Effect of incubation temperature on sex-dependent embryo mortality and morphological traits in Mallard.

Although birds have genetically determined sex, the sex ratio has been reported to deviate from parity in several studies. Temperature-dependent sex determination, which is common in reptiles, is absent in birds. However, females are able to adjust their investment into eggs according to the sex of the embryo, which may cause sex-specific embryonic mortality. Incubation temperature may also cause sex-biased embryonic mortality, and it may differentially affect the phenotype of male and female hatchlings. We aimed to investigate differences between male and female Mallard embryos regarding their egg size, mortality during incubation and hatchling phenotype in relation to incubation temperature. Mallard eggs were incubated under six constant incubation temperatures (ranging from 35.0 to 38.0 °C). Hatchlings were weighed, and their morphological traits were measured. We determined the sex of hatchlings and unhatched embryos by genetic analysis and found higher male embryonic mortality at 35.5 °C (44 males vs. 28 females) and a higher proportion of female hatchlings at 38 °C (24 males vs. 38 females); however, these results were not statistically significant. Our results suggest that Mallard females do not differentiate quantitatively between sexes during egg production. Male hatchlings were significantly larger but not heavier than females. The size difference between sexes was most pronounced at temperatures around 36 °C, which is the mean temperature of naturally incubated Mallard eggs.

Developmental basis of phallus reduction during bird evolution.

BACKGROUND: One of the most puzzling events in evolution is the reduction and loss of the phallus in birds. All birds reproduce by internal fertilization, but only ∼3% of birds have retained a phallus capable of intromission. A number of hypotheses have been proposed for the evolutionary mechanisms that drove phallus reduction; however, the underlying developmental mechanisms are unknown. RESULTS: We investigated genital development in two sister clades of birds, Galliformes (land fowl), most of which lack an intromittent phallus, and Anseriformes (waterfowl), which have well developed phalluses; and in two outgroups, Paleognathae (emus) and Crocodilia (alligators). Galliform embryos undergo cryptic development of a genital tubercle, the precursor of the phallus, but this later undergoes apoptosis, leading to regression of the tubercle. At the molecular level, a derived pattern of Bmp4 expression was identified in chick (a galliform) genital tubercles. Inhibition of Bmp signaling in chick genitalia rescues cells from apoptosis and prevents phallus regression, whereas activation of Bmp signaling in duck (an anseriform) genitalia induces a galliform-like pattern of apoptosis. Thus, distal Bmp activity is necessary and sufficient to induce apoptosis in Galloanserae genital tubercles. CONCLUSIONS: Our results indicate that evolutionary reduction of the intromittent phallus in galliform birds occurred not by disruption of outgrowth signals but by de novo activation of cell death by Bmp4 in the genital tubercle. These findings, together with discoveries implicating Bmps in evolution of beak shape, feathers, and toothlessness, suggest that modulation of Bmp gene regulation played a major role in the evolution of avian morphology.

Genital evolution: cock-a-doodle-don't.

Losing the penis in species with internal fertilization may seem paradoxical, but birds have managed to do it multiple times. A new study addresses one developmental mechanism responsible for penis reduction in birds, and opens the door to further examination of this little understood evolutionary phenomenon.

Developmental basis for telencephalon expansion in waterfowl: enlargement prior to neurogenesis.

Some altricial and some precocial species of birds have evolved enlarged telencephalons compared with other birds. Previous work has shown that finches and parakeets, two species that hatch in an immature (i.e. altricial) state, enlarged their telencephalon by delaying telencephalic neurogenesis. To determine whether species that hatch in a relatively mature (i.e. precocial) state also enlarged their telencephalon by delaying telencephalic neurogenesis, we examined brain development in geese, ducks, turkeys and chickens, which are all precocial. Whereas the telencephalon occupies less than 55 per cent of the brain in chickens and turkeys, it occupies more than 65 per cent in ducks and geese. To determine how these species differences in adult brain region proportions arise during development, we examined brain maturation (i.e. neurogenesis timing) and estimated telencephalon, tectum and medulla volumes from serial Nissl-stained sections in the four species. We found that incubation time predicts the timing of neurogenesis in all major brain regions and that the telencephalon is proportionally larger in ducks and geese before telencephalic neurogenesis begins. These findings demonstrate that the expansion of the telencephalon in ducks and geese is achieved by altering development prior to neurogenesis onset. Thus, precocial and altricial species evolved different developmental strategies to expand their telencephalon.