Ancestry of unisexual salamanders

Area of Science:

• Evolutionary Biology
• Genetics
• Herpetology

Background:

• Eastern North America harbors unisexual salamander populations with complex hybrid origins, involving nuclear genomes from multiple bisexual species.
• These hybrids exhibit varying ploidy levels (diploid to pentaploid) and known combinations, but their evolutionary origins remain largely uncharacterized.
• Reproduction in these all-female lineages necessitates sperm from bisexual species, though its incorporation into the egg varies.

Purpose of the Study:

• To investigate the evolutionary origins and maternal lineage of a complex hybrid unisexual salamander system.
• To determine the mitochondrial DNA (mtDNA) characteristics of these hybrids in relation to their nuclear genome composition and ploidy.

Main Methods:

• Sequencing of a portion of the cytochrome b gene from the mitochondrial DNA of diploid and triploid unisexual salamander hybrids.
• Analysis of mtDNA sequences to identify maternal lineages and compare them with those of sympatric bisexual species.

Main Results:

• Nearly all analyzed hybrids, irrespective of nuclear genome composition or ploidy, shared a highly similar mitochondrial genome sequence.
• This common mtDNA sequence was distinct from that of the four sympatric bisexual salamander species.
• The findings indicate a conserved, independent mitochondrial lineage within the hybrid complex.

Conclusions:

• The unisexual salamander hybrids maintain a distinct mitochondrial lineage that has evolved separately from their nuclear genomes.
• This conserved mitochondrial lineage represents the most ancient known unisexual vertebrate lineage, providing insights into their unique evolutionary history.
• The study highlights the independent evolutionary trajectory of mitochondrial genomes in hybrid vertebrates.