Positive selection drives faster-Z evolution in silkmoths

Area of Science:

• Evolutionary genetics
• Comparative genomics
• Sex chromosome evolution

Background:

• Genes on sex chromosomes (X or Z) evolve differently than autosomal genes due to hemizygosity in the heterogametic sex.
• The 'faster-X effect' can result from more efficient selection on recessive mutations or increased genetic drift due to reduced effective population size.
• Previous studies suggest faster-X in males is due to selection, while faster-Z in females is due to drift.

Purpose of the Study:

• To investigate the generality of the faster-Z effect observed in birds and snakes.
• To determine the evolutionary forces driving faster-Z evolution in the lepidopteran silkmoth, Bombyx huttoni.

Main Methods:

• Genome sequencing of the silkmoth Bombyx huttoni.
• Comparative genomic analysis to assess evolutionary rates on the Z chromosome.

Main Results:

• Silkmoths exhibit faster Z chromosome evolution.
• Unlike in birds and snakes, the faster-Z effect in silkmoths is attributed to more efficient positive selection, not increased drift.
• This indicates that female heterogamy alone does not fully explain the evolutionary dynamics of Z chromosomes in vertebrates.

Conclusions:

• Female heterogamy does not universally lead to faster-Z evolution driven by drift.
• Efficient positive selection can also drive faster-Z evolution, as seen in silkmoths.
• Multiple factors, including effective population size, influence Z chromosome evolution across taxa.