Ornithology Ex2 P3

Why do birds have small genomes?

The avian genome is small and conserved (not very variable in size).

• Birds’ coding regions are as complex as other vertebrates but have fewer repetitive “parasitic” DNA elements. Much less junk DNA.

• This compact genome makes whole-genome sequencing easier.

What explains the variation in genome size between bird species?

• Powered flight requires lightweight, efficient cells for high metabolism and oxygen use.

• Birds with more intense flight (higher metabolism) evolved smaller genomes, since smaller nuclei → smaller cells → faster oxygen diffusion.

• Within birds, genome size BEST correlates with heart index (size of heart relative to body) and body mass — both linked to flight demands.

• This supports the metabolic rate hypothesis:

  • High-energy activities (like flight) select for smaller, more efficient cells, which in turn require smaller genomes.

What are genomic “islands,” and how do they affect speciation?

• Genomic islands of divergence are regions in the genome that show high genetic difference between closely related species.

• They often contain genes involved in mate recognition, behavior, or adaptation to different environments.

• Because natural selection strongly favors these differences, these genes resist gene flow (mixing) between populations.

• Over time, this helps reinforce reproductive isolation—meaning species become less able or willing to interbreed (due to mismatched traits, preferences, or reduced hybrid fitness)—which strengthens their separation and promotes speciation.

(islands = regions that resist blending → drive speciation.)

Do birds have many or few chromosomes, and why?

• Birds have many chromosomes: a mix of microchromosomes and macrochromosomes.

• Resulted from fission of ancestral chromosomes (not fusion like in most vertebrates).

• Birds show high synteny (conserved gene order across species).

• Small chromosomes → higher recombination rates → greater genetic diversity and faster evolution.

What are germline-restricted chromosomes (GRCs)?

• Tiny chromosomes found only in germline cells (eggs and sperm).

• Removed during development from somatic cells.

• Function unclear — possibly developmental roles or reducing pleiotropy (unwanted gene effects).

• May make cells more efficient.

How do avian sex chromosomes compare to mammalian ones?

• Bird Z and W chromosomes evolved independently from mammalian X and Y — they share no genes.

• The Z chromosome in birds carries many genes and is comparable in function to the mammalian X.

• The bird Z maps to human chromosomes 5, 9, and 18, meaning those human chromosomes contain genes that were once part of the same ancestral set as bird Z — not that they’re literally shared now.

• The W chromosome (found only in females) is much smaller. It doesn’t recombine with Z, so:

  • Mutations accumulate over time.

  • Deleted genes cannot be replaced because there’s no recombination with a partner chromosome.

  • This leads to gene loss and shrinkage of the W chromosome.

Which sex is homogametic vs. heterogametic in birds?

• Females: heterogametic (ZW)

• Males: homogametic (ZZ)

Features of avian Z and W chromosomes (vs. mammals)

• Dosage compensation: keeps gene expression balanced between sexes.

  • In mammals, females silence one X entirely (X-inactivation).

  • In birds, males (ZZ) have two Z copies, females (ZW) have one — so males reduce (“downregulate”) expression from each Z-linked gene slightly to match females’ single Z.

• Downregulate Z-linked genes = lower the activity of those genes so males don’t make double the protein amount.

• The W chromosome is small, non-recombining, and accumulates harmful mutations.

• The Z chromosome is large and carries most sex-linked genes controlling sex-related traits.