic distribution and morphology have not been described in these species. Consequently, we compared the distribution and volume of the vocal brain areas: HVC, RA, Area X, and LMAN, cell density, and the morphology of astrocytes in the house wren and the rufous-tailed hummingbird. Individuals of the two species were collected, and their brains were analyzed using serial Nissl- NeuN- and MAP2-stained tissue scanner imaging, followed by 3D reconstructions of the vocal areas; and GFAP and S100β astrocytes were analyzed in both species. We found that vocal areas were located close to the cerebral midline in the house wren and a more lateralized position in the rufous-tailed hummingbird. The LMAN occupied a larger volume in the rufous-tailed hummingbird, while the RA and HVC were larger in the house wren. While Area X showed higher cell density in the house wren than the rufous-tailed hummingbird, the LMAN showed a higher density in the rufous-tailed hummingbird. In the house wren, GFAP astrocytes in the same bregma where the vocal areas were located were observed at the laminar edge of the pallium (LEP) and in the vascular region, as well as in vocal motor relay regions in the pallidum and mesencephalon. In contrast, GFAP astrocytes were found in LEP, but not in the pallidum and mesencephalon in hummingbirds. Finally, when comparing GFAP astrocytes in the LEP region of both species, house wren astrocytes exhibited significantly more complex morphology than those of the rufous-tailed hummingbird. These findings suggest a difference in the location and cellular density of vocal circuits, as well as morphology of GFAP astrocytes between the house wren and the rufous-tailed hummingbird.

Sensitive periods during the development and expression of vertebrate sexual signals: A systematic review

Many sexually selected traits exhibit phenotypic plasticity. Despite a growing appreciation for the ecological context in which sexual selection occurs, and for the role of plasticity in shaping traits associated with local adaptation and divergence, there is an important gap in knowledge about the onset and duration of plasticity in sexual trait expression. Integrating this temporal dimension of plasticity into models of sexual selection informs our understanding of the information conveyed by sexual traits and our predictions related to trait evolution, and is critical in this time of unprecedented and rapid environmental change. We conducted a systematic review of 869 studies to ask how trait modalities (e.g., visual and chemical) relate to the onset and duration of plasticity in vertebrate sexual signals. We show that this literature is dominated by studies of coloration in birds and fish, and most studies take place during the breeding season. Where possible, we integrate results across studies to link physiology of specific trait modalities with the life stage (e.g., juvenile, breeding, or nonbreeding) during which plasticity occurs in well-studied traits. Limitations of our review included a lack of replication in our dataset, which precluded formal analysis. We argue that the timing of trait plasticity, in addition to environmental context, is critical for determining whether and how various communication signals are associated with ecological context, because plasticity may be ongoing or occur at only one point in an individual's lifetime, and determining a fixed trajectory of trait expression. We advocate for careful consideration of the onset and duration of plasticity when analyzing how environmental variation affects sexual trait expression and associated evolutionary outcomes.