Testicular melanization has evolved in birds with high mtDNA mutation rates

Area of Science:

• Evolutionary Biology
• Biochemistry
• Animal Physiology

Background:

• Melanin, a pigment, is primarily in animal skin but also found in other tissues.
• Testicular melanin's presence across vertebrates is noted, yet its function and origin are unclear.
• Oxidative stress from mitochondrial DNA mutations can impair sperm viability.

Purpose of the Study:

• To investigate the hypothesis that testicular melanization is an adaptive response to oxidative stress.
• To determine if natural selection favors testicular melanin in species with high mitochondrial mutation rates.
• To explore the link between testicular melanin and sperm protection against mitochondrial DNA damage.

Main Methods:

• Examined the occurrence of testicular melanization in 134 bird species.
• Assessed mutation accumulation using cytochrome b mitochondrial DNA substitution rates.
• Controlled for confounding factors like body mass, reproductive activity, and evolutionary relationships (phylogeny).

Main Results:

• Testicular melanization was significantly associated with higher rates of accumulated mitochondrial mutations.
• Melanization of testes was more prevalent during the breeding season.
• No correlation was found between plumage melanin levels and testicular melanization.

Conclusions:

• Testicular melanization appears to be an adaptive evolutionary response to high mitochondrial mutation rates.
• Melanin in testes likely confers protection against oxidative stress, safeguarding sperm viability.
• This finding suggests melanization patterns can be influenced by historical mutation rates, independent of external coloration.