How birds become male or female, and occasionally both

March 11, 2019 by Jenny Graves, The Conversation

The highly unusual "semi-identical" Australian twins reported last week are the result of a rare event. It's thought the brother and sister (who have identical genes from their mother but not their father) developed from an egg fertilised by two different sperm at the same moment.

In humans, it's the sperm that determines whether an embryo is pushed along a male or female development pathway. But in birds, it's the other way around. Eggs are the deciding factor in bird sex.

There are other fascinating aspects of bird sex that are not shared with humans. Female birds seem to have some capacity to control the sex of their chicks. And occasionally a bird that is female on one side and male on the other is produced – as in recent reports of this cardinal in the United States.

X and Y, Z and W chromosomes

So what is it about bird chromosomes that makes bird sex so different from human sex?

In humans, cells in females have two copies of a large, gene-rich chromosome called X. Male cells have one X, and a tiny Y chromosome.

Birds also have sex chromosomes, but they act in completely the opposite way. Male birds have two copies of a large, gene-rich chromosome called Z, and females have a single Z and a W chromosome. The tiny W chromosome is all that is left of an original Z, which degenerated over time, much like the human Y.

When cells in the bird ovary undergo the special kind of division (called "meiosis") that produces eggs with just one set of chromosomes, each egg cell receives either a Z or a W.

Read more at: 

Chromosome ends hold clues to a bird's longevity

The long and short of a bird’s life may be recorded in the tips of its chromosomes, a new study suggests.

A study of Seychelles warblers living on a small island in the Indian Ocean suggests that the length of telomeres — bits of DNA that cap chromosome ends — can predict a bird’s chance of dying better than its chronological age can. Warblers with shorter telomeres were less likely to survive another year, especially if the truncation happened rapidly, David S. Richardson, a molecular ecologist at the University of East Anglia in Norwich, England, and colleagues report online November 20 inMolecular Ecology.

The study “provides very important evidence that backs up what has been found in the laboratory — changes in telomere length matter a lot,” says animal ecologist Pat Monaghan of the University of Glasgow in Scotland.

Increasing age and body mass were also linked to shorter telomeres in the birds. That result stands in contrast to a recent large study of people in northern California that found telomeres get shorter with age, but that higher body mass is associated with longer telomeres (SN Online, 11/11/12).

Like shoestring aglets, telomeres stop chromosomes from unraveling or being eaten away at the ends. Cells with very short telomeres become decrepit or die, but it has been unclear whether that has any effect on whole body.

Continued:  http://www.sciencenews.org/view/generic/id/346616/description/Chromosome_ends_hold_clues_to_a_birds_longevity