Sperm mtDNA Copy Number Is Not Associated With Midpiece Size Among Songbirds.

Area of Science:

• Evolutionary biology
• Animal reproduction
• Genetics

Background:

• Sperm morphology exhibits significant diversity across the animal kingdom, particularly within avian songbirds (infraorder Passerides).
• The sperm midpiece, crucial for motility, is formed by mitochondria, but the exact number of mitochondria per songbird sperm is largely unknown.
• Previous studies in zebra finches suggest a link between sperm midpiece length and mitochondrial DNA (mtDNA) copy number.

Purpose of the Study:

• To investigate the relationship between sperm midpiece length and mtDNA copy number in songbirds.
• To estimate the mtDNA copy number per spermatozoon across a diverse range of songbird species.

Main Methods:

• Sperm mtDNA copy number was quantified in 19 songbird species from 10 families using droplet digital PCR (ddPCR).
• Mitochondrial and nuclear DNA abundance were measured, and the ratio of mitochondrial to single-copy nuclear genes was used to estimate mtDNA copy number per sperm.
• Sperm midpiece lengths were measured to assess variation across species.

Main Results:

• A small variation in average mtDNA copy number was observed among the studied songbird species.
• The estimated mtDNA copy number per sperm was not significantly correlated with sperm midpiece length.
• Species exhibited differences in mtDNA copy number, but this variation was limited.

Conclusions:

• The hypothesis that longer sperm midpieces correlate with higher mtDNA copy numbers in songbirds was not supported.
• The findings suggest that mitochondrial genomes might be eliminated during spermatogenesis in songbirds.
• Further research is needed to understand the regulation of mitochondrial content during avian sperm development.